Investigation of the Na Intercalation Mechanism into Nanosized V2O5

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Investigation of the Na Intercalation Mechanism into NanoSized V2O5/C Composite Cathode Material for Na-Ion Batteries GHULAM ALI, Ji–Hoon Lee, Si Hyoung Oh, Byung Won Cho, Kyung-Wan Nam, and Kyung Yoon Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11954 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016

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ACS Applied Materials & Interfaces

Investigation of the Na Intercalation Mechanism into Nano-Sized V2O5/C Composite Cathode Material for Na-Ion Batteries Ghulam Alia,b, Ji–Hoon Leea , Si Hyoung Oha,b, Byung Won Choa, Kyung-Wan Namc, and Kyung Yoon Chunga,b* a

Center for Energy Convergence Research, Korea Institute of Science and Technology,

Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea b

Korea University of Science and Technology, 217 Gajeong-ro Yuseong-gu, Daejeon 305-333,

Republic of Korea c

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715,

Republic of Korea. *Corresponding author: e-mail: [email protected]

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Abstract There is a significant interest to develop high performance and cost effective electrode materials for next-generation sodium ion batteries. Herein, we report a facile synthesis method for nanosized V2O5/C composite cathodes and their electrochemical performance as well as energy storage mechanism. The composite exhibits a discharge capacity of 255 mAh g-1 at a current density of 0.05 C which surpasses that of previously-reported layered oxide materials. Furthermore, the electrode shows good rate capability; discharge capacity of 160 mAh g-1 at a current density of 1 C. The reaction mechanism of V2O5 upon sodium insertion/extraction is investigated using ex situ X-ray diffraction (XRD) and synchrotron based near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Ex situ XRD result of the fully discharged state reveals the appearance of NaV2O5 as a major phase with minor Na2V2O5 phase. Upon insertion of sodium into the array of parallel ladders of V2O5, it was confirmed that lattice parameter of c is increased by 9.09 %, corresponding to the increase in the unit cell volume of 9.2 %. NEXAFS results suggest that the charge compensation during de/sodiation process accompanied by the reversible changes in the oxidation state of vanadium (V4+ ↔ V5+).

KEYWORDS: Na-ion batteries, nano-sized V2O5, NaV2O5, X-ray diffraction, near edge X-ray absorption fine structure

Introduction Electricity produced by different intermittent renewable power sources such as wind and solar energies often relies on stationary energy storage systems (ESS). Achieving the high efficiency and superior safety is essential for large scale rechargeable batteries.1-2 Lithium-ion batteries (LIBs) are facing the issues of high production cost, limited resources and serious safety 2


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concerns thus not suitable for the use of ESS. In recent years, researchers have been investigating sodium-ion batteries (NIBs) as a potential alternative to the LIBs for the use of large-scale ESS.35

World-wide abundant resources of sodium make NIBs a potentially low cost technology.5-7

Although, sodium is the next smallest and lightest alkali-metal after lithium in periodic table,8 NIBs are still inferior to LIBs in terms of energy and power densities due to larger size and lower negative reduction potential of sodium as compared to its counterpart.9 To solve these issues, electrode material should be designed with large concentration of interstitial site, where the sodium could be reversibly occupied, as well as large open tunnels.10 As one of the strategies, a number of layered cathode materials (NaxMO2; x=0.44‒1, M=3d transition metals) have been investigated for the reversible sodium intercalation. Unfortunately, they delivered limited reversible capacity of ≤160 mAh g-1 and suffered with poor cycle and rate performance because of structural instability.8 Vanadium pentoxide (V2O5) is considered as promising active material due to its unique crystal structure with large interlayer spacing of 4.4 Å.11-12 To date, layered V2O5 has been intensively investigated in LIBs and supercapacitors.13-14 The lithium insertion mechanism of V2O5 is well understood.15 Two lithium ions can be reversibly inserted/extracted from the layered V2O5 (with a composition of Li2V2O5), resulting in a high capacity of 294 mAh g-1.11-12 It is also known that layered V2O5 is electrochemically active when the electrode was applied in NIBs but exhibiting less noticeable performances.16 Layered vanadium oxide xerogel delivers high capacity in NIBs but it shows fast capacity fade due to large lattice breathing upon de/sodiation.17 Despite having a high theoretical capacity, V2O5 is also facing barriers of poor electrical conductivity and low working potential compared to other cathodes in NIBs.15 Furthermore, sodium insertion

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mechanism in layered V2O5 is not still clear so far and urged to be investigated for finding clues for further improvement in performances. The electrochemical properties of the V2O5 can be improved through nanostructuring or/and surface modifications. The approaches to the design of nanostructured cathode materials have been found to enhance the electrochemical activity by increasing the contact area between electrolyte and electrode, resulting in shortened the diffusion length of guest species within the host electrode material.15, 18-19 Carbon is the most commonly used coating material for battery application due to various advantages such as high conductivity, low cost, superior chemical stability and wide electrochemically stable window.20 Carbon is used as conductive additive in the preparation of the electrodes in a large amount which is not so effective as compared to coating where the amount of carbon can be reduced significantly21 The combination of reduced particle size and carbon coating can improve the electrochemical performance of the electrode materials.22 In this report, we designed the novel composite electrodes which consist of V2O5 nanoparticles and carbon and investigated the electrochemical energy storage mechanism of the electrode materials. V2O5/C composite was prepared by sequential methodology; formation of nanometersized V2O5 with an aid of oxalic acid followed by mixing with the acetylene black by ball-mill process. Due to the incorporation of carbon-based material, the charge transfer resistance was significantly reduced compared to the electrode with V2O5 alone. Accordingly, the nano sized V2O5/C composite has shown a superior reversible capacity as well as high rate capability. The electrodes at fully charged-discharged states have been further investigated by ex situ XRD and the results reveal the reversible sodium de/intercalation. Ex situ TEM analysis of the fully 4


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discharged electrode shows both crystalline and amorphous phases of Na2V2O5. In addition, NEXAFS spectroscopy is employed to monitor the valence change of vanadium ions upon Na+ insertion/extraction and it is found that the redox (V4+/V5+) is responsible of the delivered capacity.

Experimental procedure Synthesis of V2O5/C composite Nano-sized V2O5 was prepared as described in an earlier report.11 In a typical synthesis, 1.0 g of micron-sized V2O5 (≥99.6%, Aldrich) was poured into 35 ml of deionized water and magnetically agitated at room temperature. Then, 2.45 g of oxalic acid (98%, Aldrich) was mixed and stirred until the color of solution turned to blue. Oxalic acid acts both as reducing and chelating agent in the reaction. The solution was placed in oven for drying at 80 °C. The dried powder was grounded and heated at 400 °C for 2 h in air. To prepare V2O5/C composite, acetylene black was mixed with V2O5 in a ratio of 1:5 in argon atmosphere, sealed and ball milled (ball/material weight ratio was 20/1) at 300 rpm for 3 h. Characterization V K-edge x-ray absorption near edge structure (XANES) spectroscopy was employed to determine the oxidation state of V during the calcination of as-synthesized samples. The assynthesized and calcinated samples were subjected to XANES analysis along with reference samples with known V3+, V4+ and V5+ oxidation states. The XANES measurements of the powder samples were performed at the 1D KIST-PAL beamline in the 2.5-GeV Pohang Light Source (ring current of 120–160 mA). The V K-edge X-ray absorption spectroscopy (XAS) data was acquired in the transmission mode using ion chamber detectors filled with high-purity 5



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nitrogen. The obtained XANES spectra were calibrated using V foil spectra collected simultaneously during the measurements and normalized to compare the energy shifts. The crystal structure of the V2O5 was characterized using powder XRD in a Rigaku X-ray Diffractometer with Cu-Kα radiations (λ=1.54 Å). The morphologies of the prepared materials were scrutinized by SEM (FE-SEM, NOVA NanoSEM200, FEI) and microstructural analyses were conducted by TEM (Tecnai G2 F20, FEI). Electrochemical tests Galvanostatic charge-discharge tests were performed using a coin cell CR 2032. The electrodes were prepared by mixing V2O5/C and polyvinylidene difluoride (PVDF) in a ratio of 93:7. Nmethyl-2-pyrrolininon (NMP) solvent was then added in an appropriate amount to achieve homogeneous slurry. The slurry was cast on Al foil and dried in oven at 80 °C. After evaporating the NMP, the electrodes were roll pressed and dried in vacuum overnight at 80 °C before making cells. The electrodes of bare V2O5 were also prepared by mixing with carbon and PVDF in a ratio of 68:25:7. 1 M NaClO4 in ethylene carbonate (EC), diethyl carbonate (DEC) and propylene carbonate in an equivalent ratio (1:1:1) was used as electrolyte and sodium metal as counter electrode in Na cell. The active material electrodes with thickness of 25 µm and average mass of 1.5 (±0.3) mg cm-2 were used for coin cells. EIS tests were conducted with the coin cells using a Biologic potentiostat/galvanostat Model VMP3 (BioLab, Inc.). Galvanostatic measurements of cells were conducted using a multichannel battery tester (Maccor 4000) at room temperature. The cell fabrication was done in an argon filled glove box (Mbraun Unilab, Germany) with controlled contents of O2 and H2O that were less than 0.1 ppm.

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Ex situ XRD and NEXAFS measurements The sample preparation of ex situ XRD, TEM and NEXAFS spectroscopic analyses was done by disassembling the cells at fully charged and discharged states in an argon filled glove box. The electrodes were collected and thoroughly washed with dimethyl carbonate (DMC) solution to remove residual salts. Here, it is worth mentioning that the Na cells were disassembled after five galvanostatic cycles for ex situ analysis. For ex situ XRD analysis, the active material was collected by scratching the electrode and then pasted onto the Kapton tape. The ex situ XRD measurements were taken on X-ray diffractometry system (R-AXIS IV++, Rigaku) at Korea Institute of Science and Technology (KIST), with MoKα radiations (wavelength of 0.7107 Å). For the sake of easy comparison, 2θ values of obtained spectra were converted to Cu-Kα radiation (wavelength of 1.54 Å). PDXL (Rigaku) software program was used to determine the different phases at charged and discharged potential states. NEXAFS spectra of V LIII,II edge and O K-edge were taken at 10D KIST bending magnet beamline at Pohang Light Source. All the measurements were performed at room temperature and the spectra were collected in a total electron yield mode under a base pressure of 3×10-10 Torr and 0.01 eV resolution.

Results and discussion To identify the oxidation state of vanadium in the as-synthesized active materials, XANES analysis is employed as shown in the Fig. 1. The XANES spectra of as-prepared and calcined (at 400°C) samples are displayed with those of standard samples of vanadium oxides, e.g., V2O3, V2O4, and V2O5. The XANES spectra show both the pre edge (marked with A in the spectra) and main absorption edge (marked with B) peaks. The weak pre-edge peak A are associated with the 7



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dipole forbidden 1s→3d electronic transition. However this transition becomes partially allowed when the p-d orbital hybridization occurs due to structural distortions in the local symmetry between the vanadium and oxygen coordination. In the V K-edge XANES, the intensity of the pre-edge has pronounced effects due to alteration of chemical coordination and the position of the pre-edge peak is associated with the oxidation state of vanadium. The inset of the Figure 1 shows the coordination of V-O bonds in V2O5 and V2O3 to understand the intensity of the preedge. V2O5 consists of square based pyramid with highly distorted environment and shows the highest pre-edge intensity. V2O3 is octahedrally connected with oxygen atoms and due to its increased local symmetry, the non-dipole transition becomes forbidden resulting in less intense pre edge peak. Already mentioned in experimental section, the size of commercial V2O5 powders was reduced to nanometer-scale with the addition of oxalic acid in aqueous medium. As the reaction proceeds, it is observed that the yellow color of the solution turns to blue implying the reduction of vanadium (as prepared).11 The intensity of the pre-edge of the as-prepared material is between the V4+ and V5+, whereas its energy position is slightly lower than that of V4+. The average oxidation state of vanadium of as prepared is estimated to be +3.86 using linear combination fitting (LCF) with standard samples. This value shows that V2O5 has reacted with oxalic acid to produce VOC2O4.23 After calcinations at 400 °C, the XANES spectrum shows entire edge shifts to higher energy, indicating increase in oxidation state. The increased intensity of the pre-edge also arises from the non-centrosymmetric environment due to the formation of V2O5 phase. However, the intensity of the pre-edge is lower in calcinated sample than the standard V2O5 indicating a decreased local asymmetry around vanadium. A slight difference in pre-edge peak position is presumably caused by the defective energy levels of the calcined

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sample. The average oxidation state of the calcinated sample is V+4.83 as estimated using LCF on main edge. The XRD patterns of nano-sized pristine V2O5 and its composite with carbon are shown in Fig. 2, where all the diffraction peaks can be indexed to orthorhombic V2O5 (ICSD reference code#98008-2152) with a space group of Pmmn. The XRD pattern of pristine material shows sharp diffraction peaks, indicating a well crystallized V2O5. The crystals are preferably grown in [110] direction as can be witnessed from the higher intensity of (110) peak as compared to (001).24 The lattice parameters and the volume of unit cell are listed in Table 1. No extra peak is detected in the calcinated sample whereas carbon composite sample shows broad diffraction peaks. The peaks broadening are related to the addition of carbon and reduction in the size of V2O5 during the ball-milling process, which is supported by considering the full width at half maximum (FWHM) of the highest diffraction peak (110); 0.16° and 0.23° for the pristine and composite materials, respectively. Accordingly, the calculated crystallite sizes of pristine and composite using Scherrer formula are 53 and 37 nm, respectively. An additional peak at 2θ value of 27.69° in the V2O5/C pattern is assigned to carbon. Fig. 3a shows the SEM image of as prepared V2O5 powder where most of the particles are in granular-shape. These granular-shaped particles are about 60 nm in diameter and twice of that in length and scattered in irregular order. The composite sample shows significant agglomeration of the particles with a varying size up to 100 nm as shown in Fig. 3b. This agglomeration presumably occurred due to the effect of ball mill and addition of carbon. Carbon contents reside between the particles and will be helpful for fast electronic transport by reducing the particleparticle interfacial resistance. Furthermore, the dimension of the particles is changed from 9



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granular-shape to nano-sized spheres after the ball milling. The reduction of particle size is already confirmed by broadening of diffraction peaks in XRD pattern of V2O5/C. These nanosized particles provide abundant electron transport sites, resulting in quick movement of electrons. The nano-sized particles also offer larger contact area with electrolyte which shortens the diffusion length and enables fast transfer of Na+ into the active material. Microstructural studies of V2O5/C nanocomposite are performed using TEM and the image is shown in Fig. 3c. The insets marked with number 1 and 2 show selected area electron diffraction (SAED) patterns of interior of V2O5 crystallite and the reflections (130) and (020) are indexed in the selected areas. Thick carbon layers are clearly recognized at the surface of the particle as marked with arrow and the SAED pattern marked with number 3 reveals the amorphous carbon rings. The electrical conductivity of the nano-sized bare V2O5 and V2O5/C are also measured using four-point probe method and the values are calculated to be 2.4 × 10-3 and 3.2 × 102 S/m, respectively. The higher electrical conductivity of the composite material is ascribed to the existence of acetylene black. Nyquist plots were obtained from the bare and the composite electrodes using electrochemical impedance spectroscopy (EIS) as shown in the Fig. 4. Both electrodes were held at open circuit potential during the measurement. The plots of two electrodes which show depressed semicircles in high and intermediate frequency and sloping lines in low frequency regions. The semicircles are related with the charge transfer process arisen at the electrolyte-electrode interface and sloping lines refer to the diffusion kinetics of Na+ ions into the active material. For evaluating the resistance component involved in the electrochemical process of V2O5 and its composite, the plot has been simulated using the equivalent circuit shown in the inset of Fig. 4. The circuit includes ohmic resistance of electrolyte (Rs), charge transfer resistance (Rct), Warburg impedance (Wc), and double layer capacitance (Cdl). V2O5/C shows a significantly lower Rct value of 217 Ω 10


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as compared to 409 Ω of its counterpart, indicating better kinetic activity and higher electronic conductivity of the electrode. Though carbon amount was same in the both of bare and composite electrodes but a clear difference in the charge transfer resistance reveals the crucial impact of carbon coating. Fig. 5a shows the charge-discharge voltage profile of V2O5/C electrode at the 1st and 10th cycle at a current density of 0.05 C (1C: 294 mA g-1) in a voltage range of 1.2-4.0 V. The galvanostatic measurements are performed by applying a constant current-constant voltage (CC-CV) mode during charging to 4.0 V. The electrode delivers an initial discharge capacity of 195 mAh g-1 in the 1st cycle which corresponds to 1.3 Na insertions per unit formula (of the parent V2O5). After a complete cycle, the charge capacity (293 mAh g-1) is higher than the discharge capacity because of the formation of the solid electrolyte interphase (SEI) layer. This is a typical phenomenon that is observed in certain electrodes during the charge-discharge cycles of Na-ion batteries.25-27 Interestingly, the discharge capacity is gradually increased in the subsequent cycles and reaches a maximum of 255 mAh g-1 in the 10th cycle. This value of discharge capacity corresponds to the 1.7 Na insertions per unit formula. The increase in capacity during the initial cycles is presumably due to the activation of the electrode, i.e., gradual increase of accessibility of electrolyte on the electrode upon cycling. The discharge capacity of the composite electrodes is considerably higher than that of previously-reported orthorhombic V2O5

16, 24

. The voltage

profile during discharge process in the 10th cycle shows two voltage plateaus around 2.7 and 2.0 V which are apparently related to the reduction of vanadium (5+ → 4.5+ → 4+). The charge curves in the 1st and 10th cycle show deviation which is related to the amount of sodium insertion/extraction. As the sodium insertion in the 1st cycle is smaller and the related high charge capacity is due to the SEI layer formation. The sodium insertion increases in the 11



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subsequent cycles where it shows two phase insertion in the 10th cycle. The charge capacity in the 10th cycle is mainly due to the sodium extraction from two phases with a minor role of SEI layer. During 30 cycles, discharge capacity shows negligible fading, e.g., only 5 % of delivered capacity was dissipated with respect to the maximum capacity of 255 mAh g-1, demonstrating a stable cycle performance. Fig. 5b shows the discharge curves of V2O5/C electrode at various current densities from 0.05 to 1.0 C. The electrode delivers average discharge capacities of 255, 226, 205 and 170 mAh g-1 at current densities of 0.05, 0.1, 0.5 and 1 C, respectively. The discharge capacity gradually increases in the initial seven cycles at a current density of 0.05 C and then it becomes stable (shown in the inset of Figure 5b). The V2O5/C electrode demonstrates a good rate capability up to current density of 1.0 C and exhibits more than one sodium ion insertion per unit formula. The discharge curves show similar profiles under all of C-rate investigated, while only the degree of the potential drop observed at the starting of discharge was varied with C-rate due to the ohmic polarization of the cell. In order to determine the structural changes upon cycling, ex situ XRD measurements are carried out with fully charged and discharged V2O5/C electrodes. Fig. 6a shows the ex situ XRD patterns of pristine (bottom), discharged to 1.2 V (middle) and recharged to 4.0 V (top) states. The details of sample preparation for ex situ XRD are discussed in the experimental section. A noticeable background in all the charged and discharged XRD patterns is observed presumably due to the Kapton tape (polyimide), which is used for the preparation of samples for ex situ investigation. The XRD pattern of the electrode material in the discharged state of 1.2 V can be indexed to NaV2O5 (peaks marked with *, PDF card no. 01-086-2185) and Na2V2O5 (peaks marked with 12


#

,

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PDF card no. 00-045-0496) as a major and minor phase, respectively, because Na2V2O5 shows less intense diffraction peaks. The existence of NaV2O5 as a major diffraction peaks indicates that the material remains crystalline with one sodium insertion and the capacity is not stabilized in the initial cycles. The lattice parameter a is decreased whereas b and c are increased with Na insertion. The values of lattice parameters and volume of NaV2O5 from the discharged XRD pattern are listed in Table 1. This shows an increment of 9.2% in the volume of the unit cell of V2O5 upon sodiation and a clear shift of (001) reflection indicates the sodium intercalation along c axis. When the material is recharged to 4.0 V, orthorhombic V2O5 (peaks marked with O) is recovered, while trace peaks of NaV2O5 is still observed. These peaks show partial irreversibility of the material and are responsible for the low capacity in the initial cycles. Further, ex situ TEM is employed to observe the phase changes upon Na insertion. Fig. 6b shows the TEM image of a particle at fully discharged potential state (1.2 V) and the inset shows the high-resolution image of the particle which reveals both the crystalline and amorphous phases. The lattice fringes with the d-spacing of 5.835 Å belong to Na2V2O5 phase. The amorphous-like phase can also be observed at the edge of the particle which verifies the less intense diffraction peaks in ex situ XRD results. There are several studies performed on cathode material in LIBs that amorphous phase allows the diffusion of lithium ions more rapidly than crystalline phase with same morphology.28 Considering the above electrochemical results, it can be concluded that the amorphous phase also contributes to the electrochemical activity. NEXAFS spectroscopy is a dynamic tool to analyze the atomic arrangement and electronic structure of specific element. The L-edge spectra of 3d transition metals possess greater information as the electronic transition occurs in dipole allowed orbitals (2p→3d) which is 13



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sensitive to probe the local structure near absorbing atom.29 From spectroscopy point of view, the study of V L-edge of V2O5 is also quite appealing due to the completely empty 3d0 orbital which can be directly probed due to the dipole allowed 2p→3d electronic transition.30 NEXAFS spectroscopy is performed to examine the changes in the local structure of V2O5 at fully discharged and charged states. Both V L-edge and O K-edge NEXAFS spectra of pristine and fully charged-discharged samples in Na half cells are shown in Fig. 7. The V L-edge of V2O5/C consists of two peaks arising from the electron transition of 2p3/2→3d, denoted at LIIIedge and 2p1/2→3d, denoted as LII-edge. These V LIII and LII edge peaks are found at energies of 519.9 and 526.3 eV, respectively, in the pristine V2O5/C electrode. The V LIII edge contains more structural information as compared to V LII edge because the later is broadened by Auger decay process into a 2p3/2 hole.31 The position of O K-edge spectra is near to vanadium L-edge spectra and shows a partial overlapping, and thus can be jointly studied. The O K-edge electron transition occurs due to dipole allowed 1s→2p transition. The splitting of O K-edge peaks in square pyramidal coordination is attributed to the roughly octahedral crystal field splitting upon hybridization of V 3d and O 2p levels thus containing powerful structural insight. Fig. 7 shows the two distinct peaks of O K-edge at 531 and 533 eV separated by the ligand field splitting and characterized by t2g and eg levels, respectively. The intensity ratio of t2g and eg levels depends on the filling of d orbitals and it is highest with d0 configuration and decreases with the increase of electrons in d orbital.31-32 When the cell is discharged to 1.2 V (middle), the spectra show a significant change in the all V L-edge and O K-edge peaks. There is a shift of 0.6 eV in V LIII edge peak to the lower energy in the fully sodiated sample as marked with the red dotted line and a clear shoulder peak is 14


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appeared on the lower energy side of V LIII-edge peak at 517.4 eV. The shift in the LIII edge peak and appearance of the shoulder peak reveal reduction in vanadium which corresponds to V4+ valence.33-35 This shows that the capacity is mainly contributed from the reduction of vanadium (to 4+). The intercalation of sodium into the structure not only causes the reduction in the vanadium but also affects the oxygen bonding greatly. The discharged spectrum of O K-edge shows the suppressed t2g level peak compared to eg one which are affected by to the change in electronic configuration of V2O5.31 This change in the intensity of orbitals peaks may be due to the filling of t2g orbital with one electron as a result of V reduction. When the cell is recharged to 4.0 V, all the V L edge and O K edge peaks are recovered with almost same energy position and intensity ratio as those of the pristine electrode. The overall electrochemical reaction mechanism of V2O5 is investigated using ex situ XRD, TEM and NEXAFS spectroscopy and summarized in Fig. 8 which displays the changes in the lattice upon Na insertion/extraction along with charge-discharge process. The crystal structures of V2O5 and NaV2O5 are constructed using the structural information from their relevant PDF card no. (as mentioned above) and their structures are drawn using VESTA.36 The crystal structure of V2O5 constructing with alternating layers of VO5 square-pyramidal. Sodium insertion/extraction occurs along with ab planes with ordered chains of layers which are connected via sharing of corners and edges. As the Na ions intercalated into the arrays of parallel ladders of V2O5, interlayer distance along c axis expands from 4.4 to 4.8 Å in the fully discharged state. Simultaneously, V2O5 is mainly transformed to the crystalline phase of NaV2O5, whereas partially amorphous Na2V2O5 is coexisted as a minor phase. The Na+ ions in NaV2O5 structure are located at Wyckoff position of 2b between the van der Waals layers and widened

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the c axis. During recharge process, the Na+ ions are extracted and V2O5 reverts to its original crystal structure along with NaV2O5 as a minor phase.

Conclusions In this work, V2O5 nanoparticles were prepared by a simple hydrothermal method and a homogenous carbon anchoring was achieved by a ball milling method. V2O5/C has been investigated as a potential cathode material for Na-ion battery where it shows an excellent electrochemical performance with a reversible capacity of 255 mAh g-1 and stability up to 30 cycles. The superior performance is attributed to the nano-sized V2O5 particles and their embedment with carbon which resides in the voids between the particles and facilitate the penetration of electrolyte, thus lowering the energy barrier. The reaction mechanism involves that the intercalation of sodium ions into bilayered structure of V2O5 induce formation of NaV2O5 and Na2V2O5 as major and minor phases, respectively. Na2V2O5 exists both in crystalline and amorphous-like phases and confirmed by ex situ XRD and TEM results. The V Ledge NEXAFS spectra reveal that the oxidation state of vanadium varies from 5+ to 4+ during the sodium intercalation.

Acknowledgements The authors would like to acknowledge the financial support from the R&D Convergence Program of NST (National Research Council of Science & Technology) of Republic of Korea and the KIST Institutional Program (Project No. 2E26330).

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Figures caption Fig. 1 XANES spectra of as prepared and after calcination at 400 °C are plotted with solid lines. The standard spectra of V2O3, VO2 and V2O5 are also plotted in dotted lines to estimate the oxidation state of the sample. The inset shows the V-O bonding of V2O5 and V2O3. Fig. 2 XRD patterns of (a) pristine and (b) V2O5/C. Fig. 3 SEM images of (a) pristine and (b) V2O5/C. (c) TEM image of carbon coated V2O5 and the insets show the SAED patterns. Fig. 4 Nyquist plot of bare and carbon composite V2O5 and the inset shows the equivalent circuit model. Fig. 5 (a) Galvanostatic charge-discharge profile of V2O5/C electrode in Na cell at current density of 0.05 C. The inset shows the discharge capacity versus number of cycles in Na cell. (b) Discharge capacity curves at various current densities of 0.05, 0.1, 0.5 and 1 C in Na cell. The inset shows the rate capability of the V2O5/C electrode. Fig. 6 (a) XRD of V2O5/C and ex situ XRD of fully charged and discharged samples. (b) TEM image of fully discharged electrode. The inset shows HR-TEM images. Fig. 7 NEXAFS of V2O5/C, fully charged and discharged samples obtained from Na cell. Fig. 8 Crystal structure of V2O5, showing Na+ de/intercalation channels along with chargedischarge process.

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Table caption

Table 1. Lattice parameters and volume of unit cell calculated for V2O5 and NaV2O5

Figure 1.

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Figure 2

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Figure 3

Figure 4

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Figure 5

Figure 6 21



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Figure 7

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Figure 8

Table 1. a (Å)

b (Å)

c (Å)

V (Å3)

Space group

V2O5

11.501

3.563

4.373

179.19

Pmmn

NaV2O5

11.369

3.624

4.822

198.67

Pmmn

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Table of Contents Entry The V2O5/C nanocomposite delivered a high discharge capacity, making it a potential candidate for NIBs.

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Investigation of the Na Intercalation Mechanism into Nanosized V2O5

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