Stereoassembled V2O5@FeOOH Hollow Architectures with Lithiation Volumetric Strain Self-Reconstruction for Lithium-Ion Storage.

Vanadium oxides have recently attracted widespread attention due to their unique advantages and have demonstrated promising chemical and physical properties for energy storage. This work develops a mild and efficient method to stereoassemble hollow V2O5@FeOOH heterostructured nanoflowers with thin nanosheets. These dual-phased architectures possess multiple lithiation voltage plateau and well-defined heterointerfaces facilitating efficient charge transfer, mass diffusion, and self-reconstruction with volumetric strain. As a proof of concept, the resulting V2O5@FeOOH hollow nanoflowers as an anode material for lithium-ion batteries (LIBs) realize high-specific capacities, long lifespans, and superior rate capabilities, e.g., maintaining a specific capacity as high as 985 mAh g−1 at 200 mA g−1 with good cyclability.


Introduction
Featuring high theoretical lithium storage capacity, considerable structural versatility and appealing electrochemical reactivity, transition metal oxides (TMOs, e.g., M x O y , M=Cu, Fe, Co, Ni, Mn, etc.) have received tremendous interest for potential energy storage applications [1][2][3][4][5]. Moreover, TMOs are endowed with enhanced safety as anode materials for lithium-ion batteries (LIBs), as the lithium dendrite growth can be inhibited at a relatively higher lithiation reaction potential. Therefore, TMOs have been widely explored and designed with expected structures, compositions, and size [6][7][8][9]. Particularly, vanadium oxides can intercalate more lithium ions owing to the distinct feature of multiple oxidation states (V 2+ to V 5+ ) [10][11][12][13][14]. In practice, despite the high theoretic capacity as both anode and cathode mate-rials for LIBs, low electrical conductivity and large volumetric strains of V 2 O 5 inevitably render poor rate capability and cycling stability [15][16][17]. To address these issues, carbon modification has been one of the most intuitive approaches to promote the electron transport for numerous TMOsbased electrode materials [18][19][20].
Alternatively, developing well-defined structures has also demonstrated great promise for boosting the electrochemical properties including enhanced capacity as well as desired stability [21][22][23][24]. For example, hierarchical assemblies consisting of nanosized building blocks, i.e., one-dimensional (1D) or two-dimensional (2D) subunits, are provided with combined merits of further enlarged contact area with electrolyte and reduced charge/mass diffusion pathways [25][26][27][28]. However, sacrificial templates associated with multistep procedures are usually involved for carefully designed complex structures. Furthermore, heterostructures through coupling versatile species are believed to offer more opportunities for various areas, which results from regulated electronic structures and intriguing synergistic effect at well-defined heterointerfaces [29][30][31][32][33]. More importantly, rational integration of functional components with consideration of their intrinsic redox properties allows tailored electrochemical behaviors for energy-based applications [34,35]. To date, efficient construction of heterostructured electrodes based on V 2 O 5 with tunable complex nanostructures remains a big challenge.
Herein, we demonstrate an efficient and facile method to stereoassemble dual-phased architectures of V 2 O 5 @FeOOH under a mild condition. Hierarchical V 2 O 5 @FeOOH heterostructures with well-defined morphology and composition are composed of highly connected ultrathin nanosheets, featuring well-defined heterointerfaces and multiple lithiation voltage plateau. Specifically, the V 2 O 5 @FeOOH with hollow nanoflower structures are well presented, enabling efficient charge transfer, mass diffusion, and self-reconstruction with volumetric strain. As a proof-of-concept demonstration, the V 2 O 5 @FeOOH hollow nanoflower anode delivers boosted Li-storage properties including high-specific capacities, long lifespans, and superior rate capabilities. Remarkably, a high-specific capacity of 985 mAh g −1 at 200 mA g −1 is achieved with good cyclability.

Results and Discussion
The facile synthetic procedure of dual-phased hollow architectures is illustrated in Figure 1. The stereoassembled V 2 O 5 @FeOOH hollow heterostructures are obtained by using commercial V 2 O 5 as the starting material under a mild condition (see the Experimental section for details). Specifically, irregular bulk V 2 O 5 powders ( Figure S1) are first dispersed in deionized water, followed by dropwise addition of hydrogen peroxide (H 2 O 2 ) at room temperature to form a uniform solution. During which, the dissolution of V 2 O 5 with the presence of H 2 O 2 results in the formation of complex vanadium-based intermediates [36]. Afterward, the self-assembly of V 2 O 5 nanosheets into well-defined hierarchical hollow architecture can be realized with the assistance of iron(III) nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O) at 50°C by controlling the reaction time ( Figures S2-S6). Importantly, Fe(NO 3 ) 3 in this case acts as the shape-directing agent to manipulate the morphology, as bare 2D nanosheets with large lateral size and smooth surface can be obtained without the addition of Fe(NO 3 ) 3 ( Figure S7). Furthermore, Fe(NO 3 ) 3 contributes to the nanoscale heterostructuring with optimized composition through in situ deposition of FeOOH as well. Lastly, a reddish-brown V 2 O 5 @FeOOH product can be harvested after a vacuum freeze-drying process.
The morphology and structure of the V 2 O 5 @FeOOH heterostructures are investigated by field emission scanning electron microscopy (FESEM). As presented by the panoramic image in Figure 2(a), these V 2 O 5 @FeOOH particles exhibit a flower-like sphere structure with high uniformity. A closer FESEM examination revealed the hierarchical architecture of V 2 O 5 @FeOOH, which are assembled from nano-sheet subunits with a small thickness of about 10 nm (Figure 2(b)). Interestingly, a well-defined hollow interior of approximately 300 nm can be clearly observed from the transmission electron microscopy (TEM) image (Figure 2(c)). At a higher magnification, the thin character of these wrinkled nanosheets is suggested by their high transparency under the electron beam (Figure 2(d)). Notably, a set of diffraction spots well fit the orthorhombic V 2 O 5 lattice structure, which is viewed along the [010] zone axis according to the selected area electron diffraction (SAED) data ( Figure S8). In the high-resolution TEM (HRTEM) image ( Figure 2 Figure 2(f), both V and Fe signals exhibit relatively higher intensity at the edge region as compared to the core region, further validating the formation of hollow structure.
It is worth noting that the configuration of V 2 O 5 @FeOOH heterostructure can be precisely tuned by varying the amount of Fe(III) precursor ( Figure 3). Interestingly, the flower-like V 2 O 5 @FeOOH spheres expanded significantly with the increasing amount of Fe(III) precursor. In sharp contrast to V 2 O 5 @FeOOH-1 that is obtained by using Fe(NO 3 ) 3 and V 2 O 5 with a mass ratio of 1 : 1 (Figures 3(a) and 3(b)), V 2 O 5 @FeOOH-2 evolves into irregular assembly of nanosheets, exhibiting open cavities as surrounding 2D building blocks tend to spread out (Figures 3(c) and 3(d)). Importantly, the hierarchical hollow structure hardly maintains when further increasing the mass ratio of Fe(NO 3 ) 3 to V 2 O 5 (V 2 O 5 @FeOOH-5). Highly interconnected 3D architectures can be obtained, which are constructed from randomly assembled nanosheets with rough surfaces (Figures 3(e) and 3(f)). For comparison, bare FeOOH particles can be obtained by the same procedure except for the presence of V 2 O 5 ( Figure S9).
The crystal structure of as-synthesized samples is then determined through X-ray diffraction (XRD) measurement ( Figure 4  . On the other hand, no characteristic peaks belonging to FeOOH are detected, which should be attributed to its amorphous feature [37]. The lower crystallinity of V 2 O 5 @FeOOH-2 and V 2 O 5 @FeOOH-5 as indicated by the disappearing of well-defined diffraction peaks further suggest the increased content of amorphous phase within the V 2 O 5 @FeOOH hybrids. The chemical composition and bonding state of V 2 O 5 @FeOOH are also investigated by Xray photoelectron spectroscopy (XPS). The high-resolution Fe 2p spectra (Figure 4(b) and Figure S10) confirm the presence of Fe species as FeOOH [38,39], with the coexistence of Fe 2+ and Fe 3+ at 710.7/724.8 eV and 712.8/727.8 eV, respectively. The peak intensity for Fe 2p 1/2 , 2p 3/2 , and satellite together enlarged from V 2 O 5 @FeOOH-1 to V 2 O 5 @FeOOH-5, in accordance with the increasing incorporation of Fe species within the heterostructure (Table S1). The V 2p region spectra of V 2 O 5 @FeOOH are analyzed, exhibiting characteristic peaks centered at 524.6 and 517.0 eV for V 5+ components of V 2 O 5 (Figure 4(c)) [40][41][42]. Besides, a negative shift of binding energy (~0.8 eV) for both V 2p 1/2 and 2p 3/2 bands are achieved as compared to bare V 2 O 5 ( Figure S11), indicating the strong electronic interaction at the V 2 O 5 /FeOOH interfaces. Meanwhile, the deconvoluted O 1s core level spectra show two dominant peaks that can be assigned to V-O and Fe-O-Fe bonds [43], which reveal varied portions of the Fe-O-Fe at 530.4 eV accordingly (Figure 4(d)).
As a proof of concept, the electrochemical properties of heterostructured V 2 O 5 @FeOOH as LIB anodes are examined ( Figure 5). The initial Coulombic efficiency (CE) is around 50% for V 2 O 5 @FeOOH (e.g., 50% for V 2 O 5 @FeOOH-1, 53% for V 2 O 5 @FeOOH-2, and 43% for V 2 O 5 @FeOOH-5), which can be attributed to the irreversible capacity loss due to the formation of solid electrolyte interphase (SEI) film on the electrode surface [44]. After a typical activation process for 50 cycles [45], remarkable reversible capacities as high as 985, 1016, and 1030 mAh g −1 at a current density of 200 mA g −1 are delivered by V 2 O 5 @FeOOH-1, V 2 O 5 @FeOOH-2, and V 2 O 5 @FeOOH-5, respectively, reaching the CE of nearly 100%. The desirable Li-storage capacities surpassing bare V 2 O 5 (635 mAh g −1 ) and FeOOH (342 mAh g −1 ) manifest the superiority of dual-phased heterostructures constructed by 2D building blocks. Different from V 2 O 5 @FeOOH-2 and V 2 O 5 @FeOOH-5 with capacity fading after ca. 100 cycles, 96% capacity is retained for V 2 O 5 @FeOOH-1 after 180 cycles, which can be further attributed to its well-defined hollow configuration. The 3D hierarchical structure integrity of V 2 O 5 @FeOOH-1 can be maintained during repeated Li + insertion and exaction as confirmed by postmortem FESEM images in Figure S12. Of note, the cycling stability of V 2 O 5 @FeOOH-1 can be further highlighted at a higher current density of 2000 mA g −1 , i.e., 494 mAh g −1 after 300 cycles with a high capacity retention of 95% ( Figure S13).
Importantly, rate performance of the V 2 O 5 @FeOOH-1 anode was investigated at different current densities from 100 mA g −1 to 3000 mA g −1 (Figure 5(b)). Reversible capacities of 992, 947, 906, 856, 714, 519, and 366 mAh g −1 are achieved at current densities of 100, 200, 300, 500, 1000, 2000, and 3000 mA g −1 , respectively. Cyclic voltammetry (CV) profiles at different scan rates from 0.2 to 1.0 mV s −1 were therefore recorded to verify the electrochemical kinetics of V 2 O 5 @FeOOH-1 for Li-storage ( Figure 5(c)). Negligible peak shifts are observed as the scan rates increase, suggesting small polarization as well as desirable kinetics of V 2 O 5 @FeOOH-1 at high rates. The relationship of log (i, peak current) versus log (v, scan rate) was plotted to give the fitted values of slope b (Figure S14), which provides insights into the charge storage mechanism according to the power-law formula i = av b [46]. Specifically, a b value of 0.5 represents an ideal diffusion-controlled process, whereas 1.0 indicates a surface capacitive-controlled one. The calculated b values are 0.74 for anodic peak 1 and 0.83 and 0.72 for the cathodic peaks 2 and 3, respectively, demonstrating a partial capacitive-controlled behavior of V 2 O 5 @FeOOH-1. Quantitative analysis further reveals that the ratios of capacitive contribution gradually improve upon increasing the scan rates ( Figure 5(d), see calculation details and Figure S15 in Supplementary Materials), which reaches 67.16% at 1.0 mV s −1 ( Figure S16). The considerable pseudocapacitive contributions can be readily attributed to the presence of abundant amorphous domains and numerous grain boundaries on the surface of highly exposed nanosheets. These electrochemically active sites are undoubtedly favorable for boosting Li-storage capacity and especially rate performance of V 2 O 5 @FeOOH-1 hybrid. Furthermore, Nyquist plots demonstrated reduced charge transfer resistances (R ct ) of V 2 O 5 @FeOOH-1 (114.7 Ω) as compared to FeOOH (125.5 Ω), and V 2 O 5 ·nH 2 O (181.0 Ω), which was fitted according to the equivalent circuit ( Figure S17 and Table S2). Moreover, the plot slope of Z′ vs. ω −1/2 can be obtained to illustrate the speed of lithium diffusion, which is calculated to be 83.7 for V 2 O 5 @FeOOH-1, 171.3 for FeOOH, and 128.2 for V 2 O 5 ·nH 2 O ( Figure S18). Therefore, the lithium diffusion coefficients at 25°C are calculated to be 5:98 × 10 −14 , 5:28 × 10 −15 , and 2:55 × 10 −14 c m 2 s −1 for V 2 O 5 @FeOOH-1, FeOOH, and V 2 O 5 ·nH 2 O, respectively. The results indicate that the V 2 O 5 @FeOOH-1 hybrid can provide more accessible pathway for charge transfer due to the sufficient heterointerface and accelerated lithium diffusion, resulting in enhanced Li-storage performance.
Additionally, Li-storage performance of V 2 O 5 @FeOOH-1 are superior to that of many other reported vanadium-based oxide materials (Table S3). The substantially optimized electrochemical properties can be ascribed to the rational construction of V 2 O 5 @FeOOH heterostructures with welldefined 3D configurations featuring intriguing interfaces and synergistic effects. Specifically, the V 2 O 5 nanosheets in the first place serve as ideal building blocks with promoted Li + /electron transport owing to their ultrathin feature. After introducing amorphous FeOOH, the Li intercalation process can be readily tailored due to the distinct lithiation voltage plateau of FeOOH and V 2 O 5 as schematically illustrated in Figure 5(e). Upon discharge, the lithiation of V 2 O 5 occurs at      a higher voltage (2.5-1.4 V) [47], during which the volumetric expansion can be effectively restrained owing to the presence of amorphous FeOOH as surface buffers. Afterwards, FeOOH further contributes considerable capacity at lower voltages (1.4-0.4 V) [48], which is also confined within the inner space provided by interconnected nanosheets. Apart from the strain self-reconstruction enabled by the synergy between V 2 O 5 and FeOOH, the hollow configuration is highly appealing for strain accommodation and cycling stability during repeated lithiation and delithiation. More importantly, extra Li + storage sites together with diffusion channels are created where the presence of amorphous FeOOH domains induce abundant grain boundaries of V 2 O 5 and substantial interfacial area, contributing to boosted reaction activity and enhanced Li-storage capacity.

Conclusions
In summary, a novel hollow V 2 O 5 @FeOOH heterostructured nanoflower has been stereoassembled via an efficient and mild method. These nanoflowers composed of wellconnected ultrathin nanosheets demonstrate superior advantages including enlarged electrode-electrolyte contact, facilitated charge transfer, and accelerated mass diffusion for LIBs. Owing to the multiple lithiation voltage plateau and well-defined heterointerfaces, the hollow V 2 O 5 @FeOOH nanoflowers deliver excellent lithium storage properties with a high reversible capacity (985 mAh g −1 at 200 mA g −1 after 180 cycles) and remarkable cycling stability (95% capacity retention at 2000 mA g −1 after 300 cycles). It is expected that the present result can be further extended to optimize other TMO-based materials and shed lights on the development of future energy applications.