Nanostructured Graphene Oxide-Based Hybrids as Anodes for Lithium-Ion Batteries

Abstract

Presently, the negative electrodes of lithium-ion batteries (LIBs) are constituted by carbon-based materials, which exhibit a limited specific capacity 372 mAh g⁻¹ associated with the cycle in the composition between C and LiC₆. Therefore, many efforts are currently made towards the technological development of nanostructured graphene materials because of their extraordinary mechanical, electrical, and electrochemical properties. Recent progress on advanced hybrids based on graphene oxide (GO) and reduced graphene oxide (rGO) has demonstrated the synergistic effects between graphene and an electroactive material (silicon, germanium, metal oxides (MO_x)) as electrode for electrochemical devices. In this review, attention is focused on advanced materials based on GO and rGO and their composites used as anode materials for lithium-ion batteries.

1. Introduction

Recently, researchers’ quest for advanced devices for storing and converting sustainable energy is increasing day by day due to the demand for renewable and clean energy resources. Among all power sources, lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and electrochemical supercapacitors (ESCs) have become the most important and efficient devices for storing the electricity produced from solar cells and windmills, and they are currently in use to solve the problem of intermittency of these renewable energy sources for their integration in grid-scale stationary storage [1,2]. Anode materials for rechargeable batteries can be classified in three main groups depending on the mechanism of lithiation [3,4,5,6,7,8,9]:

(i)

Alloying materials such as Si, Ge, Sn, Al, Bi, etc.,

(ii)

Conversion materials like transition-metal oxides (Mn_xO_y, NiO, Fe_xO_y, CuO, Cu₂O, MoO₂, etc.), metal sulfides, metal phosphides, and metal nitrides (M_xX_y; here X = S, P, N),

(iii)

Insertion materials, such as carbonaceous compounds (graphite, porous carbon, carbon nanotubes, graphene), TiO₂, Li₄Ti₅O₁₂, etc.

Presently, the negative electrodes of lithium-ion batteries (LIBs) are constituted by graphite-based materials, which exhibit a limited specific capacity of 372 mAh g⁻¹ associated with the cycle in the composition between C and LiC₆. As promising anode materials for LIBs, silicon and transition-metal oxides have many advantages over graphite currently in use, such as very high capacity, wide availability, good stability, and environmental friendliness. However, certain obstacles prevent the use of these materials, namely low electronic/ionic conductivity, large volume variations, poor cycling performance, and low faradaic efficiency in the first cycle. To solve these issues, many strategies have been proposed such as the fabrication of composite electrode including a carbon-type compound like graphene and its derivatives [10,11].

Since the first experimental investigation in 2004, mono-layer graphite, known as graphene [12], has attracted considerable interest to replace graphite due to its outstanding properties. Graphene has an exceptional chemical architecture, i.e., a two-dimensional (2D) monolayer sp² hybridized conjugated carbon atoms with a tight packing of honeycomb lattice and displays remarkable electronic, thermal, optical, and mechanical properties [13]. It is a material that has attained tremendous attention due to its large surface area of 2630 m² g⁻¹, giant electrical conductivity of 6000 S cm⁻¹, and high tensile strength of 130 GPa [14]. A large number of materials have been combined with graphene, i.e., metals, semiconductors, metal oxides, polymers, alloys, etc., and applied in energy storage devices, such as lithium-based batteries, especially lithium-ion (LIBs), lithium sulfur (Li-S), and lithium-air (Li-O₂) (for a review, see Ref. [15]). However, the 2D-sheets of graphene tend restacking and irreversible aggregation due to the strong p–p interaction, hydrophobic interaction, and van der Waals force, which induce small specific surface area and poor electrochemical performance (i.e., low Li-ion diffusion rate) [16]. These issues can be overcome by the elaboration of graphene oxide (GO) and reduced graphene oxide (rGO).

Why use reduced graphene oxide in batteries instead of just graphene? Graphene is hydrophobic and difficult to handle in chemical processes, while graphene oxide has superior hydrophylic behavior; thus, graphene oxide is easy to deposit on any surface with any methods. Due to the simple and fast reduction of GO (which results in an increase of the conductivity desirable in batteries), this suggests the possibility of producing low-cost graphene battery by using cost effective rGO. Moreover, rGO can be easily fabricated in terms of mass production [17].

Therefore, using GO or rGO, many efforts are currently being made towards the technological development of new nanostructured hybrid anode materials for LIBs, which exhibit better cycling stability than graphite. We guide the reader to older review papers on graphene-based hybrids as anode materials for prior works [11,18,19]. For a historical overview related to GO and rGO synthesis and their electrochemical applications, see Refs. [20,21].

Transition-metal oxides (MO_x) have been widely studied as elements of composite electrode due to their different oxidation states and heterogeneous electrochemical activity. The combination of a nanostructured MO_x with a graphene network has met the ever-growing requirements for energy storage applications. In such hybrids, the graphene component plays several roles including stabilization of nanosized particles, inhibition of the unwanted volumetric changes, increase of the electrical conductivity and enhancement of the mechanical strength, both are improving cyclability and rate capability. Anode hybrids have been extensively investigated including various MO_x such as MnO [22], Mn₃O₄ [23], SnO₂ [24,25], MoO₂ [26], Co₃O₄ [27], Fe₃O₄ [28], and Fe₂O₃ [29]. Single elements embedded in carbonaceous matrix have been also actively investigated such silicon [30], germanium [31,32], and tin [33]. For the works related to graphene/metal oxide composites before 2012, we just guide the reader to the review article by Wu et al. [34]. The synthesis of graphene–transition metal oxide hybrid nanoparticles and their application in various fields have been reviewed in 2017 by Jana et al. [35].

In this review, we present the recent five-year developments of graphene-supported nanocomposites for advanced energy storage devices. The electrochemical performances of several rGO-based composites are discussed in relation with their methods of preparation and morphology. Graphene-supported materials are used not only as components of electrodes for batteries, but also for supercapacitors as well. However, recent reviews have been already devoted to such electrodes for supercapacitors [36,37], so that attention in the present work is focused on anode materials for lithium-ion batteries. The paper is organized as follows. A brief introduction to graphene oxide materials is given in Section 2. In Section 3, we examine the synthesis and properties of composites of graphene oxide and reduced graphene oxide with a single element, such as silicon, germanium, or tin, the technology of which is currently developed to increase the delivered specific energy, the rate capability, and the cycle life. Similar issues are treated in Section 4 for composites with metal oxides. For each system, we highlight the synthesis, morphology (particle size, porosity, specific surface area), and electrochemical parameters (initial capacity, initial coulombic efficiency (ICE), long-term cycling, and rate capability).

2. Graphene Oxide Materials

2.1. Properties of Graphene Oxides

Graphite oxide is derived from graphite by introducing a range of oxygen-containing functional groups including carboxylic acid, phenol, hydroxyl, and epoxide [38]. These functional groups attach themselves to carbon atoms in basal planes resulting in widening of the interlayer spacing. This material with an ideal chemical formula of C₈O₂(OH)₂ has been investigated since the 1850s by the English chemist Brodie [39]. It is an insulator with a large portion of sp³ hybridized carbon atoms, which results in a sheet resistance of ~10¹² W sq⁻¹ [40] depending on the oxygen content. GO is generally prepared from natural graphite via a modified Hummers’ method [41]. The entire process is achieved within a few hours at temperature T < 45 °C. Typically, 100 gr powdered flake graphite and 50 g NaNO₃ into 2.3 L H₂SO₄ are mixed at 0 °C in an ice-bath to form a suspension and then vigorously agitated with 300 g KMnO₄. The exothermic reaction was controlled to prevent the suspension temperature lower than 20 °C. Then, the obtained brownish grey paste is diluted in water and treated with 3% hydrogen peroxide to reduce the residual KMnO₄. Finally, the yellowish-brown cake is washed in warm water and the dry from of GO is obtained by centrifugation and dehydration at 40 °C.

Reduced graphene oxide (rGO) is a layered, two-dimensional sheet of sp²-hybridized carbon with ultrahigh specific surface area and excellent electrical conductivity [42,43]. In contrast to GO, rGO transforms into semiconductor or even into graphene-like semimetal [20]. The conductivity of rGO can reach ~10³ S m⁻¹ [44]. Rao et al. [45] investigated the effect of reducing agents on the electrical resistivity: The average resistivity for GO samples treated with hydroiodic acid appear to be in the range of 0.003–0.022 Ω·cm, followed by those treated with hydrobromic acid (0.056–0.347 Ω·cm), and finally with hydrochloric acid (0.33–0.641 Ω·cm). Wang et al. [46] found that the conductivity of rGO could be tuned in the range 2–15 S m⁻¹ by changing the synthesis alkaline solution (8 ≤ pH ≤ 14). Tokarczyk et al. [47] reported that rGO samples reduced by combined thermal and pressure treatment exhibit values of surface resistivity of 1–128 W sq⁻¹. Marquez et al. [48] measured the sheet resistance of 226 W sq⁻¹ for rGO prepared by laser-assisted photothermal reduction process. The properties of rGO depend on the electronic band gap, which depends on the sp² cluster size. They are controlled by performing reversible functionalization of rGO with oxygen species. The reversible functionalization of rGO results in its partial transformation to GO such that the size of sp² clusters within sp³ matrix varies, thereby affecting the π-π band structure [49].

Three-dimensional (3D) rGO can be easily synthesized by chemical reduction of GO by using reducing agents such as hydrazine hydrate [50,51], sodium borohydride [52], ethylene glycol [53], ascorbic acid [54], and metal powders [55], which endows them with unique properties. rGO was also prepared by reduction of GO using mild green agents, i.e., extract of fruits or vegetables containing plant polyphenols [56]. During the reduction reaction, the structure of graphene (p-conjugated structure) is substantially restored by eliminating oxygen-containing groups in GO. Birch’s reaction, which entails the use of ammonia, alkali metal, and an alcohol is another efficient method to reduce GO [57]. As compared with pure graphene, many advantages endow graphene oxide (GO) and reduced graphene oxide (rGO) as a preferred conductive agent in LIBs, such as lower cost, facile preparation processes, large-scale production, and easy functionalization [58,59,60]. The 2D-layered structures of GO and rGO sheets anchored with metal oxide nanoparticles (NPs) are used to facilitate the electron transport and Li⁺ diffusion in negative electrodes of LIBs [61,62]. rGO proved to be a highly versatile conductive and structure stabilizing additive in many hybrids used in LIBs. It is believed that rGO plays an essential role to enhance the electrochemical performance of composite electrodes by serving as the electrical conductive medium for the electron transfer process, providing a synergistic effect between active particles, alleviating the agglomeration of dispersed nanocrystals during the lithiation/delithiation process, and facilitating the reaction by generating well-mixed Li₂S in rGO matrix [3,9,23,63,64]. In addition, in the particular case of lithium-sulfur batteries, which will not be considered in this review, it facilitates the reaction by generating well-mixed Li₂S in rGO matrix [65].

In September 2020, the price of GO in the form of aqueous acidic paste (C/O atomic ratio 2.5–2.6) is 1600 US$ per kg, while 50 g of rGO (fully reduced) in a volume of 4–5 L costs 1200 US$ [66].

To evidence the reduction of GO to rGO, the as-synthesized samples are currently characterized by thermogravimetry analysis (TGA), X-ray diffraction (XRD), and Raman spectroscopy. Figure 1a displays the TGA profiles of GO prepared via the Hummers’ method and rGO obtained by hydrothermal process. The initial weight loss of GO from 100 °C to 200 °C was attributed to the decomposition of residual oxygen-containing functional groups. On another hand, the rGO phase is stable up to 350 °C, which indicates that the main oxygenated functional groups were removed from the network by the reduction process during the synthesis. Above 200 °C, the loss results from the decomposition of carboxyl, hydroxyl, and epoxy groups at the edge and basal planes. Figure 1b presents the typical XRD patterns, in which the reflections from the stacked atomic planes of GO and rGO appear at the 2θ angle of ~12.5° and 24.0°, respectively, corresponding to the interlayer spacing of 7.08 Å for GO and 3.71 Å for rGO. Figure 1c shows the typical Raman spectra of GO and rGO samples, showing characteristic D band (at ca. 1348 cm⁻¹) and G band (at ca. 1588 cm⁻¹). The peak intensity ratio I_D/I_G increases upon reduction, which reveals the removal of oxygenated functional groups attached on carbon backbones. The I_D/I_G ratio in Raman spectroscopy can be used to evaluate the distance between defects in graphene, which is about 1 for pristine graphene and increases with the decrease in average size of the sp² domain and average crystalline size (L_a): L_a = (2.4 × 10⁻¹⁰) λ⁴ (I_D/I_G)⁻¹, where λ is the laser wavelength in nm [67,68]. In the present case, the calculated average crystalline sizes of GO and RGO are 22 and 13 nm, respectively, indicating the decrease in average crystalline size of RGO compared with GO.

2.2. Lithiation of rGO

The lithiation mechanism and lithium storage capacity of rGO have been the subject of numerous investigations by means of first-principle calculations and by experimental studies such as galvanostatic charge/discharge (GCD) measurements, cyclic voltammetry (CV), and differential capacity curves [69,70,71,72,73,74,75,76,77]. The theoretical capacity of graphene is 744 mAh g⁻¹ if Li can be absorbed on both sides up to the chemical formula Li₂C₆, and 1116 mAh g⁻¹ if Li can be trapped at the benzene rings up to LiC₂, but these targets have not been achieved by using pure graphene materials [78]. Pan et al. [72] have investigated the mechanism of the lithiation of the rGO anode. The GCD profiles show a first cycle lithiation peak at 800 mV and a secondary lithiation event beginning around 250 mV and intensifying down to the cutoff voltage of 10 mV. The first cycle charge capacity at a rate of C/20 was nearly 1200 mAh g⁻¹ and then dropped steadily to 350 mAh g⁻¹, where it stabilized after 20 cycles. The rate capability leads the capacity of ~200 mAh g⁻¹ at a rate of C/5 and ~100 mAh g⁻¹ at a rate of 1C. Wang et al. [79] observed a first cycle capacity of 700 mAh g⁻¹ with an immediate decrease to 100 mAh g⁻¹ in the second cycle at a rate of C/7.5. The high irreversible first cycle capacity is due to the electrochemical reactions contributing to solid-electrolyte interphase (SEI) layer formation. It is also well known that the capacity of rGO anodes is sensitive to the preparation method. Solar reduced graphene oxide (SrGO) proved to be an excellent anode material in Li-ion batteries. The SrGO material was obtained by a thermal reduction and exfoliation of GO spread over Petri disk and sun-lighted [80]. This material exhibits first cycle discharge and charge capacities as high as 1480 and 880 mAh g⁻¹, respectively. Moreover, the coulombic efficiency (CE) was found to be >95% and the specific capacity after 60 cycles was >500 mAh g⁻¹ [81].

2.3. rGO-Based Composites

Currently, the typical process to fabricate a MO_x/rGO composite consists in the preparation of GO by the modified Hummers’ method and then attach/grow MO_x nanoparticles (NPs) on the GO, followed by a reduction process to transform GO to rGO [82,83]. Various strategies for the fabrication of nanostructured composites have been proposed, including mixed model [84], encapsulated model [85], wrapped model [86], anchored model [87], sandwich-like model [88], and layer model [34]. Among them, the sandwich-like model appears to be a smart choice to take combinations of both the electrochemically active material and the rGO matrix in an ordered arrangement. Figure 2 shows the different structural models of MOx-based composites [34].

The theoretical specific capacity, C_theor, of the hypothetical metal oxide/reduced graphene oxide mixture (MO_x/rGO) can be calculated through the equation:

C_theor = C_MOx × m_MOx + C_rGO × m_rGO,

(1)

where C_MOx and m_MOx are the theoretical specific capacity and mass percentage of metal oxide (wt%), respectively, and C_rGO and m_rGO are the theoretical specific capacity and mass percentage of rGO, respectively. The value of C_rGO in the potential range of 1–3 V (744 mAh g⁻¹) is used to estimate the theoretical capacity of these composites [89].

3. Single Element/GO or rGO Composites

3.1. Silicon

Among all existing anode materials, silicon has the high theoretical specific capacity of 4200 mAh g⁻¹, which corresponds to 4.4Li⁺ per Si atom to form Li_4.4Si alloy. The huge volume change (~400%) in the lithiation/delithiation process results in loss of electrical contact of Si powders and structural cracking, implying poor cycle ability of Si anodes. To overcome this problem, the best anodes consist in Si nanoparticles dispersed in a carbon matrix, referred to as Si/C anodes. Nano-size is needed to alleviate the volume change during cycling, and the dispersion of carbon is needed to maintain electrical contact [90,91,92,93,94,95]. Since the first preparation of Si/graphene hybrid by Chou et al. in 2010 [96] constant progress in the preparation of Si/C electrodes for LIBs has been obtained through the years, even though some drawbacks still exist, such as low first coulombic efficiency [97].

As mentioned in the introduction, this review is focused on rGO-supported anodes, which are closer to commercial use than graphene-supported anodes. However, it may be useful to mention few results obtained this year on Si/graphene anodes, which illustrate the best results observed with such a composite, for comparison with the results obtained with Si/rGO anodes detailed hereunder. Jamaluddin at al. reported a Si@graphene composite with a ball-like structure [98]. The graphene sheets were obtained by an electrochemical exfoliation method from natural graphite. The Si@graphene composite was then obtained by a spray drying process, with a void space in the core-shelled ball structure inside the site. The void space helps in the accommodation of the volume change during cycling. 2882.3 mAh g⁻¹ and an initial coulombic efficiency of 86.9% at 0.2 A g⁻¹ was observed. At 0.5 A g⁻¹, the capacity maintained at 1063 mAh g⁻¹ after 100 cycles. Si/multilayer graphene (thickness controlled to 2 nm) delivered a capacity of 990 mAh g⁻¹ at 1 A g⁻¹ after 500 cycles. At 4 A g⁻¹, the capacity was 1164.5 mAh g⁻¹ [99]. This result was attributed to the formation of a silicon oxycarbide interlayer formed by a rapid cooling during the synthesis process of the composite. Both graphene and GO can also be used in the same anode in combination with Si. Spherical graphite/silicon/graphene oxides/carbon (Gr/Si/GO/C) composite was prepared by Huang et al. by electrostatic self-assembly and spray drying process [100]. This composite delivered a capacity of 1212 mAh g⁻¹ at 200 mA g⁻¹ with a capacity retention rate of 81.7% after 100 cycles.

Si@rGO composites are considered as promising anodes for the next generation of LIBs. It was shown that the particle morphology, specific surface area, and architecture of hybrids are the key issues optimizing the electrochemical performance. For example, Kim et al. have recently prepared a 2D multi-layered Si/rGO hybrid anode by direct growth of Si intercalated into a porous multi-layered rGO film, in which the porous rGO network acts as a cushion against the expansion of the Si layer during lithiation [101]. The templated self-assembly (TSA) strategy seems to be another efficient way for the preparation of Si/rGO composite [102,103,104,105]. In a typical TSA method, the self-assembled rGO/Si solution was acquired by mild stirring for 2 h and dialysis in absolute ethanol several times and then transferred to a hydrothermal reactor for reduction at 80 °C for 3 h by sodium borohydride [105].

In 2012, Chockla et al. [32] investigated the lithiation/delithiation process in binder-free rGO-supported Ge and Si nanowires (NWs) as negative electrode materials in LIBs. In the Si/Ge NWs/rGO composite, there is a competition between rGO and Si/Ge lithiation. At a high cycling rate (>0.1 C) the rGO lithiation dominates, while at low cycle rates (<0.1 C) and high Si/Ge loading only Si and Ge are lithiated. The Si NWs/rGO electrode was stabilized to a small extent but the specific capacity was low and dropped to less than 100 mAh g⁻¹ for when the rate was increased from C/20 to C/10. Wang et al. [88] developed a novel type of self-supporting binder free silicon-based negative electrode through the encapsulation of silicon nanowires (SiNWs) with dual adaptable apparels (overlapped graphene sheaths and rGO overcoats). The resulting SiNWs@G/rGO composite gave high reversible capacity of 1600 mAh g⁻¹ at 2.1 A g⁻¹ current rate and 80% capacity retention over 100 cycles. A rate capability of 500 mAh g⁻¹ at 8.4 A g⁻¹ (on the basis of the total electrode weight) was demonstrated. Two advantages of such an architecture were discussed. Firstly, sealed and adaptable coated graphene sheets avoid the direct contact of encapsulated silicon and electrolyte, enabling the structural and interfacial stabilization. Secondly, the flexible and conductive rGO network controls the pulverization of the electrode and provides the conductive homogeneity to the composite. It is generally accepted that the creation of defects in the graphitic planes of graphene by replacing the carbon atom with heteroatoms increases its electrochemical performance. [106]. Chang et al. [107] prepared a Si/rGO hybrid composite anode with over 20 layers with excellent electrochemical performance at 3C rate. A control sample of Si/rGO composite was prepared using pristine Si nanoparticles and GO (weight ratio of Si to GO = 3:2) by self-assembly process mediated by electrostatic attraction [103]. The Si/rGO composite containing 64.6 wt% Si exhibits a capacity of 512 mAh g⁻¹ after 40 cycles at current density of 200 mA g⁻¹ between voltage limits of 0.05–1.2 V vs. Li⁺/Li. In 2015, Chang et al. fabricated an anode constituted by 3D carbon-coated Si/rGO nanostructure anchored by nickel foam with carbon nanotubes using Si NPs and GO sheets in poly(methyl methacrylate solution [87]. The carbon-coated Si NPs/rGO hybrid displays a high reversible capacity up to 2700 mAh g⁻¹ at a current density of 0.05 C (130 mA g⁻¹) and 70% capacity retention (up to 1311 mAh g⁻¹) at 2.6 A g⁻¹ current rate after 900 cycles. Lin et al. [108] developed a Si@carbon/rGO composite with the assistance of polyaniline (PANI), which can glue to combine Si nanoparticles with GO and can be pyrolyzed as carbon coating layer. The Li storage performance of this assembled hybrid, which delivers a reversible capacity of 1121 mAh g⁻¹ at a current density of 0.9 A g⁻¹ over 230 cycles with CE of 81.1%, comparing well with that of the Si@carbon anode (495 mAh g⁻¹ at 0.3 A g⁻¹ after 50 cycles). A CNT/rGO/Si nanoparticle composite anode showing high rate and long cycle life was prepared by a layer-by-layer assembly process. In this structure, rGO completely wrapped the entire Si/CNT networks by different layers and CNT networks provide fast electron transport pathways. The CNT/rGO/Si-NPs composite exhibits high performances with long cycle life; it delivers a specific capacity of 455 mAh g⁻¹ at 15 A g⁻¹ after 2000 cycles, high specific charge capacity of 2250 and 650 mAh g⁻¹ at 0.2 and 15 A g⁻¹ current rate, respectively, and fast charge/discharge rates up to 16 A g⁻¹ [109].

Recently, a multi-layered structure obtained by tuning the interface chemistries of GO and siloxene sheets as anode delivered 3880 mAh g⁻¹ at 205 mA g⁻¹, and still 1040 mAh g⁻¹ after 1000 cycles at 4.1 A g⁻¹ [110]. The fabrication of Si@SnS₂/rGO composite anodes for high-capacity Li-ion batteries was achieved by Dai et al. [111]. The composite is formed by Si nanoparticles (30 nm in size) anchored on a 3D rGO network and mixed with SnS_2, which provides space-constraining effects to accommodate volume expansion and particle aggregation. The Si@SnS₂/rGO composite presents a high specific capacity of 1480 mAh g⁻¹ after 200 cycles at a current density of 0.2 A g⁻¹ and a high stability at rates of 0.2–3.0 A g⁻¹. A stable specific capacity of 425 mAh g⁻¹ was achieved after 600 cycles at a current density of 3 A g⁻¹. Deng et al. [112] reported the properties of a layer-by-layer coated nanocomposite anode fabricated by a scalable laser shock assisted roll-to-roll deposition process. The Si NPS/rGO hybrid can deliver a high specific capacity of 1956 mAh g⁻¹ at a high cyclic rate of 15 A g⁻¹ and can retain 71.3% of the initial capacity after 1000 cycles. A prototype full-cell using a slurry-cast LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) cathode with a mass loading of 7 mg cm⁻² was built to demonstrate the performance of the Si/rGO composite anode (with mass loading of 0.5 mg cm⁻²) to form the full-cell. The NMC622‖Si/rGO full-cell delivers a good areal capacity of 0.95 mAh cm⁻² at a charge/discharge rate of 0.25 mA cm⁻² and 0.70 mAh cm⁻² at 1 mA cm⁻² in a voltage window 2.0–4.3 V. In addition, a 98% capacity retention after 50 cycles at 0.25 mA cm⁻² and a high energy density of 467 Wh kg⁻¹ with an average voltage of ~3.58 V are achieved. A Si/rGO composite used as an active element of an anode with mass loading of 1.0–1.2 mg cm⁻² delivered a capacity of 894 mAh g⁻¹ over 100 cycles at 0.2 A g⁻¹, and demonstrated a high-rate capability, with a capacity of 513 mAh g⁻¹ retained at 5 A g⁻¹ [113]. Si nanoparticles veiled with ultrathin rGO film reduced directly by precoated Ni template delivered capacity of 975 and 717 mAh g⁻¹ after 300 cycles at 1 and 3 A g⁻¹, respectively [114].

Several reports are related to the Si/rGO mesoporous-like architectures [115,116,117,118,119,120]. A 3D porous micro-size pomegranate shaped nano-silicon and rGO composite delivered a capacity of 1200 mAh g⁻¹ after 300 cycles at 0.5 A g⁻¹ [121]. Si particles encapsulated by rGO and forming a mesoporous structure delivered 1548 mAh g⁻¹ at 200 mA g⁻¹ after 100 cycles, and 905 mAh g⁻¹ after 100 cycles at 1 A g⁻¹ [116]. A lower cost and scalable synthesis of Si@rGO composite was proposed by Benzait et al. [122]. First, low-cost Al–Si (80:20) powder was chosen as the precursor instead of the higher-priced Si. Then, nanostructured porous Si was obtained by chemical dealloying with etching of Al using hydrochloric acid. This Al-etching reaction produces hydrogen allowing the reduction of GO to rGO simultaneously. This process thus avoids the post-treatment usually needed to reduce GO. This composite delivered a capacity of 1080 mAh g⁻¹ at 0.2 A g⁻¹ after 250 cycles, and 840 mAh g⁻¹ at 4 A g⁻¹. Liang et al. reported the fabrication of highly porous Si@C network derived from rice husks through activation and subsequent calcinations and consolidation by rGO introduced via ultrasonication [119]. The 3D carbon-coated mesoporous Si nanospheres@graphene foam (C@Si/GF) nanoarchitectures, synthesized by a thermal bubble ejection assisted chemical-vapor-deposition and magnesiothermic reduction method, exhibits superior anode performance including a specific capacity of 1200 mAh g⁻¹ at 1 A g⁻¹ current rate and shows stable morphology after 200 cycles [120].

The Si@C/rGO product having a high specific surface area of 539 m² g⁻¹ and pore volume of 0.693 cm³ g⁻¹ achieved enhanced electrochemical properties with a stable capacity of 760 mAh g⁻¹ after 80 cycles and a capacity of 317 mAh g⁻¹ at 1 A g⁻¹ current rate. Another highly porous composite was prepared by a templated self-assembly procedure with xrGO:Si ratios (x = 5, 10, 15) and optimized by calcination. The specific surface area of 482 m² g⁻¹ for rGO/Si composite (x = 10) increased to 539 m² g⁻¹ after a calcination at 600 °C for 4 h in Ar environment. The optimized composite displayed an initial discharge capacity of 2317 mAh g⁻¹ (ICE of 93.2%) and a capacity retention of 85% after 100 cycles at 0.1 A g⁻¹ rate. At a higher current density of 2 A g⁻¹, this electrode delivered a specific capacity of 728 mAh g⁻¹ after 100 cycles [105]. Song’s group [123] prepared Si nanoparticles (Si-NPs) enwrapped in N-doped carbon and combine with N-doped rGO and CNTs by solution-mixing and subsequent carbonization process using pyrolyzing melamine formaldehyde resin (MFR). The Si-NPs/rGO/CNTs composite, in which N-doped carbon outside Si-NPs buffers the volume change, exhibited a high reversible capacity of 892 mAh g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹, good rate capability, and cycling stability.

Zhang et al. [124] proposed a facile template-sacrificing method to fabricate bowl-like Si@rGO hybrids using uniform SiO₂ spheres wrapped by GO. The reduced 3D bowl-like nanoarchitecture resulting in Si nanoparticles distributed on the flexible rGO layer exhibited a high first discharge capacity of ~1890 mAh g⁻¹ with a CE of 90.79% at a current density of 0.1 A g⁻¹. A stable specific capacity of 450 mAh g⁻¹ was achieved after 100 cycles. More recently, Majeed et al. [116] fabricated a clay-derived mesoporous Si/rGO composite using a cost-effective and scalable process (i.e., one-pot magnesiothermic reduction), in which Si nanoparticles are encapsulated by rGO. Montmorillonite clay were used as natural resource for Si. This suitable mesoporous (with 89.2 m² g⁻¹ specific surface area) anode material for LIBs shows an enhanced capacity of 1548 mAh g⁻¹ after 100 cycles at 200 mA g⁻¹ current rate.

Since 2016, a large amount of research was focused on ternary composites showing superior electrochemical performance with long-term stability. Chen et al. [125] reported the fabrication of an electrospun sandwich-structured silicon-based anode with an architecture of rGO-protected silicon/carbon nanofibers (CNF) inhibiting the fragmentation of silicon electrodes typically caused by the large volume changes during charge/discharge reactions. This composite exhibited a high specific capacity of 1055 mAh g⁻¹ up to 130 cycles at 0.1 A g⁻¹. The Si/CNFs@rGO electrode also demonstrated outstanding rate behavior with a reversible capacity of 358 mAh g⁻¹ at 5.0 A g⁻¹. Pan et al. [126] prepared a micro-sized ternary Si@C@rGO spherical composite by a scalable spray drying technique and a subsequent calcination process. The obtained Si@C@rGO anode delivered a high initial reversible specific capacity of 1599 mAh g⁻¹ at a current density of 0.1 A g⁻¹ with a good capacity retention of 94.9% at a higher current rate of 0.2 A g⁻¹. Moreover, the Si@C@rGO anode demonstrated a good rate capability with a capacity of 951 mAh g⁻¹ at 2 A g⁻¹. According to the EIS results, the incorporation of graphene inhibited the growth of the SEI layer and enhanced the cycling stability of the electrode. The Si/rGO/C composite was prepared by encapsulating graphene-coated Si nanoparticles in the interconnected carbon nanofibers based on electrospinning technology [127]. This hierarchical core-shell nanofibers Si/rGO/C NFs demonstrated excellent electrochemical performance with a reversible capacity of 1228 mAh g⁻¹ and a capacity retention of 72% after 100 cycles with a current density of 0.8 A g⁻¹. When the current rate increased to 4 A g⁻¹, the anode displayed superior rate capability with a specific capacity of 954 mAh g⁻¹. Lee and coworkers [128] synthesized rGO/Si composite via a simple dip-coating method with nickel foam as a current collector and template for the electrode fabrication. The rGO/Si hybrid anode with 0.03 wt% Si delivered a capacity of 778 mAh g⁻¹ after 100 cycles.

Porous Si/C/reduced graphene oxide (Si/C/rGO) microspheres were prepared by the Tao’s group [129] using spray drying and subsequent carbonization process with polyvinyl alcohol (PVA) as cross-linking agent. The rGO wrapped Si/C ball structure offers a buffer space for the volume change of Si during the charge–discharge process. The as-prepared Si/C/rGO anode exhibited a high reversible capacity of 928 mAh g⁻¹ after 70 cycles at a current density of 0.1 A g⁻¹, and a good rate capability. An electrostatic self-assembly method was proposed to fabricate ball-milling-silicon@carbon/rGO composite (bmSi@C/rGO) using chitosan as a charged bridge [104]. The composite with 5-nm thick carbon coating and 20.4 wt% rGO tested as anode material displayed high reversible capacity of 935 mAh g⁻¹ at 0.2 A g⁻¹ current rate in the potential range of 0.02–1.5 V vs. Li^+/Li and 71.9% capacity retention after 100 cycles. In 2017, Qin et al. [130] reported the preparation of binder-free, conductive agent-free Si/rGO anode on nickel foam via one-step hydrothermal method with subsequent post-annealing process. This anode material demonstrated a remarkable cycle ability and rate performance, i.e., reversible specific capacity of 1500 mAh g⁻¹ at 2 A g⁻¹ current rate after 2500 cycles and over 600 mAh g⁻¹ at 4 A g⁻¹ after 2500 cycles. The rate capability of 500 mAh g⁻¹ was still maintained at high current density of 16 A g⁻¹ (Figure 3).

Recently, 3D-framework Si@N-doped C/rGO composite was synthesized by a novel polymer network method using acrylic amide and N,N’-methylene bisacrylamide as the crosslinker [131]. After the crosslinking process, Si nanoparticles (30 nm in size) were embedded in a N-doped C/rGO sheet-like network. As an anode, this composite delivered a first discharge capacity of 2609 mAh g⁻¹, an ICE of 69% and demonstrated a capacity of ~479 mAh g⁻¹ after 200 cycles at 2 A g⁻¹ with a CE of 99.6%. A yolk-shell structured rGO/multi-walled carbon nanotubes (MWCNT)-based hybrid as binder-free anode material was designed and investigated for different MWCNT:rGO ratios by Toçoğlu and coworkers [132]. In this configuration, the void space between Si nanoparticles (50–150 nm) and carbon coating created by the yolk-shell architecture buffered the Si volume expansion. The rGO/MWCNT/Si@void@C free-standing composite with MWCNT:rGO of 2:1 tested at 200 mA g⁻¹ current rate in the voltage range 0.05–2.5 V delivered specific discharge capacities of 2472 and 1621 mAh g⁻¹ at the first and second cycle, respectively. After 500 cycles at the current rate of 200 mA g⁻¹, the composite anodes with MWCNT:rGO ratios of 1:1, 1:2, 2:1 delivered capacities of 820, 503, and 951 mAhg⁻¹, respectively, while the rGO/Si@void@C (without MWCNT) exhibited only 500 mAhg⁻¹. A green-synthetic spiderweb-like hybrid composed of Si nanoparticles encapsulated by a multifunctional citric acid binder (SiNPs@CA) anchored on rGO sheets through chemical bonds was characterized as anode material [133]. The SiNPs@CA@rGO anode retained a capacity of 2566 mAh g⁻¹ after 100 cycles at a current density of 1 A g⁻¹ with an ICE of 81%. Kinetic measurements by EIS and GITT methods showed that Li⁺ diffusion coefficients are in the range 10⁻⁸–10⁻¹⁰ cm² s⁻¹. Wu et al. [134] prepared ternary hierarchical Si/rGO/C composites by encapsulating Si nanoparticles into the rGO/C dual-carbon matrix via self-assembling, ball milling, polystyrene sphere (PSS) template-assisting, and spray drying techniques. When cycled at 0.4 A g⁻¹ for 500 cycles, the Si/rGO/C anode showed a capacity retention of ~75% and CE of ~99.7% retaining a specific capacity of ~602 mAh g⁻¹. Yu et al. [135] employed a synthesis process in mild conditions free of reducing agent to fabricate rGO/Si and N-doped rGO/C@Si composites. Analysis of the peak strength ratio of D- and G-Raman band (I_D/I_G) showed that rGO/C@Si had the higher degree of graphitization (I_D/I_G = 0.87) compared with rGO/Si (I_D/I_G = 1.04). Due to the increased affinity to lithium of amorphous carbon, the designed N-doped rGO/C@Si anode material (29.1 wt% GO; specific surface area of ~86 m² g⁻¹; pore volume of 0.13 cm³ g⁻¹) showed superior cycling performance, i.e., specific capacity of 1115.8 mAh g⁻¹ after 150 cycles at 0.42 A g⁻¹ and better rate capability of 1077 mAh g⁻¹ at 4.2 mA g⁻¹. Pomegranate-like Si@N-doped C/rGO composites were successfully fabricated by in-situ nitrogen doped carbon coating and freeze-drying processes [116]. The composite anode with ~84% Si (spherical nanoparticles with average diameter of 50 nm) presented an initial discharge capacity of ~1717 mAh g⁻¹, an ICE of 70.7%, and a charge specific capacity of 709 mAh g⁻¹ at 1 A g⁻¹ after 400 cycles. The capacity was still 532 mAh g⁻¹ at 3 A g⁻¹. The Si@N-doped C/rGO anode demonstrated a slightly increasing D_Li⁺ from 8.08 × 10⁻¹⁴ to 8.50 × 10⁻¹⁴ cm² s⁻¹ after 100 cycles. Recently, Zhu et al. fabricated an anode with yolk-void-shell Si-C particles strongly anchored on rGO [136]. The Si@void@C/rGO with a void size of 5 nm delivered a capacity of 1294 mAh g⁻¹ after 100 cycles at 0.5 A g⁻¹. Electrochemical performance of Si/rGO composite anodes from the recent literature are summarized in

Table 1. Electrochemical performance of Si/rGO composites. The relevant cycle number is displayed in brackets.

Material	Synthesis	Specific Capacity (mAh g−1)	Current Density (A g−1)	Ref.
SiNWs@G/rGO	sandwich procedure	1280	2.1 (100)	[88]
Si NPs/rGO	polymer-assisted	1311	2.6 (900)	[87]
Si/CNFs@rGO	layer-by-layer assembly	455	15 (2000)	[109]
CNT/rGO/Si-NPs	electrostatic method	1480	0.2 (200)	[111]
Si@C/rGO	sonication + calcination	780	1.0 (80)	[119]
Si NPS/rGO	roll-to-roll deposition	1394	15 (1000)	[112]
Si/rGO	self-assembly	1481	0.5 (50)	[102]
3D Si/rGO	evaporation	1406	0.05 (100)	[117]
Si@SnS2/rGO	electrostatic self-assembly	1055	0.1 (130)	[125]
Si@C@rGO	spray drying + calcination	1567	0.2 (100)	[126]
Si@N-doped C/rGO	polymer network method	479	2.0 (200)	[131]
Si/C/rGO	spray-drying + carbonization	928	0.1 (70)	[129]
Si/rGO/C NFs	electrospinning	884	0.8 (100)	[127]
bmSi@C/rGO	ball-milling + pyrolysis	935	0.2 (100)	[104]
Si@N-doped C/rGO	freeze drying	709	1.0 (400)	[117]
N-doped rGO/C@Si	freeze-drying + pyrolysis	1115	0.42 (150)	[135]
Si/rGO/C	self-assembling + spray-drying	602	0.4 (500)	[134]
rGO/MWCNT/Si@void@C	dispersion + ultrasonication	951	0.2 (500)	[116]
rGO/Si@void@C	dispersion + ultrasonication	500	0.2 (500)	[132]
SiNPs@CA@rGO	suspension + freeze drying	2566	1.0 (100)	[133]
Si@rGO (bowl-like)	templating	450	0.1 (100)	[124]
Si/rGO @ Ni foam	hydrothermal	1500	2 (2500)	[130]
Si@Void@C/rGO	surface carbonization + etching	1294	0.5 (100)	[136]

.

3.2. Germanium

Germanium has important advantages over Si: A better electronic conductivity (2.1 S cm⁻¹) and higher Li-ion diffusivity (6.5 × 10⁻¹² cm² s⁻¹), and a huge theoretical capacity of 1624 mAh g⁻¹; since it can be lithiated to Li₁₅Ge₄ (4.4 Li⁺ per Ge atom) at low operating voltage (0.3 V vs. Li^+/Li) [137]. Reported phases of Li–Ge alloys include Li₇Ge₁₂, LiGe, Li₁₁Ge₆, Li₉Ge₄, Li₇Ge₂, Li₁₅Ge₄, and Li₂₂Ge₅ [138]. In addition, the dilatation/contraction of the lattice upon lithiation/delithiation is isotropic (~260% volume change) so that the particles avoid cracking even at a high C-rate and big particles 620 nm in size, which result in remarkable rate capability of Ge [139]. However, Ge has a disadvantage with respect to Si: A much higher cost of production. For a recent review on Ge-nanomaterials as anodes for LIBs, see Ref. [140,141].

Since 2012, improvements on Ge anodes have been made by the fabrication of hybrids with graphene oxide using different synthesis routes. Xue et al. [142] proposed a double carbon protection strategy, in which Ge is combined with amorphous carbon and graphene. When cycled under the high current density of 3.6 mA g⁻¹, the Ge@C/rGO nanocomposites still displayed a specific capacity of 380 mAh g⁻¹ after 50 cycles, which is still higher than the specific capacity of 100 mAh g⁻¹ of Ge@C NPs hybrid. Later, another Ge@C/rGO composite was fabricated by simply distributing Ge@C on the rGO sheets using a low-pressure thermal deposition approach. Such a hybrid anode exhibited a reversible capacity of ~940 mAh g⁻¹ at a current density of 50 mA g⁻¹ after 50 cycles. Germanium-graphene composites retained a capacity of 675 mAh g⁻¹ after 400 cycles at current density to 400 mA g⁻¹ [31]. Later, Xu et al. [143] reported the fabrication of a Ge/rGO composite made of ~5 nm Ge nanoparticles uniformly distributed within a nitrogen-doped rGO carbon matrix using a PVP-assisted hydrolysis method. The Ge/rGO composite delivered an initial discharge capacity of 1475 mAh g⁻¹ and a reversible capacity of 700 mAh g⁻¹ after 200 cycles at a current density of 0.5 A g⁻¹, which demonstrates that the conductive rGO carbon matrix alleviated the pulverization problem, prevented Ge particle aggregation, and facilitated lithium-ion motion. Wang et al. [144] prepared Ge@C/rGO hybrids as anode materials for Li-ion batteries by anchoring the core-shell structured germanium@carbon (Ge@C) on the reduced graphene oxide nanosheets by the strong adhesion of polydopamine (PDA). The products delivered a reversible capacity of 1074 mAh g⁻¹ at 2C rate after 600 cycles (capacity retention of 96.5%) and 436 mAh g⁻¹ at 20C rate after 200 cycles. An advanced anode material, which consists of sub-micron Ge crystals (200–600 nm) uniformly encapsulated in a conductive rGO matrix forming a sandwich-structured rGO/Ge/rGO, was successfully prepared through a facile thermal reduction method [145]. Enhanced reversible specific capacity of 1085 mAh g⁻¹ was delivered after 500 cycles at current density of 1.6 A g⁻¹ (1C rate) with the capacity loss of ~0.017% per cycle. This rGO/Ge/rGO anode was tested in a coin-type full cell with LiNi_0.5Co_0.2Mn_0.3O₂ as cathode, which exhibited a high specific capacity of 940 mAh g⁻¹ after 100 cycles at 1C rate. Fang et al. [146] compared the performance of two Ge-based hybrids, namely Ge-rGO and Ge-rGO-CNTs. Ge nanoparticles anchored on rGO intertwined with carbon nanotubes (CNTs) yielded a capacity of 863 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 100 cycles and good rate performance of 1181, 1073, 1005, 872, 767, and 644 mAh g⁻¹ at current densities of 100, 200, 400, 800, 1600, and 3200 mA g⁻¹, respectively. As an example, Figure 4 presents the electrochemical performance of pure Ge, Ge-rGO, and Ge-rGO-CNTs nano-composite anodes.

Recently, Chen and coworkers [147] proposed an in-situ method (i.e., chemical reaction at 60 °C for 3 h of a mixture of GO suspension and Na₂GeO₃ aqueous solution added with NaBH₄ solution) for the synthesis of amorphous Ge/rGO nanocomposites containing 27–37 wt% carbon, which were transformed to crystalline phase by annealing at 650 °C in reducing Ar/H₂ atmosphere. The mesoporous Ge/rGO hybrid (34.3 wt% C) had a BET specific surface area and average pore size of 81 m² g⁻¹ and 4.2 nm, respectively. Galvanostatic measurements carried out at the current density of 0.1 A g⁻¹ in the potential range of 0.01–1.5 V vs. Li^+/Li) showed initial discharge and charge specific capacities of 2100 and 1184 mAh g⁻¹ (ICE of 56.4%) and cycling performance of 500 mAh g⁻¹ is obtained at 5 A g⁻¹ current rate after 100 cycles. It is believed that the small size (~5 nm) of Ge nanoparticles embedded in rGO sheets results in void space large enough to buffer the volume and maintain high structural stability during cycles. The full cell Ge/rGO‖LiFePO₄ performed the specific capacity of ~125 mAh g⁻¹ at 0.5C (85 mA g⁻¹) over 100 cycles. Zhao et al. [148] designed a double-layered protective structure in which the cubic hollow Ge@C hybrids are uniformly dispersed on rGO through dopamine-coating and subsequent carbothermal reduction. The Ge@C/rGO composite demonstrated the capacity of 1183 mAh·g⁻¹ at the specific current of 0.1 A g⁻¹ and 710 mAh g⁻¹ at 1 A g⁻¹ for 200 cycles. A few-layer methyl-terminated germanene-graphene nanocomposite (GeCH₃/rGO) was prepared through ultrasonic wet dispersion using GeCH₃ nanosheets (typical thickness 4–5 nm) prepared by liquid-phase exfoliation [149]. The resulting GeCH₃/rGO‖Li half-cell exhibited a remaining capacity of 1058 mAh g⁻¹ after 100 cycles at 0.2 A g⁻¹ with CE of 98%. After 500 cycles the capacities were 439 and 288 mAh g⁻¹ (CE of 98.58 and 99.31%) at current densities of 0.5 and 1.0 A g⁻¹, respectively. Electrochemical performance of Ge/rGO composite anodes from the recent literature are summarized in

Table 2. Summary of electrochemical properties of Ge-based composite anodes. Relevant cycle number is given in brackets.

Material	Synthesis	Specific Capacity (mAh/g−1)	Current Density (A/g−1)	Ref.
Ge@C/rGO	polymerization with PDA	1074	3.2 (600)	[144]
Ge@C/rGO	dispersion + thermal reduction	380	0.1 (50)	[142]
Ge@C/rGO	low-pressure thermal deposition	940	0.05 (50)	[31]
Ge-rGO-CNTs	dispersion + thermal reduction	863	0.1 (100)	[146]
Ge/rGO	PVP-assisted hydrolysis	700	0.5 (200)	[143]
rGO/Ge/rGO	carbothermal reduction	1085	1.6 (500)	[145]
Ge@C/rGO	carbothermal reduction	710	1.0 (200)	[148]
Ge/rGO	polymeric in-situ reduction	960	1.0 (100)	[147]
GeCH3/rGO	ultrasonic wet dispersion	1058	0.2 (100)	[149]
CuGeO3/rGO (30 wt%)	hydrothermal reduction	909	0.1 (200)	[150]

.

3.3. Tin

Wen and Huggins showed that tin reacted with lithium to generate different alloys within the Li–Sn phase diagram [151]. Therefore, Sn has attracted great attention as anode material due to its low cost and good electrochemical activity. Upon lithiation, Sn exhibits a theoretical reversible capacity of 994 mAh g⁻¹ according the reaction Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4) with a working potential of ~0.5 V vs. Li^+/Li [152,153]. However, during the battery cycles, Sn anodes seriously suffer from repeated large compressive and tensile stresses, which leads to fast capacity fading [154]. This problem can be solved by using Sn-graphene composites. In the early 2000s, Li and coworkers [155] investigated several nano-alloy anodes for lithium-ion batteries and showed that, with the pure nanostructured phase β-Sn-Sb prepared by co-precipitation, the reversible alloy reaction of Li with Sb(2) and Sn(3) occurs at 0.9–0.7 and 0.7–0.0 V successively. However, the use of this anode material is limited because Li alloy clusters have a very strong tendency to aggregate and form larger nano-crystallites (>20 nm) inducing a decrease of the surface energy of nano-alloy particles/clusters. In 2012, Wang et al. [156] outlined the development of Sn-based nanomaterials with a focus on their dimensionality when combined with different types of carbonaceous materials (i.e., carbon supported and carbon confined) as anode composites in LIBs. Sn/graphene composites grown by in-situ chemical reduction process (using GO and SnCl₂ precursors) exhibited lithium storage performance of 795 mAh g⁻¹ in the 2nd cycle and 508 mAh g⁻¹ in the 100th cycle, much better than that of bare Sn nanoparticles [157]. Hybridized 1D Sn nanoparticles were integrated onto the rGO surface via a hydrolysis process to form rGO-supported Sn/C nanocales (rGO/Sn@C) [158]. These rGO/Sn@C coaxial nanocables displayed an initial capacity of 760 mAh g⁻¹ with a retention of 83% after 50 cycles. Submicron metallic Sn particles encapsulated in a rGO framework were used as the electrode materials for potassium-ion batteries. The Sn@rGO anode, with abundant structural defects and a high BET surface area of 136.5 m² g⁻¹, exhibited a capacity of 120 mAh g⁻¹ after 500 cycles at 0.5 Ag⁻¹ and an excellent rate capability of 67 mAh g⁻¹ at 2 A g⁻¹ [159]. Botas et al. developed an advanced binder-free anode material composed of 3D macroporous foams formed by rGO and submicron tin-based particles [160]. Typically, Sn/rGO samples were obtained by preparing a Sn-rGO aerogel by dissolving SnSO₄ in a GO suspension, with a control at pH = 9 by addition of NH₃ solution, followed by a pyrolysis process at 800 °C in argon atmosphere. The Sn/rGO anode delivered an initial capacity of 715 mAh g⁻¹, decreasing to 234 mAh g⁻¹ after 50 cycles at 50 mA g⁻¹.

Two other metal/carbon would be worthy to be mentioned. A Ni@C composite, which consists of dispersed Ni nanocrystals (average size of 20 nm) encapsulated by carbon nanosheets was prepared via thermal decomposition of a nickel oleate complex at 700 °C. The composite adopts a bending structure with a good ordering of the carbon layers, forced by the Ni encapsulation. The electrode delivers an average capacity value around 723 mAh·g⁻¹ on cycling at moderate rate of C/4. At higher rate (1C), the average specific capacity decreases to ~205 mAh·g⁻¹ with a coulombic efficiency close to 100% after the first cycles [161]. To overcome the capacity fading of the antimony anode (theoretical capacity of 660 mAh g⁻¹, volumetric strain of 135%), a Sb/rGO anode was prepared by poly(acrylic acid) and nickel nitrate assisted electrophoretic deposition. The composite anode deposited on copper foil offers a stable discharge and charge capacity of ~498 and ~498 mAh g⁻¹, respectively, up to 100 cycles at a high current rate of 0.5 A g⁻¹ [162].

4. Metal Oxide/rGO Composites

4.1. Silicon-Based Oxide Composites

Silica suboxides (SiO_x) have been regularly studied as hopeful anode materials replacing pure Si. In 2002, the Yamamoto group demonstrated the effect of both the oxygen concentration and particle size on the cycling life and on the reversible capacity of SiO_x [163]. In 2012, the Sohn group found that SiO₂ could be activated via a simple high-energy ball-milling process and could deliver a reversible capacity of ~800 mAh g⁻¹ after 200 cycles [164]. The lithium battery with SiO_0.8 anode (50 nm particle size) can provide a reversible capacity of ca. 1600 mAh g⁻¹ over the voltage range from 0.02 to 1.4 V vs. Li⁺/Li. The success of SiO_x materials is due to their high theoretical capacity (>1600 mAh g⁻¹) and to the lithium oxygen co-ordination, which implies minimal volume change and, at the same time, lower activation energy [165].

SiO_x-based composites were synthesized by green methods such as hydrothermal route [166,167], chemical etching [168], sol-gel with subsequent annealing [169,170], solvothermal reaction [171], template route [172], and vacuum-assisted filtration and self-assembling [173,174]. Haeri et al. [169] synthesized SiO₂/GO nanohybrids through one-step sol-gel method using a mixture of tetraethylorthosilane (TEOS) and 3-aminopropyl triethoxysilane (APTES) silanes at various hydrolysis times. Structural analysis showed that amino-functionalized SiO₂ nanoparticles (~20–30 nm) were deposited on the basal plane of GO sheets. Kasimayan et al. [168] used the same method for the preparation of SiO₂@α-Fe₂O₃ core-shell decorated rGO nanocomposites with a subsequent refluxing process at 75 °C for 6 h.

Guo et al. [175] designed a SiO_x/C/rGO ternary nanocomposite using tetraethoxysilane (TEOS) as the silicon precursor. The primary SiO_x/C nanoparticles (spherical shape 50–100 nm diameter) were SiO_x particles coated with a 3-nm thick carbon layer; the disorderly stacked layer-by-layer SiO_x/C/rGO structure containing 20 wt% of amorphous carbon and rGO obtained by solvothermal reaction demonstrated very high BET specific surface area of 390.2 m² g⁻¹. Tested at 100 mA g⁻¹ between 0.01 and 1.5 V, it exhibited a capacity of 1284 mAh g⁻¹ after 100 cycles. At high rate of 3.2 A g⁻¹, the electrode retained 412 mAh g⁻¹. The electric conductivity of the SiO_x/C/RGO was 1.26 × 10⁻³ S cm⁻¹. Xu et al. [166] fabricated a hierarchical structure SiO₂@SnO₂/rGO by a facile hydrothermal process, which consisted of mono-dispersed SnO₂ nanoparticles (as small as 5 nm) on the surface of rGO sheets and SiO₂ spheres (~100 nm). The SiO₂@SnO₂/rGO electrode displayed a large initial discharge capacity of 1548 mAh g⁻¹ at a current density of 100 mA g⁻¹ and it still retained a discharge capacity of about 600 mAh g⁻¹ after 100 cycles. In this hybrid, the presence of SiO₂ nanospheres mitigated the agglomeration tendency of SnO₂ nanoparticles. Zhao et al. [167] reported ternary Co₂SiO₄/SiO₂/RGO nanocomposites that exhibited a reversible capacity of 1382 mAh g⁻¹ at 0.5 A g⁻¹ current rate and long-life span (500 cycles), which is over 20 times higher than the SiO₂/rGO counterpart. Ex situ XPS measurements evidenced the role Co, which activates and promotes the conversion of tetravalent state of Si to low valence ones along with cycling. Guo et al. [176] reported the lithium storage performance of SiO₂@rGO hybrid, which consists of 3D interconnected SiO₂ nanoparticles (<120 nm in diameter) coated by rGO. This composite was fabricated by online hydrolysis and subsequent condensation of the precursor tetraethylorthosilicate (TEOS) in the dispersed GO sol, followed by the reduction of GO using hydrazine hydrate. The SiO₂@rGO anode material delivered a reversible discharge capacity of 708 mAh g⁻¹ for the 2nd cycle and 490 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 60 cycles with CE of ~98%.

More recently, Hu et al. [168] developed a composite with core-shell Fe₃O₄@SiO₂ particles wrapped by rGO nanosheets. Meso-/micro-porosity was produced by chemical etching, forming 3D continuous channels for Li⁺ motion. The Fe₃O₄@SiO₂/GO composite delivered an initial specific capacity of 1630 mAh g⁻¹ at 0.1 A g⁻¹ rate over the potential range of 0.01–3.0 V vs. Li^+/Li along with a CE of 86% and retained the capacity of 514 mAh g⁻¹ at 5 A g⁻¹ after 1000 cycles. The fast charge/discharge process was attributed to the pseudocapacitive behavior. A multicomponent nanosheet rGO@SiO_x@NNC consisting of an inner rGO nanosheet substrate, an SiO_x intermediate layer, and an N-doped nanoporous carbon (NNC) was obtained by pyrolysis of poly(vinylpyrrolidone) [177]. The as-prepared rGO@SiO_x@NNC nanosheets with a high BET specific surface area of 360 m² g⁻¹ and pore volume of 0.6 cm³ g⁻¹ exhibited a good cycle ability of 410 mAh g⁻¹ at a current density of 1 A g⁻¹ for 200 cycles as well as a specific capacity of 179 mAh g⁻¹ at 5 A g⁻¹ after 500 cycles. The beneficial advantage of non-stoichiometric SiO_x is evidenced by the comparison with the rGO@SiO₂@C composite, which possesses the capacity of 320 mAh g⁻¹ at 1.0 A g⁻¹ after 200 cycles (Figure 5).

4.2. Tin-Based Oxide Composites

SnO₂ has been widely investigated as anode for Li-ion batteries. It is abundant in nature, environmentally friendly, and has the advantage of high theoretical capacity (782 mAh g⁻¹). In 1997, Idota et al. from Fuji Photo Film Company (Japan) replaced the graphite anode of Li-ion cells by a tin-based amorphous composite oxide (TCO), which can accept 8 moles of Li⁺ ions per unit mole [178]. Based on previous findings, the lithiation of SnO₂ occurs within three steps: Insertion (intermediate phase), conversion, and alloying/dealloying [179]. However, the poor electrical conductivity of SnO₂ and its severe volume expansion during lithiation hinder its wide scale utilization. Thus, the extensive study of SnO₂-graphene composites started in 2010–2011; the integration of SnO₂ with graphene has demonstrated improvement in cycling stability and rate capability [180,181,182,183,184].

Currently, SnO₂/graphene hybrids are fabricated using tin salts and GO via different routes, such as chemical reduction, hydrothermal growth, in situ deposition, and combined routes including calcination as the last step [25,185,186,187,188,189,190]. The most explored strategy used for the fabrication of SnO₂/rGO composite involves the hydro- or solvo-thermal deposition of very small SnO₂ nanoparticles (generally ~5 nm) on the GO layers [186,191] and its subsequent partial reduction by thermal treatments at moderate temperatures under inert atmosphere [192] or through its reaction with hydrazine [193]. For example, Hu et al. [194] designed and prepared a 3D porous rGO wrapped hollow SnO₂ nanosphere composite via a hydrothermal reaction and hydrazine reduction process. The as-developed 3D porous H-SnO₂/rGO composite was tested as anode showing a high reversible capacity of 1107 mAh g⁻¹ after 100 cycles at a current density of 0.1 A g⁻¹ and maintaining 552 mAh g⁻¹ over 500 cycles at a current density up to 1 A g⁻¹. SnO₂NCs@N-rGO hybrid anode material was fabricated by binding SnO₂ nanocrystals (NCs) in N-doped rGO sheets by means of an in-situ hydrazine monohydrate vapor reduction method [193]. It is believed that bonds formed between graphene and SnO₂ NCs limit the aggregation of in situ formed Sn nanoparticles, leading to a stable hybrid anode material with long cycle life. A simple one-pot hydrothermal method was utilized by Wang and coworkers [195] to prepare tungsten-doped SnO₂ and reduced graphene oxide nanocomposites. The W:SnO₂/rGO hybrid exhibited an excellent long-term cycling performance. After 100 cycles, the specific discharge capacity was still as high as 1100 mAh g⁻¹ at 0.1 A g⁻¹ current rate (i.e., capacity retention of 90%). It still remained at 776 mAh g⁻¹ after 2000 cycles at the current density of 1A g⁻¹. Wang et al. [196] prepared defect-rich crystalline SnO₂ nanoparticles with an average size of 3.5 nm homogeneously deposited on graphene nanosheets using a one-step microwave-assisted hydrothermal method. The SnO₂/graphene nanocomposites deliver a superior reversible capacity of 635 mAh g⁻¹ after 100 cycles and display excellent rate performance. Shao and coworkers [197] prepared a SnO₂/rGO composite with SnO₂ nanoparticles confined by multilayered and interconnected 3D spherical rGO sheets; such hybrid anode displayed a large reversible capacity over 1000 mAh g⁻¹ and long cycling life with 88% retention after 100 cycles. In situ chemical synthesis of SnO₂/rGO nanocomposites (SnO₂ crystal size of ~3–4 nm) was realized in ethanol solution. This anode material had a reversible lithium storage capacity of 1051 mAh g⁻¹ [198]. Hierarchical porous SnO₂ incorporated with rGO (HP-SnO₂/rGO) was fabricated by Chen and coworkers [199] showing mesopores of 3.3 nm and macropores of ~200 nm. The half-cell using Li‖SnO₂/rGO exhibited an excellent cycle performance of 850 mAh g⁻¹ in 100 cycles at a current density of 0.3C and rate capability of 436 mAh g⁻¹ at a current density of 4.5C. The GO/SnO₂ nanocomposite was prepared using an aqueous solution of Sn(BF₄)₂ with the addition of GO powders and different concentrations of reducing agent (i.e., 0–0.075 mol L⁻¹ Na₂S₂O₄ + 0.5 mol L⁻¹ HBF₄) [200]. Using low content of reducing agent (0.025 mole), the rGO-SnO₂ hybrid obtained by annealing at 500 °C under Ar atmosphere for 2 h exhibited SnO₂ nanoparticles of 15–20 nm in size and rGO Raman spectrum with I_D/I_G ratio of ~1.06 ± 0.01. It showed good Li storage performance with specific capacity of 378 mAh g⁻¹ at 3.2 A g⁻¹, and cyclability of 522 mAh g⁻¹ after 50 cycles at 30 mA g⁻¹ current rate.

Since 2019, advanced rGO-based anodes have shown remarkable performance. Tan et al. [201] have used combined methods including in-situ metal tin oxidation for the generation of the 2D Sn/SnO/SnO₂ structure, and spray drying and thermal annealing for the transformation to 3D blueberry-shaped SnO₂/rGO. The as-prepared hybrid containing 18.6 wt% rGO displayed a BET surface area of 218 m² g⁻¹. Tested under a current density of 500 mA·g⁻¹ in the potential window 0.01–3.0 V it exhibited a reversible capacity of 708 mAh·g⁻¹ over 150 cycles. Liu et al. [202] have developed an SnO₂/GO composite composed of SnO₂ nanoparticles (200 nm in size) uniformly anchored on the surface of GO sheets using ultrasonication and calcination processes. The anode with SnO₂:GO ratio of 4:1 exhibits a discharge capacity of 492 mAh g⁻¹ at 0.2C rate after 100 cycles. Tan et al. [201] reported the properties of a shrunken 3D dried blueberry shaped SnO₂/rGO composite synthesized by tin foil washed by sonification in anhydrous ethanol and placed in GO suspension in oil bath maintained at 90 °C. The SnO₂/rGO composite consisting of individual SnO₂ crystals (10 nm in size) aggregated to form larger cluster (100 nm) were well encapsulated by the few-layered rGO sheets. Electrochemical tests of this anode electrode demonstrated a high initial capacity of 737 and 128 mAh·g⁻¹ under a current density of 0.2 and 10.0 A·g⁻¹, respectively, over 150 cycles, and a reversible capacity of 708 mAh·g⁻¹ under a current density of 0.5 A·g⁻¹ over 150 cycles. Jiang et al. [203] developed a SnO₂/rGO nanocomposite, which showed an initial specific capacity of 1705 mAh g⁻¹ and a capacity retention of 500 mAh·g⁻¹ after 50 cycles at 250 mA g⁻¹. A free-standing SnO₂@rGO composite anode, in which SnO₂ nanoparticles were tightly wrapped within wrinkled rGO sheets, was synthesized via an anti-solvent-assisted precipitation [204]. The SnO₂@rGO anode showed significant enhanced lithium storage behavior; a high charge capacity above 700 mAh g⁻¹ was achieved with 95.6% retention after 50 cycles at a current density of 0.5 A g⁻¹. The superior performance of the free-standing SnO₂@rGO electrode was evidenced by analysis of its structure after 50 cycles (Figure 6). Briefly, results are as follows: (i) The cycled composite anode displayed compressed pores and channels, attributed to the SEI formation during cycling, (ii) XRD pattern of the cycled anode revealed the co-existence of three rGO, SnO₂, and Sn components in the fully charged state, (iii) TEM image shows that, in the assembled composite, Sn-based nanoparticles remain uniformly wrapped within rGO sheets, and (iv) the observation of lattice fringes evidences the structural stability.

Recently, rGO/SnO_x (x = 0, 1, 2) nanocomposites were prepared using a low-cost and room-temperature one-pot in-situ chemical reduction process for 30 min with NaBH₄ (0 mol–0.06 mol) as chelating agent [205]. SnO_x nanoparticles (2–4 nm) were uniformly distributed over rGO sheets. The rGO-SnO_x hybrid synthesized by using 0.04 mol NaBH₄ revealed stable specific capacity of 833 mAh g⁻¹ after 50 cycles, 767 mAh g⁻¹ after 100 cycles at current rate of 0.1 A g⁻¹, and good rate capability (481 mAh g⁻¹ at ~1 A g⁻¹). The lithium-ion diffusion coefficient in the rGO-SnO_x anode was estimated to be 2.4 × 10⁻¹⁰ m² s⁻¹, which evidences the vital role of rGO in terms of kinetics.

Several reports have shown that doping also results in an increase of lithiation rate in SnO₂. Wang et al. [206] compared the electrochemical behavior of an Fe-doped SnO₂/rGO nanocomposite with undoped SnO₂/rGO nanoparticles prepared through a wet-chemical approach. The specific capacity of the SnO₂/rGO electrode (SnO₂ particle size of 6–8 nm) reaches 905 mAh g⁻¹ after 100 cycles at 0.1 A g⁻¹, while the Fe@SnO₂/rGO nanocomposite retains a capacity of 1353 mAh g⁻¹ in similar conditions. Huang et al. [207] fabricated a P-doped SnO₂/rGO/C composite by electrospinning process of a GO/(H₃PO₄)/SnCl₄/PVP-containing precursor solution. The cycling performance is 285 mAh g⁻¹ at a rate of 0.1 A g⁻¹ after 60 cycles. Zoller et al. [208] investigated the Sb-doped SnO₂(ATO)/rGO composite synthesized via a microwave-assisted in-situ method in tert-butyl alcohol. This composite featured a specific capacity of 1226 mAh g⁻¹ at 1C rate and retained 577 mAh g⁻¹ at very high charge rate of 60C. Fluorine-doped SnO₂ was also realized through hydrothermal process, significantly improving the electrochemical properties of the F-SnO₂ NCs/rGO composite [209]. The composite delivers a reversible capacity of 1277 mAh g⁻¹ after 100 cycles at current density 0.1 A g⁻¹. The high-rate capability of 634 mAh g⁻¹ after 10 cycles at current density 5 A g⁻¹ was due to the reduced resistance induced by the F-doping that protects the electrode surface from the hydrolysis or decomposition of the LiPF₆ salt. The chemical diffusion coefficients of Li ions in F-SnO₂@rGO and SnO₂@rGO electrodes were calculated from EIS measurements as ~1 × 10⁻¹⁶ and ~4 × 10⁻¹⁷ cm² s⁻¹, respectively. More recently, F-SnO₂@rGO synthesized by hydrothermal process was mixed with artificial graphite and proposed as an anode by Zhang et al. [210]. It maintained specific capacities of 440 mAh g⁻¹ at 500 mA g⁻¹ with high capacity retention of 96.28% after 500 cycles. Electrochemical performance of SnO₂/rGO composite anodes for LIBs from the recent literature are summarized in

Table 3. Electrochemical performance of SnO₂/GO composites as anode materials for Li-ion batteries. Relevant cycle number is given in brackets.

Material	Synthesis	Specific Capacity (mAh g−1)	Current Density (A g−1)	Ref.
SnO2/rGO	in-situ hydrolysis	600	0.05 (20)	[198]
H-SnO2/rGO	hydrothermal reduction	552	1.0 (500)	[194]
PS@GO/SnO2	hydrolysis	781	0.1 (100)	[197]
SnO2/rGO	spray drying + thermal treatment	708	0.5 (150)	[201]
SnO2-x/rGO	precipitation	950	0.1 (1100)	[211]
F–SnO2/rGO	hydrothermal @180 °C/12 h	1037	0.1 (150)	[188]
F–SnO2/rGO	hydrothermal	440	0.5 (500)	[210]
HP-SnO2/rGO	hydrolysis + calcination	850	0.3C (100)	[199]
W–SnO2/rGO	hydrothermal reduction	1100	0.1 (100)	[187]
Zn–SnO2/rGO	sol-gel + freeze-drying	628	0.1 (200)	[212]
SnO2/GO	ultrasonication + calcination	492	2C (100)	[202]
SnOx/rGO	chemical reduction	767	0.1 (100)	[205]
Fe@SnO2/rGO	wet chemistry	1353	0.1 (100)	[206]
Sb-SnO2/rGO	in-situ microwave-assisted	577	60C	[211]
F-SnO2 NCs/rGO	hydrothermal	1277	0.1 (100)	[209]
SnO2/rGO	precipitation + calcination	522	0.03 (50)	[200]

.

4.3. Manganese-Based Oxide Composites

Manganese-based oxides such as MnO, Mn₃O₄, Mn₂O₃, MnO₂, and their derivatives are attractive anode materials because of their theoretical specific capacity of 755, 936, 1018, and 1232 mAh g⁻¹, respectively [213,214,215,216]. In addition, they exhibit high safety, low cost, and environmental benignity and manganese is an abundant element. The improved performance (i.e., discharge capacity, high rate capability, kinetics of inserted ions, and long-term cyclability) of the MnO₂/rGO composite electrodes as anode materials have been evidenced by several electrochemical techniques, accounting for their extraordinary cycling properties compared with that in a simple MnO₂ electrode. Generally, the volume expansion of MnO₂ during the charge–discharge process is significantly alleviated due to the uniform dispersion and firm anchoring of MnO₂ nanoparticles on the rGO surface.

4.3.1. MnO₂-Based Composite

MnO₂ has received much attention because of its low cost, ease of fabrication, environment friendliness, low operating voltage, existence of several polymorphs with tunnel-like structure (depending on the linkage of fundamental MnO₆ octahedral units) [217]. The β-MnO₂ phase crystallizes with the rutile-like tetragonal symmetry (P4₂/mnm (136) space group) and possesses 1 × 1 tunnels (0.23 nm × 0.23 nm). The α-MnO₂ phase is widely used in batteries and supercapacitors due to its large 2 × 2 tunnel structure (tetragonal, I4/m (87) space group) along c-axis with MnO₆ octahedral units, which favors short diffusion path and easy ionic transport. However, the low conductivity of α-MnO₂ (~10⁻⁵ S cm⁻¹) inhibits its application in high power devices. To overcome these drawbacks, workers have fabricated MnO₂ nanocomposites using carbon-based materials as conducting additives. Hydrothermal method is the most popular route to prepare MnO₂/rGo composite using various reaction times [218,219]. Other techniques include in-situ electrodeposition [220], spray coating [221], ionic layer adsorption and reaction (SILAR) method [222], redox reaction [223], precipitation method [224], room temperature solution method [225], and co-electrodepossition [226].

Zhang et al. [227] reported the electrochemical performance of β-MnO₂/rGO composite anode synthesized by hydrothermal route at 160 °C for 6 h in solution containing HCl. The composite product cycled in the voltage range of 0.01–3.0 V at 100 mA g⁻¹ current rate delivered an initial discharge/charge capacity of 1475/746 mAh g⁻¹ and maintained a sustainable reversible capacity of 448 mAh g⁻¹ over 50 cycles. The same group of researchers used the as-prepared composite as anode material in an all-solid-state lithium-ion battery with Li₇La₃Zr₂O₁₂ as the solid electrolyte (20 µm thick) and LiCoO₂ as the cathode material. When cycled at 2 µA cm⁻² current density, the (MnO₂/rGO)/Li₇La₃Zr₂O₁₂/LiCoO₂ cell delivered a capacity of 1.87 µAh cm⁻² [228]. Ali et al. [229] utilized a one-step electrochemical synthesis to generate MnO₂/rGO nanocomposite from Mn₃O₄ and GO starting materials. Performing a simple in-situ hydrothermal method at 160 °C for 6 h, Liu and coworkers [230] used MnO₂ nanorods (a-phase) for the fabrication of MnO₂/3D-rGO composites with a large specific surface area of ~150 m² g⁻¹. Galvanostatic discharge/charge measurements showed that the first discharge occurs as a long potential plateau at ca. 0.31 V attributed to the reduction of MnO₂ to Mn (MnO₂ + 4Li⁺ + 4e⁻ ↔ Mn + 2Li₂O) and a weak potential plateau at ca. 0.03 V assigned to the Li insertion into graphene (C + xLi⁺ + xe⁻ ↔ Li_xC). The initial discharge capacity of 1515 mAh g⁻¹ decreased to 812 mAh g⁻¹ in subsequent cycles and reached the value ~595 mAh g⁻¹ after 60 cycles at 0.1 A g⁻¹ current rate and high CE of 99%. A MnO₂ nanoflake (birnessite type)–reduced graphene oxide (MnO₂/rGO) composite was synthesized by a one-step solution method (i.e., redox reaction of KMnO₄ and rGO in 0.5 mol L⁻¹ H₂SO₄ solution) [231]. The composite yielded a reversible capacity of 1430 and 520 mAh g⁻¹ at current densities of 0.1 and 10 A g⁻¹, respectively, and a capacity retention of 1000 mAh g⁻¹ after 200 cycles at the current density of 1 A g⁻¹. Lee et al. [225] synthesized rGO/a-MnO₂ nanocomposites via a simple solution method at room temperature using 1 wt% cetyltrimethylammonium bromide (CTAB) in the rGO suspension as surfactant and protection layer to prevent direct reaction between rGO and MnO₄⁻ ions. With a MnO₂ loading of 75 wt%, the BET surface area was 324 m² g⁻¹ and the average pore size was evaluated as 3–4 nm. The discharge and charge capacities of the as prepared nanocomposite in the first cycle were ~1450 and ~850 mAh g⁻¹, respectively, at 100 mA g⁻¹ current density in the voltage range 0.01–3.0 V. Discharge specific capacities of 222 and 115 mAh g⁻¹ at a current density of 5 and 10 A g⁻¹, respectively, maintained after 400 cycles. Chae et al. [232] suggested to prepare MnO₂/rGO composites using an unrecognized chemical method by induction of intra-stacked assembly from MnO₂ NRs (birnessite d-MnO₂) and GO through polyethylenimine (PEI)-derived electrostatic modulation between both constituents. The hierarchical, 2D MnO₂/rGO hybrids (73.2:26.8 in weight) enabled highly reversible capacities of 880, 770, 630, and 460 mAh g⁻¹ at current densities of 0.1, 1, 3, and 5 A g⁻¹, respectively.

Highly performing rGO/MnO₂ mesoporous nanocomposites were fabricated in a Teflon-lined stainless-steel autoclave kept at 150 °C for 15 h using pre-synthesized a-MnO₂ nanowires (diameter of 40–50 nm and length of 5–10 µm) dispersed in solution with GO sheets [233]. The rGO/a-MnO₂ composite anode featured a reversible specific capacity of 998 mAh g⁻¹ at a current density of 60 mA g⁻¹ after 30 cycles and rate capability of 590 mAh g⁻¹ at 12 A g⁻¹ current rate. The outstanding electrochemical performance is due to the increase of the BET specific surface area of the nanowires from 37.7 to 110.7 m² g⁻¹ after graphene coverage and the inhibition of the MnO₂ nanowires agglomeration by graphene coating. Ma et al. [234] mitigated the capacity fading of MnO₂ at high cycling rates by anchoring MnO₂ nanorods (average diameter of ~100 nm and length of ~750 nm) on the surface of graphene. The composite was prepared by hydrothermal route at 160 °C for 18 h using a mixture of MnO₂ nanorods (synthesized from aqueous solution of MnSO₄∙H₂O and KMnO₄ (ratio 2:5 in wt%)), GO sheets (prepared by the modified Hummers’ method), and (3-Aminopropyl) triethoxysilane (APTES) as reducing agent. The MnO₂/rGO composite showed superior electrochemical performance with reversible capacity of 600 mAh g⁻¹ after 650 cycles at 0.5 A g⁻¹ current density. When cycled at a high rate of 5 A g⁻¹, a reversible specific capacity of 168 mAh g⁻¹ was delivered (Figure 7). Li et al. [235] designed a polypyrrole nanoparticles (~10 nm) wrapped MnO₂/reduced graphene oxide/carbon nanotubes composite (PPy/MnO₂/rGO-CNTs). In this heterostructure, CNTs act as conductive medium and rGO nanosheets work as both reactive material and carrier for in situ growth of MnO₂. The PPy/MnO₂/rGO-CNTs shell-layer anode achieved a reversible specific capacity of 1748 mAh g⁻¹ at 100 mA g⁻¹ current density after 200 cycles and still features 941 mAh g⁻¹ at 1 A g⁻¹ current rate after 1200 cycles. Flexible graphene-wrapped carbon nanotube/graphene@MnO₂ 3D multilevel porous films were fabricated via vacuum freeze-drying [236]. The outstanding nanosized pores provided vast accessible active sites and alleviated volume change. The CNT/rGO@MnO₂ porous film as an anode in lithium cell showed a high capacity of 1344.2 mAh g⁻¹ over 630 cycles at 2 A g⁻¹, and superior cycling stability of 608.5 mAh g⁻¹ over 1000 cycles at 7.5 A g⁻¹.

Kim et al. [237] employed a new strategy to construct sequentially stacked a-MnO₂/rGO composites driven by surface-charge-induced mutual electrostatic interactions. The as-prepared hybrid displayed a reversible charge/discharge capacity of 1100 mAh g⁻¹ without capacity fading after 100 cycles. A free-standing reduced graphene oxide/MnO₂-reduced graphene oxide-carbon nanotube nanocomposite was fabricated using vacuum filtration and thermal annealing [238]. The rGO/MnO₂/CNTS hybrid anode with 56 wt% MnO₂ cycled 100 times at 0.1 A g⁻¹ current rate yielded a specific capacity of 1172 mAh g⁻¹, as 71% of the initial capacity.

Layered manganese oxides (birnessite δ-MnO₂ phase) are also considered as promising cathode materials for lithium-ion and lithium-sulfur batteries, and supercapacitors. Ultrathin layered MnO₂ nanosheets were attached on the surface of rGO sheets by one-pot precipitation method to form 2D MnO₂/rGO nanocomposites used as cathode materials for Li-ion batteries [224]. The initial discharge-specific capacity of 279.7mAh g⁻¹ was maintained at 242 mAh g⁻¹ after 60 cycles at 0.1C rate. The synthesis of MnO₂/rGO nanocomposites was realized using ultrasonication with Mn powder under mildly acidic conditions, which is a efficient reducing agent for GO and the MnO₂ precursor [239]. Ternary composites of GO-polyaniline (PANI)-MnO₂ nanorods were synthesized by a two-step process: (a) Coating solution-exfoliated GO nanosheets with polyaniline and (b) dispersion of graphene oxide sheets and MnO₂ nanorods in solution by ultrasonication and self-assembly during the second aniline polymerization [240]. In such a hybrid, the PANI layer anchors MnO₂ nanorods onto the surface of GO and prevents GO sheets and MnO₂ nanorods from stacking/aggregating. Chan et al. prepared rGO-wrapped MnO₂ composite electrode by an electrodeposition method using an electrolyte formed a solution of manganese acetate with L-ascorbic acid added to a dispersed solution of GO. A 13-min electrodeposition process leads spherical shaped-MnO₂ nanoparticles (~100 nm) and absence of rGO agglomeration [241]. Recently, Yang et al. [219] demonstrated that the MnO₂/rGO/S composite prepared by one-step hydrothermal method using GO and hollow MnO₂ spheres (birnessite-type) as precursors can facilitates a fast-electronic transfer. The MnO₂/rGO/S composite exhibited a BET specific surface area and the total pore volume 15.3 m² g⁻¹ and 0.059 cm³ g⁻¹, respectively. This cathode showed a first discharge specific capacity of 1202 mAh g⁻¹ at current density of 0.2C (1C = 1675 mAh g⁻¹) and a capacity retention of 61.4% after 100 cycles at 1C rate, which corresponds to only ~0.38% decay per cycle.

The syntheses and supercapacitive behaviors of composite materials combining either GO or rGO with manganese-based oxide have been the subjects of intensive researches. The high capacitance values of MnO₂/rGO composites are most likely attributed to the synergistic effect originating from the high surface area of MnO₂ nanoparticles, the high conductivity of rGO nanosheets, and the high porosity of nanocomposites. Sawangphruk et al. [221] produced MnO₂/rGO nanocomposites on graphitized carbon fiber paper by a spray coating method. The as-prepared MnO₂/rGO composite displayed a specific surface area of 490 m² g⁻¹, which is slightly lower than that of pure rGO (510 m² g⁻¹) indicating that the composite is rather stable and still dispersed. A specific capacity of 393 F g⁻¹ at a scan rate of 10 mV s⁻¹ was obtained in 0.5 mol L⁻¹ Na₂SO₄ electrolyte. Zhu et al. assembled an interconnected rGO/b-MnO₂ binary hybrid electrode with ultrafine MnO₂ nanostructure and precisely mass-loading control for high performance ESC [242]. The typical hybrid hydrogel also showed outstanding cycling stability with 96.3% capacitance retention after 10,000 cycles. In the work of Chen et al. [243], needle-like MnO₂-GO nanocomposites were fabricated by soft-chemical route using the double-solvent system of a water-isopropyl alcohol. These composites demonstrated the specific capacitance of 197 F g⁻¹ at 0.2 A g⁻¹ current density. Wang et al. synthesized MnO₂ nanowire-GO and MnOOH nanowire-GO hybrids for ESCs using a redox reaction between KMnO₄ and GO, and hydrothermal reaction without any extra reductants, respectively. The maximum specific capacitances of 155 and 76 F g⁻¹ at current density of 0.1 A g⁻¹ were obtained, respectively, in 5% ammonia aqueous solution [223]. Lu et al. [244] demonstrated that the CNTs/RGO/MnO₂ composite (with the d-MnO₂ phase) as electrode of ESC in 2 mol L⁻¹ Na₂SO₄ electrolyte delivered specific capacitances of 404 and 82 F g⁻¹ at 1.0 and 10 A g⁻¹ current densities. The long-term charge–discharge test indicates that the specific capacitance remained 70% after 5000 cycles under current density of 30 A g⁻¹.

In the work of Hareech and coworkers, a ternary nanocomposite, which consists of MnO₂ nanorods and reduced graphene oxide sheets supported on poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) polymer, was developed for ESC applications [245]. Electrochemical studies demonstrate a specific capacitance of 633 F g⁻¹ at 0.5 A g⁻¹ current density and 100% stability over 5000 cycles at 1 A g⁻¹ rate in acetonitrile containing lithium perchlorate as electrolyte. Seredych et al. reported the preparation of MnO₂/GO composites containing only 10 wt% GO of different oxidation levels and low crystallization birnessite-type MnO₂; specific capacitance of 125 F g⁻¹ at current density of 0.1 Ag⁻¹ was obtained [246]. Nanostructured a-MnO₂/rGO composites with different weight feed ratios of graphene to MnO₂ (1:1, 1:2 and 2:1) were synthesized by coprecipitation under alkaline conditions. The a-MnO₂/rGO (1:1) hybrid, which exhibited a BET specific surface area of 174 m² g⁻¹ and ~26 nm average pore diameter had a specific capacitance of 367 F g⁻¹ at current density of 1 A g⁻¹ (i.e., 4.6-fold higher than of MnO₂) [247]. Miniach et al. [248] studied a MnO₂/rGO via a hydrothermal technique with good specific capacitance of 157 F g⁻¹ and energy density of 21 Wh kg⁻¹. Chen et al. [249] offered a rGO/MnO₂/carbon black ternary hybrid with desirable specific capacitance of 209 F g⁻¹ and higher energy density of 24.3 Wh kg⁻¹ in 0.5 mol L⁻¹ Na₂SO₄ aqueous electrolyte at room temperature. In this study, the rGO/MnO₂ hybrid films were prepared using a vacuum filtration route by the mixture of the KMnO₄ solution with the rGO dispersion through cellulose esters filter. A flexible asymmetric ESC including CuO@MnO₂/rGO and MoS₂/rGO as positive and negative hybrid electrodes, respectively, showed only 8.1% capacitance loss after 10,000 cycles and an outstanding energy density of 165.6 Wh kg⁻¹ and a power density of 1992 W kg⁻¹ [250]. PANI/MnO₂/graphene composite was prepared by pulse electrodeposition method (current density of 2 mA cm⁻², pulse frequency of 1 kHz). The γ-phase MnO₂ nanoparticles are uniformly distributed on the PANI nanorods, which increases the specific surface area [251]. A MnO₂/rGO/CNTs framework was designed to inhibit the shuttle effect of lithium-sulfur batteries. It was fabricated by hydrothermal method at 180 °C for 12 h using KMnO₄, GO, CNTs, and H₃PO₄∙H₂O. The hybrid was composed by MnO₂ short nanorods (α-phase, diameter of 50 nm, and length of ~200 nm) compounded with carbon nanotubes (diameter of 20–50 nm) and tulle-like rGO and exhibited a mesoporous structure with average pore size of ~12 nm [252]. The MnO₂/rGO/CNTs/S cathode delivered an initial specific capacity of 1010 mAh g⁻¹ at 0.5 C rate and achieved 780 mAh g⁻¹ after 200 cycles (i.e., capacity decay rate of 0.11% per cycle).

4.3.2. Mn₃O₄-Based Composite

Mn₃O₄ (hausmannite, tetragonal structure) has gained much attention as anode material due to its high reversible capacity, low oxidation potential, abundance, and environmental friendliness [253]. Mn₃O₄ can react with 8 Li ions per formula unit (i.e., conversion reaction to form lithium oxide and metal nanoparticles: Mn₃O₄ + 8Li⁺ + 8e⁻ ↔ 3Mn + 4Li₂O) leading to a theoretical specific capacity of about 936 mAh g⁻¹ [254]. For the first time, in 2010, Wang and coworkers [255] reported the deposition of Mn₃O₄ nanoparticles onto rGO sheets as anode materials for LIBs. Mn₃O₄/rGO hybrids were fabricated by two-step solution phase reaction, i.e., hydrolysis in DMF followed by a hydrothermal process in water. The authors stated that Mn₃O₄ nanoparticles were possibly grown on the oxygen functional group on GO. With the assistance of rGO, this composite exhibited a capacity of 900 and 390 mAh g⁻¹ at 40 and 1600 mA g⁻¹, respectively.

Mn₃O₄/rGO composites have been synthesized using combined techniques such as co-precipitation and subsequent flame procedure [256,257,258], hydrothermal method [259,260], in situ reduction by hydrazine vapour [261], sonication with subsequent heat treatment [262], gel formation and electrochemical reduction process [263], one-step reduction in aqueous phase [264,265], successive ionic layer adsorption and reaction (SILAR) method [222], flame plasma [256], and sonochemistry-assisted method [266,267]. Gund et al. investigated the capacitive performance of hybrid Mn₃O₄/GO composite prepared by the SILAR method, which implicates the sequential immersion (20 s duration) of the charged substrate in separately placed cationic (MnSO₄) and anionic (NaOH) solution, with the deposition of the monolayer of different species on the substrate in one cycle. The composite electrode with Mn₃O₄ nanograins of 20 nm and BET specific surface area of 94 m² g⁻¹ showed the specific capacitance of 344 and 180 F g⁻¹ at scan rate of 5 and 20 mV s⁻¹, respectively, in 1 mol L⁻¹ Na₂SO₄ electrolyte [222]. Nanosized Mn₃O₄ particles grown on rGO were prepared by precipitation method via refluxing in solution containing MnCl₂·4H₂O and KMnO₄ added to GO dispersion with various MnCl₂·4H₂O/GO ratios [268]. The Mn₃O₄/rGO hybrid with 72% Mn₃O₄ (~30 nm in size) delivered an initial discharge capacity of ~1226 mAh g⁻¹ at 120 mA g⁻¹ current rate, higher than the theoretical value of 989 mAh g⁻¹ because of high surface area and charge accumulation at active sites commonly observed in nanomaterials [269]. However, the best rate capability and cyclability were obtained with the composition of 54% Mn₃O₄ (i.e., a capacity of 531 mAh g⁻¹ was maintained after 40 cycles at 0.1 C rate) due to the minimum particle agglomeration. Seong et al. [262] compared the performance of an acid-treated rGO/Mn₃O₄ nanorods (ArGO/Mn₃O₄ NRs) composite with that of its parent rGO/Mn₃O₄ NRs synthesized by a simple mixing of GO and MnOOH (3:1 weight ratio) using sonication followed by heat treatment at 400 °C for 5 h in N₂ atmosphere. The ArGO/Mn₃O₄ NRs and rGO/Mn₃O₄ NRs had a mesoporous morphology with BET specific surface areas of 73.8 and 48.1 m² g⁻¹, respectively. Tests carried out at 200 mAh g⁻¹ current density in the potential range 3.0–0.01 V showed that rGO/Mn₃O₄ and ArGO/Mn₃O₄ electrodes deliver an initial discharge capacity of 1100 and 1130 mAh g⁻¹ and then a reversible capacity of 695 and 778 mAh g⁻¹, respectively. The first cycling capacity loss was due to the formation of the SEI and the reduction of Mn₃O₄ (Mn³⁺) to MnO (Mn²⁺), which occurs as a broad cathodic peak in the range 0.5–1.9 V. In the second cycle, the strong cathodic peak at ca. 0.35 V is assigned to the reduction of MnO (Mn²⁺) to MnO, while the anodic peak at 1.3 V is the fingerprint of the oxidation reaction of Mn to MnO.

More recently, Rosaiah et al. [270] reported the Mn₃O₄@rGO composite for ESCs and LIBs. Nithya et al. demonstrated the application of the Mn₃O₄@rGO composite for K-ion Batteries [271]. The rGO/Mn₃O₄ hybrid anode reported by Nam et al. [272] delivers a reversible capacity of 1294 mAh g⁻¹ at 100 mA g⁻¹ after 100 cycles. A low-cost and green ultrasound-assisted method was developed by Luo [273]. For the rapid synthesis of Mn₃O₄ nanosheets supported on rGO, Mn(OAc)₂ and GO obtained according to the Hummers’ method were dispersed in aqueous solution in the presence of tert-butylamine. Such anode materials yielded a high specific capacity of ~1400 mAh g⁻¹ after 40 cycles at a current density of 100 mA g⁻¹ (based on the mass of Mn₃O₄). A coprecipitation method was used to fabricate a 3D GO/Mn₃O₄ hybrid, which exhibited a large specific capacity of 949 mAh g⁻¹, CE of ~98%. After 100 cycles at 100 mA g⁻¹ current rate, the specific capacity remained at 792 mAh g⁻¹ with a CE of 98.1% [257]. Li et al. [274] took advantage of C-O-Mn chemical bonds to prepare graphene nanosheets dispersed in porous Mn₃O₄ making a 3D Mn₃O₄/rGO hybrid, which displayed excellent performance as anode of LIBs and electrode of ESCs by simply tuning the graphene content (<7 wt%). As LIB anode, with 6.9 wt% rGO, BET specific surface area of 153 m² g⁻¹, and ~5 nm pore size, the composite featured a reversible capacity of ~1500 mAh g⁻¹ at 50 mA g⁻¹, and 561 mAh g⁻¹ at 1 A g⁻¹. Zhang et al. [275] took advantage of the effective synergy between MnCO₃ and Mn₃O₄ to fabricate a multicomponent hybrid with the configuration MnCO₃/Mn₃O₄/rGO. In a typical experiment, a GO/Mn suspension (yellow color) was first obtained by mixing GO prepared by the modified Hummers’method with KMnO₄ at 0 °C for 3 h. Second, the as-prepared GO/Mn suspension was treated ultrasonically for 1 h to exfoliate the GO layers and added to Na₂CO₃ solution (1 mol L⁻¹) under stirring to adjust the pH value of the whole suspension to 11. Then, reduction of GO to rGO was obtained in an alkaline solution maintained at 95 °C for 2 h. Parents MnCO₃/rGO and Mn₃O₄/rGO composite were prepared using similar procedure changing the pH and the alkaline solution, respectively (Figure 8A). The MnCO₃/Mn₃O₄/rGO ternary composite displayed a cycling stability of 988 mAh g⁻¹ after 200 cycles at 0.1 A g⁻¹ current rate and 532 mAh g⁻¹ after 800 cycles at 1 A g⁻¹ (Figure 8B).

The Mn₃O₄/rGO designed by Shah et al. was prepared by the hydrothermal treatment at 180 °C for 18 h of an aqueous alkaline solution of MnCl₂, GO, and NaOH [276]. The presence of Mn₃O₄ nanoparticles on the surface of graphene nanosheets makes the structure with porous morphology. The rGO-Mn₃O₄ electrode tested over 5000 cycles displayed a high specific capacitance of 457 F g⁻¹ at 1 A g⁻¹ current rate. By means of a novel surfactant-free, single-step hydrothermal method, Varghese et al. [277] synthesized the Mn₃O₄ nanostructures and their rGO composites. The morphology of Mn₃O₄/rGO composite revealed that Mn₃O₄ nano-octahedrons (~100–150 nm in size) were wrapped in the graphene matrix. The Mn₃O₄/rGO‖Li half-cell showed a reversible specific capacity of 474 mAh g⁻¹ at 100 mA g⁻¹ upon 200 cycles. Further, a high capacitive contribution of ~73% (evaluated at 1.0 mV s⁻¹ sweep rate) contributed to fast Li⁺ ion kinetics of this anode.

4.3.3. MnO-Based Composites

Manganese monoxide (MnO) has drawn considerable attention as anode material for LIBs due to its high theoretical capacity of 755.6 mAh g⁻¹ and relatively low voltage hysteresis. However, poor cyclic performance and inferior rate capability caused by the severe particle agglomeration and large volume expansion are challenging issues. Exfoliated graphene oxide (EGO)/MnO composite (35/65 wt%) powders synthesized by simple solid state graphenothermal reduction process (specific surface area of ~82 m² g⁻¹ and an average pore size of ~12 nm) were formed by cubic MnO crystallites that anchored onto EG surfaces. Tested as an anode in lithium-ion batteries, the EGO/MnO composite exhibited high reversible capacity of 936 mAh g⁻¹ at a current rate of 75 mA g⁻¹ with a capacity retention of ~84% and CE of 99% over 100 cycles [278]. The reversible conversion reactions of MnO and electrochemical absorption/desorption by EGO were evidenced by cyclic voltammetry studies showing easy lithiation kinetics. An ever-increasing surface redox capacitive lithium storage device was fabricated with a sandwich nanostructure. MnO was electrodeposited on rGO, forming interconnected nanorods (15–30 nm in diameter) with random orientation covered by depositing the top GO layer. The as-prepared RGO–Mn_xO_y–GO film was then heat treated at 600 °C for 6 h in H₂/Ar (5:95) atmosphere to achieve the RGO–MnO–RGO product. The as-designed rGO-MnO-rGO sandwich nanostructures displayed a surface pseudocapacitance dynamically in equilibrium with the diffusion-controlled process leading ultrahigh-rate capability of ~332 mAh g⁻¹ at 40 A g⁻¹ and ~379 mAh g⁻¹ after 4000 cycles at 15 A g⁻¹ was achieved due to the dynamical equilibrium between surface pseudocapacitance and diffusion-controlled ion storage [279]. Cheng et al. [280] developed a MnO-based sandwiched composite within 3D graphene-based N-doped carbon (NC) networks. The NC/MnO/rGO nanohybrid was prepared by precipitation assisted by polydopamine followed by heat treatment at 700 °C for 2 h in H₂/Ar atmosphere. HRTEM analysis showed that MnO nanoparticles (40 nm in size) were completely wrapped by a uniform amorphous carbon layer with a thickness about 2 nm, and then anchored on the rGO matrix. It demonstrated high reversible capacity of 1360 mAh g⁻¹ at 0.2 A g⁻¹ over 150 cycles, excellent rate capability, and good cyclability (648 mAh g⁻¹ at 2 A g⁻¹ without fading over 600 cycles). A composite anode, which consisted of rGO-anchored MnO within an nitrogen-codoped carbon coating (rGO/MnO@NC) was prepared under solvothermal conditions (at 180 °C for 15 h) of suspension of rGH sheets with manganese acetate followed by a two-step heat treatment with dopamine to produce the rGO/MnO@NC final hybrid (at 400 °C for 2 h with a heating rate of 1 °C min⁻¹ under a pure argon flow and then at 800 °C for 2 h at a heating rate of 3 °C min⁻¹). This composite electrode presented high specific capacity of 699 mAh g⁻¹ at 0.1 A g⁻¹, and excellent cycling performance of 607 mAh g⁻¹ at 1 A g⁻¹ over 550 cycles [281].

Table 4. Comparison of the electrochemical behaviors of Mn-based oxide/rGO composite anodes of rechargeable lithium batteries. The relevant graphene content in wt% (a) and cycle number (b) are displayed in brackets.

Material (a)	Synthesis	Specific Capacity (mAh g−1)	Current Density (b) (A g−1)	Ref.
b-MnO2/rGO	hydrothermal @160 °C/6 h	448	0.1 (50)	[227]
a-MnO2/3D-rGO	in situ hydrothermal @ 60 °C/6 h	595	0.1 (60)	[230]
d-MnO2 NFs/rGO	redox reaction with KMnO4	1000	1 (200)	[231]
a-MnO2/rGO	solution method with CTAB	222	5 (200)	[225]
a-MnO2/rGO	hydrothermal @150 °C/15 h	590	12 (30)	[233]
MnO2 NRs/rGO	hydrothermal @160 °C/18 h	600	0.5 (650)	[234]
d-MnO2 NRs/GO	sonication	460	5 (80)	[232]
CNT/rGO@MnO2	vacuum freeze-drying	608	7.5 (1000)	[236]
PPy/MnO2/rGO-CNTs	vacuum filtration	941	1 (1200)	[235]
rGO/MnO2/CNTS	vacuum filtration + thermal annealing	1172	0.1 (100)	[238]
Mn3O4/rGO (10)	two-step solution-phase reaction	900	0.04 (5)	[255]
MnO/GO (9.17)	impregnation + thermal reduction	500	0.2C (32)	[282]
N-MnO/rGO (8.9)	hydrothermal + ammonia annealing	772	0.1 (90)	[283]
MnO/rGO (75)	in-situ carbothermal reduction	782	0.1 (60)	[284]
MnCO3/Mn3O4/rGO	sonication + alkaline solution	998	0.2 (100)	[275]
Mn3O4-MnO/rGO (12)	one-pot solvothermal	825	0.05 (30)	[285]
MnO/rGO (72.5)	two-step liquid phase deposition	665	0.1 (50)	[286]
MnO NWs/GO	dialysis	930	0.1 (470)	[287]
MnO NRs/GO (7.58)	in-situ reduction	705	0.05 (100)	[288]
MnO/rGO (6)	pyrolysis	950	0.1 (5)	[289]
MnO NSs/rGO	hydrolysis + calcination	648	0.75 (50)	[290]

summarizes the electrochemical performances of Mn-based oxide/rGO composite anodes of rechargeable lithium batteries.

4.4. Molybdenum-Based Oxide Composites

Since 1987, MoO₂ has been recognized as anode material for lithium-ion batteries [291]. Bulk MoO₂ possesses excellent physical properties with a metallic conductivity of ~10⁻⁴ S cm⁻¹ at room temperature and a theoretical specific capacity as high as 838 mAh g⁻¹. It adopts the distorted variant of the rutile-like structure (monoclinic, P21/c space group) in which the metal atoms occur in pairs along the c axis of the rutile pseudo-cell [292]. In the 0.01–3.0 V potential range, the lithiation of the MoO₂ electrode investigated by cyclic voltammetry occurs with two steps (first discharge): (1) The Li insertion (Li_0.98MoO₂) evidenced by the 1.52 and 1.24 V reduction peaks attributed to the monoclinic orthorhombic phase transition and (2) below 0.7 V reduction peaks assigned to the conversion of MoO₂ into elemental Mo with ~3Li uptake [293,294]. However, inherent disadvantages of bulk MoO₂ anode are the low reversible capacity due to sluggish lithiation/delithiation kinetics [295], the substantial volume change during lithiation process, which result in rapid capacity fading and poor capacity retention [296].

Graphene oxide was used as a reducing agent of MoO₃ (molar ratio of 4:1) and as a source for exfoliated graphene oxide (EGO) to fabricate EGO/MoO₂ composites with 46 wt% graphene [297]. The small particle size of ~33 nm indicates that agglomeration of MoO₂ has been controlled with the increased amount of EGO. The initial capacity of 713 mAh g⁻¹ increased slightly to 878 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹ current rate in the voltage window 0.005–3.0 V. Kinetics investigated by cyclic voltammetry revealed high values of the diffusion coefficients of Li ions in the range 6 × 10⁻¹⁰–1 × 10⁻¹¹ cm² s⁻¹ during Li_xMoO₂ lithiation (0 < x < 4). Hu et al. [298] prepared MoO₂/GO composites via hydrothermal route at 200 °C for 24 h using Mo powder and exfoliated GO produced by sonication. MoO₂ nanoparticles (5−15 nm in size) were homogeneously dispersed on GO. When tested as an anode material for lithium, the MoO₂/GO composite displayed an initial specific capacity of 780 and 500 mAh g⁻¹ at 0.1 and 0.5 A g⁻¹, respectively. The beneficial contribution of exfoliated GO was evidenced by EIS measurements, as the charge transfer resistance of 44 W for MoO₂/GO was much lower than 300 W for pure MoO₂ nanoparticles, due to more conductive pathway for Li-ion transportation. MnO₂/graphene composite was also synthesized by an effective polymer-assisted chemical reduction method, which provides nanosized MnO₂ particles homogeneously distributed on graphene nanosheets [299].

Huang and coworkers [300] fabricated a composite of MoO₂ nanoparticles (~100 nm) anchored on GO for sodium-ion battery anodes. This MoO₂/GO hybrid (15 wt% GO) yielded a discharge capacity of 483 mAh g⁻¹ (~2318 mAh cm⁻³) at the current density of 100 mA g⁻¹ and a capacity fading of 0.019% per cycle over the first 1000 cycles. Tang et al. reported the electrochemical properties of rGO-wrapped MoO₂ porous nanobelts (rGO/MoO₂ NBs) synthesized using a reduction process of the GO-MoO₃ hybrids with Ar/ethanol vapor at 400 °C for 12 h [301]. The full-scale multi-electron conversion reaction in rGO/MoO₂ NBs (with 8 wt% rGO) led to a high reversible capacity of 974 mAh g⁻¹ at the current density of 60 mA g⁻¹ after the 6th cycle and, after initial activation, a specific capacity of 420 mAh g⁻¹ was retained at the end of 1900 cycles at 5 A g⁻¹ rate. Zhu et al. [302] demonstrated that rGO/MoO₂ nanobelt composite (with 19.4 wt% rGO) synthesized by a simple thermal reduction of MoO₃ and GO at 550 °C for 2 h in a 10% H₂/Ar atmosphere had a BET specific surface area and pore volume of 17.68 m² g⁻¹ and 0.09 cm³ g⁻¹, respectively. This self-assembled hybrid showed attractive electrochemical performance as LIB anodes with specific discharge capacity reaching 584 mAh g⁻¹ after 100 cycles at 1 A g⁻¹.

In 2017, Ju and coworkers [303] designed a TiO₂/rGO/MoO₂@Mo composite electrode for ESC. This architectural nanostructure with 3D walnut-shape synthesized by one-step hydrothermal method was composed of amorphous TiO₂ decorating the rGO/MoO₂ surface grown on Mo foam. It exhibited a specific capacitance of 1636 F g⁻¹ at 1.25 A g⁻¹ with only 3.5% capacitance loss after 5000 cycles. Tightly anchored MoO₂ nanoparticles (10–20 nm in size) deposited on the basal plane of rGO were fabricated using a green supercritical methanol (scMeOH) route. When tested as an anode in lithium cells, the composite with 37 wt% MoO₂ loading (BET surface area of 14.3 m² g⁻¹, 15% porosity) delivered a reversible capacity of 793 mAh g⁻¹ at 50 mA g⁻¹ current rate [304]. Graphene oxide flexibly supported MoO₂ porous hybrids were constructed by decomposition of (NH₄)₂MoO₄/GO preforms with the percentage of GO at 4.3, 15.2, and 20.8 wt%. The lithium storage performance was investigated at the current density of 0.1 A g⁻¹ using various MoO₂/GO architectures showing specific capacities of 901, 1127, and 967 mAh g⁻¹ after 100 cycles for micrometer MoO₂/GO, MoO₂/GO nanohoneycomb, and layered MoO₂/GO, respectively. The MoO₂/GO nanohoneycomb delivered the capacity of 461 mAh g⁻¹ at 5 A g⁻¹ current rate with CE of 99.3% [305]. Wang et al. [306] synthesized 2D layered mesoporous-MoO₂/rGO composites (m-MoO₂/rGO) through a novel strategy using silica KIT-6/rGO as a template and ammonium molybdate as a precursor via a nanocasting method with subsequent heat treatment heated up to 600 °C for 6 h at a rate of 2 °C min⁻¹ in a mixed gas flow of H₂/Ar (10:90). Analyses of the morphology validated the ordered mesoporous structure with a BET specific surface area of 86 m² g⁻¹, a pore size of 3–4 nm in a large domain, and a pore volume of 0.19 cm³ g⁻¹. The m-MoO₂/rGO (1:1) electrode delivered an initial discharge capacity of 1160 mAh g⁻¹ at current density of 0.1 A g⁻¹ with a capacity loss of 31.9% during the first cycle and a reversible capacity is 801 mAh g⁻¹ after 50 cycles.

In 2018, Chen et al. [307] investigated a hierarchical MoO₂@rGO (~10 nm MoO₂ particle size and 21.1 wt% rGO) composite prepared by hydrothermal route at 200 °C for 2 days, which exhibited outstanding electrochemical performance with a reversible capacity of 708 mAh g⁻¹ at a current density of 0.5 A g⁻¹ after 50 cycles. A capacity of 473 mAh g⁻¹ was maintained when cycled at 2 A g⁻¹ current rate. Li et al. [308] utilized Mo particles as both the reductant for GO and as the Mo source to prepare MoO₂/rGO hybrid anodes. The MoO₂/rGO composite with Mo:GO of 3:1 was used as an efficient cathode catalyst for the oxygen reduction reaction (ORR). Recently, Liu et al. [309] reported the direct growth of MoO₂/rGO hollow sphere composites through hydrothermal technique. This advanced anode materials tested in potassium-ion batteries delivered a reversible specific capacity of 219 mAh g⁻¹ after 200 cycles at 50 mA g⁻¹ current rate. When cycled at 0.5 A g⁻¹, a high charge specific capacity of 104 mAh g⁻¹ was achieved after 500 cycles. Li et al. [310] prepared the MoO₂/Mo-GO hybrids using a freeze-drying technique, which exhibited a high ICE of 97%. A capacity of 550 mAh g⁻¹ ws delivered by the MoO₂/Mo-GO anode materials after 150 cycles at 0.1 A g⁻¹ rate. Devina et al. [311] used the same technique to prepare MoO₂/rGO composite attached with Mo₂C during carbothermal hydrogen reduction. The MoO₂/Mo₂C/rGO product displayed an increased initial coulombic efficiency (77%), long-term cyclability (500 mAh g⁻¹ at 50 mA g⁻¹ after 150 cycles), and high-rate performance (200 mAh g⁻¹ at 1 A g⁻¹) compared with those of the MoO₂/rGO composite (53%; 280 mAh g⁻¹ at 50 mA g⁻¹ after 150 cycles; 120 mAh g⁻¹ at 1 A g⁻¹).

Recently, the fabrication of submicrometer-sized MoO₂@MoS₂/rGO composite with a hierarchical core-shell structure was suggested by Zheng et al. [312]. The three-step synthesis includes: (i) The preparation of the MoO₂@MoS₂ precursor using a solid-state reaction (i.e., MoO₃ and sulfur heated at 600 °C for 1 h in a gas stream of Ar/H₂ (90:10)), (ii) a hydrothermal process at 180 °C for 4 h using a GO suspension, and (iii) a final heat treatment at 400 °C for 2 h in Ar/H₂ flowing gas. The electrochemical tests of the MoO₂@MoS₂/rGO anode showed an initial reversible capacity of 833 mAh g⁻¹ with ICE of 80.6% and improved cycling stability (733 mAh g⁻¹ after 80 cycles at 0.2 mA g⁻¹). The good rate capability with a capacity of 645 mAh g⁻¹ at 4 A g⁻¹ was also evidenced. An assembled MoO₂@MoS₂/rGO‖LiCoO₂ full cell maintained a reversible capacity of 694 mAh g⁻¹ (based on the mass of MoO₂@MoS₂/rGO) at 0.2 mA g⁻¹ after 40 cycles. Electrochemical performance of MoO₂/rGO composite anodes for LIBs from the recent literature are summarized in

Table 5. Electrochemical performance of MoO₂/GO composites as anode materials for Li-ion batteries. The relevant cycle number is displayed in brackets.

Material	Synthesis	Reversible Capacity (mAh g−1)	Current Rate (A g−1)	Ref.
MoO2/Gr	layer-by-layer assembly	676	48 (100)	[313]
MoO2/Gr	sonication in water (11.2)	597	1000 (70)	[26]
MoO2/rGO	solvothermal in ethanol (10.0)	714	100 (30)	[314]
MoO2/rGO	hydrothermal @200 °C/24 h	503	100 (30)	[298]
MoO2/rGO	hydrothermal @400 °C/3 h (10)	1009	100 (60)	[315]
MoO2/rGO	solid state reaction (22.0)	640	200 (50)	[316]
MoO2/GO	thermal reduction @550 °C/2 h	752	100 (100)	[317]
MoO2/Gr	hydrothermal @180 °C/26 h (33)	769	540 (83)	[318]
MoO2/N-rGO	hydrothermal @180 °C/24 h	400	1000 (5)	[319]
MoO2/exfol-rGO	solid-state graphenothermal (46)	878	100 (100)	[297]
MoO2 NSs/rGO	in situ reduction of MoO3	1003	100 (100)	[320]
MoO2/GO	solvothermal @160 °C/16 h (10)	500	800 (30)	[299]
MoO2/rGO	solid-state reaction @500 °C (15)	276	100 (1000)	[300]
MoO2/rGO	hydrothermal @200 °C/2 days (21)	708	500 (50)	[307]
MoO2/rGO	thermal reduction @550 °C (19)	584	1000 (100)	[308]
MoO2/rGO/NBs	surfactant-free self-assembly	420	5000 (1900)	[302]
MoO2 NBs/rGO	supercritical methanol route	793	50 (50)	[304]
MoO2/Mo-GO	freeze-drying (5)	550	100 (150)	[310]
MoO2/rGO	solid state reaction @500 °C (15)	1127	100 (150)	[305]
MoO2 NBs/rGO	freeze-drying (8)	420	5000 (1900)	[301]
m-MoO2/rGO	nanocasting (50)	801	100 (100)	[306]
MoO2@MoS2/rGO	hydrothermal @180 °C/4h	733	200 (80)	[312]

.

Xu et al. [314] used a solvothermal treatment of a GO suspension with (NH₄)₆Mo₇O₂₄ and HNO₃ in ethanol at 160 °C for 16 h. The MoO₂/GO with 10 wt% GO tested in lithium half-cell delivered an initial capacity of 800 mAh g⁻¹ at current density of 0.8 A g⁻¹ corresponding to Li uptake of 3.43 moles. The capacity retention was 63% after 30 cycles.

4.5. Titanium-Based Oxide Composites

4.5.1. TiO₂-Based Composites

TiO₂ has been widely investigated as anode material of LIBs due to its strong redox ability, relative non-toxicity, good stability, low cost, and abundance [321]. The insertion of Li⁺ ions in the TiO₂ framework occurs at about 1.5 V with the electrochemical process TiO₂ + xLi⁺ + xe⁻ ↔ Li_xTiO₂ (0 ≤ x ≤ 1) providing a theoretical capacity of 170 mAh g⁻¹ (for 1 Li⁺ uptake) almost equal to Li₄Ti₅O₁₂. However, pure TiO₂ suffers from poor electronic conductivity (~10⁻¹²–10⁻⁷ S cm⁻¹) and low diffusion rate (10⁻¹⁵–10⁻¹¹ cm² s⁻¹) [322,323], which affects the cycling performance and rate capability. To overcome these drawbacks, researchers have tried to combine TiO₂ with high conductive carbonaceous substances such as graphene [324], GO [325], and rGO [326,327,328,329]. In this context, Wang et al. reported that the sandwich structure formed by carbon-coated mesoporous TiO₂ nanocrystals and rGO sheets with a high surface area of ~209 m² g⁻¹, and a large pore volume of ~0.68 cm³ g⁻¹ gives a capacity retention of 110 mAh g⁻¹ after 100 cycles at 0.2 A g⁻¹ current rate [330].

Several effective strategies proved that the TiO₂/GO and TiO₂/rGO composite materials exhibit enhanced electrochemical properties in comparison with pure TiO₂. In most cases, the carbonaceous support results in several advantages: It provides fast transmission channels for electronic, maintains the structural stability of the hybrid, and increases the conductivity in the presence of reduced graphene oxide. The presence of carbon also provokes a reduction of the TiO₂ particles size during synthesis, which can shorten the transport paths for both Li⁺ ions and electrons and can lead to more electroactive sites due to the enlarged electrode–electrolyte contact area. In addition, the Ti-C bonds in the hybrid are crucial for fast interfacial charge transfer [331]. For instance, Etacheri et al. identified the formation of Ti³⁺ -C bonds between TiO₂(B) and rGO in chemically bonded mesoporous TiO₂(B) nanosheets to rGO sheets prepared by a photocatalytic reduction method, using EPR and XPS measurements. These TiO₂(B)/rGO nanohybrids demonstrated superior rate capability with 80% of the initial capacity retained after 1000 cycles at high rate of 40C [332].

Hydrothermal method is the most popular technique to prepare TiO₂/rGO nanocomposites [333]. TiO₂/rGO nanocomposites with small particle size were prepared via a mild hydrothermal route at 160 °C for 12 h with titanium tetrabutoxide (3 mL) and GO as the precursors in a mixture of ethanol (2.5 mL) and hydrochloric acid (3.5 mL); after heat treatment at 350 °C the final product with blue-black color was obtained [334]. This TiO₂/rGO nanocomposite delivered a first discharge capacity of 275 mAh g⁻¹ corresponding to a nominal degree of lithiation of x = 1.64 in Li_xTiO₂ and it retained a capacity of 112 mAh g⁻¹ after 100 cycles at a high charge rate of 1 A g⁻¹. Zhen et al. [327] fabricated rGO nanosheet supported anatase TiO₂ fiber bundles by hydrothermal method at 180 °C for 20 h using the mixture of the TiCl₃ and NaF in NaOH solution added to GO into HCl solution. This TiO₂/rGO composite showed a mesostructure with a broad pore size distribution from 5 to 50 nm, average ~21 nm pore diameter and higher specific surface area of 83.8 m² g⁻¹. The galvanostatic charge–discharge patterns carried out under 200 mA g⁻¹ current rate in the voltage range from 3 to 0.01 V vs. Li⁺/Li showed a capacity retention of 235 mAh g⁻¹ after 1000 cycles. He et al. [328] used a mild seed-assisted hydrothermal method at 110 °C for 12 h with Ti(OBu)₄ titanium source to synthesize a sandwich-like nanocomposite consisting of rutile TiO₂ nanorods (<25 nm in size) onto rGO. The composite with specific surface area of 43 m² g⁻¹ demonstrated reversible capacities of 145 mAh g⁻¹ (1 C) and 94 mAh g⁻¹ (5 C) retained up to 100 cycles (1C = 168 mAh g⁻¹), better than that of cells composed of nanosized TiO₂, i.e., 80 mAh g⁻¹ at 2C rate (8 nm grain size, 5 mg cm⁻² loading) [335]. TiO₂ nanorods (~5 nm in size, anatase phase) anchored on rGO composite were fabricated by hydrothermal method (180 °C for 12 h) after annealing treatment using GO solution, titanium trichloride (TiCl₃), and sodium hydroxide (NaOH) aqueous solution. The resultant R-TiO₂/rGO composites (BET surface area of ~149m² g⁻¹, average pore diameter of ~18 nm) were explored as anode material for LIBs displaying reversible capacities of 276 and 151 mAh g⁻¹ at 1C and 10C (1 C = 168 mA g⁻¹), respectively, with an excellent cycle ability (~90 mAh g⁻¹ at 10 C after 1000 cycles in the potential range 0.01–3 V) [336]. Li and coworkers [337] reported in situ growth of anatase TiO₂ nanoparticles on nitrogen-doped reduced graphene oxide (N-rGO) via hydrothermal route at 200 °C for 20 h. The properties of the TiO₂/N-rGO composite were compared with those of pure TiO₂ and TiO₂/rGO prepared in similar conditions. However, it was noted that the presence of rGO results in a decrease of the particle size from 30 nm in TiO₂ to 10–15 nm in TiO₂/rGO. The average reversible capacities of bare TiO₂, TiO₂/rGO, and TiO₂/N-rGO were 194, 210, and 226 mAh g⁻¹, respectively, at a current rate of 0.2 C. The authors discussed the effects of nitrogen doping on the electrochemical performance, in terms of defects in pyridinic graphene sheets suitable sites for lithium-ion storage. The nitrogen doping improved the conductivity by opening the bandgap and the disordered carbon structure increases the Li:C ratio. Geng et al. [338] synthesized TiO₂/rGO anode materials using hydrothermal method at 150 °C for 14 h. In a typical process, titanium oxysulfate dissolved in ethyl alcohol was mixed with multilayers GO sonically dispersed in water and hydrazine hydrate as reducing agent. The discharge specific capacity of TiO₂/rGO composite was 236 mAh g⁻¹ after 100 charge–discharge cycles at 0.1 A g⁻¹ rate and the coulombic efficiency was 98.5%.

In 2017, Mondal et al. [339] developed a large-scale synthesis for TiO₂-rGO nanocomposite hollow spheres by an aerosol-assisted spray-drying technique (using an aqueous titanium ammonium peroxo-carbonate complex (TAPCC) solution) with subsequent calcination. The porous TiO₂-rGO nanohybrid was composed of TiO₂ particles (10–20 nm) dispersed on the surface of rGO (10%) with a BET specific surface area of 86 m² g⁻¹. Specific discharge capacities of 265 and 274 mAh g⁻¹ at 18.8 mA g⁻¹ current rate were delivered by the synthesized TiO₂-rGO hollow spheres with rGO content of 10 and 20 wt%, respectively, against 236 mAh g⁻¹ for the pristine sample with a 2D flake-like morphology. Li et al. [329] prepared TiO₂/rGO hybrids using an ethanol-type hydrothermal method at 180 °C for 12 h followed by a freeze-drying process. Samples consisted of TiO₂ spherical particles (10 nm diameter, anatase phase) combined with the rGO matrix. The TiO₂/rGO assembly exhibited high rate capability of ~270 mAh g⁻¹ at 0.1 A g⁻¹, and excellent cycling capacity of ~180 mAh g⁻¹ at 0.5 A g⁻¹ after 2000 cycles. The improvement of these electrodes was explained in terms of fast pseudocapacitive charge storage process. Enhancement of the transport of Li⁺ ions in TiO₂/rGO composite electrode was evaluated by EIS measurements showing a diffusion coefficient of 1.14 × 10⁻¹¹ cm² s⁻¹, against 7.4 × 10⁻¹³ cm² s⁻¹ for pure TiO₂ electrode. A TiO₂(B)–CNTs–graphene ternary was synthesized by a three-step growth procedure, i.e., dispersion of Go and CNTs in ethanol, precipitation with titanium isopropoxide and ammonium hydroxide followed by a hydrothermal process at 200 °C for 48 h and a calcination at 350 °C for 5 h [340]. The TiO₂(B)–CNTs–GO sample with a 11.2% weight content carbon, including graphene and CNTs, had a BET surface area of 65.5 m² g⁻¹. It exhibited electrochemical activity within the voltage range of 0.01–3 V with a specific capacity of 388 mAh g⁻¹ in the 2nd cycle at 0.1C rate and a rate capability of 76 mAh g⁻¹ at 10C. A capacity of 190 mAh g⁻¹ was maintained after 200 cycles at 1C rate.

Recently, the hollow TiO₂@rGO hybrid spheres as anode materials for LIBs by Zhao et al. exhibited excellent electrochemical performance, i.e., high-rate and long-life [333]. The composite with a BET surface area of 58.7 m² g⁻¹ was in-situ constructed by using TiO₂ spheres in GO suspension as a precursor and a simple in-situ hydrothermal route with subsequent annealing at 600 °C for 3 h under an Ar atmosphere. It exhibited a specific capacity of 254 mAh g⁻¹ at 1C rate after 600 cycles (1C = 188 mAh g⁻¹; voltage rand 0.01–3.0 V vs. Li⁺/Li) and superior rate capacity of 159 mAh g⁻¹ at 10C after 3000 cycles. Fang et al. [341] prepared a two-dimensional TiO₂/rGO composite (particle size of 15–25 nm) by a facile hydrothermal method at 160 °C for 16 h using Ti₂C and GO as precursors. HRTEM images showed a large amount of sandwich layer-space and TiO₂-rGO interfaces, which are very advantageous for electron transfer and ion transport. The average specific capacities of 302, 200, 170, 148, and 128 mAh g⁻¹ were obtained at the current density of 0.5, 2.0, 10, 20, and 50C (1C = 168 mA g⁻¹), respectively. Even at the large current density of 100C, the TiO₂/rGO anode yielded a capacity of 111 mAh g⁻¹. At the current density of 1.68 A g⁻¹, a capacity of 130 mAh g⁻¹ was achieved, and an outstanding cycling retention of 86% remained after 1000 cycles. Quantification of the capacitive and diffusion contributions made from cyclic voltammetry data demonstrated that, at high currents, the capacity control was the main reaction power, i.e., accounted for 69.6% of the total charge (see Figure 9). TiO₂/rGO was also used as the anode material for a K-ion cell showing remarkable capacity retention of 83% after 1000 cycles at 1 A g⁻¹ current rate. Farook et al. used a microwave hydrothermal process to obtain a TiO₂–rGO nanocomposite consisted of TiO₂ particles with a size of ~100 nm, uniformly distributed on the rGO sheets. As an anode for LIBs, it delivered a capacity of a capacity of 250 mAh g⁻¹ ± 5% at 0.2C for more than 200 cycles. A capacity of 100 mAh g⁻¹ with >99% coulombic efficiency was recorded up to 20C [342]. Subasi et al. could increase the capacity of TiO₂–rGO owing to a surface modification of the nanocomposite by the use of H₂O₂ [343]. This composite, with TiO₂ 20 nm-thick synthesized through a sol-gel method, delivered 291 mA h g⁻¹ at a current of 100 mA g⁻¹.

GO can be photocatalytically reduced by the TiO₂ nanoparticles under the UV illumination [344,345]. Qiu et al. [345] prepared the TiO₂/GO hybrids by sonication and reduced GO into rGO using the photocatalytic property of TiO₂. The suspension was illuminated under the UV light of a 1000-W exon lamp for 2–5 h producing TiO₂ nanoparticles uniformly anchored on the surface of rGO sheets. The TiO₂/rGO nanocomposites exhibited the first discharge and charge capacities of 546 and 296 mAh g⁻¹ and preserved a cycling stability of ~215 mAh g⁻¹ at 200 mA g⁻¹ after 100 cycles. Tong et al. [346] fabricated 3D-mesoporous TiO₂/GO microspheres synthesized through an UV-assisted method of reduced graphene oxide with hydrazine. This composite possessed large surface area of 258 m² g⁻¹ and a narrow pore size of 7.8 nm. These TiO₂/GO microspheres achieved an initial discharge capacity of 220 mAh g⁻¹ with 84% (~185 mAh g⁻¹) capacity retention over 100 cycles at 0.2C rate, and 141 mAh g⁻¹ at 1C rate. A 3D TiO₂–carbon–rGO composite was fabricated and post-treated with UV irradiation (254 nm) for 0.5 h. A specific discharge capacity up to 191 mAh g⁻¹ was obtained after 100 cycles at a current rate of 0.2C (40 mA g⁻¹) for composite with 158 m² g⁻¹ [347].

Ren et al. [348] constructed TiO₂-carbon-rGO composites by anchoring TiO₂ nanoparticles (10–15 nm in size, anatase phase) encapsulated in carbon shells onto rGO sheets (15 wt%). This hybrid showed a BET specific surface area of 167.2 m² g⁻¹ and a typical bi-modal mesoporous structure with pore width of 3.8 and 32.5 nm and average pore width of 7.5 nm calculated using BJH model. The total pore volume with pore width from 1.7 to 300 nm was 0.26 cm³ g⁻¹. Such a morphology useful for the facile diffusion of Li⁺ ions in electrodes provided excellent electrochemical performance. Tested as the anode materials in Li-ion cell, the TiO₂-carbon-rGO composites exhibited an initial specific discharge capacity of 188 mAh g⁻¹ and a superior capacity retention of 158 mAh g⁻¹ after 100 cycles at 1 A g⁻¹ current rate. Cao et al. [89] investigated the lithium-ion insertion/extraction behavior in TiO₂/rGO nanocomposites synthesized using tetrabutyl titanate (Ti(OBu)₄) as the titanium source via a solvothermal route as anode materials for lithium-ion batteries. The hybrid nanocomposite featured a reversible capacity of 153 mAh g⁻¹ for after 100 cycles at a high charge rate of 5C (1000 mA g⁻¹). Madhusanka et al. [349] reported the electrochemical properties of TiO₂/rGO composite prepared by sonication for 5 min of a water/ethanol solution containing rGO, TiO₂ powder, Triton X-100 surfactant, and sodium carboxymethyl cellulose. TiO₂/rGO electrodes (loading of 4–6 mg cm⁻²) delivered a first discharge capacity of 159 mAh g⁻¹ and retained 143 mAh g⁻¹ after 100 cycles at current density of 100 mA g⁻¹ with a coulombic efficiency of 99.5%. Mesoporous anatase TiO₂ sheets/rGO sandwich-like nanocomposites were fabricated by acid-assisted Ti(OBu)₄ hydrolysis and subsequent thermal reduction process [350]. Three samples with different proportion of rGO were obtained by changing the volumes of Ti(OBu)₄ and concentrated H₂SO₄. The optimal sample (9.6 nm particle size, ~43 m² g⁻¹ BET specific surface area) delivered an initial discharge capacity of 357 mAh g⁻¹. The reversible lithium-storage capacity reached 161 mAh g⁻¹ after 50 cycles with ~70% retention at a current rate of 170 mA g⁻¹ (0.5C). Park et al. have synthesized rGO-coated TiO₂ microcones as composite anode materials via simple anodization and cyclic voltammetry [351]. The obtained TiO₂/rGO hybrid showed a capacity of 157 mAh g⁻¹ at 10C rate and sustained 1000 cycles with only 0.02% capacity fading per cycle. The hierarchically porous TiO₂/rGO hybrid anode material designed by Yu and coworkers [352] exhibited a reversible capacity of 250 mAh g⁻¹ at 1C rate (1 C = 335 mA g⁻¹) in the voltage range of 1.0–3.0 V. After 1000 cycles, stable capacities of 176 and 160 mAh g⁻¹ were delivered at 5C and 500 cycles at 10C, respectively. An ultrathin mesoporous TiO₂/rGO composite was obtained by Liang et al. via a layer-by-layer process [353]. Ti₃O₇²⁻ nanosheets obtained by exfoliation of Na₂Ti₃O₇ were used as the precursor for TiO₂. Coprecipitation of GO and Ti₃O₇²⁻ nanosheets in presence of HCl resulted in the formation of restacked H₂Ti₃O₇/GO. Then, annealing at 450 °C for 5 h facilitated the phase transition from H₂Ti₃O₇ to TiO₂ and the reduction of graphene oxide. The resulting mesoporous TiO₂/rGO composite delivered a capacity of 350 mAh g⁻¹ at a current density of 0.2 A g⁻¹, and a capacity of 245 mAh g⁻¹ maintained over 1000 cycles at 1 A g⁻¹.

Several studies were attempted to enhance the performance of TiO₂/rGO composites by doping with nitrogen. As experimental results, both electronic and ionic transport were significantly improved [337,354,355]. TiO₂/N-rGO hybrid was prepared via a facile one-pot hydrothermal method, in which ethylene glycol and ammonia are used as the reducing agent and nitrogen precursor, respectively [354]. The TiO₂/N-RGO nanocomposite exhibited superior electrochemical behaviors with reversible capacity of 126.8 mAh g⁻¹ at 10C rate and outstanding capacity retention of 118.4 mAh g⁻¹ after 100 cycles. In addition to the beneficial effect on ionic and electronic transport, the nitrogen doping enhances the nucleation and growth kinetics of TiO₂ nanoparticles uniformly anchored on the N-rGO surface. Li et al. [337] reported that the introduction of nitrogen on the rGO framework leads to more defects. A disordered structure is thus formed to accommodate more lithium ions. Another nitrogen-doped TiO₂/rGO hybrids was synthesized by hydrothermal reaction and post-annealing at 400 °C for 3 h [355]. The as-prepared samples contained anatase TiO₂ nanoparticles (~20 nm) uniformly anchored on GO lamella. The N-TiO₂/rGO electrode delivered a discharge capacity of 318 mAh g⁻¹ at 0.1 A g⁻¹ and maintained 117 mAh g⁻¹ at 5 A g⁻¹. A high reversible capacity of 210 mAh g⁻¹ was preserved after 2000 cycles at 0.5 A g⁻¹. Li⁺ ion diffusion coefficient evaluated from EIS measurements for N-TiO₂/rGO was 3.8 × 10⁻¹⁵ cm² s⁻¹, which is higher than that of the TiO₂/rGO electrode (5.5 × 10⁻¹⁶ cm² s⁻¹). Electrochemical analysis finds the number of active Li-ions is increased from the formation of N-Ti-O and N-C bonds.

Ternary Si@TiO₂@rGO composite with sandwich-like structure was prepared via simple sol-gel combined with graphene encapsulation process of GO, with subsequent annealing process at 600 °C for 2 h in N₂ atmosphere to reduce GO. The composite consisted of Si nanoparticles (spherical, 50 nm diameter) covered by uniform TiO₂ shells during the sol-gel process to fabricate the Si@TiO₂ composite, which were then encapsulated by flexible graphene sheets (15 nm thick) to produce the Si@TiO₂@rGO hybrid. Due to the synergetic effects of the stable TiO₂ (anatase phase) coating and the flexible graphene network, the as-synthetized Si@TiO₂@rGO anode delivered a charge specific capacity of 1135 mAh g⁻¹ under current rate of 0.2 A g⁻¹ after 100 cycles with an initial CE of 76.9%. A charge specific capacity of 568 mAh g⁻¹ was retained at current rate of 3 A g⁻¹ (Figure 10). Results of EIS measurements in the 100^th cycle showed higher diffusion coefficient of lithium ion of 8.0 × 10⁻¹¹ cm² s⁻¹ than D_Li+ = 5.7 × 10⁻¹² cm² s⁻¹ for the fresh Si@TiO₂@rGO anode, which suggests a better lithium-ion transfer after the initial cell formation [356]. Electrochemical performance of TiO₂/rGO composite anodes for LIBs from the recent literature are summarized in

Table 6. Electrochemical properties of TiO₂/GO composites. Relevant cycle number is given in brackets.

Material	Synthesis	Specific Capacity (mAh g−1)	Current Rate (mA g−1)	Ref.
TiO2/rGO	hydrothermal 160 °C/12 h	112	1700 (100)	[334]
TiO2/GO	UV photocatalysis	130	200 (100)	[345]
TiO2/rGO	UV photocatalysis	215	200 (100)	[345]
TiO2/N-doped rGO	hydrothermal 200 °C/20 h	187	-	[337]
TiO2/rGO	solvothermal with Ti(OBu)4	153	1000 (100)	[89]
TiO2/rGO	hydrothermal 150 °C/14 h	236	100 (100)	[338]
TiO2/rGO	hydrothermal 180 °C/20 h	235	200 (1000)	[327]
TiO2-carbon-rGO	hydrothermal 160 °C/24 h	158	1000 (100)	[348]
TiO2/rGO	sonication for 5 min	143	100 (100)	[349]
R-TiO2/rGO	hydrothermal 180 °C/12 h	90	1680 (1000)	[336]
TiO2/rGO	hydrothermal 160 °C/16 h	130	1680 (1000)	[341]
TiO2/rGO	hydrothermal 180 °C/12 h	180	500 (2000)	[329]
TiO2-carbon-rGO	UV photocatalysis (254 nm)	191	40 (100)	[347]
TiO2/rGO	thermal reduction	161	170 (50)	[350]
TiO2(B)–CNTs–GO	hydrolysis + hydrothermal	190	1C (200)	[340]
TiO2/rGO (9.9% rGO)	hydrothermal + annealing	254	168 (600)	[333]
TiO2/rGO (10% rGO)	aerosol-assisted spray-drying	174	1C (200)	[339]
N-doped TiO2/rGO	hydrothermal + annealing	210	100 (2000)	[355]
Si@TiO2@rGO	sol-gel	1135	200 (100)	[356]
TiO2/rGO	solvothermal alcoholysis	176	1675 (1000)	[352]
TiO2/rGO	microwave hydrothermal process	250	0.2C (200)	[342]
TiO2/rGO	coprecipitation Ti3O72− + GO	245	1000 (1000)	[353]

.

4.5.2. Li₄Ti₅O₁₂-Based Composites

Li₄Ti₅O₁₂ (LTO) can interact with Li ions by an intercalation/deintercalation process up to 3Li delivering a theoretical capacity of 175 mAh g⁻¹ with negligible volume expansion (<0.2%). However, this spinel framework has a poor electrical conductivity, which can be mitigated through the introduction of carbonaceous material [357,358].

Since 2011, great efforts have been devoted for the in-situ synthesis of LTO anode materials combined with graphene-based compounds. For example, Shen et al. [359] employed a three-step process with the hydrolysis of tetrabutyl titanate in the presence of GO to produce TiO₂/GO hybrid, which was then transformed into Li-Ti-O/rGO by a hydrothermal method, and finally to LTO/rGO by a post-heating treatment. It is noteworthy that the specific capacity of 82.7 mAh g⁻¹ was obtained at a high rate of 60C. Kim et al. [360] utilized a similar microwave assisted, two-step method with titanium ethoxide, LiOH, and GO to obtain Li-Ti-O/rGO hybrid, which was then transformed to LTO/rGO by post annealing. The phase-pure LTO/rGO nanocomposite delivered a reversible capacity of 168 mA h g⁻¹ at 1 C-rate and a remarkable rate capability with 59% capacity retention at 50 C-rate.

Chen et al. [361] prepared a nanohybrid electrode of mesoporous single crystalline Li₄Ti₅O₁₂ deposited on rGO (MSCs-LTO/rGO) via a simple hydrothermal reaction of TiO₂/rGO and LiOH with subsequent annealing in Ar at 600 °C. This MSCs-LTO/rGO anode yielded a specific capacity of 171 mAh g⁻¹ at 0.5C rate with CE of 95.6%, a rate capability of 132 mAh g⁻¹ at high rate of 40C and 85% capacity retention after 2000 cycles. A scalable synthesis of LTO/rGO composite was proposed by Ni et al. [362]. The as-prepared hybrid presented LTO nanoparticles uniformly immobilized on the rGO sheets and showed outstanding performance, i.e., good cycle ability and excellent rate capability with specific capacities of 154 and 149 mAh g⁻¹ at 10C and 20C rate, respectively. Cycle stability of LTO/rGO after 200 cycles at 5C (98.9% capacity retention) was much better than that of bare LTO (93%). Di et al. fabricated an LTO/rGO through a simple hydrothermal process, which demonstrated a capacity retention of 86.6% after 2000 cycles at 5C [363]. The capacity delivered at 50C and 100C were still 132 and 97.5 mAh g⁻¹, respectively. Chen et al. reported that even a small amount of rGO (1.2 wt%) effectively prevents grain growth and particles aggregation of LTO and enhances the specific surface area of the anode material [364]. The Li‖LTO/rGO half-cell could deliver the specific capacity of 187 mAh g⁻¹ at 1C rate and the high-rate discharge capacity of 128 mAh g⁻¹ at 80C operating for 2000 cycles. Zhang et al. fabricated a Li₄Ti₅O₁₂ microspheres/rGO composite by hydrolysis of titanium butoxide with subsequent hydrothermal reaction at 180 °C for 36 h of TiO₂ precursor with GO and LiOH, followed by a heat treatment at 600 °C in Ar atmosphere [365]. The 3 wt% rGO-coated LTO composite presented a high reversible Li-ion insertion capability with a specific discharge capacity of 131.6 mAh g⁻¹ at 5C rate and 97% capacity retention after 500 cycles. A porous Li₄Ti₅O₁₂/N-doped rGO composites was synthesized, while using cyanamide as the nitrogen source prevented the rGO restacking and increased the electronic conductivity to 1.6 S cm⁻¹ [366]. The Li₄Ti₅O₁₂/N-doped rGO composite with a BET specific surface area of 35.8 m² g⁻¹ demonstrated a specific discharge capacity of 117.8 mAh g⁻¹ at 30C rate. The work by Cao et al. [367] shows that LTO/rGO composite grown via one-pot way (hydrothermal process at 180 °C for 36 h) displays higher rate capacity and larger discharge capacity than the bare LTO sample. The small rGO ratio-containing composite (~6.2 wt%) delivered a reversible specific capacity of 196.4 mAh g⁻¹ at 1C rate and good cycle ability with 98.1% retention after 100 cycles. The rate capability tests showed discharge capacities of 175, 167, and 160 mAh g⁻¹ at 10C, 15C, and 20C, respectively. After 500 cycles at 20C, the discharge capacity retained 154.5 mAh g⁻¹ (96.5% retention). Ge et al. [368] reported the synthesis of LTO/GO and LTO/rGO composites by a facile one-step hydrothermal route at 180 °C for 24 h using Ti(SO₄)₂ raw material. The reduction of GO to rGO was realized by the hydroxyl groups of glucose, which prevented LTO particle aggregation and led to a mesoporous structure (~5 nm pore size). The reversible specific capacity of 193 mAh g⁻¹ at 0.5C rate was delivered by this nanocomposite anode and a capacity of 168 mAh g⁻¹ was retained at superior rate of 30C (Figure 11).

The LTO/rGO composite with a “fishnet-like” hierarchical porous structure was fabricated via a gas-foaming method [369]. LTO particles of 50–100 nm were obtained with the NH₄Cl concentration of 1 mg mL⁻¹. Electrochemical tests demonstrated a specific capacity of 176.6 mAh g⁻¹ at a rate of 1C, a rate capability of 167 and 143 mAh g⁻¹ at 3C and 10C, respectively, and a capacity retention of 95.4% after 100 cycles at 10C. Pender et al. [370,371] reported the fabrication and electrochemical performance of metal-foil free Li₄Ti₅O₁₂ (LTO) and LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) electrodes supported on conductive and porous reduced graphene oxide/poly(acrylic acid) (rGO-PAA) aerogels. This 3D nanocomposite had a low density (~5 mg cm⁻³) and high porosity (99.6% void space) [370]. When applied as a free-standing, 3D current collector in LIBs, the highly porous rGO-PAA substrate increased the effective electrode−electrolyte contact area 2.5- fold (16 mg cm⁻² loading) by increasing the electrode surface roughness and thus improving the lithiation/delithiation kinetics of a commercial LFP cathode [372]. The use of rGO-PAA instead of conventional metal-foil (CMF) showed a 25% increase in energy density over similar electrodes built with CMF current collector, i.e., high volumetric energy densities of 1723 Wh L⁻¹ for NMC and 625 Wh L⁻¹ for LTO [371].

Recently, an in-situ supramolecular self-assembly assisted synthesis combining spray drying and high-temperature calcination was employed to prepare Li₄Ti₅O₁₂-carbon-rGO microspheres for LIBs. During the sol formation, GO was connected with Ti(OH)₄ coated by polydopamine [373]. The as-prepared composite consisted of small size LTO particles (~50 nm) wrapped in 3D rGO nanosheets. The LTO/C/rGO hybrid anode exhibited a reversible capacity of 184 mAh g⁻¹ at 1C rate and a capacity retention of 94.5% after 500 cycles at 20C rate. These results compared well with those reported by Chen et al. on LTO/CNTs/graphene composite prepared by ball-milling, which exhibits a discharge capacity of 132 mAh g⁻¹ at 20C rate [374].

More recently, the ternary microspherical C/LTO/rGO, which consisted of ethylene glycol-derived in-situ carbon-coated Li₄Ti₅O₁₂ nanoparticles and rGO sheets, was synthesized by polyol-mediated spray drying method [375]. The as-fabricated C/LTO/rGO composite tested as anode material for LIBs showed 94% capacity retention after 3000 cycles at 10C and 93% capacity retention after 1000 cycles at 60 °C. This electrode was also studied as anode for sodium-ion batteries showing 95% capacity retention after 1000 cycles at 5C for Na⁺ storage at room temperature. An LTO/rGO nanolayered composite was synthesized by electrophoretical co-deposition onto Cu substrate using a suspension of 2D-lithium titanate hydrate and GO nanosheets followed by a controlled annealing at 600 °C for 6 g using a ramping temperature of 5 °C min⁻¹ [376]. Galvanostatic experiments performed at 1C (175 mA g⁻¹) showed a retention capacity of ~135 mAh g⁻¹ after 150 cycles. The rate capability test demonstrated a specific capacity of 50 mAh g⁻¹ at high rate of 50C.

4.6. Iron-Based Oxide Composites

4.6.1. Fe₃O₄-Based Composites

Magnetite (Fe₃O₄) proved to be a rechargeable conversion electrode material that reacts with 8 Li ions per formula unit leading a theoretical specific capacity of about 926 mAh g⁻¹ [377]. While cells using iron oxides present voltage hysteresis from 0.7 to 1.0 V, the use of rGO reduces the overpotential to ~0.7 V. In 2010, the study of a micro-structured graphene nanosheets/Fe₃O₄ composite has shown the superior performance with the addition of graphene with a reversible specific capacity approaching 1026 mAh g⁻¹ after 30 cycles at 35 mA g⁻¹ and 580 mAh g⁻¹ after 100 cycles at 700 mA g⁻¹ [62]. Since 2011, numerous works have been devoted to the investigation of GO-ferrite composite. Fe₃O₄/rGO composites are not only mostly synthesized using the popular hydrothermal method with subsequent annealing process [378,379,380,381,382] but also employing other techniques such as chemical precipitation [383,384,385], one-pot microwave reduction [386], electrophoretic deposition [387], and redox reaction [388].

Li et coworkers prepared Fe₃O₄/rGO nanocomposites through a chemical co-precipitation of Fe²⁺ and Fe³⁺ in the presence of GO in alkaline solution. After a thermal reduction, the composite was composed of Fe₃O₄ nanoparticles homogeneously dispersed on the rGO sheet surface [383]. Tested in half-cell, this composite displayed a reversible capacity of about 200 mAh g⁻¹ even at a high charge/discharge rate of 10C (9260 mA g⁻¹). The rGO/Fe₃O₄‖LiNi_1/3Mn_1/3Co_1/3O₂ full-cell exhibited a capacity of 80 mAh g⁻¹ when discharged at 0.2C rates over 10 cycles. Zhu et al. [384] reported the fabrication of Fe₂O₃/rGO composite using a facile two-step synthesis by homogeneous co-precipitation of FeCl₃ in a suspension of GO platelets with urea, with subsequent reduction of the GO with hydrazine under microwave irradiation. The composite has a curled morphology consisting of a thin, wrinkled, “paperlike” structure. Such rGO platelets decorated with Fe₂O₃ nanoparticles (~60 nm in diameter) showed discharge and charge capacities of ~1355 and 982 mAh g⁻¹, respectively, based on the total mass of the composite, at a current density of 100 mA g⁻¹ over 50 cycles. A three-dimensional Fe₃O₄/rGO composite was constructed by anchoring Fe₃O₄ nanoparticles encapsulated within carbon shells onto reduced graphene oxide sheet [58]. The two-step synthesis consisted in the mixture of an aqueous solutions of Fe(NO₃)₃∙6H₂O and polyvinylpyrrolidone (PVP, M_W = 40,000) with the aqueous suspension of graphene oxide (GO) and an aqueous solution of glucose and urotropine (1:1) followed by a calcination to 500 °C in Ar flow. The final black powders exhibit a BET specific surface area of 104.2 m² g⁻¹. This Fe₃O₄/rGO composite, tested in Li half-cell, displayed an initial specific capacity of 1426 mAh g⁻¹ and superior reversible capacity of 842 mAh g⁻¹ after 100 cycles at 200 mA g⁻¹ current rate.

A Fe₃O₄/rGO composite was fabricated by a microwave-assisted combustion synthesis of porous Fe₃O₄ particles followed by decoration of Fe₃O₄ by rGO. The material was porous with specific surface area and pore volume of 81.87 m² g⁻¹ and 0.16 cm³ g⁻¹, respectively [389]. Electrochemical results revealed a reversible capacity of 612, 543, and 446 mAh g⁻¹ at current rates of 1C, 3C, and 5C, respectively, with a coulombic efficiency of 98% after 50 cycles, which emphasized the importance of the composite porosity and the improved electronic conductivity by rGO grafting. A Fe₃O₄/rGO composite was prepared by Liang et al. via coprecipitation, with Fe₃O₄ nanoparticles of size 9nm [390]. As an anode containing 44.9 wt% Fe₃O₄, it delivered a capacity of 300 mAh g⁻¹ at 1 A g⁻¹ after 100 cycles. Kumar et al. [386] designed and synthesized an interconnected hybrid network constituted by faceted Fe₃O₄ nanoparticles embedded in 3D rGO nanosheets by one-pot microwave approach. The electrochemical properties of these as-synthesized Fe₃O₄/rGO hybrids were analyzed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy. These hybrids demonstrated superior performance as electrostatic capacitors (ESC) with a specific capacitance of 455 F g⁻¹ at the scan rate of 8 mV s⁻¹. Moreover, the 3D composite exhibited good cycling stability with a retention of 91.4% up to 9600 cycles. Liang et al. [391] investigated the effect of Fe₃O₄ loading on the cycling behavior of Fe₃O₄/rGO hybrid anode for LIBs. The composite was prepared by co-precipitation method using Fe₃O₄ nanoparticles (grain size of ca. 9 nm) and different amount of GO followed by reduction. The best cycling capacity of 300 mAh g⁻¹ at 1 Ag⁻¹ current density was obtained after 100 cycles for the Fe₃O₄/rGO composite electrode containing 44.9 wt% Fe₃O₄. Fe(OH)₃ nanoparticles were embedded in rGO nanosheets via a pre-reduction with hydrazine followed by a hydrothermal process at 180 °C for 12 h and a subsequent heating at 500 °C for 2 h in an Ar atmosphere with a heating rate of 5 C min⁻¹, to yield Fe₃O₄/rGO composite with Fe₃O₄ nanoparticles of ~6 nm in size. The as-prepared samples showed a uniform porous morphology with a high BET specific surface area of 114.7 m² g⁻¹. Galvanostatic charge–discharge measurements showed that the Fe₃O₄/rGO‖Li half-cell delivers a reversible specific capacity of 1108 mAh g⁻¹ at a current density of 0.5 A g⁻¹ even after 400 cycles and maintains a reversible capacity of 593 mAh g⁻¹ at 5 A g⁻¹ current rate [378]. The Fe₃O₄/rGO composites consisting of well-crystallized Fe₃O₄ nanoparticles (10–30 nm in diameter) are tightly and homogeneously anchored on the flexible graphene substrate were synthesized through a redox reaction between highly oxidized GO and ferrous salt (FeSO₄·4H₂O) under basic atmosphere (ammonia), followed by a stabilization treatment ranging from 5 min to 5 h [388]. The optimized Fe₃O₄/rGO composite obtained after 2-h annealing delivers a reversible capacity of 1024 mAh g⁻¹ at 1 A g⁻¹ current rate and retains 584 mAh g⁻¹ at 5 A g⁻¹ even after 450 cycles (Figure 12). A super-mesoporous Fe₃O₄/rGO anode material was prepared by a poly(methacrylic acid) (PMAA)-induced self-assembly process, which provided a sample with high porosity by optimizing the dispersion state of PMAA-stabilized Fe₃O₄/GO sheets at different surrounding pH values. The mesoporous composite showed a high specific surface area of 338.8 m² g⁻¹ with a large amount of 10–40 nm mesopores. The corresponding Fe₃O₄/rGO anode delivered specific-charge capacity of 740 mAh g⁻¹ after 100 cycles at 0.5 A g⁻¹ and decreases to 200 mAh g⁻¹ at 10 A g⁻¹ [391].

Several studies are related to Fe-based ternary composite electrodes showing superior electrochemical properties. The Fe₃O₄/CNTs/rGO composite electrode as a binder-free anode was prepared by electrophoretic deposition. This robust sandwich-like architecture consisted of Fe₃O₄ nanoparticles interconnected by CNTs between rGO layers [387]. The resultant Fe₃O₄/CNTs/rGO hybrid electrode demonstrated enhanced electrochemical properties with high reversible capacity of 540 mAh g⁻¹ at a high current density of 10 A g⁻¹, and a remarkable capacity retention of 1080 mAh g⁻¹ after 450 cycles at 1 A g⁻¹. Xiao et al. [392] constructed a multilayered composite rGO/Fe₃O₄@polydopamine (PDA) hydrogel, in which the PDA layer plays the role of artificial SEI onto Fe₃O₄ nanoparticles. The electrochemical tests of the rGO/Fe₃O₄@PDA‖Li half-cell showed high specific capacity of 1358 mAh g⁻¹ after 300 cycles at 1 A g⁻¹ current rate, satisfying rate capability (~600 mAh g⁻¹ at 3C), and fast charge/discharge rate (712 mAh g⁻¹ after 2000 cycles at 3 A g⁻¹). Rice crust-like structure of thin carbon layer (~2 nm thick) coated Fe₃O₄ nanoparticles (~15 nm) uniformly covering rGO sheets was obtained via solvothermal process with subsequent annealing at 600 °C for 1 h under flowing Ar gas (100 sccm) [393]. This Fe₃O₄@C/rGO composite showed a specific capacity of ~970 mAh g⁻¹ at 0.2 A g⁻¹. At higher current rate (5 A g⁻¹), it exhibited a capacity of 420 mAh g⁻¹, which is much higher than that of the composite Fe₃O₄/rGO without carbon coating. The Fe₃O₄@C/rGO anode cycled 1000 times at 5 A g⁻¹ delivered 594 mAh g⁻¹ with a CE of 99.4%. The authors stated that the charge/discharge process occurs with two reactions: A conversion reaction (above 0.8 V) and a surface adsorption (below 0.8 V), which is attributed to the pseudocapacitance contribution calculated at 89%, originating from the carbon coated nanostructure. A ternary Fe₃O₄-MoO₃/rGO nano-composite prepared by microwave hydrothermal method was deposited on carbon cloth (CC) [394]. The detailed morphological studies showed that Fe₃O₄-MoO₃ nanoparticles were decorated on thin sheets of rGO with a resolution of 50 nm. The as-prepared Fe₃O₄-MoO₃/rGO-@CC electrode showed a specific capacitance of 1666 F g⁻¹ at a current density of 2.5 A g⁻¹ and a capacitance retention of 95% after 5000 cycles. A NiO/Fe₃O₄/rGO composite obtained via a solvothermal route consisted of Fe₃O₄ nanospheres (average size ~150 nm) and rod-shaped NiO particles (about 300 nm) distributed on the surface of rGO. The porous NiO/Fe₃O₄/rGO sample exhibits a BET specific surface and average pore diameter of 497 m² g⁻¹ and 1.2 nm, respectively. The initial discharge capacity was 841 mAh g⁻¹ and maintained 647 mAh g⁻¹ after 200 cycles at the current density of 0.5 Ag⁻¹. Rate capability tests showed that the discharge capacity was still 582 mAh g⁻¹ after 200 cycles at 1 A g⁻¹ [395].

Recently, Xiao et al. [396] reported the fabrication of multi-layered graphene oxide-Fe₃O₄@polydopamine-polyacrylamide (rGO-Fe₃O₄@PDA-PAM) composites through a green and scalable strategy. In this assembly, Fe₃O₄ nanoparticles (NPs) were confined by the PDA shell and rGO sheets. Gao et al. [397] developed a simple one-step hydrothermal method to prepare Fe₃O₄/FeS/rGO composite with a uniform deposition of octahedral Fe₃O₄/FeS particles on the rGO. This structural design delivered a reversible capacity of 744 mAh g⁻¹ at 0.1C rate after 50 cycles showing excellent interface stability. Using broccoli-like multichannel FeCO₃ microspheres as raw material, the Fe₃O₄ nanoflakes/rGO composites were realized via a low temperature hydrolysis reaction in the presence of graphene and hydrazine. The Fe₃O₄ NFs/rGO electrode exhibited a satisfied capacity of 925.3 mAh g⁻¹ with 68.8% retention and retained a capacity of 636.1 mAh g⁻¹ after 1000 cycles at 0.5 A g⁻¹ [380].

Fe₃O₄ was intercalated into rGO and conductive carbon layers to form a 3D sandwich-like C/Fe₃O₄/rGO architecture. The composite, prepared by one-pot solvothermal process, exhibited excellent electrochemical performance as anode in LIB with specific capacity of 844 mAh g⁻¹ at 0.2 A g⁻¹ after 300 cycles and high-rate cyclic performance (i.e., 363 mAh g⁻¹ at 4 A g⁻¹ after 300 cycles. Meanwhile, when used as ESC electrode, the C/Fe₃O₄/rGO hybrid showed a pseudocapacitance of 77% when scan rate is 5 mV s⁻¹ [398]. Liang and Lu developed a new technique for the fabrication of Fe₃O₄/rGO nanocomposites, namely the gamma-irradiation method, in which the iron(III) hydroxide and GO are reduced to Fe₃O₄ and rGO by the reducing species generated from the radiolysis of the solvent [399]. The electrochemical performances of the Fe₃O₄/rGO composite containing various amounts of Fe₃O₄ evaluated in coin-type cells yielded good cycling stability at a low current density of 50 mA g⁻¹. The specific discharge capacity of with 78.8 wt% Fe₃O₄/rGO remained at 568.6 mAh g⁻¹ after 100 cycles, while the 74.7 wt% Fe₃O₄/rGO electrode exhibited a sustained discharge capacity is 738.5 mAh g⁻¹ after 100 cycles at current density of 500 mA g⁻¹. Copper-ferrite-based composites, namely CuFeO₂/rGO and Cu/CuFeO₂/rGO were proposed by Wang et al. [400] as high-performance anode materials for LIBs. A four-step synthesis consisted in: (i) The formation of the CuFeO₂ delafossite crystals in an alcoholic solution containing Fe(NO₃)₃ and Cu(NO₃)₂ dissolved species, and (ii) the mixture of the GO suspension and the CuFeO₂-like solution with control of the pH at 12 by 6 mol L⁻¹ NaOH, (iii) an hydrothermal process at 180 °C for 12 h, and finally (iv) a calcination in a tube furnace at 400 °C for 2 h under a N₂ atmosphere. The TEM images of the as-prepared hybrids revealed that ultrafine nanoparticles (15–25 nm in diameter) decorated the graphene nanosheets. When tested in the voltage range 0.02–3.0 V vs. Li^+/Li at 200 mA g⁻¹ current rate, the sheet-on-sheet CuFeO₂/rGO electrode displayed a reversible capacity of 587 mAh g⁻¹ after 100 cycles, while the Cu/CuFe₂O₄@rGO hybrid, containing combined nano-copper and rGO, manifested a superior rate capability of 723 and 560 mAh g⁻¹ at 0.8 and 3.2 A g⁻¹, respectively and a cyclability of 1102 mAh g⁻¹ after 250 cycles at 800 mA g⁻¹ rate.

Table 7. Summary of electrochemical performances of iron oxide/rGO composites. Relevant cycle number is displayed in brackets.

Material	Synthesis	Specific Capacity (mAh g−1)	Current Density (A g−1)	Ref.
Fe3O4/rGO	co-precipitation + annealing	200	9.26	[383]
Fe3O4/rGO	microwave-assisted combustion	446	5C (50)	[389]
Fe3O4/rGO	coprecipitation	300	1.0 (100)	[390]
Fe3O4/rGO	hydrothermal + heating	593	5.0 (400)	[378]
Fe3O4/rGO	redox reaction	584	5.0 (450)	[388]
Fe3O4/CNTs/rGO	electrophoretic deposition	1080	1.0 (450)	[387]
rGO/Fe3O4@PDA	hydrothermal + calcination solvothermal	712	3.0 (2000)	[392]
Fe3O4@C/rGO	hydrothermal + calcination	594	5.0 (1000)	[393]
Fe3O4/FeS/rGO	hydrolysis	744	1C (50)	[397]
Fe3O4 NFs/rGO	gamma-irradiation method	636	0.5 (1000)	[380]
78.8 wt%Fe3O4/rGO	gamma-irradiation method	568	0.05 (100)	[399]
74.7 wt%Fe3O4/rGO	solvothermal	738	0.5 (100)	[399]
NiO/Fe3O4/rGO	electrostatic assembly	580	1.0 (200)	[395]
CP/Fe3O4/rGO	solvothermal	1160	0.5 (1000)	[401]
Fe3O4/rGO	PMAA-induced self-assembly	992	0.15 (100)	[402]
Fe3O4/rGO	hydrothermal + calcination	740	0.5 (200)	[391]
CuFeO2/rGO	one-pot solvothermal	587	0.2 (100)	[400]
C/Fe3O4/rGO		844	0.2 (300)	[398]

summarizes the electrochemical performances of iron oxide/rGO composites.

4.6.2. Fe₂O₃-Based Composites

Fe₂O₃ (a-phase) has the advantages of rich ores, low price, and high theoretical capacity of 1007 mAh g⁻¹ according the lithiation process, that is the conversion reaction Fe₂O₃ + 6Li⁺ + 6e⁻ ↔ 3Li₂O + 2Fe⁰. In the anodic polarization process, the broad anodic peak at around 1.55 V is related to the reverse de-conversion process of Li₂O with metallic iron. Since 2011, a-Fe₂O₃/rGO hybrids were recognized as important electrode materials for energy storage devices [403,404,405,406,407]. For example, Wang and his collaborators were one of the first researchers who demonstrated the enhanced performances of a-Fe₂O₃/rGO nanocomposites as anode materials for LIBs, showing a high discharge capacity of 660 mAh g⁻¹ during up to 100 cycles at the current density of 160 mA g⁻¹ [408].

In 2015, Wang et al. [409] fabricated cells with 3D mesostructured Ni scaffolded Fe₂O₃ electrodes. The voltage hysteresis was 0.62 V at 100 mA g⁻¹ and was further reduced to 0.42 V at elevated temperature (45 °C). Zhu et al. [384] prepared Fe₂O₃/rGO composite using a co-precipitation/microwave technique. The as-prepared electrode exhibited a specific capacity of 1355 mAh g⁻¹ after 50 cycles at 100 mA g⁻¹. Liu et al. [410] built a nanocomposite of a-Fe₂O₃ nanoplates imbedded in graphene networks using a one-step hydrothermal method with water and glycerin as hydrothermal solvents. In this hybrid, the a-Fe₂O₃ nanoplates, with a size of 20–30 nm, were well wrapped and closely connected by the flexible conductive network. The a-Fe₂O₃/rGO composite anode demonstrated high discharge capacities of ~896 mAh g⁻¹ up to 200 cycles at 5C rate and ~429 mAh g⁻¹ up to 1000 cycles even at a 10 C rate. Lee et al. [411] prepared a-Fe₂O₃ hollow nanobarrels (a-HNBs, ~200 nm in diameter, ~360 nm in length) deposited on rGO using a one-step microwave-assisted hydrothermal process. The as-synthesized a-HNBs/rGO composite yielded superior electrochemical properties such as a reversible capacity of 1279 mAh g⁻¹ at 0.5C rate after the first cycle, a long-term cycle ability of 478 mAh g⁻¹ after 1000 cycles, and a high rate capability of 403 mAh g⁻¹ even at 10C rate. The g-Fe₂O₃/RGO composite electrode materials was prepared by the reduction of few layer GO to rGO by iron metal in aqueous solution under ambient conditions; the final product was obtained by calcination at 600 °C for 2 h under N₂ atmosphere [412]. The g-Fe₂O₃ nanoparticles formed by the oxidation of iron were uniformly dispersed on the surface of rGO sheets. The g-Fe₂O₃/RGO composite anode, tested in the voltage range 0.01–3.0 V at 500 mA g⁻¹ current rate, delivered an initial specific capacity of 850 mAh g⁻¹, maintained at of ~800 mAh g⁻¹ after 300 cycles. When cycled at 15 A g⁻¹ performance, the capacity remains stable at 200 mAh g⁻¹.

Recently, Liu and coworkers [413] focused on the development of facile and low-cost synthetic method to grow Fe₂O₃/rGO hybrids. They used FeCl₃ as the raw material and adopted different one-step hydrothermal methods (i.e., hydrothermal in-situ method, PVP-assisted hydrothermal in-situ method, or oleic acid-assisted hydrothermal in-situ method). A comparison of the electrochemical performance of the Fe₂O₃/rGO anodes prepared by these three processes shows that the oleic acid-assisted hydrothermal method is best. With this process, the initial specific discharge capacity was 1431 mAh·g⁻¹ at a current density of 100 mA·g⁻¹, and, remained at 478 mAh·g⁻¹ after 50 cycles. Within similar conditions, capacities of 435 and 225 mAh·g⁻¹ were delivered by the anodes prepared by hydrothermal in-situ and PVP-assisted hydrothermal in-situ method, respectively.

4.6.3. FeO-Based Composites

Exfoliated graphene oxide (EGO)/iron (II) oxide (FeO) composite was prepared by graphenothermal reduction process at 650 °C for 5 h in argon [414]. This composite displayed a measured BET of 11 m² g⁻¹, pore diameter of 10.6 nm and average pore volume of ~0.064 cm³ g⁻¹. Electrochemical tests showed a reversible capacity of 857 mAh g⁻¹ at a current rate of 50 mA g⁻¹ in the voltage range 0.005–3.0 V vs. Li⁺/Li with a high coulombic efficiency of 98% up to 60 cycles. As a cathode material for LIBs, LiFePO₄/graphene oxide (LFP/GO) composite was prepared by spray-drying and annealing processes, in which nanoparticles were wrapped homogeneously and loosely with the graphene 3D network facilitating electron migration throughout the secondary particles and fast Li⁺ diffusion in the voids. The LFP/GO composite could deliver a specific capacity of 70 mAh g⁻¹ at 60C discharge rate. After 1000 cycles, the capacity decay rate of <15% was measured under 10C charging and 20C discharging [415]. One type of Li-ion storage material with fast faradaic surface redox reaction offers high power density but limited in some extent of energy contribution. Qi et al. demonstrated that in-situ pulverized FeOOH rods on rGO sheets exhibit a pseudocapacitance storage behavior (82% of the total capacity). The FeOOH/rGO composite achieved specific capacities of 1135 and 783 mAh g⁻¹ at current rate of 1.0 and 5.0 A g⁻¹, respectively, over 200 cycles [416].

4.7. Vanadium-Based Oxide Composites

Vanadium oxides (VO_x) exist in various compositions (e.g., V₂O₅, V₃O₇, V₄O₉, V₆O₁₃, VO₂, V₂O₃, etc.), depending on the oxidation state (5⁺ to 2⁺) of vanadium. They have attracted considerable attention as potential electrode materials for energy storage devices (LIBs, SIBs, ESCs) [417].

4.7.1. VO₂-Based Composites

Since the 1980s, vanadium oxides VO_{2 + y} (0 ≤ y ≤ 0.5) are known for lithium storage applications [418]. VO₂(R) is the most stable phase, which crystallizes in the rutile-like structure (monoclinic, C2/m space group). Nanocrystalline VO₂(B) is a metastable, which crystallizes in the monoclinic structure with a fibrous morphology owing to its V₄O₁₀-type double layers to form V-O tunnels perpendicular to the c-axis for rapid lithium-ion insertion/deinsertion process. Typically, VO₂/rGO composites are synthesized by either freeze-drying or a solvothermal method. In the freeze-drying, a dispersed suspension is obtained by mixing graphene oxide and ammonium vanadate in water and freeze in liquid nitrogen; finally, the VO₂/rGO composite is grown after calcination at 500 °C for 5 h under Ar conditions. During this process, NH₄VO₃ is converted into VO₂ (NH₄VO₃ + C → NH₃ + VO₂ + CO₂ + H₂O) with simultaneous reduction of GO to rGO [419]. In the solvothermal method, the VOC₂O₄ clear dark blue solution formed by the mixture vanadium pentoxide and oxalic acid (molar ratio 1:3) in water is added to a GO dispersion and hydrothermally treated in butanol at 200 °C for 8 h. Finally, the VO₂/rGO composite is obtained after alcohol washing at 80 °C for 8 h [420]. In a two-step hydrothermal reaction, well-dispersed VO₂(B) nanorods with diameter of 40 nm and length of 150 nm are grown using V₂O₅ as raw material and rod-like vanadyl ethylene glycolate as intermediate product, separately. The 3D VO₂(B)/rGO composite is obtained by adding GO. Electrochemical test shows that the close connection between rGO sheets and VO₂(B) NRs improved the cycling stability [421].

VO₂(B)/rGO nanocomposite was synthesized via a one-step microwave-assisted solvothermal process fromVOC₂O₄ obtained by dissolution of V₂O₅ in the presence of oxalic acid [422]. The VO₂(B)/rGO hybrid reveals good performance as an anode in lithium-ion batteries with an initial capacity of 400 mAh g⁻¹ and a reversible specific capacity of 200 mAh g⁻¹ over 400 cycles in the potential range 3.0–0.05 V vs. Li⁺/Li. Nethravathi et al. [423] performed a hydrothermal reaction of a mixture of a colloidal dispersion of GO and NH₄VO₃, which yielded a VO₂(B)/GO hybrid. The nanotubular morphology of VO₂(B) wrapped between graphene sheets improved the charge transfer and achieved ~450 mAh g⁻¹ at 40 mA g⁻¹ current rate. Song et al. [424] investigated an rGO supported VO₂(M) nanocomposite prepared via a one-step sol-gel assisted hydrothermal process. A GO-dispersed V₂O₅ gel was reduced with an appropriate hydrazine monohydrate (N₂H₄∙H₂O) as strong reductant and hydrothermally treated at 220 °C for 24 h. From this synthesis, the as-prepared M-phase VO₂ nanoparticles (20 50 nm in size) well dispersed in rGO exhibits the monoclinic (P2₁/c space group) structure. The discharge capacity of VO₂(M)/rGO electrode is 283 mAh g⁻¹ at a current density of 60 mA g⁻¹ over 200 cycles. The rate capability showed a capacity of 210 mAh g⁻¹ at 300 mA g⁻¹. Choi et al. [425] prepared VO₂-decorated rGO balls using a one-pot spray-pyrolysis process from a colloidal spray solution of well-dispersed GO and NH₄VO₃ followed by post-treatment at 300 °C in an air atmosphere. The as-prepared VO₂(B) nanorods (~200 nm in diameter) are encapsulated by rGO nanosheets. The rGO/VO₂ composite with 5 wt% graphene showed an initial discharge and charge capacities of 148 and 135 mAh g⁻¹, respectively, at 300 mA g⁻¹ current rate. A free-standing VO_2.07/rGO film was fabricated through hydrothermal process using NH₄VO₃∙and GO in 1 mol L⁻¹ HNO₃ aqueous solution [426]. The as-prepared nanoribbons (30 nm thick, 50 µm long) were annealed at 300 °C in N₂ atmosphere and tested as cathode material (mass loading of 1.2 mg cm⁻²) in Li cell. The VO_2.07/rGO intertwining network exhibits a reversible specific capacity of 160 mAh g⁻¹ and good cycling stability of 133 mAh g⁻¹ after 200 cycles at a current density of 70 mA g⁻¹ in the voltage range between 2.0 and 3.5 V.

Xiao et al. [427] prepared VO₂/rGO composites by a simple one-step hydrothermal method. Samples consist of VO₂ flakes anchored with rGO. When used as anode for Li-ion battery, this VO₂/rGO composite yields a high capacity of ~622 mAh g⁻¹ at 100 mA g⁻¹ and maintains a capacity of 256 mAh g⁻¹ after 300 cycles at 2 A g⁻¹ with capacity retention of almost 100%. Yan et al. [420] encapsuled VO₂ nanospheres (average diameter of ~500 nm) into crumpled rGO using a one-step rapid and scalable solvothermal approach at 200 °C for 8 h. The as-prepared sample has a BET specific surface area of 130.4 m² g⁻¹. The sodiation reaction of VO₂/rGO composites tested in the voltage range of 3.0 to 0.01 V vs. Na⁺/Na results in a reversible capacity of 383 mAh g⁻¹ at 0.1 A g⁻¹ and 214 mAh g⁻¹ at 4 A g⁻¹, and long-term stability over 2000 cycles at 4 A g⁻¹ with a capacity fade of 0.013% per cycle. Dai et al. [419] demonstrated a highly reversible zinc/vanadium dioxide system, where freestanding reduced graphene oxide/vanadium dioxide (rGO/VO₂) composite films were used as the cathodes of Zn-ion batteries. When cycled at 4 A g⁻¹, the rGO/VO₂ hybrid exhibit an excellent discharge capacity of 240 mAh g⁻¹ with a coulombic efficiency ~100% and ~100% capability retention over 1000 cycles.

Graphene woven VO₂ nanoribbons synthesized using in-situ hydrothermal method show high discharge capacity (380 mAh g⁻¹) and 99% capacity retention after 50 cycles [428]. Dai et al. [419] investigated the electrochemical performance of VO₂/rGO films in Zin-ion batteries. The composites synthesized by freeze-drying method delivers a specific capacity of 276 mAh g⁻¹ at a current rate of 0.1 A g⁻¹ in the potential range of 0.3–1.3 V (energy density of 65 Wh kg⁻¹) and exhibits excellent capacity retention of 99% after 1000 cycles. A considerable capacity of 120 mAh g⁻¹ was still obtained at high rate of 35 A g⁻¹. Recently, Cui et al. [429] designed a cathode material for Zn-ion batteries consisting in VO₂(B) nanobelts uniformly deposited on to rGO nanoflakes via a one-step hydrothermal route. The VO₂(B)/rGO composite presents a specific capacity of 456 mAh g⁻¹ at 0.1 A g⁻¹. Cycling performance shows an excellent capacity retention of 72% after 1600 cycles at 5 A g⁻¹. Zhao et al. [430] investigated VO_x/rGO composites as cathode materials for Li-ion batteries using a facile hydrothermal method at 190 °C for 48 h. Black precursor heat treated at 500 °C for 2 h produced a V₂O₅/rGO composite (BET surface area of 64 m² g⁻¹), while treat at 550 °C for 2 h in N₂ atmosphere resulted in a VO₂/rGO hybrid material (BET surface area of 35 m² g⁻¹) with the VO₂(R) phase. The obtained VO₂ had a uniform lamellar 3D structure with thickness of 20 nm. The VO₂/rGO cathode exhibited an initial specific capacity of 115 mAh g⁻¹ with a retention of 74% after 100 cycles.

VO₂(B)/rGO composite formed by the assembly of VO₂(B) nanoparticles and rGO were also used as electrodes for ESCs [431,432,433,434,435,436,437]. VO₂(B) nanobelts/rGO composite films were fabricated as electrodes for high-performance flexible all-solid-state supercapacitors [421]. The assembly composed of nanobelts with length of 3-5 µm and width of 40-150 nm, provide the maximum power density of 7152 W kg⁻¹ at the energy density of 3.13 Wh kg⁻¹. Lee et al. examined the effect of the ultra-large graphene transverse dimension (47 ± 22 µm) on the electrochemical properties of VO₂(B)/rGO composites for ESC. The hybrid electrode with ultra-large rGO performs lower sheet resistance (0.57 kW sq⁻¹) than the composite with rGO of 0.8 µm lateral size. Excellent capacitive performance of 769 F g⁻¹ was reported [431]. A 2D nanocomposite was built by the uniform distribution of 3D VO₂(B) flowers on N-doped GO (N-GO) [436]. The anchoring of the VO₂(B) flowers to N-GO results in high surface area of 71 m² g⁻¹ and shows enhanced electrochemical performance, i.e., after 50 cycles, the hybrid retained 251 mAh g⁻¹ (60% retention). GO-coated VO₂(B) nanomaterials were synthesized from a VOSO₄ sol using a hydrothermal approach [437]. The functionalized GO-coated VO₂(B) exhibits an urchin-like spherical shape (10–50 nm). Investigated as cathode materials deposited on stainless-steel mesh within a voltage range 1.5–4.0 V vs. Li/Li⁺ at current density 100 mA g⁻¹, these GO-coated VO₂(B) nanomaterials exhibit a discharge capacity of 240 mAh g⁻¹. Cycling performance of 220 mAh g⁻¹ was achieved after 20 cycles (10% capacity loss). Hierarchical VO₂(B)/rGO composite, synthesized through the hydrothermal route at 180 °C for 24 h, was investigated as an anode and activated carbon on carbon cloth (CC) as a cathode in battery-supercapacitor hybrid (BSH) device taking advantage of the mixed valency of V (V³⁺ and V⁴⁺) inside the as-prepared VO₂ matrix, which facilitates redox reaction in working potential of 0.01–3.0 V vs. Li^+/Li. The VO₂(B)/rGO heterostructure yields a high specific capacity of 1214 mAh g⁻¹ at current rate of 0.1 A g⁻¹ after 120 cycles, with a high rate capability and stability [434]. The kinetic activation of the composite toward the electrochemical reaction during the activation period for Li-ion penetration resulting in the unconventional increase in the specific capacitance was observed with the increase in the number of cycles. Lv and coworkers [435] constructed a porous-type VO₂(B)/rGO framework formed of flexible rGO sheets and VO₂(B) nanobelts intertwined together, which exhibits a specific capacitance of 353 F g⁻¹ at 1 A g⁻¹ current rate. The increased transport of ions and electrons by the porous structures allows a capacitance retention of 78% after 10,000 cycles at 10 A g⁻¹. Recently, Kang et al. [438] developed a mesoporous VO₂(B)/rGO composite, which had facile access of Li ions to storage site. The composite electrode with a large specific surface area and pore volume of 335 m² g⁻¹ and 3.09 cm³ g⁻¹, respectively, achieves a specific capacity of 226 mAh g⁻¹ at 50 mA g⁻¹ current density and superior capacity retention of 67.5% over 40 cycles at current density up to 2000 mA g⁻¹ (Figure 13). Electrochemical performance of VO₂/rGO composites as anodes or cathode for LIBs from the recent literature are summarized in

Table 8. Summary of the electrochemical performance of VO₂(B)/rGO composites. Relevant cycle number is given in brackets.

Material	Synthesis	Specific Capacity (mAh g−1)	Current Density (mA g−1)	Ref.
Cathode
VO2(B)/rGO	hydrothermal 150 °C/12 h	250	50 (50)	[440]
VO2(B)/GO	sonication + hydrothermal	350	40 (20)	[436]
VO2.07/rGO	hydrothermal 180 °C/20 h	133	70 (200)	[426]
VO2(R)rGO	hydrothermal 190 °C/48 h	85	20 (100)	[430]
VO2(B)/rGO	hydrothermal 180 °C/24 h	130	1000 (500)	[438]
VO2(B)/rGO	single-step hydrothermal	250	5000 (1600)	[429]
Anode
VO2(B)/rGO	microwave-assisted solvothermal	400	200 (400)	[422]
VO2(M)/rGO	sol-gel assisted hydrothermal	283	60 (200)	[424]
GO-coated VO2(B)	sol-gel + calcination	220	100 (20)	[437]
VO2(B)/rGO	hydrothermal 180 °C/24 h	1214	100 (120)	[434]

.

Although the present work focuses on the graphene-based composite electrodes for Li-ion batteries, it should also be noted that VO₂/rGO was also successful as a cathode for lithium sulfur batteries. The chemistry of such batteries is different, since the major difficulty is to solve the so-called shuttle effect of lithium polysulfides, but a VO₂/rGO proved to be able to solve this problem. The polar VO₂ can chemically anchor and catalyze the conversion of polysulfides and thus avoid their dissolution, while the rGO provides efficient transport pathways. The rGO/VO₂/S cathode with rGO wrapped yolk–shell VO₂ sphere hybrid host delivered a capacity of 718.6 mAh g⁻¹ initially, and 516.1 mAh g⁻¹ over 400 cycles at 1C [439].

4.7.2. V₂O₃-Based Composites

V₂O₃ has been considered as anode material because of its high theoretical capacity of 1070 mAh g⁻¹ and low discharge potential, but its application is hindered by the low conductivity and drastic volume change [441,442]. Leng et al. [443] prepared 3D V₂O₃/rGO gels through a hydrothermal reaction and annealing process using GO and NH₄VO₃. These composites show a discharge capacity of 786 mAh g⁻¹ under 0.07 A g⁻¹ current density and good cycle stability. V₂O₃/rGO composites were fabricated by through a facile solvothermal method and annealing process; V₂O₃ nanoparticles (NPs, sizes of 5–40 nm) are uniformly dispersed on rGO sheets without aggregation [444]. The V₂O₃/rGO nanocomposites achieves a reversible specific capacity of 823 mAh g⁻¹ under the current density of 0.1 A g⁻¹ in the potential range 0.01–3.0 V and maintains 407 mAh g⁻¹ under the high current density of 4.0 A g⁻¹. Using vanadyl oxalate (VOC₂O₄) and GO prepared by a modified Hummers’ method as starting precursors, V₂O₃/rGO composites were synthesized via solvothermal process in ethylene glycol and hydrothermal route at 200 °C for 12 h with subsequent annealing at 600 °C for 4h in H₂/Ar (8:92) mixed atmosphere. Another specimen V₂O₃/GO was obtained by heat treatment in 100% Ar ambient (i.e., without reducing atmosphere) [445]. The V₂O₃–rGO electrode yields a stable capacity of 380 mAh g⁻¹ after 50 cycles at a current density of 0.1 A g⁻¹, much higher than 280 mAh g⁻¹ of V₂O₃–GO. The rate capability as anode electrode provided capacities of 150, 108, and 97 mAh g⁻¹ at current densities of 3, 9, and 15 A g⁻¹, respectively. Liu and coworkers [446] performed an in-situ synthesis of V₂O₃ nanorods adhered to rGO via hydrothermal route and heat-treatment. Electrochemical features endow V₂O₃/rGO composite as advanced anode material for LIBs with good cycling stability, i.e., 675 mAh g⁻¹ after 300 cycles at 0.5 A g⁻¹ and excellent rate capability, i.e., 428 mAh g⁻¹ at 2 A g⁻¹. Xiao and coworkers [444] have successfully prepared V₂O₃/rGO nanocomposites through a solvothermal method and annealing process. The V₂O₃ nanoparticles (NPs, 5–40 nm in size) were uniformly dispersed and encapsulated by rGO sheets, which inhibits volume changes, improves the ion, and electronic conductivity of highly resistive V₂O₃ and avoids NPs aggregation. The electrochemical properties of V₂O₃/rGO investigated in the potential range 0.01–3.0 V show a reversible specific capacity of 823 mAh g⁻¹ under the current density of 0.1 A g⁻¹, and 407 mAh g⁻¹ under the high current density of 4.0 A g⁻¹.

4.7.3. V₂O₅-Based Composites

Vanadium pentoxide is a typical layered material, which is crystallized with the orthorhombic structure (Pmmn space group). V₂O₅ is a promising cathode material because of its 2D architecture, abundant sources, and high theoretical specific capacity (294 mAh g⁻¹ for 2Li uptake into g-Li_xV₂O₅) but shows limiting electrochemical performance in regard to the low ionic diffusivity (10⁻¹²–10⁻¹³ cm² s⁻¹) and moderate electrical conductivity (10⁻²–10⁻³ S cm⁻¹) [447]. Liu et al. [448] fabricated V₂O₅/rGO composites with two different morphologies, i.e., nanosheet or nanoparticle-assembled subunits (50–100 nm in-plane size) by adjusting the solvothermal solution as cathode materials for lithium-ion batteries. In the voltage range of 2.5–4.0 V vs. Li⁺/Li, the nanosheet-assembled V₂O₅/GO hybrid delivered an initial capacity of 142 mAh g⁻¹ at 2C rate and retained 85% of their original capacity at 1C rate after 500 cycles, while the nanoparticle-assembled V₂O₅/GO hybrid retained only 80 mAh g⁻¹. The V₂O₅/rGO composites exhibited better rate capability and long-term cycling stability; they delivered capacities of 122 mAh g⁻¹ at 64C rate. A two-step solvothermal method followed by vacuum annealing was employed to prepare mesoporous hybrids of V₂O₅ nanoparticles anchored on rGO. This hybrid obtained by slow hydrolysis of vanadium oxytriisopropoxide (C₉H₂₄O₄V) had a hierarchical structure with 20–30 nm V₂O₅ nanoparticles uniformly deposited on rGO nanosheets, leading to a surface area of 24 m² g⁻¹ and average pore size of 20 nm. The V₂O₅/rGO mesoporous hybrids were used as electrodes in symmetrical ESC (specific capacitance of ~466 F g⁻¹ in 1 mol L⁻¹ Na₂SO₄ electrolyte) and cathode of lithium battery (specific capacity of 295 and 132 mAh g⁻¹ at C/9 and 10C rate, respectively). They showed excellent cycling stability with discharge capacity retention of ~83% after 150 cycles at 1C rate [449]. Luo et al. prepared the GO/V₂O₅-coated LiNi_0.5Co_0.2Mn_0.3O₂ (GO/VO/NCM) cathode material by a wet chemical method [450]. The composite consisted of a V₂O₅ inner layer of 5 nm and a GO outer layer of 5–9 nm thick. The double-layer-coated NCM cathode material (initial discharge capacity of ~180 mAh g⁻¹) showed improved electrochemical performance with a capacity retention of 74.2% after 50 cycles in a range of 2.5–4.5 V vs. Li⁺/Li at 55 °C. Liu et al. developed a method to introduce GO sheets into V₂O₅ nanoribbons via sol-gel route [451]. The 2 wt% GO containing composite delivered a specific capacity of 315 mAh g⁻¹ at 1C rate and showed good cyclability over 200 cycles with a capacity decay of 0.13 mAh g⁻¹ per cycle. For the micro-nanosized V₂O₅/rGO composites that were fabricated by the hydrothermal treatment of the mixture of V₂O₅, cetyltrimethyl ammonium bromide, and GO dispersed in aqueous solution kept at 190 °C for 48 h [430], the initial charge capacity was 235 mAh g⁻¹ at 20 mA g⁻¹ current rate and 235 mAh g⁻¹ was retained after 100 cycles. A simple solvothermal technique was used to prepare self-assembled V₂O₅ nanosheets (NSs)/rGO nanocomposite with high specific area of 40 m² g⁻¹. The V₂O₅ NSs/rGO anode cycled at 3 and 15 A g⁻¹ current densities delivered specific capacities of 138 and 76 mAh g⁻¹, respectively. When discharged at 2C rate a discharge capacity of 102 mAh g⁻¹ was obtained after 160 cycles [452]. Highly porous polycrystalline V₂O₅/rGO hybrids were grown through heterogeneous nucleation of V₂O₅ nanoparticles (10–50 nm) on rGO sheets by using a solvothermal method followed by an annealing process [453]. In the voltage range of 4.0–2.0 V, the cathode performance of the as-prepared V₂O₅/rGO (46 wt%) was 102 mAh g⁻¹ at a high current density of 5700 mA g⁻¹ after 2000 cycles. A V₂O₅-reduced graphene ball composite was formed by the post-annealing of VO₂/rGO hybrid at 300 °C in air [425]. Hu and coworkers [454] synthesized uniform wrinkle V₂O₅/GO flexible thin films via a drop-casting technique with subsequent annealing process. The optimized films with mass ratio (8:1) of V₂O₅ and GO exhibited a specific capacitance of 397 F g⁻¹ at a current density of 1 A g⁻¹ and cycling stability with 98% retention over 2000 cycles at 5 A g⁻¹ rate. With 5 wt% graphene, the initial discharge capacity of V₂O₅/rGO was 282 mAh g⁻¹ with a coulombic efficiency of 99%. This cathode material delivered a capacity of 214 mAh g⁻¹ at 1 A g⁻¹ after 100 cycles. V₂O₅/rGO composites were formed by wet mixing the electrospun V₂O₅ nanowires (100–200 nm in diameter and µm length) [455]. The V₂O₅/rGO composite with 1 wt% rGO showed the 1st and 60th discharge capacities of 225 and 125 mAh g⁻¹, respectively, at C/5 rate. Recently, Kurc et al. [456] investigated the impact of the addition V_xO_y nanoparticles (50–150 nm) in TiO₂/rGO composite electrode synthesized by solvothermal method. Incorporation of 4.5 wt% V_xO_y in TiO₂/rGO was beneficial in terms of the cell stability. This electrode showed a reversible capacity of 182 mAh g⁻¹ at 50 mA g⁻¹ current density and 118 mAh g⁻¹ at 500 mA g⁻¹. A full cell with LiFePO₄//V_xO_y-TiO₂/rGO electrochemical chain showed good cycle ability while providing a capacity of 120 mAh g⁻¹. The cell did not lose significant capacity upon extended cycling. Electrochemical performance of V₂O₅/rGO composites as electrodes for LIBs from the recent literature are summarized in

Table 9. Comparison of the V₂O₅/rGO-type materials with preparation methods. Relevant cycle number is given in brackets.

Material	Synthesis	Specific Capacity (mAh g−1)	Current Rate (mA g−1)	Ref.
V2O5/GO/NCM	wet chemistry	125	55	[450]
V2O5/rGO	solvothermal solution	122	64C (500)	[448]
V2O5/rGO	two-step solvothermal	205	1C (120)	[449]
V2O5/GO (2 wt%)	sol-gel	240	1C (200)	[451]
V2O5/rGO (3.2%)	hydrothermal	175	20 (100)	[430]
V2O5 NSs/rGO	solvothermal	141	600 (160)	[452]
V2O5/rGO	solvothermal	102	5700 (200)	[453]
V2O5 NRs/rGO	hydrothermal + reflux	140	150 (100)	[457]
V2O5/rGO	post-annealing of VO2/rGO	214	1000 (100)	[425]
V2O5/rGO	electrospinning	125	C/5 (60)	[455]
VxOy-TiO2/rGO	solvothermal	180	50 (20)	[456]

.

Yan et al. [458] reported the fabrication of a crumpled rGO (c-rGO) encapsulated 3D hollow V₂O₅ nano/microspheres using one-step solvothermal process followed by subsequent annealing. The as-prepared c-rGO/V₂O₅ composite containing 5 wt% rGO delivered reversible capacities of 289 and 163 mAh g⁻¹ at current densities of 0.1 and 5.0 A g⁻¹, respectively, as well as a capacity retention of about 94% after 200 cycles at 2 A g⁻¹ in the potential range 2.0–4.0 V vs. Li⁺/Li. Wang et al. [459] investigated the V₂O₅ nanobelts/rGO composites as positive electrode for aqueous zinc-ion battery (ZIB). While the commercial V₂O₅ powders show poor cycle stability and low specific capacity of 30 mAh g⁻¹ after 200 cycles, V₂O₅/rGO nanocomposites delivered a high specific capacity of 135 mAh g⁻¹ after 200 cycles at a current density of 0.1 A g⁻¹. The rate capability test yielded a capacity of 113 mAh g⁻¹ when a current density of 1 A g⁻¹ was applied. Du et al. [460] developed GO/V₂O₅ composites as cathode materials for rechargeable magnesium batteries via solvothermal synthesis using vanadium oxytriisopropoxide and graphene oxide. The composites attain a high discharge capacity of 178 mAh g⁻¹ at 0.2C rate and 140 mAh g⁻¹ after 20 cycles in the potential range 1.0–2.8 V. The GO/V₂O₅ composite can host 0.6 Mg ions per formula unit.

The V₃O₇/GO nanocomposites, with the V₃O₇ nanobelts of 12 nm in width and several µm in length were prepared by a hydrothermal method. The electrochemical tests of these electrode in Li cells showed a first discharge capacity of 352 mAh·g⁻¹at the current density of 100 mA·g⁻¹ and a good cycling performance with 87% and 91% capacity retention upon 50 cycles at the current density of 400 and 800 mA·g⁻¹, respectively [461].

Numerous reports are related to the V₂O₅/rGO composites applied as electrodes of electrochemical ESCs. Ramadoss et al. reported the fabrication of a V₂O₅/rGO electrode, which displayed a maximum specific capacitance of 250 F g⁻¹ at 5 mV s⁻¹ scan rate retaining 95% of its initial value after 5000 cycles [462]. A bifunctional composite hydrogel containing rGO nanosheets and V₂O₅ nanobelts (rGO/V₂O₅) was successfully prepared by a hydrothermal method at 180 °C for 12 h with GO and aqueous V₂O₅ precursors as raw materials followed by a freeze-drying process [463]. The rGO/V₂O₅ composite exhibited outstanding performance for ESCs, providing a specific capacitance of ~320 F g⁻¹ at a current density of 1 A g⁻¹ for V₂O₅ mass loading of 67.3 wt%. Foo et al. [464] eliminated the need for current collectors or additives and reducing resistance (sheet resistance 29.1 Ω sq⁻¹) by constructing V₂O₅/rGO free-standing hybrid electrodes for ESC applications. The effective exfoliation of rGO achieved high areal capacitance of 511 mF cm⁻² coupled with high mass loadings. An asymmetric flexible supercapacitor based on the rGO/V₂O₅-rGO configuration delivered a low equivalent series resistance (ESR) of 3.36 Ω. Zhang et al. [465] prepared V₂O₅-rGO composites using a two-step reduction method with V₂O₅ nanowires (as long as ~1 mm) as the skeleton and rGO nanosheets as the additive. The product had an electrical conductivity of 1.62 S cm⁻¹, which is higher than that of the pure V₂O₅ nanowire (0.049 S cm⁻¹). Studied as cathode for Li-ion battery, the free-standing V₂O₅/rGO composite showed a reversible specific areal capacity of 885 μAh cm⁻² at 0.09 mA cm⁻². When tested at a high current rate of 0.9 mA cm², it gave excellent cycling performance with a retention of ~69% after 1000 cycles.

Table 10. Comparison of the V₂O₅/graphene composites as electrodes for electrochemical supercapacitors (ESCs).

Material	Synthesis	Capacitance (F g−1)	Rate (mA g−1)	Electrolyte	Ref.
V2O5/rGO	solvothermal	537	1000	8 M LiCl	[466]
VO2/rGO	hydrothermal	225	250	0.5M K2SO4	[467]
VOx NTs/rGO	hydrothermal	210	1000	1M Na2SO4	[468]
VO2 NBs/rGO	hydrothermal	426	1000	0.5M K2SO4	[469]
V2O5 NWs/rGO	hydrothermal	80	500	1M LiTFSI	[470]
VOx/rGO	solvothermal	183	500	0.5M Na2SO4	[471]
V2O5/rGO	hydrothermal	130	100	1M LiClO4	[464]
V2O5/rGO	coprecipitation	484	500	0.5M K2SO4	[472]
V2O5/rGO	solvothermal	450	500	1M Na2SO4	[449]
V2O5 NRs/rGO	sol-gel	218	5000	1M Na2SO4	[473]

summarizes the electrochemical performance of V₂O₅/graphene composites as electrodes for supercapacitors

4.7.4. VPO₄-Based Composites

Among the polyanionic anode materials, vanadyl phosphate, VPO₄, is considered as a promising anode material because of the excellent chemical activity of vanadium with a relatively high theoretical specific capacity (550 mAh g⁻¹) [474]. The lithiation/delithiation mechanism is a conversion reaction involving three electrons. Combined with rGO, VPO₄ as electrode of both LIBs and ESCs has shown significant improvement in their electrochemical performances [475,476]. Lu et al. [476] developed a 3D hydrangea-like vanadium phosphate microspheres embedded in the interlaced multiporous rGO scaffold (VPO₄/rGO) to form self-supporting porous anodes for lithium-ion batteries. The VPO₄/rGO samples were synthesized by in-situ solvothermal reduction method using a mixture of GO and VOPO₄∙2H₂O acting as reductant and dispersant followed by a hydrothermal treatment at 180 °C for 24 h. The porous microspheres with 80% VPO₄ had a specific surface area of ~87 m₂ g⁻¹ and half pore width of ~35 nm. The as prepared VPO₄/rGO-80% electrode displayed a fast-charging specific capacity of 537 mAh g⁻¹ at 0.1 A g⁻¹ (weight based on whole electrode), and long-life performance of 318 mAh g⁻¹ after 3000 cycles at 2 A g⁻¹ (Figure 14). Through differentiating the contribution from pseudocapacitance and diffusion-controlled process, the authors demonstrated that the charge–discharge mechanism was dominated by the pseudocapacitance.

4.8. Cobalt-Based Oxide Composites

4.8.1. Co₃O₄-Based Composites

Co₃O₄ has attracted extensive interest due to its high specific capacity of 890 mAh g⁻¹ (i.e., electrochemical conversion reaction Co₃O₄ + 8Li⁺ + 8e⁻ ↔ 3Co + 4Li₂O to form lithium oxide and metal nanoparticles). However, similarly to other TM oxides, Co₃O₄ suffers from large volume expansion/contraction and loss of interparticle contact due to electrode pulverization upon lithiation [477].

The most common technique utilized for the synthesis of Co₃O₄/rGO composite is the hydrothermal method, which follows the preparation of a primary solution of GO with the cobalt precursor. For example, Kumar et al. [478] fabricated a ternary Co₃O₄/rGO/CNTs nanocomposite for supercapacitor application using a three-step procedure: (i) Mixture of Co(NO₃)₂·6H₂O with a GO/CNTs dispersion and urea as reducing agent for GO with adjustment of the pH at 10 using ammonia, (ii) hydrothermal process at 180 °C for 8 h and (iii) after washing and drying then an annealing treatment at 300 °C in N₂ environment for 3 h yields the final product. Another popular technique of fabrication is the one-pot microwave-assisted synthesis. Typical experiments are carried out in a domestic microwave at a power of 1600 W for 30 min [479].

A rough comparison of early reports indicates that the Co₃O₄/rGO composite displayed superior electrochemical performances. The Co₃O₄/rGO sheet-on-sheet nanocomposite prepared by microwave-assisted synthesis presents a capacity of 931 mAh g⁻¹ at a large current rate of 5C (4450 mA g⁻¹) and better cycling behavior with reversible capacity of ~1065 mAh g⁻¹ after 30 cycles [480]. Yang et al. [481] have described a self-assembly approach to prepare tailored graphene-encapsulated, mesoporous Co₃O₄ microspheres. The nanocasting method produces a composite with full wrapping by graphene shells, which delivers a first discharge capacity of 1533 mAh g⁻¹ and rapidly stabilizes. A high reversible capacity of about 820 mAh g⁻¹ was achieved after 35 cycles at a current of 100 A g⁻¹. Wu et al. [482] synthesized the composite of Co₃O₄ nanoparticles (10–30 nm in size) anchored on conducting rGO sheets via a one-step approach. The Co₃O₄/rGO hybrid anode displays a discharge capacity of 935 mAh g⁻¹ was obtained after 30 cycles. Zhang et al. [483] also reported the deposition of Co₃O₄ and the exfoliation of graphene sheets in a one-step electrochemical process. The Co₃O₄/rGO composite as an anode material yields a reversible discharge capacity of about 880 mAh g⁻¹ for 40 cycles at 0.3C rate.

Using a convenient PVP (polyvinylpyrrolidone)-assisted method under reflux conditions, Pan et al. [484] prepared a Co₃O₄/rGO composite, which consists of uniform Co₃O₄ nanoparticles (~100 nm) tightly enwrapped by rGO in the presence of the PVP surfactant. The Co₃O₄ NPs/rGO/PVP composite exhibits superior Li-battery performance with large discharge capacity of 860 mAh g⁻¹ during 120 cycles at 40 mA g⁻¹ rate. Mesoporous Co₃O₄ nanosheets (Co₃O₄-NSs) and nitrogen-doped reduced graphene oxide (N-rGO) were prepared through a facile hydrothermal approach, and the N-rGO/Co₃O₄-NSs composite was fabricated using an infiltration procedure. The 2D/2D nanostructured mesoporous N-rGO/Co₃O₄-NSs composite was evaluated in a half-cell as an anode material delivering a reversible capacity of 1305 and 1501 mAh g⁻¹ at a current density of 80 mA g⁻¹ for the 1st and 50th cycles, respectively [27]. Lou et al. [485] prepared a Co₃O₄/rGO anode material with a specific surface area of 149.5 m² g⁻¹ composed of ultra-small Co₃O₄ nanoparticles (~12.5 nm in size) anchored on rGO nanosheets. The Co₃O₄/rGO containing ~40 wt% rGO nanocomposite shows excellent electrochemical performances with a high reversible discharge capacity of 830 mAh g⁻¹ after 75 cycles at 200 mA g⁻¹, and a reversible capacity of 681 mAh g⁻¹ after 30 cycles at 200 mA g⁻¹ and 100 consecutive cycles at 500 mA g⁻¹. Rate capability measurements displays reversible specific capacities of about 1153, 961.0, 851, and 736 mAh g⁻¹ after each eight cycles at 50, 100, 200, and 500 mA g⁻¹, respectively.

Cho et al. [486] highlighted the enhanced conversion reaction of the Co₃O₄ NFs@rGO composite composed of 1D Co₃O₄ nanofibers (NFs, 150–200 nm in diameter) covered with low content (~3.5 wt%) rGO ultrathin sheets (~3 nm) by electrostatic self-assembly. To ensure efficient graphene wrapping, the surface of the Co₃O₄ NFs was functionalized to amine group poly(allylamine hydrochloride) (PAH). The Co₃O₄ NFs/rGO composite presents a high reversible capacity of 615 mAh g⁻¹ and 92% capacity retention after 400 cycles at 4.0 Ag⁻¹ and displays excellent rate capability. Kesavan et al. [487] fabricated a nanocomposite anode composed of hollow Co₃O₄ nanospheres (h-Co₃O₄ NSs) combined with CNTs and rGO using a sonication method followed by a hydrothermal treatment. The h-Co₃O₄ NSs (27 nm in size) were prepared by self-assembly process using poly(styrene–b–2-vinylpyridine–b–ethylene oxide) micelle as a soft template and (NH₄)₂Co(SO₄)₂·6H₂O as cobalt source. The nanocomposite h-Co₃O₄ NSs/CNT/rGO electrode exhibits higher charge/discharge capacities of ~943 mAh g⁻¹ when compared to pristine h-Co₃O₄ NSs-based electrode (~678 mAh g⁻¹) after 50 cycles. Park et al. [488] utilized a one-pot microwave-assisted hydrothermal method for the preparation of Co₃O₄/rGO nanocomposites. The Co₃O₄ nanoparticles (10–30 nm in size) uniformly decorated the surface of rGO sheets, which act as 2D template. The Co₃O₄/rGO hybrid anode containing 79 wt% Co₃O₄ was tested galvanostatically in the voltage range 0.01–3.0 V using a 2032-type coin cell. It delivers a first discharge capacity of 947 mAh g⁻¹ at 1 A g⁻¹ current rate. The excellent cyclability was observed after 100 cycles at 0.5 A g⁻¹ rate, and the specific discharge capacity is still as high as 683 mAh g⁻¹, which is almost the second discharge capacity (685 mAh g⁻¹). The suitability of Co₃O₄/rGO hybrid as cathode material of rechargeable aluminum-ion batteries has been demonstrated by Zhang et al. [489]. The composite was synthesized by a spontaneous self-assembly process using precursor made from the coprecipitation of the Co²⁺ - hexacyanocobaltate (II) metal-organic framework. The self-assembled Co₃O₄/rGO products were further annealed at 500 °C in Ar atmosphere for 2 h. After the initial activation phase, an Co₃O₄/rGO‖Al cell retains discharge capacity of 168 mAh g⁻¹ and coulombic efficiency of 76% over 500 cycles of charge/discharge.

More recently, Mussa et al. [490] used a microwave irradiation technique to prepare Co₃O₄/rGO/hexagonal boron nitride (h-BN) nanocomposites as anodes for high-temperature LIBs. The addition of h-BN not only enhances the thermal stability but also increases the specific surface area (191 m² g⁻¹) of composites. The Co₃O₄/rGO/h-BN electrode cycled at 208 mA g⁻¹ in the voltage range 0.5–2.7 V displays a first specific capacity of 758 mAh g⁻¹ maintained at 266 mAh g⁻¹ after 100 cycles higher than 186 mAh g⁻¹ for the Co₃O₄/rGO hybrid.

The use nitrogen-doped rGO (N-rGO) is a promising strategy for the development of anode materials for LIBs with high electrochemical performance [491,492]. Li et al. [492] designed a Co₃O₄/N-rGO composite, in which ultrafine Co₃O₄ nanocrystals (5–10 nm) are homogeneously attached without aggregation to N-rGO sheets by hydrothermal method. In this synthesis, ammonia acts as nitrogen dopant. The charge/discharge cycling performance of the Co₃O₄/N-rGO anode shows a capacity of 882 mAh g⁻¹ at the 6th cycle at 400 A g⁻¹, which gradually decreases down 766 mAh g⁻¹ at the 50th cycle. Rate capability tests show discharge capacities of 1100, 950, 860, 670, and 460 mAh g⁻¹ at different current densities of 100, 200, 400, 1000, and 2000 mA g⁻¹, respectively.

Hierarchically well-developed porous graphene nanofibers comprising N-doped graphitic carbon (NGC)-coated cobalt oxide hollow nanospheres were tested as anodes for high-rate Li-ion batteries [493]. This nanostructured composite delivers a discharge capacity of 823 mAh g⁻¹ after 500 cycles at 3.0 A g⁻¹ current density with a low decay rate of 0.092% per cycle. The rate capability is also noteworthy as the structure exhibits high discharge capacities of 422 and 222 mAh g⁻¹ at current densities of 50 and 100 A g⁻¹, respectively. Co₃O₄/rGO composite powder with the nanobubble-decorated rGO sphere structure was prepared by spray pyrolysis with a reduction process under a mixture of H₂ and Ar gas (i.e., nanoscale Kirkendall diffusion process [494]. Tested as anode for Li-ion battery at the current density of 2 A g⁻¹ for 200 cycles before and after the nanoscale Kirkendall diffusion, the Co₃O₄/rGO composite powders show discharge capacities of 932 and 1156 mAh g⁻¹, respectively, with capacity retention of 89% and 99%, respectively, at the second cycle.

4.8.2. CoO-Based Composites

Cobalt monoxide including nanoparticles, nanorods, and mesoporous nanowires have been under prime study because its theoretical capacity of 716 mAh g⁻¹ (following the conversion reaction CoO + 2Li⁺ + 2e⁻ ↔ Co⁰ + Li₂O) and its long cycling stability when combined with rGO [495]. Fu and coworkers [496] elaborated a series of CoO/rGO composites with different proportions via an in-situ hydrothermal method and investigated them as an additive for the nickel-based alkaline secondary battery cathode. The Ni electrode including 5 wt% of CoO/rGO (5:5) delivered a high discharge capacity 208.6 mAh g⁻¹ at 10C rate.

Bindumadhavan et al. [497] fabricated a cobalt oxide decorated reduced graphene oxide (CoO/rGO) nanocomposites prepared by simultaneous reduction of GO and Co²⁺ in the presence of NaBH₄. The addition of 15 wt% rGO led to a narrow size distribution of ultra-fine CoO NPs. For loading ratios of rGO of 5, 15, 25, and 35 wt%, the BET specific surface area of CoO/rGO increases to 81, 87, 94 and 117 m² g⁻¹, respectively. Later, the same group has developed a facile and simple route for the preparation of ultrafine CoO/rGO nanohybrids with tunable particle size and crystallinity. The homogeneous and ultrafine CoO NPs (4.5 nm in size) were intimately attached onto rGO surface after an optimized calcination time of 60 min forming CoO/rGO flakes ~30 nm thick. These CoO/rGO composite anodes delivered a specific capacity of 520 mAh g⁻¹ at current rate of 2.0 A g⁻¹, and the capacity was recovered to 900 mAh g⁻¹ at 150 mA g⁻¹ after 60 cycles [498]. The CoO/rGO composite developed by Wang et al. [499] was synthesized using a hydrothermal process (160 °C, 6 h) of a GO suspension mixed with a Co(NO₃)₂ solution followed by an annealing at 500 °C for 2h under nitrogen ambient. The as-fabricated composite anode exhibited a first discharge capacity of 1312 mAh g⁻¹ at a current density of 100 mA g⁻¹. The results of long-term cycling demonstrated a capacity of 557 mAh g⁻¹ at a high current density of 10 A g⁻¹ after 300 cycles.

The CoO-based composite anode proposed by Sun et al. [500] was composed of CoO-Co₃O₄ heterostructure nanoribbons (~50 nm) sandwiched between rGO nanosheets through a solvothermal technique and a sintering process at 350 °C in N₂ atmosphere for 1 h. The CoO-Co₃O₄/rGO composite with a specific surface area of 150 m² g⁻¹ and average pore size of 4.2 nm delivered a reversible capacity of 994 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 200 cycles and presented good cyclability of 395 mAh g⁻¹ after 500 cycles even at a high charge–discharge rate of 1 A g⁻¹. The rate capability of the CoO-Co₃O₄/rGO anode carried out at 100, 200, 500, 1000, 2000, 5000, and 100 mA g⁻¹ after each 10 cycles at each current density was 1210, 1060, 890, 730, 578, 392, and 915 mAh g⁻¹, respectively. Cao et al. [501] assembled a CoO/rGO hybrid for LIBs by using oleic acid as particle shape controller. The composite with 19.wt% carbon had a BET specific surface area of 183.6 m² g⁻¹ and pore size in the range 5–10 nm. It delivered a highest capacity of 1309 mAh g⁻¹ after 100 cycles at 0.1 A g⁻¹ current rate. The diffusion coefficient of Li ions in the CoO/rGO network determined from EIS measurements was 4.37 × 10⁻¹⁵ cm² s⁻¹ (Figure 15).

Leng et al. [502] reported the synthesis of rGO/CoO nanowires (diameter of 10–20 nm and length of several µm) mutually supporting porous structure using an in situ method by annealing self-assembled GO/Co(CO₃)_0.5(OH)·0.11H₂O nanowires in N₂ environment. The as-prepared CoO/rGO anode material (BET specific surface area of 66.1 m² g⁻¹) presented a high reversible capacity of 994 mAh g⁻¹ even after 100 cycles with 96.3% capacity retention of the first discharge capacity, and superior rate capability (262 mAh g⁻¹ 10 A g⁻¹). This electrode sustained a capacity of 520 mAh g⁻¹ at a current rate of 3 A g⁻¹ after 750 cycles. More recently, Yin et al. [503] designed a new CoO/Co₂B/rGO nanocomposite anode with large capacitive contribution for high-efficiency and durable lithium storage. The composite was synthesized by a combination of precipitation method and pyrolysis. Initial discharge/charge capacities of 889/669 mAh g⁻¹ with a high ICE of 75.28% at 0.1C rate are delivered for the CoO/Co₂B/rGO composite with primary particles of 20–60 nm. The proportion of the capacitive contribution deduced from cyclic voltammetry is calculated to be 89.2%.

Table 11. Electrochemical performance of Co/rGO composite anode materials. Relevant cycle number is given in brackets.

Material	Synthesis	Specific Capacity (mAh g−1)	Current Density (mA g−1)	Ref.
Co hollow cube/rGO	sacrificial-template + annealing	1170	150 (60)	[504]
m-CoO NRs/rGO	hydrothermal + calcination	960	100 (50)	[505]
CoO/3D rGO hydrogel	hydrothermal + calcination	962	200 (80)	[506]
Co/rGO	simultaneous reduction	690	600 (60)	[497]
CoO/rGO flakes	simultaneous reduction	900	150 (60)	[498]
CoO-Co3O4/rGO	solvothermal + sintering	994	100 (200)	[500]
CoO/Co2B/rGO	precipitation + pyrolysis	276	10C	[503]
CoO/rGO	one-pot in situ solution	577	100 (435)	[507]
CoO/CuO/rGO	hydrothermal	1364	200 (100)	[508]
CoO/rGO	hydrothermal + calcination	557	10000 (300)	[499]
CoO/rGO	hydrothermal (oleic acid)	1039	100 (100)	[501]
CoO/rGO NWs	in situ self-assembly	520	3000 (750)	[502]

summarizes the electrochemical performance of CoO/rGO composites as anodes for LIBs from the recent literature.

4.9. Copper-Based Oxide Composites

Song et al. [509] manufactured a three-dimensional CuO/GO aerogel cylindrical composite as a super-capacitor electrode material through a one-step self-assembly hydrothermal procedure at 180 °C for 8 h using Cu(NO₃)₂ as Cu precursor and citric acid as reducing agent. The as-prepared product exhibited specific capacitance of 211 F g⁻¹ at current density of 1 A g⁻¹. Liu et al. [510] reported the synthesis of sponge-like porous GO/CNTs/CuO hybrid aerogel as anode material for lithium-ion batteries. The samples were prepared as a flocculent suspension obtained by adding a GO suspension and oxidized water-soluble CNTs treated with nitric acid in a CuO colloidal solution followed by a hydrothermal reduction at 180 °C for 10 h. The galvanostatic charge–discharge tests carried out in the voltage range of 0.01–3.0 V under a constant current density of 67 mA g⁻¹ (0.1 C) demonstrated the excellent cyclic retention of the binder-free GO/CNTs/CuO hybrid with the specific capacity of 636.9 mAh g⁻¹ after 80 cycles. Zhu and coworkers [511] designed three-dimensional CuO/PANI/rGO ternary hierarchical architectures (where PANI is polyaniline) using an in-situ polymerization method combined with hydrothermal route at 180 °C for 12 h. The CuO/PANI/rGO hybrid was reversibly cycled as electrode for supercapacitor in a high voltage region up to 1.2 V with 97.4% specific capacitance retention after 10,000 cycles. It exhibited a maximum specific capacitance of 634 F g⁻¹ and a high energy density of 126.8 Wh kg⁻¹ with a power density of 114.2 kW kg⁻¹ at a current density of 1.0 A g⁻¹. Zhao et al. [512] reported the synthesis of CuO/GO composites in the water–N,N-dimethylformamide without an extra reducing agent, which provided mesoporous sample with high BET surface area of 235 m² g⁻¹. Zhu et al. [513] reported the synthesis of CuO/GO hybrids by aligning CuO nanoparticles onto GO sheets in the water–isopropanol media. The authors suggested that the formation mechanism of these CuO/GO nanocomposites occurred as intercalation and adsorption of Cu²⁺ ions onto the GO sheets, followed by the nucleation and growth of the CuO crystallites. The CuO/rGO nanocomposite anode was prepared via microwave-assisted method within short reaction times (<20 min) and subsequent sintering at 500 °C for 5 h by Rai et al. [514]. The first discharge capacity of the nanocomposite (40–65 nm particle size and specific surface area of 37.7 m² g⁻¹) with 10 wt% graphene was reported at 1043 mAh g⁻¹ with reversible capacity retention of 516 mAh g⁻¹ after 45 cycles at 0.1 mA cm⁻² over the potential range of 0.01–3.0 V vs. Li⁺/Li. These results compared well with the theoretical specific capacity of 670 mAh g⁻¹ based on the electrode conversion reaction, i.e., CuO + 2Li⁺ + 2e⁻ ↔ Cu⁰ + Li₂O. Xu and coworkers [515] reported the fabrication of core–shell structured Cu₂O@GO composite consisting of Cu₂O microspheres encapsulated by GO nanosheets. These anode materials delivered a reversible capacity of 458 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles and a reversible capacity of 240 mAh g⁻¹ after 200 cycles even at a high charge–discharge rate cycling of 1 A g⁻¹. The rate capability evaluated by the ratio of capacity at 100/1000 mA g⁻¹ was 49% for the Cu₂O@GO hybrid, against 25% for a bare Cu₂O anode.

In 2016, Sun et al. [516] reported the synthesis of Cu₂O-CuO-RGO composites as anode materials by a self-assembly solvothermal method in a water-ethanol mixture, which delivered a reversible capacity of 842 mAh g⁻¹ after 80 cycles at a current density of 100 mA g⁻¹. The capacity measured every 10 cycles at current densities of 100, 200, 500, 1000, 2000, 5000, and 100 mA g⁻¹ were 840, 723, 511, 402, 336, 224, and 703 mAh g⁻¹, respectively. These greatly improved electrochemical behaviors are closely related to the sandwich-like and mesoporous structure of the composite, which had a high specific surface area of 132.1 m² g⁻¹ and typical pore sizes in a wide range (2.5–20 nm). Sheikhzadeh et al. [517] fabricated nanocomposite foam layer of CuO/GO by the deposition of GO particles on electroplated CuO foam layers through electrophoretic deposition providing samples with BET surface area of 99.2 m² g⁻¹ and average pore diameter of 29 nm. By embedding GO particles on CuO during 7 min, a specific capacitance of 334 F g⁻¹ was obtained. A 3D Ni-Cu foam/rGO composite was prepared by the electrochemical deposition of rGO nanosheets on Ni-Cu foam (0 to −1.5 V vs. SCD) and the subsequent reduction by cyclic voltammetry under different sweep rates in the range of 10–100 mV s⁻¹ [518]. The Ni-Cu foam/rGO hybrid deposited electrochemically at the sweep rate of 50 mV s⁻¹ presented the specific capacitance of 1380 F g⁻¹ at a discharge rate of 2 A g⁻¹ and an energy density of 30 Wh kg⁻¹ at a power density of 10.2 kW kg⁻¹.

4.10. Nickel-Based Oxide Composites

Nickel monoxide has received widespread attention because of its high theoretical specific capacity of 718 mAh g⁻¹ for 2 Li⁺ uptake from the conversion reaction NiO + 2Li⁺ + 2e⁻ ↔ Ni⁰ + Li₂O. In addition, it is a low-cost, safe, and environmentally friendly compound. Several synthesis techniques are used to fabricated NiO/rGO composites including hydrothermal method [519], thermal decomposition [520], solvothermal [521], in situ ultrasonic agitation [522,523], and heterogeneous assembly approach with subsequent in-situ thermal reduction [524].

Liu et al. reported the formation of hierarchical NiO/rGO nanohybrids with tunable microstructures through the thermal decomposition of Ni(OH)₂/rGO formed by the electrostatic attraction between exfoliated Ni(OH)₂ and GO nanosheets [520]. Ren et al. [525] reported the hydrothermal synthesis and lithium storage performance of binary NiO NWs/rGO and ternary NiO/rGO/PPy hybrids as anode materials for LIBs. The ternary composite was prepared by chemical oxidation polymerization of pyrrole monomer with a template of the binary hybrid. The NiO/rGO/PPy hybrid delivered the first discharge capacity of 1080 mAh g⁻¹ at 0.2C rate and the first charge capacity reached 830 mAh g⁻¹. After 40 cycles, the discharge capacity is maintained at 499 mAh g⁻¹. Li et coworkers [519] designed a hybrid NiO/rGO structure with flower-like NiO wrapped homogeneously by rGO sheets via a facile and controllable hydrothermal treatment of the suspension formed with Ni(NO₃)₂·6H₂O, CO(NH₂)₂, NH₄F, and GO and subsequent annealing at 400 °C for 2 h in Ar ambient. The NiO/RGO nanocomposites exhibited a reversible specific capacity of 702 mAh g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. NiO-ZnO/rGO composites with NiO-ZnO nanoflakes were obtained by thermal annealing of sonically mixed Ni (OH)₂-Zn (OH)₂/rGO precursor [526]. The as-prepared porous anode material, which exhibited a specific surface area of 110.49 m² g⁻¹ and pore size distribution in the range 30–80 nm, delivered a reversible capacity of 1017 mAh g⁻¹ at a current density of 100 mA g⁻¹ after 200 cycles and a specific capacity of 458 mAh g⁻¹ at 500 mA g⁻¹ even after 400 cycles. A specific capacity of 185 mAh g⁻¹ was reached at a current density of 2000 mA g⁻¹ (Figure 16). The double-layer NiFe₂O₄@NiO-hollow-nanosphere (mean size of 14 nm) decorated rGO composite was prepared by spray pyrolysis with nanoscale Kirkendall diffusion. The first discharge capacities of the nanosphere-decorated rGO composite powders with filled NiFe₂O₄/NiO and hollow NiFe₂O₄@NiO, at a current density of 1 A g⁻¹, are 1168 and 1319 mAh g⁻¹, respectively, which decrease to 597 and 951 mAh g⁻¹, respectively, after 100 cycles. For repeated lithium-ion discharge at the high current density of 4 A g⁻¹ for the 400th cycle, a capacity of 789 mA h g⁻¹ is delivered by the NiFe₂O₄@NiO/rGO hybrid [527].

Recently, Tian et al. [521] used metal organic frameworks as NiO precursor for the fabrication of NiO/rGO anode, which provided a modest specific capacity of 440 mAh g⁻¹ at 1C rate after 200 cycles. In situ ultrasonic agitation method followed by heat calcination in N₂ atmosphere was carried out for the synthesis of NiO nanoflakes/rGO hybrid, which delivered reversible capacity of 825 mAh g⁻¹ after 50 cycles at 100 mA g⁻¹, higher than that of the pure NiO flakes (197 mAh g⁻¹) [518]. Ma and coworkers [523] studied the lithium storage properties of NiO/rGO composites derived from different degrees of GO through an ultrasonic agitation technique. The reversible capacity of these anode materials increased gradually to 1046 mAh g⁻¹ and then declined to 956 mAh g⁻¹ after 50 cycles at 100 mA g⁻¹ current rate. Another Ni-based complex with spinel structure was optimized by Li et al. [528], namely the NiFe₂O₄/rGO composite, which was fabricated via a hydrothermal-annealing procedure. The NiFe₂O₄/rGO with NiFe₂O₄ nanoparticles 20 nm diameter retained a specific capacity of 1105 mAh g⁻¹ at a current density of 100 mA g⁻¹ over 50 cycles.

5. Anode Materials for Na-Ion Batteries

To obtain anodes with high performances, it is important to exploit the entire graphene layer—which provides favorable sites for energy storage—and to construct the overall conductivity of the graphene networks. In this regard, CNTs can bridge the vertical defects for electron transfer in the graphene layers and increase the distance between graphene sheets. Therefore, when CNTs and graphene are combined with transition-metal chalcogenides (TMCs), synergistic effects on the anodes are expected by the bridging graphene and the suppression of the restacking of graphene sheets during repeated charge and discharge processes. Golden bristlegrass-like unique nanostructures comprising rGO matrixed nanofibers entangled with bamboo-like N-doped CNTs containing CoSe₂ nanocrystals at each node (N-CNT/rGO/CoSe₂) are designed as anodes for high-rate sodium-ion batteries (SIBs). During a simple etching process, the defects in the N-CNTs allow the complete phase conversion of Co into CoSe₂ through the efficient penetration of H₂Se gas inside the CNT walls. The N-CNT/rGO/CoSe₂ hybrid, tested as anode material for sodium-ion batteries (SIBs), delivers a discharge capacity of 264 mAh g⁻¹ after 10,000 cycles at a high current density of 10 A g⁻¹, and the capacity retention of 89% at the 100th cycle. The rate capability tests show discharge capacities of 395, 263, and 151 mAh g⁻¹ at 1, 10, and 20 A g⁻¹ current densities, respectively [529].

Mesoporous WSe₂/rGO composites as anode materials for SIBs were prepared by spray pyrolysis and subsequent selenization. The composite particles exhibit both well-dispersed rGO nanosheets and well-faceted WSe₂ nanocrystals with plenty of folded edges. The WSe₂/rGO composite and bare WSe₂ deliver discharge capacities of the 238 and 36 mAh g⁻¹, respectively, for the 100th cycle at a current density of 0.5 A g⁻¹. The WSe₂/rGO composite shows much lower onset potential and larger current density (36.5 mA cm⁻² at η = 300 mV) than those of the bare WSe₂ particles (0.61 mA cm⁻² at η = 300 mV) [530]. Through the treatment of Fe₃O₄/rGO powders under H₂Te gas, the FeTe₂/rGO hybrid powders were prepared via spray pyrolysis. Electrochemically tested as the anode materials for sodium-ion batteries (SIBs), the FeTe₂/rGO composite containing 5 wt% rGO delivered discharge capacities of 493 and 293 mAh g⁻¹ for the 1st and 80th cycles, respectively. The Na-ion storage mechanism for FeTe₂ in SIBs is described by FeTe₂ + 4Na⁺ + 4e⁻ ↔ Fe + 2Na₂Te (i.e., the discharge process forms metallic Fe nanocrystals and Na₂Te by a conversion reaction) [531]. Hybrid of graphene-wrapped NiSe₂/C porous nanofibers (NiSe₂/rGO-C) were prepared via electrospinning and a subsequent selenization process with optimum amount of amorphous carbon obtained from the polystyrene to polyacrylonitrile ratio of 1:4. The electrochemical tests in sodium cell at a current density of 200 mA g⁻¹ show initial discharge capacities of the NiSe₂-rGO-C composite (nanofibers of 27 nm) and bare NiSe₂ nanofibers of 717 and 755 mAh g⁻¹, respectively, which decrease to 468 and 35 mAh g⁻¹ after 100 cycles [532]. A 1D nanohybrid comprised of conductive graphitic carbon (GC)-coated hollow FeSe₂ nanospheres decorating reduced graphene oxide (rGO) nanofiber (FeSe₂@GC–rGO) was designed as an efficient anode material for SIBs [533]. The hollow nanosphere FeSe₂@GC–rGO hybrid nanofibers deliver a discharge capacity of 412 mAh g⁻¹ at a current density of 1 A g⁻¹ for the 150th cycle bare, which is much higher than 63 mAh g⁻¹ for FeSe₂ nanofibers. Even at high current density of 10 A g⁻¹, a discharge capacity of 352 mAh g⁻¹ was achieved, which arose from the synergetic effects of the FeSe₂ hollow morphology and highly conductive rGO matrix. FeSe_x/rGO composite, in which hollow FeSe_x nanoparticles are uniformly distributed throughout the rGO matrix, was prepared by spray pyrolysis applying the nanoscale Kirkendall diffusion process. The powders had mixed crystal structures of FeSe and FeSe₂ phases. The FeSe_x/rGO composite has superior sodium-ion storage properties compared to those of the Fe₂O₃/rGO hybrid with similar morphological characteristics. The discharge capacities of the FeSe_x/rGO and Fe₂O₃/rGO composites deliver discharge capacities of 434 and 174 mAh g⁻¹, respectively, after 200 cycles at 0.3 A g⁻¹ current rate. The FeSe_x/rGO hybrid had a high discharge capacity of 311 mAh g⁻¹ for the 1000th cycle at a high current density of 1 A g⁻¹ [534].

6. Concluding Remarks

In this review, attention was focused on advanced anode composite materials showing efforts currently made towards the technological development of nanostructured electrodes. For the sake of comparison and discussion, the best electrochemical properties obtained with the components have been reported. The advantages and disadvantages of a series of anode materials are highlighted.

The constant progress experienced these last five years evidenced here demonstrates that the rGO-based composites are good candidates as anode materials for rechargeable batteries providing high specific capacity that allows high energy density. They compete with the conventional graphite anodes in terms of rate capability, cycle ability, and safety. Owing to their very good mechanical properties in addition to their good electrical conductivity, remarkable improvement of the electrochemical properties of the anodes are observed with amounts of rGO as small as 1–5 wt%. For mobile applications, such as batteries for EVs, and for static utilization, to buffer the intermittence problem and integration of the production of wind and solar plants to the grid energy storage, the rGO-based hybrids are efficient to improve the structural stability of the electrochemically active materials.

The choice of rGO instead of graphene, which exhibits superior electrical conductivity, is guided by its great hydrophylic behavior and easy deposition on any surface with any method. The simple and fast preparation of rGO by reduction of GO suggest its low-cost and scalable production. Chemical reduction of GO can be eco-friendly achieved using reducing mild agents derived from natural fruits or vegetables. In addition, the oxygen functionalities anchored on the surface of rGO render ability to functionalize graphene with desirable properties and engineer them into diverse morphologies. In contrast, the preparation of graphene without defects is not trivial and is costly in terms of mass production (one needs a CVD system and carbon precursor gases (i.e., CH₄). Actually, mass production remains a bottleneck for the production of high-quality graphene derivatives. The reduction of GO to rGO is thus considered as the most promising route for mass production in the near future, sustained by many applications, not only in the framework of energy storage, but also for biosensors, organic solar cells, actuators, and touch panels [535]. In addition, the performance of Si@graphene and Si @rGO anodes reported in this review for comparison do not evidence a clear advantage to graphene. Interestingly, a more detailed comparison between graphene and rGO-based nanoplatforms on electrochromic switching kinetics show that improved electrochromic switching kinetics is observed on reduced graphene oxide in comparison to the pristine graphene sheets, possibly due to the more heterogeneous electron transfer in graphene [536]. This phenomenon might also explain the remarkable results obtained with rGO-supported materials as anodes for the LIBs.

Owing to their excellent electrochemical performance, composites formed with a single element (silicon, germanium, tin) or with metal oxides MO_x with M = Si, Ti, Mo, Mn, Co, Ni, Cu, etc., have been the most studied hybrid anodes, which can deliver high specific capacity (at least twice the graphite value of 372 mAh g⁻¹). However, anode materials that are electrochemically active by conversion reaction suffered for a long time from the huge variation of volume during cycling. The superior performance of composite in batteries originates from the addition of reduced graphene oxide and their architecture (i.e., from 1D to 3D), which boost the reaction kinetics and electrode stability. The introduction of rGO facilitates the continuous and rapid electron transport and prevent the collapse of nanostructures and aggregation of the active materials. In addition, the strategies adopted in the design of nanocomposites are effective methods to mitigate the large volume changes and low electrical conductivities often met in the bare anodes.

Herewith, we have given many examples according to which such electrodes are better performing. For each composite, we have identified, as possible as it was, the electrochemical properties in relation with its morphology (i.e., particle size, specific surface area, porosity, etc.). Alloying materials such as Si, Ge, SiO_x, and SnO₂ and compounds based on conversion reaction MO_x can provide much larger capacities, and energy densities than the Ti-O oxides based on insertion reaction (i.e., TiO₂, Li₃TiO₄, Li₄Ti₅O₁₂). Si, SnO₂, and Mn-based oxides are the most promising elements of this family, while germanium is expensive and not abundant in nature. The big challenge of such nanostructured composite anodes is now the cost, and a scalable production. With the composites of rGO with metal oxides being presently prepared only at the laboratory scale, hopefully the recent results suggest that the one-pot in-situ hydrothermal process is a viable method for the large-scale production for battery electrodes. Today, this technology has the capability to fabricate many composites, which can deliver a capacity superior to 500 mAh g⁻¹ and even more for thousands of cycles. Therefore, the rGO holds great promise for use as composite element in electrode for advanced rechargeable batteries such as lithium-sulfur batteries, lithium-air batteries, aluminum-ion batteries, etc. Undoubtedly, the future of rGO-based composites as anode materials for energy storage devices remains very promising and exciting.