Fabrication of Various V2O5 Hollow Microspheres as Excellent

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Fabrication of Various V2O5 Hollow Microspheres as Excellent Cathode for Lithium Storage and the Application in Full Cells Xiaochuan Ren, Yanjun Zhai, Lin Zhu, Yanyan He, Aihua Li, Chunli Guo, and Liqiang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03257 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016

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ACS Applied Materials & Interfaces

Fabrication of Various V2O5 Hollow Microspheres as Excellent Cathode for Lithium Storage and the Application in Full Cells Xiaochuan Renab, Yanjun Zhaia, Lin zhua, Yanyan Hea, Aihua Lia, Chunli Guob and Liqiang Xua* a

Key Laboratory of Colloid & Interface Chemistry (Shandong University), Department of

Chemistry, Ministry of Education, Jinan 250100, China b

Country College of Materials Science and Engineering, Taiyuan University of Technology,

Taiyuan, Shanxi 030024, P. R. China *Corresponding authors: [email protected] Keywords: V2O5; Full cell; Hierarchical; Multi-shell; High cycle performance

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ABSTRACT: Vanadium pentoxide (V2O5) has attracted interesting attention as cathode material for LIBs because of its stable crystal structure and high theoretical specific capacity. However, the low rate performance and poor long-term cycling stability of V2O5 limit its applications. In order to improve its battery performance, various V2O5 hollow microspheres including yolk-shell structure, double-shell structure, tribble-shell structure and hierarchical hollow superstructures have been selectively prepared. The obtained hierarchical V2O5 hollow microspheres (HVHS) exhibit a high capacity of 123 mA h g−1 at 20 C (1C = 147 mA g−1) in the range of 2.5 to 4.0 V and 73.5 mA h g−1 can be reached after 3000 cycles. HVHS also display good cycling performance in the range of 2.0 to 4.0 V. Moreover, The V2O5//Li4Ti5O12 full−cell was successfully assembled, which exhibit an excellent performance of 139.5 mA h g−1 between 1.0 and 2.5 V at a current density of 147 mA g−1, and a high capacity of 106 mA h g−1 was remained after 100 cycles, indicating the good cycling performance and promising application of the full cell.

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1. INTRODUCTION The rational design and development of high−performance electrode materials for the energy storage devices is of great importance to meet the increasing demand on portable electronics, electric vehicles and large scale energy storage.

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Hollow microarchitecture materials have been

studied as a type of promising material for energy storage in recent years. Their distinct structures provide an increased surface−to−volume proportion and reduced transport distance for both mass and charge transport.

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For the lithium−ion batteries, unique structure of the electrode material is

particularly prominent to improve the performance by shortening the diffusion distance of Li +, buffering the volume change during charge and discharge process as well as promoting the permeation of electrolyte. As a result, many kinds of hollow structures have been fabricated, such as hollow nano−cocoons,

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hollow nano−boxes

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and hollow spheres

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. The strategies for preparing

hollow structure inorganic materials are various, such as self−assembly techniques and template method, whereas it still remains a great challenge to rationally design and facilely prepare the hollow structures with high performances in lithium storage devices. 9 Vanadium pentoxide (V2O5), as a kind of promising cathode material, has drawn much attention due to its abundance, low cost and excellent capacity. However, its low rate performance and poor cycling stability limit its application due to its low conductivity, poor structural stability and slow electrochemical kinetics during the charge−discharge process.10,11 Therefore, various nanostructures of the V2O5 have been synthesized to improve the electrochemical performances, such as nanobelt arrays,

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porous V2O5 hierarchical octahedrons,

V2O5 sheet network structure,

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polygonal nanoscrolls,

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and two−dimensional

. In particular, V2O5 with interior hollow structure, such as yolk−shell

multi−shell structure,

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and hollow microclews

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have aroused great interest

because of their advantageous features for charge−discharge reaction. Moreover, hierarchical hollow superstructures, which are self−assembled and transformed from nanostructures, not only retain their advantage of hollow features, but also provide larger active area and higher volumetric energy density for LIBs. However, the synthesis procedures and the reactant materials for fabrication of hollow structure materials are usually complex and expensive and the sizes of products are always as large as several micrometers, which produce adverse effects on development of high performance V2O5 electrode materials. Herein, we rationally designed and prepared different kinds of uniform hollow V2O5 microspheres, such as multiy-shell microspheres (yolk-shell structure, double-shell structure and 3



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triple-shell structure) and hierarchical hollow superstructures through a new facile template−free method. The diameter of the products are hundreds of nanometers. The various V2O5 hollow structures can be simply controllable fabricated by adjusting the using amounts of HNO3 and PEG−400 as well as the solvothermal reaction duration. The results of electrochemical measurements show that hierarchical V2O5 hollow superstructures (HVHS) exhibit better electrochemical performance than muilty−shell hollow V2O5 microspheres. Furthermore, full−cell has been assembled by using the product (LiV2O5) obtained from the electrochemical lithiation HVHS as cathode together with spinel Li4Ti5O12 anode, which dispalys excellent cycling performance and high capacity, indicating its great potential application as electrode for LIBs. 2. EXPERIMENTAL SECTION 2.1. Preparation of hierarchical V2O5 hollow superstructures. In a typical synthesis, 2 mmol NH4VO3 (0.234 g) were dispersed in 40 mL of absolute ethanol, which was then added into 3 mL polyethyleneglycol−400 (PEG−400). After stirring for 30 minutes, 1 mL concentrated nitric acid was slowly added into the above solution under vigorous stirring for several hours. After that, the resulting solution was transferred into a 60 mL Teflon−lined stainless steel autoclave and kept at 180 °C for 20 h, and then it was cooled down to the room temperature. The resulting precipitate was collected by suction method and washed with deionized water and absolute ethanol for three times before drying overnight at 60 °C in air. Finally, the hierarchical V2O5 hollow superstructures were obtained after annealing at 350 °C for 3 h in air with a heating rate of 1 °C min−1. Different amounts of PEG−400 and HNO3 were added into 40 mL of absolute ethanol while keeping other parameters constant to study the effect of PEG−400 and HNO3 on the morphology evolutions of the products. Morever, different products were also prepared with different solvothermal reaction durations in the range of 5−30 h to study the time−dependent structural evolution. 2.2. Preparation of muilty−shell hollow V2O5 microspheres. The muilty−shell hollow V2O5 microspheres were preprared using the similar method, while the addition amount of HNO3 is 3mL with other parameters unchanged. 2.3. Materials characterization. Crystallographic phases of all the products were collected by powder X−ray diffraction (Bruker, D8−Advanced XRD) with Cu Kα radiation ( λ = 1.5406 Å). The morphologies of products were observed on a field emission scanning electron microscope (FESEM, ZEISS SUPRA−55) and a transmission electron microscope (TEM, JEM−1011). HRTEM images 4


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were examined by a high resolution transmission electron microscope (HRTEM, JEOL−2100) operating at 200 kV. Nitrogen sorption isotherm was examined on a Micromeritics Automatic Surface Area Analyzer (Gemini 2360, Shimadzu). 2.4. Electrochemical measurements. The electrochemical tests were examined by CR2032 coin cells. The electrode was made by pressing mixed slurry that consisted of 70 wt% active materials (V2O5), 20 wt% acetylene black, and 10 wt% poly(vinylidenefluoride) (PVDF) onto an aluminum foil. The resulting foil was pressed and punched into the discs with a diameter of 12 mm before drying in vacuum at 60 °C for 10 h to use as the cathode. The loading of the active material was about 1−2 mg cm−2 on each of discs. The coin cells assembly were carried out in an argon−filled glove box (Mikrouna, Super 1220/750/900). The electrolyte used in the cells was a mixture of 1 M LiPF6 in an ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1 by volume). Lithium foil was used as counter electrode and reference electrode and a Celgard 2300 microporous polypropylene membrane was used as the separator. Galvanostatic discharge/charge cycles were performed by Land CT2001A battery cycler at room temperature in the voltage range of 2.5−4.0 V (vs Li/Li+) at different current densities. Cyclic voltammetry (CV) profiles were carried out by a CHI 760E electrochemical workstation in the potential window of 2.5 to 4.0 V and Electrochemical impedance spectra (EIS) were acquired over a frequency range of 100 kHz to 0.01 Hz. Full cell was constructed using commercial Li4Ti5O12 as anode and HVHS as cathode. The CV and cycling performance tests were measured within the potential window of 1.0−2.5 V. The mass matching between cathode and anode is based on the performance of HVHS and Li 4Ti5O12 in half−cell system under the same current density. The best mass ratio is about 1:0.84. In order to ensure the efficient utilization of the cathode materials, the ratio is settled about 1: 1.26 (i.e. the mass of anode is in excess of about 1.5−fold.) Before the full cell is assembled, the HVHS cathode is electrochemically lithiated by discharging to 2.5 V vs Li in a half cell. 3. RESULTS AND DISCUSSION

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Figure 1. XRD pattern and the corresponding crystal structure (A), FESEM images (B, C) and TEM images (D, E) of the hierarchical V2O5 hollow superstructures. HRTEM image (F) of part of a HVHS microsphere. The XRD pattern of the obtained hierarchical V2O5 hollow superstructures (HVHS) is shown in Figure 1A. All the diffraction peaks are perfectly indexed to the orthorhombic phase V2O5 with high crystallinity (JCPDS card No. 41−1426). The calculated lattice constants are 11.51 Å, 3.56 Å and 4.37 Å for a, b and c, respectively, which are very close to the standard JCPDF card (a=11.516Å, b=3.5656Å and c=4.3727Å), indicating its good crystallinity. The crystallite size of V2O5 are also calculated by Debye−Scherrer’s formula from the half−width of the peaks of (001), (110) and (301) and the calculated crystallite size is ~ 67 nm. The FESEM images (Figure 1B, C) show that the V2O5 microspheres with diameters in the range of 600−800 nm are assembled by many small nanoplates, indicating their uniform morphologies. From the TEM image (Figure 1D, E), the interior hollow structure can be observed clearly. The HRTEM image (Figure 1F) shows visible lattice spacing of 2.86 Å, which is in accordance with (301) lattice plane of orthorhombic V2O5. The morphology of the precursor has also been characterized by FESEM and TEM (Figure S1). Before the annealing process, the precursor is composed of uniform flower−like microspheres with a size of ~ 1 μm (Figure S1A). The magnified FESEM image (Figure S1B) displays that the surface of the microspheres consists of thin nanosheets. The TEM images shown in Figure S1C and D also confirm this result. The TEM images again present the hollow interior structure of precursor. After 6


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the calcination process, the microspheres maintained the interior hollow structures, while a notable change of the surface of microspheres was observed. After annealing, the self−assembled nanosheets on the surface of microspheres split into small nanoplates owing to the thermal decomposition and evolution from the precursor into crystalline V2O5. Several comparison experiments were designed and launched to understand the formation processes of the nanostructures. As the quantity of HNO3 and PEG−400 was 1.0 and 3.0 mL, respectively, different morphologies of products were obtained along with the reaction time increased from 5 to 10, 20, and 30 h (Figure S2). With the prolonging of reaction time, the structure transformed from solid sphere to hollow sphere accompanied with the growth of nanosheets upon the microspheres’ surface. In addition, the amounts of PEG-400 and HNO3 can influence the morphologies of the products. The presence of PEG−400 is more likely to promote the formation of nanosheets on the surface of microspheres (Figure S3), while the HNO3 is more influential for the formation of ball-like structure. In the absence of HNO3, only irregular shaped products were obtained (Figure S4A, B). In addition, the self-assembled solid microspheres composed of nanosheets were formed in the absence of HNO3 and PEG-400 (Figure S4C, D). When the amount of HNO3 was increased to 3.0 mL, triple−shelled uniform microspheres without apparent nanosheets covering around the surface were obtained (Figure S6C1, C2). However, when HNO3 was replaced by other kinds of strong acid (such as HCl and H2SO4) and kept the same hydrogen ion concentration, the morphologies of the products showed different shapes and varied sizes (Figure S5), revealing that NO3− ion played an important role for formation of these special morphologies rather than H+. The effect of concentrations of NH4VO3 for the morphologies of products was judged for comparison (Figure S7). The relatively high concentration of HNO3 compared to NH4VO3 will really have significant effect on the generation of ball-like structure. On the contrary, the flower−like microspheres will form under the assistant of the PEG-400. Besides, the reaction temperature during the solvothermal process can influence the morphologies of products. Along with the temperature's enhancement, morphologies can change from nanorods to microspheres, while large messy nanoplates or irregular aggregated particals will be obtained at a higher temperature under the different amount of HNO3 (Figure S8). The time dependent comparison experiments were also carried out to study the formation processes of multi−shelled structures when the addition amount of HNO3 is 3 mL (Figure. S6). During the reaction process, the average diameter of outer shell (~ 600 nm) has no significant difference. The interior structure transformed from solid spheres to multi−shell hollow structure combined with continuous growth of 7



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interior spheres. To sum up, the evolution process of the V2O5 related precursors under different reactive conditions are illustrated in Table S1.

Scheme 1. Schematic illustration of the formation process of flower−like structure (Route A) and multi−shelled microspheres (Route B).

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Figure 2. XRD pattern (A), TEM images (B) and FESEM images (C, D) of the obtained triple−shelled V2O5 microspheres. Scheme 1 displays the schematic illustration of the formations process of flower−like microspheres (Route A) and multi–shelled microspheres (Route B). When the concentration of HNO3 is lower, at the initial reaction stage, the amount of HNO3 is sufficient and the solid microspheres are formed at first. As the reaction progresses, part of the HNO3 may be consumed and nanosheets gradually form under the assistance of PEG-400, which wrap around the solid microspheres to decrease the surface energy (Route A, Stage I). The shape evolution from the solid structures to the interior hollow microspheres may be attributed to the Ostwald−ripening process (for details, see the Supporting Information), which is generally associated with the growth of exterior shell by consumption of the interior material and the subsequent re−crystallization process (Route A, Stage II).

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With extended solvothermal reaction, the nanosheets surrounded the

surface of outer shell grow adequately (Route A, Stage III). However, when the amount of HNO3 is adequate, it will restrain the formation of nanosheets. On the contrary, the repetitive Ostwald−ripening happens on the pre−generated solid interior cores, leading to the formation of 9



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multi−shelled structures (Route B, Stage II and III). Therefore, the hollow interior structures could be effectively customized by simply limiting the reactant concentration and the reaction duration. Furthermore, the triple−shelled V2O5 hollow microspheres (defined as “VHS”) were also obtained by annealing the triple−shelled precursor in air at 350 °C. The XRD diffraction pattern peaks of VHS (Figure 2A) can be also perfectly indexed to the orthorhombic phase V2O5 (JCPDS card No. 41−1426). The morphology and microstructure of VHS were further investigated by FESEM and TEM (Figure 2B). The images confirm that the multi−shelled structure is well preferably maintained after calcination, indicating the structural stability feature. In Figure 2C and D, it can be clearly observed the high uniform of VHS in size and no visible shrinkage or structural deformation after thermal treatment.

Figure 3. Electrochemical characterizations of the hierarchical V2O5 hollow superstructures (HVHS), triple−shelled V2O5 hollow microspheres (VHS) and commercial V2O5. (A) CV curve of HVHS at 2.5−4 V at a scan rate of 0.1 mV s−1. (B) Charge−discharge curves of HVHS at a current 10


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density of 1 C (147 mA g−1). (C) Rate performance of HVHS, VHS and commercial V2O5 at 2.5−4 V. (D) Cycling performance of HVHS, VHS and commercial V2O5 in 2.5−4 V at 10 C. (E) Schematic of HVHS during lithiation and delithiation. (F) Cycling performance and coulombic efficiency of HVHS at 20 C. The electrochemical properties of HVHS are first examined by cyclic voltammetry. Figure 3A shows the initial four consecutive cyclic voltammograms (CVs) of HVHS under the scan rate of 0.1 mV s−1. Two cathodic peaks at 3.38 and 3.18 V (vs. Li/Li+) during the cathodic scan indicate the phases evolutions from α−V2O5 to ε−Li0.5V2O5, and then to δ−LiV2O5. Two anodic peaks at 3.25 and 3.43 V are associated with the Li+ de−intercalation process. It is noticeable that the two potential separations between the anodic and cathodic peaks are only 0.07 and 0.05 V, which are apparently smaller than the reported works, indicating the small polarization and good reversibility of HVHS. Figure 3B displays the discharge and charge curves of 1st, 2nd and 10th for HVHS cathodes at a current density of 1C (1C=147 mA g−1). Two plateaus observed related to the Li+ insertion/extraction process are coincident well with the CV results. The discharge specific capacity can achieve 146.8 mA h g−1 for the first cycle, which is close to the theoretical capacity of 147 mA h g−1. The discharge curve after tenth cycle is nearly coincident with the first one, confirming the good reversibility of HVHS. The rate performances (Figure 3C) are compared among the HVHS, VHS and commercial V2O5. The specific discharge capacity of HVHS are 146.8, 141.7, 132.4, 122.4 and 107.2 mA h g−1 at 1 C, 2 C, 5 C, 10 C and 20 C, respectively. As the current density was reset back to 1 C, 95% of the original capacity can be recovered. Apparently, HVHS display large specific capacities compared to VHS and commercial V2O5. Furthermore, HVHS show superior cycling performance. When the current density is 10 C, HVHS can achieve 83 % capacity retention with the specific capacity decreasing from 124.5 to 103.5 mA h g−1 after 1000 cycles, which is much higher than VHS (68.6 %) and commercial V2O5 (70.5 %), respectively (Figure 3D). Even at a high current density of 20 C, the specific capacity of HVHS still keep as high as 73.5 mA h g−1 after 3000 cycles (Figure 3F). In addition, HVHS exhibit high reversible capacity and good stability at 1 C after the rate test (124 mA h g−1 after 350 cycles, Figure S10). The performance of HVHS is apparently enhanced compared to the previously reports (see Table S2 in the supporting information). Moreover, the morphology of HVHS after discharge/charge are also presented in Figure S11 to confirm their stability further. It can be seen that the hollow structure can be well maintained after 60 cycles. 11



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Even after 500 cycles, the hollow structures still keep intact. The electrochemical performances of HVHS are also measured at the range of 2.0 to 4.0 V (Figure S12). From the CV curve (Figure S12A), the third cathodic peak at 2.21 V is associated with the intercalation reaction of the second Li+, causing the generation of γ-Li2V2O5, while the first anodic peak at 2.59 V denotes the reverse transformation reaction. The long-term cycling performance at 1 A g−1 is also provided in Figure S12B. The capacity of HVHS decreases slightly in the first few cycles, which is caused by the escape of loose active materials from the current collector. After the first 10 cycles (149 mA h g−1 at the 10th cycle), a very stable discharge capacity with less than 0.07% decay per cycle is delivered (130 mA h g−1 after 200 cycles). Its good performance can be attributed to the special hierarchical hollow structure, which can buffer the self−expansion and self−shrinkage during the charge and discharge processes (Figure 3E). To understand why HVHS display such excellent performance, additional measurements have been performed for comparison. First, based on the nitrogen adsorption–desorption measurement results (Figure S13A and B), the Brunauer–Emmett–Teller (BET) surface area and pore volume of HVHS is 20.6 m2 g−1 and 0.15 cm3 g−1, respectively, while those of VHS are only 10.2 m2 g−1 and 0.05 cm3 g−1, respectively. Therefore, HVHS display a higher surface area than VHS, so that HVHS show the larger contact area between the electrode material and electrolyte. Meanwhile, HVHS exhibit a special structure composed of many two-dimensional nanoplates, which ensure much shorter lithium diffusion distance during lithiation and delithiation process. Second, the overpotential, noted as ΔV(Q/2), is measured based on the difference of charge/discharge potential at the condition of half reversible capacity (Figure S13C).

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The overpotential of HVHS is much

lower than VHS (121 mV) and commercial V2O5 (158 mV), which is only 70 mV. Third, from the electrochemical impedance spectra (EIS) after 10 cycles (Figure S13D), the charge transfer resistance (Rct) of HVHS demonstrates a much lower resistance (81 Ω) than VHF and commercial V2O5, indicating a significantly reduced charge−transfer resistance of HVHS at the electrode/electrolyte interface. It can be understanding from above results that HVHS exhibit excellent reversibility and outstanding cycling performance, suggesting the fast kinetics for lithium ion intercalation−deintercalation during electrochemical reaction process.

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Figure 4. (A) CV curves of HVHS, commercial Li4Ti5O12 and HVHS//Li4Ti5O12 full−cell at a scan rate of 0.1 mV s−1. Charge−discharge curves of HVHS, Li4Ti5O12 (B) and HVHS//Li4Ti5O12 full−cell (C) at a current density of 147 mA g−1. (D) Cycling performance and coulombic efficiency of HVHS//Li4Ti5O12 full−cell in 1−2.5 V at the current density of 147 mA g−1. In order to further demonstrate the performance of HVHS in practical cells, a full−cell was assembled by lithiated HVHS as cathode electrode and commercial spinel Li4Ti5O12 as anode electrode. The FESEM and cycling performance of commercial spinel Li4Ti5O12 are shown in Figure S14. The potential window of the full−cell is determined by the CV curves of V2O5 and Li4Ti5O12 in half−cell configurations as shown in Figure 4A. Based on the calculation from CV curves of half-cell, the oxidation peaks of full−cell during charging process will arise in 1.75 and 13



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1.93 V, and the reduction peak will arise in 1.55 and 1.75 V. Therefore, we choose the range of 1.0 to 2.5 V as the full−cell testing potential window. The measured CV curve of full−cell exhibits two pair of peaks and their positions are in accordance with the calculation values. The electrochemical performance of full cell is investigated by the galvanostatic charge/discharge cycles. The first discharge/charge curve at the current density of 147 mA g−1 displays two prominent plateaus (Figure 4C), which is in good agreement with the discharge/charge curve of HVHS in half−cell system and the CV curve measured in full−cell. The initial discharge capacity is about 133.5 mA h g−1. After 14 cycles, the discharge capacity reaches the largest value (139.5 mA h g−1) and still exhibit 105 mA h g−1 after 100 cycles with the capacity retention about 80%. The Coulombic efficiency is >99% except for the first few cycles, indicating its good cycling stability. The full−cell performance is significantly better than the previous reports under the same testing condition, such as the reported discharge capacity of 75 mA h g−1 at 20 mA g−1 23 and 119 mA h g−1 at 20 mA g−1 24. The good performance of full cell confirms the great potential of HVHS as cathode material in lithium storage devices. 4. CONCLUSION In summary, various uniform hollow V2O5 microspheres with complex interior and exterior structure have been successfully prepared through a new facile template−free solvothermal method. The hierarchical V2O5 hollow microspheres exhibit better electrochemical performance than multi−shell hollow V2O5 microspheres, which can deliver a very high capacity of 147 mA h g−1 between 2.5 and 4.0 V at a current density of 1C (147 mA g−1). Even at a high current density of 10 C, a capacity of 103 mA h g−1 can remain reached after 1000 cycles. Furthermore, the V2O5//Li4Ti5O12 full−cell exhibits a high capacity of 139.5 mA h g−1 at a current density of 147 mA g−1 between 1 and 2.5 V and 106 mA h g−1 can be still delivered after 100 cycles. The outstanding performance of full cell indicates the great potential of HVHS as cathode material in lithium ion battery.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The characterization results including：TEM images of the products obtained at different reaction condition; XRD patterns of multi-shelled structure microspheres at different reaction durations; cycling performance of HVHS after rate test; TEM images of the cycled composite; N2 14


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adsorption/desorption isotherm of HVHS and VHS; Electrochemical impedance spectra of V2O5/Li cells at fully charged stage after 10 cycles; CV curve and cycling performance of HVHS in the potential range from 2.0 to 4.0 V; comparisons between the HVHS and previously reported materials.

Acknowledgements Financial support from the National Natural Science Fund of China (Nos. 21201129 and 21471091), the Major Project for Science & Technology of Shanxi Province (Grant Nos. 2013011012-3) and the Fundamental Research Funds of Shandong University (no. 2015JC007) are gratefully acknowledged.

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Figures

Figure 1. XRD pattern and the corresponding crystal structure (A), FESEM images (B, C), TEM images (D, E) of the hierarchical V2O5 hollow superstructures. HRTEM image (F) of part of a HVHS microsphere.

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Scheme 1. Schematic illustration of the formation process of flower−like structure (Route A) and multi−shelled microspheres (Route B).

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Figure 2. XRD pattern (A), TEM images (B) and FESEM images (C, D) of the triple−shelled V2O5 microspheres.

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Figure 3. Electrochemical characterizations of the hierarchical V2O5 hollow superstructures (HVHS), triple−shelled V2O5 hollow microspheres (VHS) and commercial V2O5. (A) CV curve of HVHS at 2.5−4 V with a scan rate of 0.1 mV s−1. (B) Charge−discharge curves of HVHS at current density of 1 C (147 mA g−1). (C) Rate performance of HVHS, VHS and commercial V2O5 at 2.5−4 V. (D) Cycling performance of HVHS, VHS and commercial V2O5 in 2.5−4 V at 10 C. (E) Schematic of HVHS during lithiation and delithiation. (F) Cycling performance and coulombic efficiency of HVHS at 20 C.

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Figure 4. (A) CV curves of HVHS, commercial Li4Ti5O12 and HVHS//Li4Ti5O12 full−cell at a scan rate of 0.1 mV s−1. Charge−discharge curves of HVHS, Li4Ti5O12 (B) and HVHS//Li4Ti5O12 full−cell (C) at a current density of 147 mA g−1. (D) Cycling performance and coulombic efficiency of HVHS//Li4Ti5O12 full−cell in 1−2.5 V at the current density of 147 mA g−1.

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Table of Contents Graphic

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Fabrication of Various V2O5 Hollow Microspheres as Excellent

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