Review—Two-Dimensional Layered Materials for Energy Storage Applications

Li-ion batteries (LIBs) are promising energy storage systems, widely used for portable consumer electronics. Generally, lithium metal oxides (e.g. LiCoO₂, LiFePO₄, LiNi_xCo_yMn_1-x-yO₂, and LiNi_0.5Mn_1.5O₄) and V₂O₅ etc. are favorable cathodes for present LIB technology^41–46 due to their high chemical potential (vs. Li/Li⁺). On the other hand, graphite is a frequently used anode material for LIBs because of its favorable layered structure, natural abundance and low cost.^47,48 Nevertheless, graphite is unable to fulfill the increasing demands for high-energy power sources owing to its low capacity (372 mAh g⁻¹). Recently, several 2D TMDCs (including MoS₂, MoSe₂, WS₂, WSe₂, VS₂, VS₄ and TiS₂) have been extensively explored as LIB anodes due to their favorable layered structure for ion/electron migration and chemical stability.^{11,25,27,29,30,34,36,49–54,47–51} For example, Figure 2a shows the high resolution transmission electron microscopy (HRTEM) images for the anodes composed of mesoporous MoS₂, MoSe₂, WS₂ and WSe₂ respectively.²¹ Their charge-discharge curves in Figure 2b, and the cycling performance, coulombic efficiency (0.1 C) and rate capabilities and capacity retention (from 0.1 to 2 C) in Figure 2c have shown promises in a high capacity anodes (>600 mAh g⁻¹) .²⁷ However, the deprived electrical conductivity, aggravated stability during cycling, pulverization of electrodes and restacking of subunits for the pristine TMDC's are severe issues which deteriorate desirable electrochemical performances. To conquer these issues, the involved intercalation, conversion and oxidation reactions must be controlled. For examples, several conductive materials including conductive polymers and allotropes of carbon are proposed to construct hybrid electrodes.^{12–14,27–39,55} Layer-by-layer stacked hetero-structures of graphene-TMDCs is also suggested as an effective way to overcome above mentioned issues.^{14,31,33–35}

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Sodium ion batteries (SIBs) have emerged as an appealing alternative to LIBs because the resources of sodium metal are in principle unlimited in nature. Moreover, the ion transport mechanism of sodium resembles to lithium, thus the knowledge accrued for the development of LIBs could be directly applied to the SIBs. A variety of cathode materials have been reported for SIB^56–58 application but identifying an appropriate negative electrode still remains as one of the prime challenges and hindrances toward the realization of Na ion full battery. Graphitic and non-graphitic carbons are not suitable anodes for SIBs as they are not capable to store sufficient sodium ions. The limited cycle life caused by the larger size of sodium ion and electrolyte decomposition is also a major issue.^59–62 Recently, TMDCs are being considered as an efficient and alternative anode for SIBs.^35,63–65 Theoretical studies predicted that the binding between Na and MoS₂ is thermodynamically advantageous; thus, MoS₂ can be a favorable choice for sodium ions storage.^{35,58,64–66} A recent report by Wang et al. proved that the exfoliated MoS₂-C composites exhibit a high capacity nearly 400 mAh g⁻¹ at 0.25 C (100 mA g⁻¹) and a long cycle life.⁶⁷ Remarkably high capability ∼290 mAh g⁻¹ was also attained at high rate of 5 C.⁶⁷ Very recently, Singh et al.³⁵ reported on the fabrication of a freestanding electrode synthesized by using acid treated MoS₂/Graphene composites for SIB anode applications as shown in Figure 2d.³⁵ In the hybrid structure, graphene provides stability and superior conductivity networks allowing Na ions to undergo insertion reaction to the TMDCs.

Supercapacitors are promising energy storage systems for futuristic energy consumption devices with fast power delivery or uptake, exceptional power densities and better cyclability.^68–70 Supercapacitors are categorized into two subgroups; (i) electric double layer capacitor (EDLC) and (ii) pseudo-capacitor, which works on the principle of rapid charge transfer through the redox reaction. The supercapacitors based on nanostructured TMDCs have been investigated extensively, as their inherent layered structures with sufficiently enough interlayer space with large surface areas can facilitate faster and easier ion diffusion/electron transport. For example, Figure 2e represents the low-magnification SEM image and Figure 2f shows the charge-discharge curves at different discharge currents for WS₂/RGO hybrids in a supercapacitor assembly.³² Nanostructured MoS₂ has been exotically used as capacitor electrode because it exhibits different oxidation states (Mo²⁺ to Mo⁶⁺) which play roles in redox reaction.^71,72 Another TMDC such as Titanium disulfide (TiS₂) also exhibits superior charge storage behaviors.⁷³ Very recently, Chhowalla and co-workers have successfully reported that chemically exfoliated MoS₂ nanosheets with the presence of 2D metallic 1T phases, which is intrinsically hydrophilic and with high electrical conductivity, can be efficiently intercalated with different ions, such as H⁺, Li⁺, Na⁺ and K⁺, resulting in ultrahigh capacitance ranging ∼400–700 Fcm⁻³.⁷⁴

The increasing demand of the high performance LIB anodes have led to extensive efforts toward discovery of new materials such as MXenes.^20–24 Since their discovery, MXenesare being explored to used as LIB anodes owing to their promising features such as excellent electrical conductivity, exceptional mechanical properties, sufficiently large electrochemically active surfaces, and tunable bandgap energies.^20–24 In addition, the structure of layered MXenes not only allows the intercalation of a variety of organic and inorganic molecules/ions but also can disperse easily in aqueous solutions (hydrophilic). MXenes promise improved electrochemical performance for LIBs since the intercalation of Li⁺ can be accomplished at low voltages (−0.2 V to 0.6 V) with low diffusion barriers. However, a trade-off between the low diffusion barriers and reversible capacity of LIBs with MXenes should be taken into account. Density functional theory (DFT) calculation predicts that the higher formula unit of MXenes leads to the fast diffusion rate but low reversible capacity than that of lower formula unit, and vice versa.²⁰ For instance, V₄C₃O₂ exhibits a low diffusion barrier (0.42 eV) but its capacity is significantly dragged down (148 mAh g⁻¹). By contrast, V₂CO₂ exhibits a higher diffusion barrier (0.82 eV) but its capacity is much improved (276 mAh g⁻¹).⁷⁵ Additionally, the lowest diffusion barrier (0.07 eV) has been reported for Ti₃C₂, which is significantly lower than graphite (0.3 eV).²³ Despite the aforementioned advantages of MXenes, their intercalated structures and involved mechanism need to be further explored. The study by M. Naguib et al.⁷⁶ has been done to evidence the intercalation of Li⁺ within the layers of exfoliated Ti₂C as the anode in LIBs. Recently, Zhang et al. reported on the synthesis of Sn⁴⁺ ion decorated highly conductive Ti₃C₂ MXene and its promising application for lithium-ion anodes.⁷⁷ Typical SEM and HRTEM images of the exfoliated Ti₂C₃ nanosheets produced by HF treatment of Ti₂AlC are shown in Figures 3a–3d. The HRTEM images in Figures 3c–3d confirms good crystallinity of the basal planes of the Ti₃C₂ phase after exfoliation. Figures 3e–3f show the galvanostatic charge/discharge profiles of the PVP-Sn(IV)@Ti₃C₂, which are measured between 0.01 and 3 V at a current density of 216.5 mAcm⁻³ (0.1 A g⁻¹), and rate performance of nanocomposite electrodes. Figures 3g–3h cycling performance and coulombic efficiency at a current density of 216.5 mA cm⁻³ (0.1 A g⁻¹) and cycling performance of PVP-Sn(IV)@Ti₃C₂ at a current density of 500 mA g⁻¹.⁷⁷ To date, only few MXene materials have been studied for LIBs, such as: Ti₂CTx, Nb₂CT_x, Ti₃C₂T_x, and V₂CT_x, where T_x represents the functional surface termination groups. Considering that there are more than 70 reported members belonging to MAX phases, further improvement in electrochemical performance of LIBs with various MXenes are expected.

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The layered structure of MXenes which can facilitate the Na⁺ storage, has been considered as promising anodes for SIBs due to their better stability at high C-rates. However, the sodiation-desodiation mechanism within the layers of MXenes is still not fully understood. Furthermore, only few MXene materials have been used for SIB anodes, such as Ti₃C₂T_x as shown in Figures 3i–3k.⁷⁸ This study further reports that a small fraction of Na⁺ remains in the structure of MXenes event after full desodiation, resulting in the increase in interlayers distance from 9.7 Å to 12.5 Å.⁷⁸ This is because the trapped Na⁺, in conjunction with penetrated solvent molecule lead to expansion of interlayer distance during the initial cycles of sodiation-desodiation by pillaring and swelling mechanisms. Further structural change in layered MXenes was not observed upon subsequent sodiation-desodiation cycles, resulting in good capacity retention. However, formation of SEI on the anode of SIBs still remains a predictable issue which needs to be investigated.

MXenes have also been considered as promising electrodes for supercapacitors owing to their large volumetric capacitances at high current densities. Comparing to conventional intercalation compounds, MXenes offer better stability with a variety of cations. Intercalation mechanism within the layered MXene has not been fully understood. Recently, it is reported that small cations can intercalates spontaneously into the interlayer spaces of MXenes, whereas the intercalation of larger cations involves the swelling of interlayer.⁷⁹ Among the members of MXenes family, Ti₃C₂T_x is widely studied for supercapacitor owing to its large volumetric capacitances at high current densities and spontaneous intercalation for a variety of cations. Recently, the surface modification of Ti₃C₂T_x to form TiO₂/Ti₃C₂ hybrid structures has been reported as shown in Figures 3l–3o.⁸⁰ In the report, TiO₂ nanoparticles have been incorporated into the surface of Ti₃C₂ nanosheets and then used as an supercapacitor electrode, yielding significantly improved specific capacitance and cycling stability (92% of its initial capacitance).⁸⁰ The nanocrystalline TiO₂ is believed to improve the accessibility of MXenes for intercalation of cations, resulting in a higher capacity than that of un-modified MXenes.