Journal of Colloid and Interface Science 418 (2014) 74–80

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Facile synthesis of hierarchical and porous V2O5 microspheres as cathode materials for lithium ion batteries Hong-En Wang a,⇑, Dai-Song Chen a, Yi Cai a, Run-Lin Zhang a, Jun-Meng Xu a, Zhao Deng a, Xian-Feng Zheng a, Yu Li a, Igor Bello c, Bao-Lian Su a,b,⇑ a b c

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Laboratory of Inorganic Materials Chemistry, University of Namur, 61 rue de Bruxelles, B-5000 Namur, Belgium Vacuum Electronics Canada, 1735 Kirkpatrick Way, London, Ontario, Canada

a r t i c l e

i n f o

Article history: Received 21 September 2013 Accepted 4 December 2013 Available online 11 December 2013 Keywords: Vanadium pentoxide Microspheres Hierarchical structures Cathode Lithium ion batteries

a b s t r a c t Hierarchical and porous V2O5 microspheres have been fabricated by a refluxing approach followed by annealing in air. The resulting porous V2O5 microspheres typically have diameters of 3–6 lm and are constructed of intertwined laminar nanocrystals or crosslinked nanobricks. It is found that the vanadyl glycolates rinsed with water have pronounced pore structures than that rinsed with ethanol alone. In addition, the configuration of the vanadyl glycolates microspheres can be tuned during the refluxing along with stirring. The possible formation processes of the vanadyl glycolates and V2O5 products have been discussed based on the experimental data. Electrochemical tests indicate that the hierarchical and porous V2O5 microspheres exhibit relatively high and stable Li+ storage properties. The porous V2O5 microspheres assembled by intertwined nanoparticles maintain reversible Li+ storage capacities of 102 and 80 mAh g1, respectively; whilst the porous V2O5 microspheres assembled by crosslinked nanobricks maintain reversible Li+ storage capacities of 100 and 85 mAh g1 over 100 cycles at current rates of 0.5 and 1 C, respectively. The superior Li+ storage performance of the hierarchical and porous V2O5 microspheres could mainly be ascribed to the improved electrode/electrolyte interface, reduced Li+ diffusion paths, and relieved volume variation during lithiation and delithiation processes. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Vanadium pentoxide (V2O5) is an important functional semiconductor material with diverse applications in catalysis [1], sensors [2], electrochromic devices [3], as well as supercapacitors [4–9] and lithium ion batteries (LIBs) [10–12] because of the ease to uptake molecules or ions into its orthorhombic layered structure. As a cathode material in LIBs, V2O5 can yield a maximum theoretic capacity of 440 mAh g1 based on the intercalation of three lithium ions [13,14], which is about twice higher than that of LiCoO2 (140 mAh g1) [15], LiMn2O4 (148 mAh g1) [16], and LiFePO4 (170 mAh g1) [17,18]. However, the application of V2O5 electrodes in rechargeable LIBs has been deferred by its poor structural stability [19], low electronic conductivity [20], and slow Li+ insertion/extraction kinetics [21]. Because of the high surface area and reduced Li+ diffusion distance, nanostructured vanadium oxides have been widely ⇑ Corresponding authors. Address: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China. Fax: +86 27 87879468. E-mail addresses: [email protected] (H.-E. Wang), [email protected] (B.-L. Su). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.12.011

investigated to enhance the electrochemical kinetics. During the past decades, various nanostructured forms of V2O5 materials [22], such as nanoparticles [23,24], nanowires [25], nanorods [26], nanoribbons [27], nanobelts [28], and nanotubes [29], have been synthesized by a variety of methods to enhance the Li+ insertion/extraction kinetics and reduce Li+ diffusion paths. However, considerable capacity fading is still observed in low-dimensional V2O5 nanostructures owning to the possible aggregation of nano-entities induced by their high surface energies [25]. Nevertheless, lower volumetric energy densities are usually expected due to the loose packing of such low-dimensional nanostructures. In this regard, three-dimensional (3D) hierarchical microstructures have attracted increasing attention because such complex architectures can better maintain structural stability during electrochemical cycling and increase the energy density [30–32]. Among these 3D architectures, porous and/or hollow microspheres have shown promising Li+ storage performances because of the improved electrolyte impregnation, reduced polarization of electrolyte in the active layer, high flexibility to accommodate volumetric change during Li+ insertion/extraction, and ultrahigh particle packing density [33–38]. Several routes have been proposed to prepare porous and/or hollow V2O5 microspheres. Wan et al. [33] reported the preparation

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of V2O5 hollow microspheres assembled by nanorods with poly(vinylpyrrolidone) structure-directing agent. Xue et al. [34] prepared uniform yolk–shell V2O5 microspheres by solvothermal reaction of vanadium acetylacetone dissolved in N,N-dimethylformamide and subsequent calcinations in air. Yin et al. [35] synthesized monodisperse porous V2O5 microspheres using vanadium isopropoxide by a sol–gel route followed by a controlled annealing process. Cao et al. [37] prepared highly porous V2O5 microspheres by a polyol-mediated solvothermal process. Lou et al. [38] synthesized hollow VO2 microspheres with multi-interiors by a solvothermal reaction and then converted them into hollow V2O5 microspheres. However rapid and economic synthesis of V2O5 microspheres with well-defined spherical geometry and tunable pore structures on a large scale remains challenging. In this work, hierarchical and porous V2O5 microspheres with tunable porosity and morphology were synthesized on a largescale by a rapid refluxing approach at atmospheric pressure from low-cost starting materials followed by calcinations in air. Electrochemical tests reveal that the prepared V2O5 microspheres possess relatively high and stable Li+ storage properties. 2. Experimental All chemicals were analytical grade and used as received. The synthesis of hierarchical V2O5 microspheres includes the fabrication of hierarchical VEG precursor samples and subsequent conversion into V2O5 samples via annealing in air. 2.1. Synthesis of hierarchical VEG and V2O5 microspheres In a typical procedure, 1 mmol ammonium metavanadate (NH4VO3) was added in 20 mL ethylene glycol. The resulting mixture was heated to 70 °C with simultaneous stirring to obtain a yellow sol which was then transferred into a 100 mL glass flask and refluxed at 170 °C for 2 h to acquire dark blue vanadyl glycolates (VEG) precipitates. The precipitates were then rinsed with absolute ethanol, and then dried in an oven at 60 °C for 8 h. For comparison, the VEG precursor was also rinsed with ethanol and water, respectively. In addition, the influence of stirring during refluxing on the structure of the resulting VEG precursor was also studied. To prepare the final V2O5 products, the VEG precursors were calcined in a furnace at 500 °C for 1 h with a temperature ramping rate of 2 °C min1 and then naturally cooled to room temperature. 2.2. Physical characterization and electrochemical performance investigation Crystal structures of the samples were identified by powder Xray diffraction (XRD) employing a Bruker D8 Advanced diffractometer with Cu Ka radiation (k = 1.54056 Å). Surface morphologies of the samples were studied with a Hitachi S-4800 scanning electron microscope (SEM) equipped with a field emission gun (FEG), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) using a JEM-2100F transmission electron microscope. Specific surface area and pore diameter distribution were determined by the method of N2 adsorption/desorption isotherms carried out at 77 K using a NOVA 1200e Surface Area & Pore Size Analyzer (Quantachrome Instruments). Prior to the adsorption experiments, the samples were degassed at 150 °C for 2 h. Thermal gravimetric analysis was performed on a SETARAM Labsys Evo S60/58458 thermal analyzer with volumetric air flow of 20 cm3 min1 and a heating rate of 5 °C min1. For electrochemical performance evaluation, coin-type (CR2032) lithium-half cells were assembled and tested under

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galvanostatic conditions. The working electrode was made by blending a powder mixture consisted of V2O5, carbon black, and polyvinylidene difluoride with a weight ratio of 7:2:1 in Nmethyl-2-pyrrolidone. The well-blended slurry was uniformly coated on an Al foil, and then dried in a vacuum oven at 120 °C for 12 h. The lithium half cells were assembled with lithium foils as counter and reference electrodes, 1 M LiPF6 dissolved in ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1 as the electrolyte. All the operation was carried out in an Ar glovebox where water vapor and oxygen contents were lower than 0.1 ppm. Galvanostatic charge–discharge trials were carried out at room temperature on a LAND battery testing system between 2.5 and 4 V (vs. Li+/Li) at current rates of 0.5 and 1 C, respectively. (1 C corresponds to a current density of 150 mA g1).

3. Results and discussion 3.1. Physical characterization of the as-synthesized samples Fig. 1a shows a representative XRD pattern of the precursors synthesized by static refluxing at 170 °C for 2 h and collected after rinsing with ethanol, which can be well indexed to the known vanadyl glycolate (VEG) with monoclinic structure (space group: C2/c; JCPDS card No. 49-2497) [37,39,40]. These analytical data clearly signify that during the refluxing process NH4VO3 had been reduced by ethylene glycol and VEG (VO(OCH2CH2O)) were formed [41]. Low-magnification SEM image (Fig. S1a in the Supporting Information) indicates that the vanadyl glycolates precursors comprise a large number of dispersed microspheres with rough surfaces and sizes typically ranging from 3 to 6 lm. The magnified SEM image in Fig. 1b presents the well-defined spherical shape of a single particle with rough surface, while the high-magnification SEM image of a part of this particle (inset of Fig. 1b) shows more details of its surface texture made of curled nanoparticles forming a porous structure. The SEM image of a fractured microsphere (Fig. S1b in the Supporting Information) reveals the interior of such particles is likely hollow. In addition, the shell comprises curved elongated particles forming channels, which are preferentially oriented in radial directions. The shell thickness of the fractured microsphere is estimated to be about 1.2 lm. Interestingly, it is found that rinsing the VEG precursors with additional water results in some microspheres with abundant macropores (Figs. 1c and S1c in the Supporting Information), which might arise from the partial dissolution of VEG molecules in water. In addition, the stirring during refluxing exerts significant role on the morphology of the resulting VEG precursors. Large quantities of spherical microparticles aggregated by packed nanostrips are thus produced via a refluxing along with stirring (Figs. 1d and S1d in the Supporting Information). These results indicate that the morphologies and pore structures of the VEG precursors can be conveniently tuned by suitably adjusting the rinsing and stirring conditions during the refluxing reactions. The as-synthesized VEG precursors can be converted into corresponding V2O5 products by a simple calcination process in air. Thermal gravimetric analysis (Fig. S2 in the Supporting Information) reveals that the VEG precursors start to decompose around 200 °C accompanied by a fast weight loss. A maximum weight loss (27%) of the VEG is observed at around 280 °C, which corresponds to the formation of VO2 [39,41]. On further heating to 600 °C, there is a gain in weight due to the oxidation of VO2 into V2O5 phase. Fig. 2a shows the XRD pattern of the precursors of Fig. 1a subjected to calcination in air at 500 °C for 1 h. The sharp XRD diffraction peaks can be indexed to the pure orthorhombic phase V2O5 (space group: Pmmn (59); JCPDS card No. 41-1426)

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Fig. 1. XRD pattern and SEM images of the VEG precursors synthesized under different experimental conditions: (a and b) static refluxing and rinsing with ethanol, (c) static refluxing followed by rinsing with ethanol and water in sequence, (d) refluxing with stirring and rinsing with ethanol.

Fig. 2. XRD pattern and SEM images of the V2O5 product synthesized by annealing the corresponding VEG precursors in air: (a and b) static refluxing and rinsing with ethanol, (c) static refluxing and rinsing with ethanol, water in sequence and (d) refluxing with stirring and rinsing with ethanol.

[14,25], indicating that the precursors have been completely transformed into well crystallized V2O5 phase during the annealing process. The transition from the VEG precursor to crystalline V2O5 product was also visually observed by the color variation of the samples from dark blue (VEG precursors) to orange (V2O5 products). Low-magnification SEM image (Fig. S3a in the Supporting Information) reveals that the V2O5 sample comprises microspheres

with density nearly equal to that in VEG precursors, suggesting that the spherical morphology has been well preserved during the calcination process. The magnified SEM image in Fig. 2b indicates that each V2O5 microsphere has rough and textured surfaces with porous structures. High-magnification SEM image taken from a portion of a single microsphere (inset of Fig. 2b) clearly reveals that the surface is constructed of laminar nanoparticles (with size

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ca. 200 nm and thickness of ca. 30 nm) kinked together to form a porous structure. The SEM image of two fractured V2O5 microspheres (Fig. S3b in the Supporting Information) shows they contain hollow shells with spherical shape. In contrast, some macroporous V2O5 microspheres are obtained via annealing the VEG precursors synthesized by static refluxing and rinsing with water, as shown in Figs. S3c (Supporting Information) and 2c. These macroporous microspheres are constructed of interconnected round nanoparticles with diameters of ca. 160 nm. In addition, spherical V2O5 microparticles assembled by loosely crosslinked nanobricks (with lateral size of ca. 200 nm and thickness of ca. 80 nm) are resulted after calcining the VEG precursors prepared by refluxing with stirring, as depicted in Figs. S3d (Supporting Information) and 2d, suggesting that the VEG nanostrips decompose and break into V2O5 nanobricks during the annealing process. Analysis of transmission electron microscopy micrographs discloses further microstructural information of the hierarchical V2O5 microspheres synthesized by static refluxing and rinsing with ethanol, followed by annealing in air. Fig. 3a shows the TEM micrograph of two V2O5 particles intergrown together. Clearly, these particles take spherical shapes and have hollow shell structures as indicated by the different contrast between the interior and peripheral regions, which is consistent with the SEM analysis (Fig. S2b in the Supporting Information). The TEM micrograph taken from the edge of the microsphere (Fig. 3b) reveals the shell of the microsphere consists of interconnected laminar nanoparticles (sizes of 200–400 nm) bound by chemical bonds, which form voids among adjacent nanoparticles. The HRTEM micrograph shown in Fig. 3c abstracted from one nanoparticle shows clear two-dimensional lattice fringes. The lattice spacing of 3.43 and 5.78 Å can be indexed to the (1 1 0) and (2 0 0) facets of

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orthorhombic structured V2O5. The Fast Fourier transform (FFT) pattern derived from the HRTEM image (Fig. 3d) indicates nanoparticles inherently have single crystalline structures. N2 sorption isotherms were further used to study the porosity of the prepared V2O5 samples. The N2 adsorption–desorption curves of the V2O5 microspheres (Fig. S5a in the Supporting Information) can be identified as type IV isotherms with a hysteresis hoop, which is characteristic of mesoporous structures. The BET specific surface area was determined to be 21.5 m2 g1. This value is slightly lower than the value for porous V2O5 microspheres reported by Yin et al. [35] and is comparable to those of Cao et al. [37]. The pore size distribution calculated from desorption isotherm using the Barrett–Joyner–Halenda (BJH) method (Fig. S5b) indicates the presence of mesopores centered at 3.7 nm with narrow distribution and macropores with wide distribution ranging from 20 to 110 nm and centered at 50 nm. 3.2. Formation mechanism analysis of the VEG precursors and V2O5 products The experimental trials reveal that the ‘‘stirring’’ during the refluxing process and the ‘‘rinsing with water’’ excert important influences on the morphology and pore characteristics of the resultant VEG precursors, as well as the final V2O5 products. During the refluxing process, the V5+ ions were reduced to V4+ ions with the consequent formation of vanadyl glycolates (VO(CH2CH2O), VEG) and the evolution of N2 gas molecules. The overall reaction equation between NH4VO3 and ethylene glycol can be formulated as follows [37,39]:

NH4 VO3 þ HOCH2 CH2 OH ! N2 " þVOðOCH2 CH2 OÞ þ H2 O

Fig. 3. (a and b) TEM images, (c) HRTEM micrograph and (d) FFT pattern of the V2O5 product prepared by static refluxing and rinsing with ethanol followed by annealing in air.

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A series of intermediate products have been collected and characterized by SEM analyses to trace the morphology evolution of vanadyl glycolates precursors. It is found that dark blue precipitates were immediately produced only after refluxing at 170 °C for 20 min. SEM images of the vanadyl glycolates collected after refluxing for 20 min and 30 min (Fig. S4a and b, Supporting Information) indicate that the resulting products comprise highly porous microspheres constructed of irregular primary particles. In contrast, the vanadyl glycolates obtained after refluxing for 60 min contain microspheres with less porous structures and are constructed by larger primary particles with smooth particle surfaces. The above experimental results suggest that a very short nucleation step followed by rapid aggregation into spherical microparticles was involved as schematically illustrated in Fig. 4a and b. Meantime, the evolution of N2 molecules or the etching effect of the water formed by the reaction between NH4VO3 and ethylene glycol might account for the formation of porous vanadyl glycolates microspheres. As the reaction further proceeds, the starting reactant NH4VO3 will be gradually consumed and the formed spherical vanadyl glycolates aggregates start to crystallize. Cao et al. recently synthesized hollow V2O5 microspheres by a solvothermal reaction of NH4VO3 in ethylene glycol and proposed that the N2 microbubbles served as a template directing the formation of hollow nanostructures. However, N2 bubbles can be easily released in our open refluxing process which make them difficult to behave as a template herein. Therefore, we infer that an ‘‘inward-outward’’ Ostwald ripening process was more likely to account for the formation of such hollow vanadyl glycolates microspheres as graphically illustrated in Fig. 4c. In this case, the surface of vanadyl glycolates crystallize preferentially due to the high interface energy at the liquid ethylene glycol and solid vanadyl glycolates, as well as the more flexible rearrangement of structural units located at surfaces. As a result, the vanadyl glycolates gradually hold a rigid surface and further crystallization starts to consume the inside amorphous parts. Meantime, the remnant N2 bubbles trapped inside the vanadyl glycolates microspheres would be released during this process and crush into some pore channels. On the other hand, vanadyl glycolate is an alkoxide and paramagnetic, which is sensitive to water. Therefore, selectively washing it with mixture of ethanol and water can cause the material to decompose and pronounced pore structures can be attained. During the calcinations in air, the vanadyl glycolates precursor is decomposed and V2O5 is finally produced at 500 °C in air, as shown in the follow equation

Fig. 4. Schematic illustration of the possible formation processes of the hierarchical and porous VEG precursors and V2O5 products.

VOðOCH2 CH2 OÞ ! V2 O5 þ CO2 " þH2 O " : The evolution of CO2 and H2O molecules and structural arrangement of V2O5 molecular units during the annealing process result in a porous shell constructed of intertwined V2O5 nanoparticles (Fig. 4d). Under stirring during the refluxing process, one-dimensional chain-like vanadyl glycolates nanostructures can be evolved because of the oriented particles’ coupling. However, these onedimensional structures also tend to aggregate to reduce their total surface energies (Fig. 1d). During calcinations in air, V2O5 crystallites can take place on more than one site, leading to the fracture of the original one-dimensional structure. In addition, the internal stress caused by the structural rearrangement of V2O5 crystallites as well as the release of CO2 and vapor also contribute to the breaking of ribbon-like structures because vanadium glycolates and V2O5 have discrepancy in lattice parameters and particle densities (Fig. 2d). 3.3. Electrochemical performance of the hierarchical and porous V2O5 microspheres To demonstrate the possible structural advantages, we have evaluated the electrochemical lithium ion storage properties of the porous V2O5 microspheres as cathode materials for LIBs. The process of Li+ ion insertion into and extraction from the V2O5 framework can be expressed as follows þ

V2 O5 þ xLi þ xe $ Lix V2 O5 : Fig. 5a shows the representative voltage-capacity profiles of the porous V2O5 microspheres synthesized by static refluxing and rinsing with ethanol, followed by annealing. It is found that two voltage plateaus located at ca. 3.3 and 3.1 V, respectively, can be observed during the discharge process at 0.5 C, which correspond to the phase transitions from a-V2O5 to e-Li0.5V2O5, and then dLiV2O5, respectively [31,35]. The first discharge and charge capacities of the porous V2O5 microspheres at 0.5 C are found to be 141 and 140 mAh g1, respectively, with a Coulombic efficiency of 99.3%. The two discharge plateaus of the porous V2O5 microspheres slightly decline at 1 C due to the slightly increased polarization. Accordingly, the initial discharge and charge capacities of the V2O5 samples are reduced to 128 and 125 mAh g1, respectively. In contrast, the porous V2O5 microspheres synthesized by refluxing with stirring followed by annealing exhibit similar discharge and charge behaviors at 0.5 C. The first discharge and charge capacities of the porous V2O5 samples are 135 and 133 mAh g1, respectively, with a Coulombic efficiency of 98.5%. However, the discharge plateaus of the V2O5 sample are significantly lowered at 1 C owing to the increased polarization induced either by the larger crystallite size or the poor interparticle contact. Fig. 5c shows the cycling performances of the porous V2O5 microspheres synthesized by static refluxing and annealing. It reveals that the porous V2O5 microspheres cycling at a current rate of 0.5 C show relatively stable capacity retention, although the capacity reduces to 102 mAh g1 over 100 cycles, with a capacity retention rate of 72%. In addition, the cycling performance of the porous V2O5 samples slightly degrades at a current rate of 1 C. The lithium ion storage capacity of the V2O5 sample drops to 80 mAh g1, with a retention rate of 65%. The cycling performance of the porous V2O5 microspheres synthesized by refluxing with stirring followed by calcinations is shown in Fig. 5d for comparison. Similarly, the porous V2O5 sample also shows relatively stable capacity retention. The lithium ion storage capacity of the V2O5 samples retains 100 mAh g1 over 100 cycles at 0.5 C, with a retention rate of 75%. Further, the V2O5 samples cycling at 1 C exhibit an initial lithium storage capacity of 113 mAh g1, and retain

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Fig. 5. Initial discharge–charge profiles and cycling performances of the porous V2O5 microspheres synthesized under different conditions: (a and c) static refluxing and annealing; (b and d) refluxing with stirring and annealing.

a capacity of 85 mAh g1, with a retention rate of 75%. The electrochemical lithium storage properties of the hierarchical and porous V2O5 microspheres herein are superior to that of nanostrips [39], nanotubes (30 mAh g1 over 50 cycles) [29], and comparable to that of porous monodisperse V2O5 microspheres (which has an initial capacity of 143 mAh g1 and decreases to 132 mAh g1 over 110 cycles at 0.2 C) [35]. The relatively high and stable lithium storage properties of the hierarchical and porous V2O5 microspheres can mainly be attributed to their fascinating structural characteristics. Firstly, the V2O5 microspheres are constructed of interconnected laminar nanoparticles forming a hierarchical and porous architecture, which maintains large electrode/electrolyte contact interface and facilitates fast Li+ insertion and extraction, as well as reduces the Li+ diffusion lengths. Secondly, the large V2O5 microparticles with well-defined spherical shape can pack tightly and facilitate the electrode film fabrication process. Thus, a higher volumetric energy density can be expected. Last but not least, the porous and hollow V2O5 microspheres can effectively relieve the unexpected volume change during the electrochemical lithiation and delithiation processes, which can reduce the internal stress and avoid the pulverization of the electrode materials during extended electrochemical cycling tests. The electrochemical properties of the V2O5 microspheres can further be improved from several aspects for practical large-scale applications. First of all, the crystallite size and shape of the primary nanoparticles as well as the pore structure of the V2O5 microspheres might be tuned by varying the refluxing temperatures and time, concentration of starting reactants, and post-treatment temperatures and atmospheres. In addition, the electronic conductivity of the V2O5 microspheres can potentially be enhanced

via suitable surface coating with carbon or conductive polymers [8], as well as the fabrication of carbon/V2O5 hybrid composites [42–44]. 4. Conclusions Hierarchical and porous V2O5 microspheres have been fabricated on a large-scale by a facile and fast refluxing reaction, followed by calcinations. The microspheres are constructed of intertwined laminar nanoparticles or crosslinked nanobricks forming interpenetrating three-dimensional hierarchical and porous microstructures. The hierarchical and porous V2O5 microspheres possess relatively high and stable Li+ storage properties when evaluated as cathode materials in lithium ion batteries due to their improved electrode/electrolyte interface, reduced diffusion lengths for lithium ions as well as facile strain relaxation during the electrochemical lithiation and delithiation processes. The current synthetic approach can potentially be extended to the synthesis of other hierarchical and porous transition metal oxides. The prepared hierarchical V2O5 microspheres may also find potential applications in other technical fields such as supercapacitors and catalysis. Acknowledgments This work was financially supported by Chinese Ministry of Education in a framework of the Changjiang Scholar Innovative Research Team Program (IRT1169). B.L. Su acknowledges the Chinese Central Government for an ‘‘Expert of the State’’ position in the Program of the ‘‘Thousand Talents’’, the Chinese Ministry

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of Education for a Changjiang Scholar position at the Wuhan University of Technology and a Clare Hall visiting fellow to the Department of Chemistry, University of Cambridge. H.E. Wang acknowledges the financial support by the National Natural Science Foundation of China (Grant No. 51302204). Y. Li acknowledges Hubei Provincial Department of Education for the ‘‘Chutian Scholar’’ program. This work is also financially supported by ‘‘the Fundamental Research Funds for the Central Universities’’ (2012II-004 & WUT: 2013-IV-098) and self-determined and innovative research funds of the SKLWUT (2013-ZD-6). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.12.011. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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