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Reticular V2O5•0.6H2O Xerogel as Cathode for Rechargeable Potassium Ion Batteries Bingbing Tian, Wei Tang, Chenliang Su, and Ying Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15407 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Applied Materials & Interfaces

Reticular V2O5·0.6H2O Xerogel as Cathode for Rechargeable Potassium Ion Batteries

Bingbing Tian,†,§ Wei Tang,& Chenliang Su,†,§ and Ying Li*,†

†

International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology (ICL-2D MOST), Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China §

Department of Chemistry, Centre for Advanced 2D Materials (CA2DM) and

Graphene Research Centre, National University of Singapore, 3 Science Drive 3, Singapore, Singapore 117543 &

Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore

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Abstract: Potassium ion batteries (KIBs), due to its low price, may exhibit advantages over lithium ion batteries (LIBs) as potential candidates for large-scale energy storage systems. However, due to the large ionic radius of K-ions, it is challenging to find a suitable intercalation host for KIBs then the rechargeable KIBs electrode materials is still largely unexplored. In this work, a reticular formation V2O5·0.6H2O xerogel was synthesized via a hydrothermal process as cathode materials for rechargeable KIBs. Compare with the orthorhombic crystalline V2O5, the hydrated vanadium pentoxide (V2O5·0.6H2O) exhibits an ability to accommodate larger alkali metal ions of K+ due to enlarged layer space by hosting structural H2O molecules in the interlayer. With adopting of H2O into the V2O5 layers, its potassium electrochemical activity is significantly improved. It exhibits an initial discharge capacity of ~224.4 mA h g-1 and a discharge capacity of ~103.5 mA h g-1 even after 500 discharge/charge cycles at a current density of 50 mA g-1, which is much higher than that of V2O5 electrode without structural water. Meanwhile, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) combining with Energy Dispersive Spectroscopy (EDS) techniques are carried out to investigate the potassiation/depotassiation process of the V2O5·0.6H2O electrodes, which confirmed the potassium intercalation storage mechanisms of this hydrated material. The results demonstrate that the interlayer spacing enlarged V2O5·0.6H2O is a promising cathode candidate for KIBs. Keywords: hydrated vanadium pentoxide, xerogel, interlayer spacing, ionic radius, potassium ion batteries, cathode materials

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1.

INTRODUCTION Recently, increasing demand for large power supplies of the electric vehicles and

distributed power storage caused rapidly increasing demand for LIBs.[1-9] However, the high cost and scarcity of lithium resources (0.0017 wt% in Earth's Crust) cause a concern about the heavy reliance on LIBs.[8,10] It calls for alternative energy storage devices that are based on earth abundant metal ion batteries, among which, sodium ion batteries (NIBs) and potassium ion batteries (KIBs) attract increasing attentions due to the abundance of sodium and potassium (2.3 wt% and 1.5 wt%, respectively, in Earth’s crust),[11] and their similar electrochemistry as LIBs.[12-26] Significant progress has been made recently on new-type electrode materials for NIBs.[27-38] However, studies on rechargeable KIBs electrode materials, especially intercalation-type cathodes in nonaqueous electrolytes, are still largely unexplored except Prussian blue (PB) cathodes.[12,20,39,40] Recently, Ji’s group firstly reported electrochemical potassium insertion in graphite as anode in a non-aqueous electrolyte, which opens up a new paradigm toward rechargeable KIBs.[10] Xue et al.[20] proposed a low-cost and high-energy cyanoperovskite KxMnFe(CN)6 (0≤x≤2) as potassium cathode in KIBs, which delivers an attractive specific capacity of 142 mAhg-1. However, the intercalation-type metal oxide cathodes (i.e. MnO2, TiO2, V2O5), which are often used for LIBs or NIBs,[41-47] still cannot be achieved in KIBs due to low electrochemical activity associated with large size of K-ions. Vanadium pentoxide (V2O5) has advantages of abundance, low cost and better safety, which make it a promising cathode candidate for rechargeable LIBs and NIBs.[46-53] Recently, Wang et al.[54] prepared V2O5·nH2O nanoflakes as a highly reversible cathode material, which shows excellent electrochemical performance with either monovalent or multivalent cation (Li+, Na+ and Al3+) intercalation. Canepa et al.[55] systematically reviewed the analysis of the increasing volume of multivalent cathode and anode research and the relevant mechanisms of multivalent ions (such as Mg2+, Ca2+, Zn2+, etc.) insertion and migration. According to our knowledge, despite a lot of efforts made on development of V2O5, very little is known about the attempt to

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use V2O5 as cathode for KIBs. Actually, due to the larger ionic size of K+, it cannot be electrochemically intercalated into the layers of crystalline V2O5 with limited layer distance. Confinement of coordinative species into interlayers of layered materials has been investigated to extend the layer space.[56-58] As also explained in ref 55, although incorporating shielding water molecules in the structure is a commonly adopted strategy to enhance ion mobility in V2O5·nH2O, its specific reaction mechanisms are not clear. Ceder et. al.[59] has theoretically predicted that structural water resident in V2O5 layers can effectively increase the electrochemical activity of Mg2+ intercalation. Hydrated vanadium pentoxide (V2O5·0.6H2O) nanoribbons has been investigated as a reversible K+ host in aqueous electrolyte, which can deliver a good asymmetric capacitive behavior.[60,61] However, vanadium(V) oxide forms soluble species such as H2VO4- or HVO42- in the aqueous solution. The intermediate species, i.e., vanadium(III) oxide, generated during the charge/discharge process also form soluble vanadate ions in H2O solution.[62,63] To our best knowledge, no investigation is reported on successful application of vanadium pentoxide to potassium storage in organic electrolyte systems, and its detailed reaction mechanisms are still unexplored. Herein, a V2O5·0.6H2O xerogel was prepared via a hydrothermal synthesis process. With confined residence of water molecules in V2O5 layers, the layer space of V2O5·0.6H2O xerogel can be remarkable expanded, enabling a highly reversible intercalation/extraction of K+ ions. The V2O5·0.6H2O xerogel cathode delivers an excellent potassium storage ability with an initial discharge specific capacity of 224.4 mA h g-1 at the current density of 50 mA g-1, which is much higher than that of orthorhombic crystalline V2O5 electrode. Moreover, V2O5·0.6H2O xerogel cathode exhibits a good long-term cycling performance used in KIBs, whose structural stability was investigated by ex-situ XRD technique.

2.

EXPERIMENTAL SECTION Materials synthesis: The method used for hydrated vanadium pentoxide

(V2O5·0.6H2O) xerogel preparation was a hydrothermal reaction according to previous work.[64-67] In brief, commercial vanadium(V) oxide powder (0.364 g, 4


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99.99% trace metals basis, Sigma-Aldrich) was placed in 30 mL distilled H2O under vigorous magnetic stirring for 30 min at room temperature, then 6 mL H2O2 (Sigma-Aldrich, 30 wt%) was added to the mixed solution and kept continuously stirred for another 60 min to form an orange transparent solution. Then the resultant solution was transferred into a 100 mL autoclave and stored in a blast air oven at 210 o

C for 5 days for hydrothermal synthesis. After naturally cooled to room temperature,

the jelly-like yellow brown product was washed with ethanol and distilled H2O, respectively, for three times. Finally, the resulting product was dried at 80 oC in a vacuum oven overnight to get the fuscous V2O5·0.6H2O xerogel. For comparison, the crystalline α-V2O5 was obtained after annealing the V2O5·0.6H2O xerogel at 400 oC in a furnace in air for 6 hours, with a heating rate of 2 oC min-1. Characterization: The as-prepared V2O5·0.6H2O xerogel was characterized by powder X-ray diffraction (XRD, BRUKER D8 diffractometer with a Cu anticathode), Fourier transform infrared spectroscopy (FTIR, transmission mode, OPUS/IR PS15 spectrometer, Bruker), Thermal gravimetric analysis (TGA, Discovery TGA Thermo-Gravimetric Analyzer, TA Instruments), X-ray photoelectron spectroscopy (XPS, Omicron EAC2000-125 analyzer, 10−9 Torr, Al Kα monochromatized radiation, hν=1486.6 eV), Field-Emission Scanning Electron Microscope (FE-SEM) and Energy Dispersive Spectroscopy (EDS) analysis on the SEM (JEOL JSM-6701F), and Transmission Electron Microscope (TEM, JEOL JEM-3011). For analyses in electrochemical process, the V2O5·0.6H2O xerogel-based electrodes were submitted galvanostatic charge/discharge. The cycling was stopped at selected stages and these cells were disassembled in Ar filled glove box and the samples were rinsed with dimethyl carbonate (DMC, Sigma-Aldrich) then dried with Ar flow in the glove box, before carrying out XRD, XPS and EDS analyses. Electrochemical measurements: CR2016-type coin cells were assembled to analyze the electrochemical behaviors. The prepared electrode material (V2O5·0.6H2O xerogel or crystalline V2O5) were mixed with super P as a conductive additive and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 7:2:1 in N-Methyl-2-pyrrolidone (NMP), and stirred overnight to obtain a slurry. The slurry 5



was coated, then pressed onto an aluminum foil substrate as electrode. The as prepared electrode was dried in a vacuum oven at 80 °C for more than 24 h. Then the electrode was cut into circular discs with a diameter of 14 mm (with a loading mass of ~2.2 mg/cm2). Potassium metal discs were used as anode electrodes. The electrolyte was 0.8 M KPF6 (99.5% trace metals basis, Sigma-Aldrich) in a 1:1 (volume ratio) mixture of ethylene carbonate (EC, 98%, Sigma-Aldrich) and diethyl carbonate (DEC, anhydrous, ≥99%, Sigma-Aldrich). The separator was Whatman Glass Microfibre Filter. The cells were assembled in an argon gas filled glovebox, and galvanostatic charge/discharge tests were performed in the voltage range of 1.5 to 4.0 V (vs K/K+) at the current densities of 50 mA g-1 on a LANHE (Wuhan, China) battery tester. An Ivium-n-Stat multi-channel electrochemical analyser was used to perform the cyclic voltammetry (CV). Cyclic voltammograms (CVs) were recorded at a scanning rate of 0.1 mV s−1 in the potential range of 1.5-4.0 V, starting from open circuit potential into the cathodic direction.

3.

RESULTS AND DISCUSSION The structure and composition of the as-prepared products were evaluated by

XRD, FTIR, TGA and XPS (Figure 1). The powder XRD patterns of the hydrothermal reaction product present a high intensity peak (001) and several low intensity peaks, corresponding to low degree crystallinity of the hydrated vanadium pentoxide xerogel. The reflection peak of (001), which is related with the distance of vanadium pentoxide layers, appears on a lower angle (2 Theta =6.665o) due to the expansion of the lattice along the c-axis (Figure S1, Supporting Information).[47,68] After calcined at 400 oC for 6 hours, the product shows a good crystallinity of orthogonal symmetric V2O5 (JCPDS card no.: 41-1426), and the (001) reflection associated to the shrinkage of interlayer spacing shift to 2 Theta = 20.275o. The spacing between adjacent layers of the two products is 13.26 and 4.38 Å, respectively. The extended interlayer spacing of hydrated vanadium pentoxide xerogel is associated with H2O molecules trapped in the layers, which are not entirely removed in the drying process. The existence of H2O in hydrated vanadium pentoxide xerogel is also 6


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confirmed by the FTIR spectra (Figure 1b). The strong IR absorbance signals of hydrated vanadium pentoxide xerogel located at around 3350 and 1610 cm-1 are assigned to O-H stretching and bending modes, respectively, of surface and/or intercalated structural water. Predictably such space enlargement by intercalation of structural H2O may be available for the insertion of large cations. Thermal gravimetric analysis (TGA, Figure 1c) shows that the hydrated vanadium pentoxide xerogel exhibits obvious weight loss (~16.5%) compared with the calcined samples. The weight loss of 8.6 wt% (below 100 °C) is attributed to the loss of physisorbed water. The molar ratio of the structural water to V2O5 was estimated to be ~0.6 by the weight loss above 150 oC (~5.0%).[56,57] Herein, the hydrated vanadium pentoxide abbreviated as V2O5·0.6H2O. To further identify the chemical compositions of the hydrated vanadium pentoxides xerogel (V2O5·0.6H2O), XPS is performed on V2O5·0.6H2O and crystalline V2O5. The XPS survey spectra (Figure S2) collected from these two vanadium oxide products are confirmed by the presence of V and O, without any other impurities except C 1s signals indicative of carbonaceous contaminations that routinely observed on metal oxide surfaces.[69-72] The V 2p3/2 core level peaks of V2O5·0.6H2O and V2O5 are decomposed into only one major peak located at 517.3 eV assigned to V(V) oxide. This demonstrate that the structure water is mainly intercalated in the layers of V2O5, implementing trifling impact on valence state of V. Detailed crystal structures of the as-prepared V2O5·0.6H2O xerogel are also characterized by FESEM, TEM and selected area electron diffraction (SAED) analyses (Figure 2 and Figure S3). FESEM images of the V2O5·0.6H2O xerogel shown in Figure 2a and b exhibit the typical reticular structure morphology. The reticular structure is composed of interconnected network of ribbons with plenty of interspace therein. As shown in the TEM image (Figure 2c), the sample surface is covered with bull-like bubbles, which are caused by the erosion effects in the hydrothermal process. SAED pattern (Figure 2c, inset) revealed that polycrystalline structure of V2O5·0.6H2O is identical even with structural H2O intercalated into layers. As shown in the lattice resolution TEM image (Figure 2d), the (001) crystal plane can 7



be identified (~11.5 Å), which is approximately in agreement with the value calculated from XRD. Additionally, nitrogen adsorption-desorption of V2O5·0.6H2O xerogel was further measured to indicated a BET surface area of 5.617 m2 g-1. To explore the feasibility of using V2O5·0.6H2O xerogel as cathode electrodes in KIBs, CV and galvanostatic charge/discharge cycling in the potential range of 1.5 4.0 V were carried out and presented in Figure 3. The CV curves corresponding to the first potassiation/depotassiation cycle of the V2O5·0.6H2O electrode display a cathodic peak at 3.31 V, indicating intercalation of K+ ions into the interlayers of the V2O5·0.6H2O. Then in the anodic process, a peak located at 3.53 V indicates K+ ions extraction from the hydrated vanadium oxide layers. This pair of pronounced redox peaks is similar to that of transformations from V2O5 to Li0.5V2O5 in LIBs,[73,74] indicating a similar reversible reaction process in KIBs. In contrary, the CV curves of V2O5 electrode show no obvious insertion/extraction of K+ ions. Consequently, V2O5 electrode can only deliver an initial discharge capacity of 44.3 mA h g-1 when discharged at a current rate of 50 mA g-1. On the other hand, the V2O5·0.6H2O xerogel with enlarged layer space can realize a highly K+ electrochemical activity by delivering a discharge capacity up to 224.4 mA h g-1 at the same testing condition. The cycling performances and evolution of coulombic efficiency (a ratio of discharge capacity/charge capacity is defined in this work) up to 500 cycles of V2O5·0.6H2O electrode are shown in Figure 3c. While a high capacity retention ratio of 78.3% is observed for V2O5·0.6H2O xerogel after 100 cycles, the V2O5 electrode showed an inferior cycling performance and an ultra-low discharge capacity (as low as 8.6 mA h g-1 at the 100th cycle). Moreover, the hydrated vanadium pentoxide electrode delivers a capacity retention ratio of 46.1%, even after a long cycling of 500 cycles. It is also interesting to note that the discharge capacity of the V2O5·0.6H2O electrode decreased in the first 50 cycles then slowly increased with further cycling. The capacity fade in the early stage of cycles may be caused by the presence or co-intercalation of water in the electrode, which may induce an activating process of the electrochemical reaction. Wangoh et al.’s work[75] on Li intercalated V2O5 aerogel cathodes indicates that the tightly bound water that is necessary for maintaining the aerogel structure but is also 8


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inherently responsible for capacity fading. The hydroxyl species, i.e. LiOH, is retained after the first cycle, and its accumulation upon cycling is considered responsible for the gradual capacity fading. The capacity increase in the late stage is most likely caused by pulverization of the cathode electrode with repeating insertion/extraction of K+ in the V2O5·0.6H2O interlayers, leading to increase of specific surface area, enabling shorter diffusion lengths and improving reaction kinetics. Smaller nanoparticles will better accommodate strains induced by potassium insertion, also enhance the supercapacitive properties of electrodes.[76] Recently, Wu’s group[77] found the special role of electrolyte type in potassium secondary batteries systems.

It

is

reported

that

a

potassium

bis(fluoroslufonyl)imide

(KFSI)-dimethoxyethane (DME) electrolyte forms a uniform SEI on the surface of potassium enabling reversible potassium plating and stripping electrochemistry with high efficiency (∼99%). The KFSI-DME electrolyte shows excellent electrochemical stability up to 5 V (vs K/K+) which enables good compatibility with high-voltage cathodes. This presented us a clue to develop compatible electrolyte for more stable KIBs systems, which may also enables the hydrated vanadium pentoxide electrodes for higher coulombic efficiency and better cycling performance in KIBs. The rate performance of V2O5·0.6H2O electrode at different galvanostatic current densities is also characterized, as shown in Figure 3d. The capacity fades quickly when increase the current densities (10 to 50 mA g-1) in the first 30 cycles. This is related to the inferior kinetics caused by large size of K-ions. However, at the high current rate of 100 to 1000 mA/g, it shows a good cycling stability but quite low specific capacity, which may be related to the surface charge storage (pseudocapacitive) properties of the electrode.[76] Impressively, its capacity increased to about 200 mA h g-1 when cycled at 10 mA g-1 even after 70 cycles, indicating a good reversibility of V2O5·0.6H2O electrode in KIBs. In order to investigate the reaction mechanism of potassium ion insertion into V2O5·0.6H2O electrode, an ex-situ XRD measurement on the electrodes at different electrochemical discharge/charge states was performed to monitor the structural change of V2O5·0.6H2O (Figure 4). When discharged to 2.5 V or 1.5 V, the (001) 9



diffraction peak shifts toward higher angle, indicating shrinkage of the interlayer spacing. This is inconsistent with the observed phenomenon of insertion cations into layered materials, which is due to K-ions intercalation enhanced the coordination reaction of the stacked layer then made the interlayer spacing smaller.[56,57] No new phase was detected at fully discharged state, as observed previously after intercalation of Li and Na ions.[56] In the following charging process (recharged to 2.5 or 4.0 V), the extraction of K ions restored the layer structure of hydrated vanadium oxide. Moreover, the (001) diffraction peak of V2O5·0.6H2O remain intact even after 10 full cycles of potassiation/depotassiation (charged to 4.0 V), indicating a good reversibility. To shed more light on the reaction mechanism and surface terminations, the electrodes after electrochemical treated were analysed by XPS. The XPS K2p & C1s and O1s & V2p core level spectra for the pristine V2O5·0.6H2O electrode and disassembled V2O5·0.6H2O electrodes of discharged to 1.5 V and charged to 4.0 V are shown in Figure 5. The binding energies are calibrated by setting the C1s hydrocarbon (-CH2-CH2-) peak at 284.8 eV. The C1s signals of the pristine sample (Figure 5a) originate from the conductive carbon (Super P) and contaminants routinely present on the extreme surface when exposed to the ambient air.[69-72] Four-component peak at 284.8, 286.2, 288.3 and 290.5 eV are assigned to −CH2−CH2−, C−O, O=C−O or O=C−N bonds and C-F bonds, respectively. The K2p signal is not observed on the pristine electrode surface. After discharged to 1.5 V, the K2p & C1s core level spectra obtained on the electrode shows three-component peak with contributions at 284.8 eV, 286.1 eV and 287.8 eV assigned to −CH2−CH2−, C−O, and O=C bonds, respectively, and the spin-orbit doublet, K2p3/2 at 294.2 eV and K2p1/2 at 296.9 eV, which can be assigned to intercalated K+ ions in V2O5·0.6H2O electrode (K2V2O5·xH2O). The electrode surface after one full cycle (recharged to 4.0 V) shows no change in the carbon spectra. However, the reduced intensity of K2p spectra indicates partially extract of K ions from V2O5·0.6H2O electrode. The O1s signal of the pristine electrode is also decomposed into four components at 529.6, 530.8, 532.4 and 534.0 eV (Figure 5b). The major component on the pristine electrode 10


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at 529.6 eV is assigned to oxygen anions in vanadium oxide matrix. Then the minor component at 530.8 and 532.4 eV could be ascribed to organic contaminations (C−O and O=C−O bonds) on the surface. The component at 534.0 eV, clearly detected on the electrode surface (relative intensity: ~0.064), suggests presence of the adsorbed water molecules. As reported by Charles and co-workers,[78] the structural water within the interlayers of potassium-intercalated disordered vanadium oxide (KVO) not only stabilizes the structure, but also enhances the electrochemical performance in aqueous electrochemical energy storage devices. However, the increased peak intensity assigned to hydroxyl groups during electrochemical potassium insertion and extraction may indicate the formation of electrochemically inactive KOH on the electrode surfaces.[75] The XPS O1s core level spectra maintain a stable chemical composition during the following potassiation/depotassiation process excepted slightly different in relative intensity. The V2p signal presents the 3/2-1/2 spin orbit doublet. With potassiation (discharged), the V2p3/2 peak shifts to a lower binding energy (517.1 eV to 516.2 eV), corresponding to a transition process of V5+ to V4+ along with intercalation of K+ ions. This is in consistent with the electrochemical intercalation process of V2O5·0.6H2O to K2V2O5·xH2O, also in agreement with the previously reported XPS results during lithiation/delithiation.[75] Moreover, the V2p3/2 peak shifts back to 516.9 eV with the further depotassiation (recharged) process, indicating reversible K+ ions insertion/extraction during discharge/charge. The similar reversibility was also confirmed by EDS analysis on the SEM for the pristine, discharged and recharged electrodes (Figure 6). Mg is coordinated by two lattice oxygen and four oxygen from cointercalated H2O, as theoretically predicted by Ceder et. al.[59]. Probably a similar reaction mechanism is taking place when K+ intercalation into the hydrated vanadium pentoxide. In conclusion, it should be emphasized that the reversible insertion/extraction of K+ ions with vanadium pentoxide is realized by a hydration process (V2O5·0.6H2O, interlayer spacing expanded), as confirmed by XRD, XPS and EDS analyses. The structural water effect not only maintains the xerogel structure of the hydrated vanadium pentoxide during electrochemical process, but also enhances its electrochemical performance in KIBs. 11



4.

CONCLUSIONS In summary, interlayer spacing expanded vanadium pentoxide (V2O5·0.6H2O)

xerogel is synthesized by a hydrothermal method. Due to its enlarged interlayer spacing with structural water molecules resident in the layers, the electrochemical activity of V2O5·0.6H2O cathode used for KIBs can be significantly improved, exhibiting a discharge capacity of 224.4 mA h g-1 at the current density of 50 mA g-1. Moreover, the V2O5·0.6H2O cathode maintains a good stability during long term cycling up to 500 cycles as also illustrated by ex-situ XRD study. This work demonstrates that V2O5·0.6H2O cathode with expanded layer space is a promising candidate for KIBs, which opens a door of constructing high capacity potassium storage materials by adjusting the interlayer spacing.

ASSOCIATED CONTENT

Supporting Information

Supporting Information available: Structural models, Detail experimental section, XPS, Low-magnification TEM image, discharge-charge curves, CV curves, cycling performance.

AUTHOR INFORMATION

*Corresponding author: Ying Li, Tel: (86) 18617186368, Fax: (86)-755-26536206 E-mail: [email protected]

Notes

The authors declare no any competing financial interest.

ACKNOWLEDGMENTS

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This work is supported by National Natural Science Foundation of China (Grant No. 21506126), the Science and Technology Planning Project of Guangdong

Province

(Grant

No.

2016B050501005),

the

Educational

Commission of Guangdong Province (Grant No. 2016KCXTD006), Shenzhen Science

and

Technology

Research

Foundation

(Grant

No.

JCYJ20150324141711645), the Natural Science Foundation of Shenzhen University (Grant No. 85302-000130, 827-000113 and 827-000059) and Shenzhen Peacock Plan (Grant No. KQTD2016053112042971).

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Figure 1. Comparison of (a) XRD patterns, (b) FTIR spectra, (c) TGA profiles and (d) XP V 2p spectra of the V2O5·0.6H2O xerogel and crystalline V2O5.

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Figure 2. (a) Low and (b) high magnification SEM images, (c) high resolution TEM image and SAED pattern (c, inset), and (d) lattice resolution TEM image of the as-prepared V2O5·0.6H2O xerogel.

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Figure 3. Comparison of (a) the first CV curves at a scan rate of 0.1 mV s-1, (b) the initial galvanostatic discharge and charge cycles and (c) cycling performances of the V2O5·0.6H2O and V2O5 electrodes at a current density of 50 mA g-1 in the potential range of 1.5 - 4.0 V, (c) also presents the Coulombic efficiency of V2O5·0.6H2O electrode at the current density of 50 mA g-1 during 500 cycles, (d) rate performance of V2O5·0.6H2O electrode at different galvanostatic current densities.

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Figure 4. Ex-situ XRD patterns of the pristine V2O5·0.6H2O electrode, V2O5·0.6H2O electrodes discharged to 2.5 and 1.5 V, recharged to 2.5 and 4.0 V and after 10 full discharge/charge cycles.

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Figure 5. XPS (a) K2p & C1s, (b) O1s & V2p core level spectra for the pristine V2O5·0.6H2O electrode and disassembled V2O5·0.6H2O electrodes of discharged to 1.5 V and recharged to 4.0 V.

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Figure 6. SEM images of the V2O5·0.6H2O electrodes: (a) pristine, (b) discharged to 1.5 V, and (c) recharged to 4.0 V. EDS patterns are presented in the images (insets).

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Reticular V2O5Â·0.6H ... - ACS Publications

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