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.
Featuring high theoretical lithium storage capacity, considerable structural versatility and appealing electrochemical reactivity, transition metal oxides (TMOs, e.g., , M=Cu, Fe, Co, Ni, Mn, etc.) have received tremendous interest for potential energy storage applications [1–5]. 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 [6–9]. Particularly, vanadium oxides can intercalate more lithium ions owing to the distinct feature of multiple oxidation states (V2+ to V5+) [10–14]. In practice, despite the high theoretic capacity as both anode and cathode materials for LIBs, low electrical conductivity and large volumetric strains of V2O5 inevitably render poor rate capability and cycling stability [15–17]. To address these issues, carbon modification has been one of the most intuitive approaches to promote the electron transport for numerous TMOs-based electrode materials [18–20].
Alternatively, developing well-defined structures has also demonstrated great promise for boosting the electrochemical properties including enhanced capacity as well as desired stability [21–24]. 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 [25–28]. 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 [29–33]. 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 V2O5 with tunable complex nanostructures remains a big challenge.
Herein, we demonstrate an efficient and facile method to stereoassemble dual-phased architectures of V2O5@FeOOH under a mild condition. Hierarchical V2O5@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 V2O5@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 V2O5@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.
2. Results and Discussion
The facile synthetic procedure of dual-phased hollow architectures is illustrated in Figure 1. The stereoassembled V2O5@FeOOH hollow heterostructures are obtained by using commercial V2O5 as the starting material under a mild condition (see the Experimental section for details). Specifically, irregular bulk V2O5 powders (Figure S1) are first dispersed in deionized water, followed by dropwise addition of hydrogen peroxide (H2O2) at room temperature to form a uniform solution. During which, the dissolution of V2O5 with the presence of H2O2 results in the formation of complex vanadium-based intermediates . Afterward, the self-assembly of V2O5 nanosheets into well-defined hierarchical hollow architecture can be realized with the assistance of iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O) at 50°C by controlling the reaction time (Figures S2-S6). Importantly, Fe(NO3)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(NO3)3 (Figure S7). Furthermore, Fe(NO3)3 contributes to the nanoscale heterostructuring with optimized composition through in situ deposition of FeOOH as well. Lastly, a reddish-brown V2O5@FeOOH product can be harvested after a vacuum freeze-drying process.
The morphology and structure of the V2O5@FeOOH heterostructures are investigated by field emission scanning electron microscopy (FESEM). As presented by the panoramic image in Figure 2(a), these V2O5@FeOOH particles exhibit a flower-like sphere structure with high uniformity. A closer FESEM examination revealed the hierarchical architecture of V2O5@FeOOH, which are assembled from nanosheet 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 V2O5 lattice structure, which is viewed along the  zone axis according to the selected area electron diffraction (SAED) data (Figure S8). In the high-resolution TEM (HRTEM) image (Figure 2(e)), the crystalline nanosheet shows typical lattice spacings of 0.23 nm and 0.20 nm, corresponding to the (113) and (006) planes of V2O5·1.6H2O. Meanwhile, notably discontinuous lattice fringes can be observed, which can be readily attributed to the vertical coverage of amorphous FeOOH domains. Moreover, scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) elemental mapping images of V2O5@FeOOH corresponding to the dark-field image (Figure 2(g)) reveal the homogeneous distribution of V, Fe, and O (Figure 2(h)). As demonstrated by the EDS line scan profile in 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 V2O5@FeOOH heterostructure can be precisely tuned by varying the amount of Fe(III) precursor (Figure 3). Interestingly, the flower-like V2O5@FeOOH spheres expanded significantly with the increasing amount of Fe(III) precursor. In sharp contrast to V2O5@FeOOH-1 that is obtained by using Fe(NO3)3 and V2O5 with a mass ratio of 1 : 1 (Figures 3(a) and 3(b)), V2O5@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(NO3)3 to V2O5 (V2O5@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 V2O5 (Figure S9).
The crystal structure of as-synthesized samples is then determined through X-ray diffraction (XRD) measurement (Figure 4(a)). For V2O5@FeOOH-1, the diffraction peaks at 8.3°, 25.9°, 31.1°, 34.2°, 46.4°, and 50.3° can be well assigned to (001), (101), (004), (103), (006), and (200) reflections of orthorhombic structured V2O5·1.6H2O (JCPDS no. 40-1296). On the other hand, no characteristic peaks belonging to FeOOH are detected, which should be attributed to its amorphous feature . The lower crystallinity of V2O5@FeOOH-2 and V2O5@FeOOH-5 as indicated by the disappearing of well-defined diffraction peaks further suggest the increased content of amorphous phase within the V2O5@FeOOH hybrids. The chemical composition and bonding state of V2O5@FeOOH are also investigated by X-ray 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 Fe2+ and Fe3+ at 710.7/724.8 eV and 712.8/727.8 eV, respectively. The peak intensity for Fe 2p1/2, 2p3/2, and satellite together enlarged from V2O5@FeOOH-1 to V2O5@FeOOH-5, in accordance with the increasing incorporation of Fe species within the heterostructure (Table S1). The V 2p region spectra of V2O5@FeOOH are analyzed, exhibiting characteristic peaks centered at 524.6 and 517.0 eV for V5+ components of V2O5 (Figure 4(c)) [40–42]. Besides, a negative shift of binding energy (~0.8 eV) for both V 2p1/2 and 2p3/2 bands are achieved as compared to bare V2O5 (Figure S11), indicating the strong electronic interaction at the V2O5/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 , 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 V2O5@FeOOH as LIB anodes are examined (Figure 5). The initial Coulombic efficiency (CE) is around 50% for V2O5@FeOOH (e.g., 50% for V2O5@FeOOH-1, 53% for V2O5@FeOOH-2, and 43% for V2O5@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 . After a typical activation process for 50 cycles , 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 V2O5@FeOOH-1, V2O5@FeOOH-2, and V2O5@FeOOH-5, respectively, reaching the CE of nearly 100%. The desirable Li-storage capacities surpassing bare V2O5 (635 mAh g−1) and FeOOH (342 mAh g−1) manifest the superiority of dual-phased heterostructures constructed by 2D building blocks. Different from V2O5@FeOOH-2 and V2O5@FeOOH-5 with capacity fading after ca. 100 cycles, 96% capacity is retained for V2O5@FeOOH-1 after 180 cycles, which can be further attributed to its well-defined hollow configuration. The 3D hierarchical structure integrity of V2O5@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 V2O5@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 V2O5@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 V2O5@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 V2O5@FeOOH-1 at high rates. The relationship of log (, peak current) versus log (, scan rate) was plotted to give the fitted values of slope (Figure S14), which provides insights into the charge storage mechanism according to the power-law formula . Specifically, a value of 0.5 represents an ideal diffusion-controlled process, whereas 1.0 indicates a surface capacitive-controlled one. The calculated 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 V2O5@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 V2O5@FeOOH-1 hybrid. Furthermore, Nyquist plots demonstrated reduced charge transfer resistances () of V2O5@FeOOH-1 (114.7 Ω) as compared to FeOOH (125.5 Ω), and V2O5·nH2O (181.0 Ω), which was fitted according to the equivalent circuit (Figure S17 and Table S2). Moreover, the plot slope of vs. can be obtained to illustrate the speed of lithium diffusion, which is calculated to be 83.7 for V2O5@FeOOH-1, 171.3 for FeOOH, and 128.2 for V2O5·nH2O (Figure S18). Therefore, the lithium diffusion coefficients at 25°C are calculated to be , , and for V2O5@FeOOH-1, FeOOH, and V2O5·nH2O, respectively. The results indicate that the V2O5@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 V2O5@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 V2O5@FeOOH heterostructures with well-defined 3D configurations featuring intriguing interfaces and synergistic effects. Specifically, the V2O5 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 V2O5 as schematically illustrated in Figure 5(e). Upon discharge, the lithiation of V2O5 occurs at a higher voltage (2.5-1.4 V) , 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) , which is also confined within the inner space provided by interconnected nanosheets. Apart from the strain self-reconstruction enabled by the synergy between V2O5 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 V2O5 and substantial interfacial area, contributing to boosted reaction activity and enhanced Li-storage capacity.
In summary, a novel hollow V2O5@FeOOH heterostructured nanoflower has been stereoassembled via an efficient and mild method. These nanoflowers composed of well-connected 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 V2O5@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.
4. Material and Methods
4.1. Preparation of V2O5@FeOOH Hollow Heterostructures
0.364 g commercial V2O5 powder was mixed with 25 mL DI water and 5 mL H2O2 (30 wt%) under stirring for 2 h to form a dark-red solution at room temperature. Fe(NO3)3·9H2O was then added to the above mixture under stirring, accompanied with water bath at 50°C overnight. The reddish-brown precipitates were collected by centrifugation and washed with water and ethanol several times. Finally, V2O5@FeOOH was obtained by a freeze-drying process. By varying the amount of Fe(NO3)3·9H2O, a series of V2O5@FeOOH heterostructures were prepared and denoted as V2O5@FeOOH-1, V2O5@FeOOH-2, and V2O5@FeOOH-5. Take V2O5@FeOOH-2 as an example, the mass ratio of Fe(NO3)3·9H2O to V2O5 is 2 : 1.
4.2. Preparation of V2O5·nH2O
0.364 g commercial V2O5 was dispersed in 25 mL DI water under stirring, followed by the addition of 5 mL H2O2 (30%) to form a dark-red solution. After aging at room temperature for 2 h, the mixture was maintained at 50°C under stirring overnight to form a hydrogel. Finally, V2O5·nH2O was collected after a freeze-drying process.
4.3. Preparation of FeOOH
0.364 g Fe(NO3)3·9H2O was first dissolved in 25 mL DI water containing 5 mL H2O2 (30%) under stirring at room temperature. The solution was then maintained at 50°C overnight. The precipitates were rinsed with water and ethanol several times by centrifugation. Finally, the as-obtained product was collected after freeze-drying overnight.
4.4. Materials Characterization
FESEM (JEOL, JSM-7600F) and TEM (JEOL, JEM-2100F) coupled with EDS spectroscopy were used to investigate the morphology, structure and composition of the as-obtained samples. Crystal phases of the obtained samples were identified using XRD (Rigaku, SmartLab with Cu Kα radiation). XPS (Thermo-VG Scientific, ESCALAB 250) was employed to characterize the compositions and valence states of the products.
4.5. Electrochemical Measurements
The electrochemical measurements of the as-prepared active materials were performed according to previously reported procedures . Specifically, CR2032-type coin cells were assembled in an argon-filled glove box with the contents of moisture and oxygen less than 0.5 ppm. 70 wt% of the product (e.g., V2O5@FeOOH-1, FeOOH, and V2O5·nH2O) was mixed with 20 wt% multiwalled carbon nanotube and 10 wt% polyvinylidene difluoride into NMP to prepare the working electrode. The as-obtained slurry was uniformly pasted on the Cu foil with a mass loading of about 1 mg cm−2 and dried under vacuum at 60°C for 24 h to remove the solvent. For the LIBs test, the lithium metal foil was used as the counter/reference electrode, 1.0 M LiPF6 dissolved into a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (EC/DMC/EMC, 1 : 1 : 1, ) was used as electrolyte, and Celgard 2400 membrane was used as the separator. The galvanostatic charge-discharge tests at various current densities were conducted with a battery testing system (NEWARE, CT-4008) under a voltage range of 0.01 to 3.0 V. The CV curves were obtained on a Bio-logic (VMP-300) electrochemical workstation.
Conflicts of Interest
The authors declare no competing financial interest.
K. Rui, J. X. Zhu, and W. Huang proposed and supervised the project. Y. Zhang carried out most of the experimental work and wrote the original draft of the manuscript. A. M. Huang and K. Hu carried out the morphological characterization. Y. Ding, W. H. Shi, X. H. Cao, and H. J. Lin analysed and discussed the data. All authors discussed the results and commented on the manuscript.
This work was financially supported by the National Natural Science Foundation of China (51872139, 51902158, 21706128), the NSF of Jiangsu Province (BK20170045), the Recruitment Program of Global Experts (1211019), the “Six Talent Peak” Project of Jiangsu Province (XCL-043, XCL-021), and the Natural Science Foundation of Jiangsu Higher Education Institutions (19KJB430002).
Figure S1: SEM images of commercial V2O5 powder. Figure S2: SEM images of the vanadium-based intermediate formed immediately after introducing Fe(NO3)3 at 50°C. Figure S3: SEM images of the vanadium-based intermediate obtained after reacting with Fe(NO3)3 at 50°C for 3 h. Figure S4: SEM images of the vanadium-based intermediate obtained after reacting with Fe(NO3)3 at 50°C for 6 h. Figure S5: SEM images of the vanadium-based intermediate obtained after reacting with Fe(NO3)3 at 50°C for 12 h. Figure S6: The XRD patterns of the vanadium-based intermediate. Figure S7: SEM, TEM images, and corresponding elemental mapping images of pure V2O5·nH2O. Figure S8: the SAED pattern for V2O5@FeOOH-1. Figure S9: SEM, TEM images, and corresponding elemental mapping images of pure FeOOH. Figure S10: Fe 2p XPS spectrum of FeOOH. Figure S11: XPS analysis of pure V2O5·nH2O. Figure S12: SEM images of V2O5@FeOOH-1 after 180 cycles at a current density of 200 mA g−1. Figure S13: cycling performance of V2O5@FeOOH-1, V2O5·nH2O, and FeOOH at a current density of 2000 mA g−1. Figure S14: value determined by the relationship between scan rate and peak current. Figure S15: plots of vs. for calculating constants and at different potentials. Figure S16: capacitive contribution to charge storage of V2O5@FeOOH-1 at 1.0 mV s−1. Figure S17: Nyquist plots of V2O5@FeOOH-1, FeOOH, and V2O5·nH2O. Figure S18: the relationships between and in the low-frequency region of V2O5@FeOOH-1, FeOOH, and V2O5·nH2O. Table S1: the molar ratio of V and Fe for V2O5@FeOOH-1, V2O5@FeOOH-1, and V2O5@FeOOH-5 calculated based on XPS results. Table S2: fitted equivalent resistances for EIS results of V2O5@FeOOH-1, FeOOH, and V2O5·nH2O. Table S3: comparison of lithium storage properties of various vanadium-based oxide materials. (Supplementary Materials)
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