Article pubs.acs.org/cm
Amorphous Vanadium Oxide/Molybdenum Oxide Hybrid with ThreeDimensional Ordered Hierarchically Porous Structure as a HighPerformance Li-Ion Battery Anode Di Zhao,† Jinwen Qin,† Lirong Zheng,‡ and Minhua Cao*,† †
Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China ‡ Institute of High Energy Physics, the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Transition metal oxides as anode materials for lithium ion batteries (LIBs) generally suffer from significant capacity fading due to their chemical and mechanical degradations upon extended cycling. In this work, a threedimensional (3D) ordered hierarchically porous amorphous hybrid based on vanadium oxide and molybdenum oxide (3DOHP-a-VOx/MoOy) was first constructed and investigated as an ideal anode material for LIBs. The valence states of V and Mo in this hybrid were determined by X-ray absorption nearedge structure (XANES) measurements. The as-synthesized 3D-OHP-a-VOx/MoOy exhibits significantly improved lithium storage performance in terms of specific capacity, cycling stability, and rate capability compared to single-component a-VOx, aMoOy, and highly crystalline VOx/MoOy hybrid (c-VOx/MoOy). The enhanced lithium storage performance of 3D-OHP-a-VOx/ MoOy probably benefits from its amorphous nature, synergistic effect between a-VOx and a-MoOy, and 3D hierarchically porous structure. To the best of our knowledge, our result is the best among the as-reported molybdenum oxides and vanadium oxides for energy storage applications. This strategy in the current work offers a new perspective in designing high-performance anode materials for LIBs.
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INTRODUTION
processes, thus leading to a significant improvement of the electrochemical performance.4,7 Of all the TMOs, vanadium (molybdenum) oxides have attracted considerable attention as host materials for LIBs owing to their high melting point and high thermal stability.8−13 Moreover, it has been demonstrated that the mixed-valence vanadium (molybdenum) oxides can provide readily accessible redox couples, which make them attractive for reversible delithiation/lithiation.14,15 However, like other metal oxides, the drastic volume expansion in crystalline vanadium (molybdenum) oxides that occurs after lithiation can leads to rapid capacity fading, which greatly limits their use in practical Li-ion devices.9,16,17 The possible reason for this result is that the Li+ storage sites are confined only to the crystallographic sites due to the highly crystalline nature of these materials.18 To address this problem, amorphous structure seems to be an effective resort, because there will be less structural confinement to the lithiation/delithiation reaction of lithium ions in the amorphous framework.6,18,19 For example, Oh and coworkers prepared amorphous MoO2, which exhibits an
Rechargeable lithium ion batteries (LIBs) are generally considered to be the predominant power source for various portable electronic devices.1 With the rapid development of current electric vehicles (PEVs) and hybrid electric vehicles (HEVs), LIBs with high energy density and long cycle life are especially required. Since Poizot et al.2 first reported transition metal oxides (TMOs) as anode materials for LIBs, TMOs have been widely investigated due to their higher specific capacities and better safety. However, most TMOs generally suffer from large volume expansion during cycling, which unavoidably results in pulverization of the electrodes and loss of electrical contact between anode particles and current collector, thus leading to performance deterioration. To solve this issue, considerable strategies have been proposed, such as reducing particle size, pore engineering, and modification by doping or coating to improve their electronic conductivity.3−7 Actually, to achieve high-performance lithium storage, the combination of the above two or three strategies often is more effective. For example, the nanosized materials with highly porous structure could not only effectively accommodate severe volume change3,6 but also greatly shorten the transport path lengths of lithium ions and electrons during the delithiation/lithiation © 2016 American Chemical Society
Received: January 29, 2016 Revised: June 2, 2016 Published: June 2, 2016 4180
DOI: 10.1021/acs.chemmater.6b00414 Chem. Mater. 2016, 28, 4180−4190
Article
Chemistry of Materials exceptionally high Li+ storage capacity due to its structural defects, which can further serve as reversible Li+ storage sites for LIBs.18 Kang et al. synthesized amorphous ant-cave MoO3− C microballs, which show a high discharge capacity of 733 mAh g−1 even after 300 cycles.19 In addition, other amorphous mixed-valence metal oxides and hydroxide materials [VOx,20 MnOx,21 SnOx,22 GeOx,7 Ni(OH)2,23 CoSnO3,24 ZnSnO3,25 FePO4,26 etc.] have also been used for LIBs or electrochemical capacitors, all of which deliver good electrochemical behavior owing to their unique ability of mitigating mechanical degradation, high specific surface area, lower lithiation/ delithiation overpotential, and narrow potential hysteresis. Moreover, it has been addressed that, compared to a singlecomponent system, the hybrid materials can exhibit some new properties. For example, the hybrid materials display a high capability to buffer the volume expansion during the delithiation/lithiation processes and to provide more active sites for lithium.27−29 Furthermore, some mutually beneficial reactions may happen between different components under certain conditions. Up to now, various hybrid materials, such as ZnO-NiO,28 ZnO-Fe2 O 3 , 30 ZnO-TiO 2 , 31 ZrO 2 −V 2 O 5 , 32 Co3O4−CoO@C,33 TiO2@α-Fe2O3,34 etc., have been developed as high-performance electrode materials for LIBs. However, we note that, despite tremendous research efforts that have been devoted to investigating a wide variety of hybrid materials for electrochemical performances in recent years, the hybrid materials are still limited to highly crystalline ones. In this regard, if the hybrid materials are controlled to be amorphous, high-performance anode materials would be expected. In 2007, Shi et al.35 prepared an amorphous Li2OCuO-SnO2 hybrid with a porous, spherical, multideck-cage morphology by an electrostatic spray deposition technique. When used as an anode material for LIBs, the hybrid displays high reversible capacity, which is due to the amorphous nature of Li2O and CuO components in this hybrid and the hollow porous structure. Inspired by these studies, we have sought to revisit molybdenum oxides and vanadium oxides that have been the subject of many studies as high capacity electrode materials for LIBs, as pseudocapacitive electrode materials, or both. Herein, we construct a three-dimensional (3D) ordered hierarchically porous amorphous hybrid for achieving highperformance lithium storage by hybridizing amorphous VOx with amorphous MoOy (donated as 3D-OHP-a-VOx/MoOy) via freeze-drying method followed by subsequent calcination treatment. The resultant 3D-OHP-a-VOx/MoOy exhibits ordered hierarchically porous structure with relatively high surface area. Unlike most of ordered porous nanostructures previously reported that were prepared mainly by template methods, the current approach features template-free. As a result of the unique microstructure and the composition, 3DOHP-a-VOx/MoOy displays exceptionally high reversible capacity, outstanding rate capability, and long cycling stability when evaluated as an anode material in LIBs. This work provides an ingenious strategy to prepare other metal- and metal oxide-based materials with tunable compositions.
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heating and vigorous stirring. Then, this solution was subjected to freeze-drying for 48 h to yield a precursor. Finally the typical sample (3D-OHP-a-VOx/MoOy) can be obtained by annealing the precursor at 550 °C for 3 h at N2 atmosphere. For comparison, we also prepared control samples of singlecomponent a-VOx, c-V2O3, a-MoOy, and c-MoO2. For a-VOx and cV2O3, 2 mmol of VO(acac)2 was added to 15 mL of distilled water to give a transparent solution. Then this solution was subjected to freezedrying for 48 h to yield a precursor. Finally a-VOx and c-V2O3 were obtained by annealing the precursor at 300 and 500 °C for 3 h at N2 atmosphere, respectively. Similarly, a-MoOy and c-MoO2 could be obtained by annealing the freeze-dried precursor containing 1 mmol of MoO2(acac)2 at 300 and 500 °C for 3 h at N2 atmosphere, respectively. Materials Characterizations. The samples were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray spectroscopy (EDS) on a Philips Tecnai G2 F20 transmission electron microscope with an EDS spectrometer. Element mapping images were collected with a scanning transmission electron microscope (STEM) (FEI Technai G2 F20). The X-ray powder diffraction (XRD) pattern was recorded on a D8 Advance (Super speed) XRD diffractometer (Bruker). The X-ray photoelectron spectroscopy (XPS) experiments were recorded on the ESCALAB 250 spectrometer (PerkinElmer). The carbon content was tested by an elemental analyzer (Vario EI) using the combustion method. The Brunauer−Emmett−Teller (BET) specific surface areas were performed at 77 K in a Belsorp-max surface area detecting instrument. The X-ray absorption near edge structure (XANES) measurements were undertaken at Beamlines 1W1B at Beijing Synchrotron Radiation Facility (BSRF) using transmission and fluorescence modes. Inductively coupled plasma spectrometry (ICP) was measured on a Jarrel-ASH (ICAP-9000). Electrochemical Measurements. The electrochemical behavior was tested at room temperature using coin cells (CR2025) on LAND CT2001A in the voltage range of of 0.01−3.00 V vs Li+/Li. The asprepared samples were used as active materials. For the working electrode preparation, the active material, conductive carbon black, and sodium carboxymethyl cellulose (CMC) with a weight ratio of 70:10:20 was mixed and ground in a mortar. Deionized water was used as the solvent to make the homogeneous slurry. The as-resultant slurry was uniformly pasted on a Cu foil and then dried at 120 °C for 36 h in a vacuum oven. The cell assembly was performed in an Ar-filled glovebox. The loading of the active material was about 0.64 mg cm−2. The theoretical specific capacity of 3D-OHP-a-VOx/MoOy in our experiments was calculated based on the total weight of a-VOx, aMoOy, and carbon. The used electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC)/ diethyl carbonate (DEC) (1:1:1, in vol %). A lithium foil was used as counter electrode. Cyclic voltammetry (CV) was measured by a CHI760E electrochemical workstation with a scan rate of 0.5 mV s−1. The impendence spectra were performed using a sine wave with amplitude of 5 mV at the frequency range from 100 kHz to 0.01 Hz. To collect the active materials after cycling for the measurements of XRD, SEM, TEM, and XPS, the electrodes were disassembled from the cells in an Ar-filled glovebox and then rinsed with absolute alcohol and DMC (Sigma-Aldrich, 99%) to remove the residual electrolyte.
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RESULTS AND DISCUSSION The overall synthesis route of 3D-OHP-a-VOx/MoOy is depicted schematically in Scheme 1. First, an aqueous solution containing VO(acac)2 and MoO2(acac)2 was freeze-dried to form a loose VO(acac)2-MoO2(acac)2 precursor, in which VO(acac)2 and MoO2(acac)2 were homogeneously mixed. Then, the precursor was annealed at 550 °C in N2 atmosphere to form the final VOx/MoOy sample. During the heat treatment process, a large amount of gases (CO and CO2) were released due to the decomposition of organic groups, thus also leading to the formation of 3D ordered hierarchically porous structure.
EXPERIMENTAL SECTION
Synthesis of 3D-OHP-a-VOx/MoOy Hybrid. All chemical regents used in this work were analytical grade and without further purification. In a typical synthesis, 2 mmol of vanadyl acetylacetonate [VO(acac)2] (Aldrich) and 1 mmol of molybdenyl acetylacetonate [MoO2(acac)2] (Aldrich) were added to 15 mL of distilled water (Beijing Chemical Reagent Ltd.) to give a transparent solution through 4181
DOI: 10.1021/acs.chemmater.6b00414 Chem. Mater. 2016, 28, 4180−4190
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their respective precursors at 300 °C, while c-V2O3 and c-MoO2 were formed at 500 °C. However, for dual-component VOx/ MoOy, 550 °C still results in the formation of amorphous product (Figure S3). Clearly, the dual-component system has higher crystalline temperature. We speculate that, during the annealing process, VO(acac)2 and MoO2(acac)2 might restrain with each other, so the crystallization temperature is increased accordingly. Important information on the electronic state and the composition of the typical sample can be further provided by XPS measurements. Before XPS tests, we etched the sample for 600 s because it still contains physi- and chemisorbed water (Figure S4). As can be seen from Figure 2a, a typical survey
Scheme 1. Illustration of the Formation Process of 3D-OHPa-VOx/MoOy and Its Application in LIBs
The XRD pattern (red curve) of the typical sample exhibited a broad-hump shape, indicating the amorphous structure To further determine its chemical composition, we annealed this amorphous sample at 750 °C for 2 h in N2 atmosphere, and the corresponding XRD pattern is shown in Figure 1b.
Figure 2. XPS spectra of 3D-OHP-a-VOx/MoOy after etching treatment for 600 s. (a) Survey spectrum, (b) O 1s, (c) V 2p, and (d) Mo 3d.
XPS spectrum involves several distinct peaks at 516 (V 2p), 632 (V 2s), 233.9 (Mo 3d), 398.0 (Mo 3p3/2), 415.6 (Mo 3p1/2), and 531.7 (O 1s) eV, the characteristic peaks of vanadium oxides and molybdenum oxides.8,36 Figure 2b shows the O 1s core-level spectra, in which the peak at 530 eV is typical metal− oxygen bonds (Mo−O and V−O in this sample),8 while the peak at 531.7 eV may be attributed to the “O−” species that could compensate some deficiencies on the subsurface of oxide materials.37 Figure 2c shows the high-resolution V 2p XPS spectrum, in which the binding energies at 516.9 and 524.3 eV could be ascribed to V5+ in V2O5,38 515.9 and 523.3 eV to V4+ of VO2, 515.1 and 522.4 eV to V3+ of V2O3, and 513.9 and 521.2 eV to V2+ of VO.36 These results confirmed the presence of mixed-valence vanadium. As reported by other studies, mixed-valence vanadium oxides (V2+, V3+, V4+, and V5+) are beneficial for charge storage in a wide range of potential windows and are promising electrode materials for LIBs or pseudocapacitors.15 Figure 2d presents high-resolution Mo 3d XPS spectrum. The three doublets at 235.9/232.8, 234.3/231.2, and 233.2/230.1 eV can be assigned to Mo6+ (3d5/2/3d3/2), Mo5+ (3d5/2/3d3/2), and Mo4+ (3d5/2/3d3/2), respectively,8,39,40 and the last doublet at 231.6/228.5 eV could be ascribed to Mo2+.39 According to these results, the typical sample also has multiple oxidation states of Mo. In order to further verify this fact, we performed XANES analysis for the 3D-OHP-a-VOx/ MoOy sample. Here, we used MoO2 and MoO3 as the standards, which are known to have Mo(IV) and Mo(VI) oxidation states, respectively. The Mo K-edge spectrum for
Figure 1. (a) XRD patterns of 3D-OHP-a-VOx/MoOy, a-VOx, and aMoOy. (b) XRD pattern of 3D-OHP-a-VOx/MoOy after annealing in N2 atmosphere and standard XRD patterns of V2O5, MoO2, and Mo4O11.
Different from the unannealed sample, the annealed one exhibited obvious diffraction peaks, indicating a phase crystallization process accompanied by a phase change during the calcination treatment. These peaks can be assigned to tetragonal V2O5 (JCPDS no. 45-1074), monoclinic MoO2 (JCPDS no. 78-1069), and orthorhombic Mo4O11 (JCPDS no. 65-0397). Thus, it can be seen that the resultant sample is a crystalline hybrid, which was donated as c-VOx/MoOy. Meanwhile, this result also confirms the fact that the typical sample may be an amorphous hybrid based on VOx and MoOy. For comparison, single-component amorphous and crystalline samples (a-VOx, c-V2O3, a-MoOy, and c-MoO2) were also prepared via a similar two-step process (Figure 1a and Figures S1 and S2). a-VOx and a-MoOry were obtained by annealing 4182
DOI: 10.1021/acs.chemmater.6b00414 Chem. Mater. 2016, 28, 4180−4190
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Figure 3. (a) XANES spectra corresponding to Mo K-edge of 3D-OHP-a-VOx/MoOy. (b) Enlarged XANES spectra of main absorption edge area of red square in (a). (c) Correlation of the half step energy of the Mo K edge with the oxidation state of various molybdenum species. (d) XANES spectra corresponding to V K-edge of 3D-OHP-a-VOx/MoOy. (e) Enlarged XANES spectra of main absorption edge area of red square in (c). (f) Correlation of the half step energy of the V K edge with the oxidation states of various vanadium species.
Figure 4. (a−d) Typical TEM images of 3D-OHP-a-VOx/MoOy hybrid. (e, f) N2 adsorption−desorption isotherms of 3D-OHP-a-VOx/MoOy hybrid and corresponding pore size distribution curve. (g) Typical HRTEM image, (h) SAED pattern, (i) STEM elemental mapping images, and (j) EDS spectrum of 3D-OHP-a-VOx/MoOy hybrid. The inset in (j) shows element contents.
oxidation state, the XANES peak exhibits a sequential shift toward higher energies. Meanwhile, from the enlarged XANES main absorption edge area (Figure 3b), Mo in 3D-OHP-aVOx/MoOy presents an average valence state between +4 and +6, but this valence state is closer to +4, which agrees with the value from the XPS analysis.41 Likely, the typical V pre-edge
each sample was collected at room temperature. Besides, the energy was calibrated using a glitch in the Io relative to the absorption edge of the Mo foil. Figure 3a shows the typical Mo pre-edge XANES spectra. The Mo pre-edge features can be clearly observed at around 19 990 eV in all Mo compounds. Moreover, it can be observed that, with the increase of the Mo 4183
DOI: 10.1021/acs.chemmater.6b00414 Chem. Mater. 2016, 28, 4180−4190
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(Figure 4j, the Cu signal comes from Cu grid). In order to determine the exact carbon amount in 3D- OHP-a-VOx/MoOy, CHN element analysis was conducted. The result revealed that the carbon content is 16.08 wt %. Moreover, the carbon contents of the other control samples were also tested, and they are 15.86 wt % for c-VOx/MoOy, 17.85 wt % for a-VOx, 15.26 wt % for a-MoOy, 13.82 wt % for c-V2O3, and 13.51 wt % for cMoO2 (Table S1). It has been demonstrated that the highly flexible carbon matrix in electrode materials could buffer the large mechanical strain during cycling, maintain the integrity of the whole electrode, and at the same time can effectively improve electrical conductivity.25,44 In addition, by inductively coupled plasma spectrometric (ICP) analysis, the molar ratio of V to Mo in 3D-OHP-a-VOx/MoOy is determined to be 1.7:1, which is very close to the result (1.77:1) from EDS measurement. If it is assumed that 3D-OHP-a-VOx/MoOy can be reduced to metallic V and Mo when this hybrid is discharged to 0 V vs Li/Li+, the theoretical capacity of this hybrid is 1051 mAh g−1, in which the contribution of VOx is 732 mAh g−1 and MoOy is 319 mAh g−1.45,46 Likely, the theoretical capacities of a-VOx and a-MoOy are 1187 and 1026 mAh g−1 according to their valence states, respectively. In a word, such a unique hierarchically porous structure, in association with the amorphous nature and the combination of mixed-valence vanadium oxide and molybdenum oxide, will probably endow 3D-OHP-a-VOx/MoOy hybrid with highperformance lithium storage. To fully clarify the electrochemical performance of asprepared 3D-OHP-a-VOx/MoOy hybrid as an anode material for LIBs, we first investigated its cyclic voltammetry (CV), which was tested in the cutoff voltage of 0.01−3 V (vs Li/Li+) at a scan rate of 0.5 mV s−1. Figure 5a shows CV curves from
features were also tested (Figure 3d,e), indicating that its average valence state is between +3 and +4. Moreover, based on the correlation of the half step energy of the V (Mo) K edge with the oxidation state of various V (Mo) species, we calculated the valence states of V and Mo in 3D-OHP-a-VOx/ MoOy.42 As shown in Figure 3c,f, V and Mo of 3D-OHP-aVOx/MoOy presented average valence states of +3.3 and +4.3, respectively. Based on these results, x and y in 3D-OHP-aVOx/MoOy are determined to be 1.65 and 2.15, respectively. Thus, it can be deduced that the typical sample (3D-OHP-aVOx/MoOy) is a hybrid composed of amorphous VOx and MoOy. Furthermore, the amorphous control samples (VOx and MoOy) were also analyzed by XPS and XANES spectra (Figures S5−S7), all of which demonstrated their amorphous nature and mixed-valence states (a-VOx: V3.5+; a-MoOy: Mo5.3+). The morphology and microstructure of the 3D-OHP-a-VOx/ MoOy hybrid were investigated using SEM and TEM techniques. Figure S8 presents SEM images of the 3D-OHPa-VOx/MoOy hybrid, from which it can be seen that the hybrid macroscopically exhibits sheet-like structure and some humps can be observed on the surface of the sheets. In fact, the inner of these humps is hollow, which has been confirmed by TEM images. As shown in Figure 4a, the low-magnification TEM image clearly shows the inner structure of the sheets, and relatively ordered macroporous structure was evidently observed. The high-magnification TEM images display that the average pore diameter of the macropores is about 100 nm and the pore walls are composed of loose nanoparticles (Figure 4b−d). To gain further insight into the porous nature and the pore size distribution of the 3D-OHP-a-VOx/MoOy hybrid, N2 sorption measurements were performed (Figure 4e). Clearly, the isotherms can be identified as a combination of types II and IV, suggesting a hierarchically porous structure. The corresponding pore size distribution is shown in Figure 4f. Two well resolved peaks mainly center at 45 and 95 nm, and the other peaks are in the size range of 2.43−25 nm, which are consistent with the sizes observed from TEM images. The macropores may be possibly as a result of the release of some gases formed from the decomposition of VO(acac)2 and MoO2(acac)2, while the mesopores may originate from the stacking of the nanoparticles consisting of the porous wall. Although the macropores do not contribute to many more specific surface areas, they do provide good electrolyte access.43 Furthermore, the Brunauer−Emmett−Teller (BET) surface area of 3D-OHPa-VOx/MoOy is 38.13 m2 g−1, which is higher than 28.1 and 15.2 m2 g−1 for a-VOx and a-MoOy, respectively (Figure S9). As shown in Scheme 1, such a porous structure not only can effectively shorten the diffusion path of lithium ions and electrons in active material but also can provide a large volume for electrolyte storage and ensure Li+ diffusion in channels across the 3D-OHP-a-VOx/MoOy anode, thus significantly improving rate capability and cycling performance. The amorphous nature of 3D-OHP-a-VOx/MoOy can be further revealed by high resolution TEM images. As shown in Figure 4g no discernible lattice fringes were observed, in good agreement with the above XRD analysis. The selected area electron diffraction (SAED) pattern manifests a dispersed and very ambiguous halo (Figure 4h), further confirming this fact. The elemental mappings (Figure 4i) of 3D-OHP-a-VOx/MoOy hybrid reveal that V, Mo, O, and C are homogeneously distributed, and their mass percentages are 30.21, 32.12, 25.42, and 12.23 wt %, respectively, based on the EDS spectrum
Figure 5. CV curves of the first six cycles of the electrodes: (a) 3DOHP-a-VOx/MoOy, (b) a-MoOy, and (c) a-VOx; (d) the discharge and charge voltage profiles for all the electrodes during the first cycle at a rate of 0.2 A g−1.
the first to sixth cycles of the 3D-OHP-a-VOx/MoOy electrode. In the first cathodic scan, the inconspicuous peak at about 1.33 V, which disappears in subsequent cycles, could be attributed to the formation of irreversible solid electrolyte interface (SEI) layers or a small amount of inserted lithium in the amorphous sample without any change in the pristine structure.10,24 Meanwhile, the other peak at 0.95 V can be assigned to the 4184
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Figure 6. Electrochemical performance of the as-prepared electrodes: (a) cycling performance of 3D-OHP-a-VOx/MoOy, a-MoOy, a-VOx, and cVOx/MoOy electrodes at a scan rate of 0.2 A g−1 and the corresponding Coulombic efficiency of 3D-OHP-a-VOx/MoOy; (b) cycling performance of 3D-OHP-a-VOx/MoOy electrode at different current densities; (c) rate performance at different current densities from 0.1 to 18 A g−1; (d) longterm cycling performance of 3D-OHP-a-VOx/MoOy electrode at a high current density of 4 A g−1; the inset in (d) is TEM image of 3D-OHP-aVOx/MoOy electrode after 400 cycles at a current density of 4 A g−1.
subsequently the capacity starts to steadily rise, and finally a capacity as high as 1380 mAh g−1 can be retained after 50 cycles. The phenomena of the capacity increase upon extended cycling is very common in porous transition metal oxide electrodes.20,22,34,49 It may be ascribed to the gradual access of more electrolyte into the porous materials and the gradual activation process of the conversion reactions between MoOy (VOx) and Mo (V) as well as the increased Li+ diffusion kinetics.20,22,50 In addition, the in situ formed Mo (V) nanoparticles during the lithiation process might act as efficient catalysts to promote the reversible formation/decomposition of a polymeric gel-like film and the decomposition of Li2O, which may also be responsible for the capacity-rising phenomenon.34,49,51,52 The initial Coulombic efficiency is 72%, and such a large capacity loss (28%) may attribute to the formation of SEI film on the surface of the electrode, which can be seen in most anode materials. As reported before, higher pore volume and more structural defects will make more active sites to expose in the electrolyte, thus resulting in the formation of thicker SEI film and larger irreversible capacity loss. This phenomenon is unavoidable in porous electrode materials for LIBs.47,48 However, from the fourth cycle, the Coulombic efficiency rapidly increases to 100% and the reversible capacity gradually increases as well, which probably results from the gradual access of much more electrolyte to the meso- and macropores of the active material (3D-OHP-a-VOx/MoOy hybrid) during the cycling processes.21 Furthermore, to confirm the important role of the amorphous structure, we transform 3D-OHP-a-VOx/ MoOy to highly crystalline c-VOx/MoOy and its lithium storage was also tested with the conditions same as those of 3D-OHPa-VOx/MoOy. As shown in Figure 6a, compared to 3D-OHP-aVOx/MoOy, c-VOx/MoOy presents a relative lower discharge capacity of 721 mAh g−1 in the first cycle. In the following cycles, the capacity also undergoes gradual increase and finally stabilizes at about 1000 mAh g−1 after 50 cycles, and this behavior is very similar to the 3D-OHP-a-VOx/MoOy hybrid.
conversion reactions for the formation of metallic Mo, V, and Li2O.45−47 Such a low potential (
