Extraction Mechanism in a Layered

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Probing the sodium insertion/extraction mechanism in layered NaVO anode material 3

Ghulam Ali, Mobinul Islam, Hun-Gi Jung, Kyung-Wan Nam, and Kyung Yoon Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03571 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Applied Materials & Interfaces

Probing the sodium insertion/extraction mechanism in layered NaVO 3 anode material Ghulam Ali,† Mobinul Islam,†,‡ Hun-Gi Jung,†,‡ Kyung-Wan Nam,§ Kyung Yoon Chung†,‡,* †

Center for Energy Storage Research, Korea Institute of Science and Technology (KIST),

Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea. ‡

Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology (UST), 217 Gajeong-ro Yuseong-gu, Daejeon 34113, Republic of Korea. §Department

of Energy and Materials Engineering, Dongguk University-Seoul, 04620 Seoul,

Republic of Korea *E-mail: [email protected]

ABSTRACT For the realization of sodium-ion batteries (SIBs), high-performance anode materials are urgently required with the advantages of being low-cost and environment-friendly. In this work, layeredtype NaVO3 is prepared by the simple solid-state rout with a rod-like morphology and used as an anode material for SIBs. NaVO3 electrode exhibits a high specific capacity of 196 mAh g-1 during the 1st cycle and retains a capacity of 125 mAh g-1 at the 80th cycle with a high coulombic efficiency of >99%, demonstrating high reversibility. The sodium diffusion coefficient into NaVO3 is measured using electrochemical impedance spectroscopy (1.368 × 10-15 cm2 s-1), galvanostatic intermittent titration technique (1.15715 × 10-13 cm2 s-1), and cyclic voltammetry (2.7935 × 10-16 cm2 s-1). Furthermore, the reaction mechanism during sodiation/desodiation process is investigated using in-situ XRD and X-ray absorption near edge structure analysis 1 ACS Paragon Plus Environment

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which suggest the formation of amorphous-like phase and reversible redox reaction of V4+↔V5+, respectively. KEYWORDS: NaVO3, solid-state method, kinetic study, sodium diffusion coefficient, amorphouslike

INTRODUCTION Sodium-ion batteries (SIBs) have captivated significant interest especially for the use of largescale applications such as electrical energy storage systems (ESS). The advantages of sodium over lithium mainly include its low cost and worldwide abundant resources which are the most important aspects to develop large-scale rechargeable batteries.1-5 The practical implications of SIBs are facing challenges to design high-performance electrode materials. Many of the electrode materials which already investigated in lithium-ion batteries (LIBs), can be transplanted in SIBs for the Na+ de/intercalation. The positive electrode materials of SIBs are broadly investigated and have made great progress compare to their counterpart.6-10 The development of anode material is more challenging for SIBs as commonly used anodes in LIBs such as graphite and high capacity silicon does not de/intercalate sodium ions.11 Moreover, the anode materials in SIBs show poor cyclability due to the larger size of sodium ions (1.03 Å) which results in sluggish diffusion kinetics of sodium during reversible de/insertion.1,12 Hard carbon is extensively studied in SIBs but it delivers limited reversible capacities between 100-300 mAh g-1.13-14 Other carbonaceous materials such as graphene oxide, amorphous carbon, petroleum-coke carbon15, polyparaphenylene carbon16, soft carbon17 etc, delivers even lower capacities which are insufficient to use for the practical application. Several other types of anode

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materials are investigated with intercalation, conversion, alloying and amorphous reaction mechanisms. Each has their own dis/advantages such as intercalated anode materials (Ti-based materials)18-20 reveal high structural stability but exhibit limited capacities of 100-200 mAh g-1, anode materials with conversion reaction (Fe3O4, NiCo2O4, FeS2, Ni3S2)21-24 exhibit large capacities of 150-400 mAh g-1 but suffer with poor cyclability, alloying mechanism (Sn, Sb and P)25-27 also show high capacities but suffer from poor cyclability because of the large volume expansion/contraction during Na+ insertion/extraction process. Thus, there is need to find appropriate anode materials with high and stable capacities which could be used for SIBs. Layered vanadium-based materials are known for multiple lithium intakes and exist with different compositions and oxidation states such as V2O5 (5+),28-29 VO2 (4+),30 LiVO3 (5+),31 V6O13 (multiple from 2+ to 5+),32 LiV3O8 (5+),33 and NaVO3 (5+).34 These materials are wellknown to deliver large capacities in LIBs as cathode and anode materials based on multiple redox reactions of vanadium. Recently, NaVO3 was synthesized by sol-gel method and investigated as a high-performance anode where it delivered a capacity of 356 mAh g-1 in LIBs.34 NaVO3 is isostructural with CaMg(SiO3)2 which belongs to pyroxene family where SiO3 chains are held together by metal ions.35 Similarly, VO3 chains formed VO4 tetrahedra which are located at alternate channels with NaO6 octahedral in NaVO3 structure.34 Despite high capacities in LIBs and their structural advantages, vanadium-based layered materials have been less explored in SIBs. Further, the zero oxidation state of vanadium was not found during charge/discharge process because of the strong interaction of V-O bonds, thus do not form metallic V, suggesting less volume variation compare to typical conversion-based electrodes.36-37 In this work, layered NaVO3 is synthesized using the simple solid-state route and the electrochemical properties are investigated as an anode material for SIBs. A comprehensive 3 ACS Paragon Plus Environment


kinetic study using different techniques is conducted to reveal the sodium diffusivity in NaVO3 electrode. Furthermore, sodium insertion/extraction mechanism was investigated to probe the structural changes using in-situ XRD and TEM techniques and the oxidation state of the material was determined using X-ray absorption near edge structure (XANES) spectroscopy.

EXPERIMENTAL Na2CO3 (>99.0%) and V2O5 (>99.6%) were purchased from Sigma and used without any further modification. NaVO3 was synthesized by the solid-state process by mixing the stoichiometric ratio of Na2CO3 and V2O5. The mixture was well grounded with a motor and pestle for ~30 min and transferred to an alumina boat. The powder was heated at 600 °C for 6 h at a temperature ramp of 3 °C/min in the air in a box furnace. The obtained powder was analyzed with XRD (Xray diffractometer from Rigaku equipped with radiations from Cu source with a wavelength of 1.54 Å), FE-SEM (NanoSEM-FEI NOVA200), and high-resolution TEM (FEI Tecnai G2-F20). X-ray photoemission spectroscopy (XPS) spectrum of the NaVO3 was acquired with PHI 5000 VersaProbe (ULVAC-PHI) under high vacuum conditions (6.8 ×10-8 pa). Al-Kα X-ray source (1486.6 eV) was used as a monochromatic to select the energy and the carbon 1s peak (284.6 eV) was used for internal calibration. The peaks were fitted using the Multipak software. Coin-type CR2032 cells were used to evaluate the electrochemical properties of NaVO3 electrodes. NaVO3 powder, acetylene black, and polyvinylidene difluoride (PVDF) were mixed in a ratio of 7:2:1 to prepare the working electrodes. It is worthy to mention here that the mixing of NaVO3 and acetylene black were done using ball-milling (material/ball weight ratio was 1/20) process for 3 h in an argon atmosphere with a speed of 300 rpm. N-methyl-2-pyrrolidinone


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(NMP) solvent was used to get uniformly mixed slurry which was then spread on Cu foil and dried in an oven at 80 °C. The cells were fabricated with NaVO3 as a working electrode, glass fiber as a separator, and sodium metallic foil as a counter electrode. 1 M NaClO4 dissolved in an equal weight ratio of ethylene carbonate, diethyl carbonate, and propylene carbonate was used as the electrolyte. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) tests were conducted using a Biologic potentiostat instrument (VMP3 BioLab, Inc.). Galvanostatic intermittent titration technique (GITT) was measured at Maccor instrument by applying the repeated current pulses for 10 min at a current density of 11 mA g-1 followed by relaxation for 60 min. The relaxation time was set longer in order to achieve the equilibrium potential for the given electrode. GITT measurements were taken over a complete galvanostatic cycle. In-situ XRD was conducted using a laboratory made coin cell with a 3 mm hole in disk, lower and upper cap and the cell was cycled using a multi-channel Wonatech (WBCS3000K8) instrument. XRD measurements were taken using the X-ray diffractometer (Rigaku R–AXIS IV++) at Korea Institute of Science and Technology (KIST). The diffractometer is equipped with Mo–Kα radiations with a wavelength of 0.7107 Å and the data is recorded on an image plate with an exposed time of 300 sec. After processing the data from the 2D image to 1D XRD pattern, the 2θ values were changed from Mo–Kα to Cu–Ka radiations in order to compare the data with the standard. For ex-situ analysis, the samples were prepared by disassembling the cells at various dis/charged voltages. The electrodes were immersed in dimethyl carbonate solution to remove any surface salts contents. X-ray absorption near edge structure (XANES) measurements of V K-edge (5465 eV) were performed at 1D KIST, 7D XAFS, and 8C Nano XAFS beamlines at Pohang Light 5 ACS Paragon Plus Environment


Sources (PLS-II). The energy was selected using a Si (111) double-crystal monochromator and the signals were detuned to ∼40% of the maximum intensity in case of 1D and 7D beamlines and a pure vanadium foil was used as a reference to calibrate the spectra. All the XANES measurements were performed in vacuum and the XANES spectra of electrodes were recorded in transmission mode. The resulting spectra were analyzed using ATHENA package.38 The extended X-ray absorption fine structure (EXAFS) data was extracted by using Hanning window function with k2 weighted in the k-ranges of 2.5-11 Å-1. It is worthy to mention here that the plotted data is converted without phase corrected so there is a shift (~0.5 nm) in the observed bond lengths.

RESULTS AND DISCUSSION The structure of the as-synthesized material is determined using XRD as shown in Figure 1. All the XRD peaks and lattice parameters of NaVO3 are found to match with PDF card No. 01-0722272 and the main peaks are indexed for clarity. NaVO3 crystallizes in the monoclinic C12/C1 space group and the lattice parameters are calculated to be a = 10.5692 Å, b = 9.4973 Å, c = 5.8772 Å, β = 108.35° and V = 559.939 Å3. The material was found to have a high peak (220) over other planes which is presumably due to the preferred orientation of the crystals. The crystallite size was calculated using Scherrer formula from the three major peaks i.e. (200), (220), and (310) and its value is calculated to be 49 nm. The crystal structure of NaVO3 along b-axis is shown in the inset of Figure 1 where vanadium is tetrahedrally bonded with oxygen and sodium ions are located at interstitial sites between the V-O tetrahedral layers which are separated by a distance of 4.6763 Å. The large interlayer spacing could be useful for sodium insertion/extraction process. The crystal structure of NaVO3 consists of alternate channels where 6 ACS Paragon Plus Environment

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VO3 chains are connected via oxygen at the corner to form the basic structural unit of VO4 tetrahedra.

Figure 1. Powder XRD pattern of NaVO3 where the diffraction peaks are indexed with respective hkl values. The inset shows crystal structure of NaVO3 along b-axis.

The morphology of NaVO3 powder is observed using SEM as shown in Figure 2a where most of the particles are grown with a rod-like morphology. The length and width of these rods are in the range of 1-4 µm and 100-700 nm, respectively. Some agglomerated particles were also observed with random shapes. TEM images (Figure 2b) shows the scattered NaVO3 particles with different shapes and sizes. In order to clearly observe the morphology, a single rod-like particle with submicron dimension is shown in the inset of Figure 2b. 7 ACS Paragon Plus Environment


Figure 2. (a) SEM image of NaVO3 powder. (b) TEM image of NaVO3 and the inset shows the single rod-like morphology.

Further, the chemical composition of NaVO3 was analyzed using XPS to study the electronic configuration and the survey spectrum is shown in Figure 3a. Prominent peaks of V 2p, O 1s, and Na 1s orbitals were detected in the XPS survey spectra and plotted in Figure 3 (b-d). The peaks at 517.1 and 524.5 eV (Figure 3b) correspond to the binding energies of the V 2p3/2 and V 2p1/2, respectively and their energy positions are indicative of V5+ oxidation state.39 The peaks were deconvoluted to get insight into V 2p bond and three different peaks at 517.1 (V 2p3/2), 523.6 (V 2p1/2), and 524.9 eV (V 2p1/2) were observed which correspond to V-O bonds.40 The deconvolution of Na 1s peak (Figure 3c) at energies of 1071.2 and 1072 eV represents the different interaction of sodium with alternate bonds in the structure.41 The deconvolution of O 1s peak (Figure 3d) shows two peaks at 530.2 and 530.7 eV, which are typical characteristic peaks


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of metal-oxygen bonds.42 Thus XRD and XPS confirm the successful formation of the NaVO3 compound.

Figure 3. (a) XPS survey of NaVO3 powder where prominent peaks from V2p, O1s, and Na1s are marked at their respective energies. (b) V2p, (c) Na1s, and (d) O1s peaks are XPS spectra.

The electrochemical properties of NaVO3 electrodes were measured using half-cell. Figure 4a shows the cyclic voltammogram for the three cycles measured in a voltage range of 0.01-3.0 V at a scan rate of 0.1 mV s-1. In the first cycle, two distinct peaks at 1.2 and 0.2 V appears during



reduction presumably attributed to the irreversible reactions at the surface of the electrode with electrolyte and formation of solid electrolyte interphase (SEI) layer, respectively. A pair of sharp redox peak was observed at ~0.1 V and could be related to the insertion/extraction of sodium in the NaVO3. Second and third cycles almost overlap, demonstrating the reversibility of the material. Figure 4b shows the galvanostatic dis/charge profile of NaVO3 electrode at a current density of 9 mA g-1 in a voltage range of 0.01-3.0 V. The material exhibits a high discharge capacity of 359 mAh g-1 in the first cycle with a sloping profile. A charge capacity of 196 mAh g-1 was recorded in the 1st cycle which corresponds to a coulombic efficiency of 55%. The low value of coulombic efficiency belongs to the formation of SEI layer and its value improved in the subsequent cycles. The second cycle delivers discharge and charge capacities of 223 and 196 mAh g-1, respectively which corresponds to an improved coulombic efficiency of 88%. The specific capacity decreases continuously in the subsequent cycles as shown in Figure 4c. However, coulombic efficiency was improved and the electrode exhibits a charge capacity of 125 mAh g-1 in the 80th cycle with a coulombic efficiency of >99%. The rate capability of NaVO3 was measured at various current densities in a voltage range of 0.01-3.0 V as shown in Figure 4d. The electrode delivered charge capacities of 164, 137, 93 and 51 mAh g-1 at current densities of 11, 22, 110 and 220 mA g-1.


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Figure 4. (a) CV of NaVO3 electrode during 1st, 2nd, and 3rd cycle at a scan rate of 0.1 mV s-1. (b) Charge-discharge profile of NaVO3 electrode at 1st and 2nd cycles at a current density of 9 mA g-1. (c) Cycle test at a current density of 9 mA g-1 in a voltage range of 0.01-3.0 V. (d) Rate capability test in a voltage range of 0.01-3.0 V at the current densities of 11, 22, 110 and 220 mA g-1.

The electrode reaction insights and sodium diffusivity in the layered NaVO3 electrode are investigated using CV, GITT, and EIS techniques. CV provides the information of the measurement of the current flowing through the electrode material as a function of externally controlled potential. These current signals provide detailed information about the reaction 11 ACS Paragon Plus Environment


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associated with electron transfer steps. The sodium diffusion coefficient (DNa) is calculated from CV measurements by assuming that the Na+ diffusion in the solid phase is a semi-infinite or finite diffusion process where the peak current (Ip) is proportional to the square root of the scan rate (v) and it follows the Nernstain equation.43 Figure 5a shows CV curves of NaVO3 electrode measured at scan rates of 0.05, 0.1, 0.2, and 0.5 mV s-1 in a voltage range of 0.01-3.0 V vs. Na/Na+. In the 1st cathodic scan at a rate of 0.05 mV s-1, the peak at ~0.7 V could be attributed to the formation of SEI layer. The anodic scan shows two broad peaks at 0.3 and 2.4 V which could be related to the oxidation of vanadium. The peak current and gap between cathodic and anodic scans increases with increasing the scan rates which indicates an increased polarization due to kinetic limitations related to the Na+ diffusion in the NaVO3 particles. Figure 5b shows the relationship graph between Ip and the square root of v. The linear relationship between two indicates the sodium diffusion in the NaVO3 electrode is rate-determining step. The DNa can be calculated by Cottrell Eq. (1). /

Ip = 2.69 × 10 A / /

(1)

Where A represents the area between electrode and electrolyte, CNa is the bulk concentration of the Na+ ions and its values is determined by assuming the concentration of sodium in NaVO3 electrode, and n is the number of charge transfer which depends on the cathodic respective anodic reaction. By substituting all these values in Eq. 1, the DNa is calculated to be 2.7935 × 1016

cm2 s-1. The calculated value of DNa for NaVO3 is lower than the previously reported layered

oxide anode where DNa is calculated to be 7.9× 10-13 cm2 s-1 in case of layered NaxMoO2.44 The lower values of DNa in NaVO3 particles could be the reason of less specific capacities at higher current densities as the electrode face more polarization in large-size particles because of the relatively higher time constant for Na+ diffusion.45 12 ACS Paragon Plus Environment

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Figure 5. (a) Cyclic voltammograms of NaVO3 electrode at various scan rates and (b) peak current as a function of square root of scan rate. (c) GITT curve plotted with sodium diffusion coefficient in NaVO3 and (d) linearly fitted potential response as a function of square root of



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time. (e) EIS graphs of as-fabricated and after the 80th cycle of NaVO3 electrode and the inset shows the equivalent circuit. (f) Real parts of complex impedance plotted as a function of ω-1/2.

Furthermore, to understand the kinetic performance in NaVO3, DNa is calculated using GITT measurements during sodium insertion/extraction process. GITT is developed by Weppner and Huggins to measures the diffusion coefficient based on chronopotentiometry in a mixed conductor by assuming the diffusion process in one-dimensional in a solid solution. GITT also combines both the steady-state and transient measurements as well as thermodynamic data.46 A constant current flux is applied to the host electrode materials to insert/extract the sodium ions for a limited time period and this process changes the overall composition and cell potential. Subsequently, the change in the cell potential is recorded for each current pulse and the diffusion coefficient is determined. The GITT discharge/charge profile of NaVO3 electrode is shown in Figure 5c in the potential range of 0.01-3.0 V. During consecutive current pulses applied to the electrode, the insertion of Na+ ions into NaVO3 influence the electrode potential and the resulting potential response is calculated to be a linear function of the square root of pulse time (√τ) as shown in Figure 5d. The slope and a linear fit R2 value of the fitted line are calculated to be 0.106 and 0.982, respectively. DNa can be calculated using Eq. (2) by supposing that sodium transport follows Fick’s law.

DNa+ =

﴾

﴿ ﴾

∆ ∆

﴿

(2)

Where τ is the current pulse time, VM (cm3 mol-1) is the molar volume of NaVO3 electrode, mB (g) is the active material weight, MB (g mol-1) is the molecular weight, and S (cm2) is the electrode surface area. ∆ and ∆ are measured for each step and denotes the change in the steady state 14 ACS Paragon Plus Environment

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voltage at plateau and total change in cell voltage during pulse time, respectively. The DNa values are calculated for each pulse by measuring and substituting the above parameters in Eq. (2). The DNa value is calculated to be 1.15715 × 10-13 cm2 s-1 at the start of discharge process and it decreases with the increase in the insertion of sodium ions into electrode material and the values is calculated to be 2.02546 × 10-17 cm2 s-1 at the last step of discharge process. The DNa value is calculated to be 5.66377 × 10-14 cm2 s-1 at the first step of charge process and it also shows a decreasing trend with the sodium extraction process from electrode material and the values is calculated to be 2.25694 × 10-16 cm2 s-1 at the end of charge process. The electrode material has shown low values of DNa towards the discharging and charging depths which indicates that sodium ions face high polarization at the end of each process. This phenomenon could be ascribed to the fact that the electrochemical reactions in these regions are severely sluggish accompanied by the strong interactions of sodium ions with other surrounding ions.47 To further probe the sodium diffusivity before and after cycling in NaVO3 electrode, EIS was conducted in a sodium-half cell at open circuit potential (OCP). EIS is a powerful tool to follow the kinetics of sodium insertion/extraction into electrode material where a collective response of kinetic processes is measured as a cell impedance. Further, the collective response consisted of interfacial and diffusion processes can be separated on the frequency scale. Moreover, EIS also offers an easy way to identify the side reactions.48 The Nyquist plots of freshly prepared and after 80th cycle cells are shown in Fig. 5e. An equivalent circuit is used to fit the curves using Zview software and the circuit diagram is shown in the inset of Figure 5e. The EIS curves composed of semi-circles at a high-frequency range which are related to the charge-transfer resistance at the electrolyte-electrode interface and a straight sloping line at the low-frequency range, corresponding to the sodium diffusion into active material. The circuit is composed of 15 ACS Paragon Plus Environment


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ohmic resistance of electrolyte (Rs), charge transfer resistance (Rct) at the interface, a constant phase element (CPE) of the double layer, Warburg impedance (Zw) which is related to the diffusion process, and a double layer capacitance (C). The as-fabricated layered NaVO3 electrode shows low Rct value of 27 Ω, indicating better kinetic activity while the electrode shows high Rct value of 153 Ω after 80th cycle. Moreover, sodium ions transport inside the NaVO3 electrode was also calculated using EIS spectra. The DNa was calculated from the sloping line in the Nyquist plot, according to the Eq. (3). !"!

= ! ! & ! & # $ % ' (

(3)

Where T represents the absolute temperature (295.15° K), R represents the gas constant (8.314 J mol-1 K-1), A denotes the surface area (1.13 cm2), C represents the concentration of sodium ions calculated from the ratio of tapped density and molecular weight, F indicates Faraday’s constant (96486 C mol-1), n represents number of electrons associated with reaction, and σ is the Warburg factor calculated based on the relation between ω-1/2 and Zreal (Figure 5f). The DNa was calculated to be 1.368 × 10-15 cm2 s-1 and 4.952 × 10-16 cm2 s-1 for as-fabricated and after 80th cycle, respectively. Overall, sodium transport was measured lower after cycling mainly due to increased polarization and structural changes.


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Figure 6. Charge-discharge profile (left) during in-situ XRD measurement and the corresponding XRD patterns (right).

The reaction mechanism of the electrode material has been explored using in-situ XRD, ex-situ TEM and XANES. Figure 6 shows the charge/discharge profile (left) and their corresponding XRD patterns (right). The ball-milling of the prepared material prior making electrodes results in the lattice disorder and mismatch in the relative intensity of the XRD peaks which can be observed in the in-situ XRD of the electrode material. The ball-milling is a well-known technique to change the particle size, surface area, microstructure, etc. of the solid materials.49 The cell diffraction and Cu peaks were eliminated to clarify the diffraction pattern from electrode material. As the sodiation process starts, a significant gradual reduction in the intensity of all the 17 ACS Paragon Plus Environment


diffraction peaks observed. This behavior of decreasing the peaks intensity is associated with the sloping trend of discharge profile which leads to the amorphous-like structure. The peak intensities continuously decrease during sodium extraction process, suggesting the amorphouslike phase of the NaVO3 anode at the end of the 1st cycle. For a better understanding of microcrystalline structure, we have conducted ex-situ TEM. Figure 7a displays the high-resolution (HR-TEM) image of the pristine material where it shows good crystallinity and the lattice fringes corresponding to d-spacing of 0.519 and 0.687 nm are observed which belongs to (110) and (200) planes, respectively. Figure 7b shows the HR-TEM image of the NaVO3 anode at fully discharged state to 0.01 V. The image reveals amorphous-like microstructure with no visible fringes. Figure 7c shows the HR-TEM image of fully charged sample to 3.0 V which again shows amorphous-like structure. Overall, HR-TEM results show that material transforms to the amorphous-like structure during first discharge process and once it loses its crystallinity, it remains as an amorphous-like phase. It is difficult to extract the structural and Na+ insertion/extraction mechanism information from the amorphous phase but the material shows high capacity and stable cycle performance. However, the amorphous type anodes are previously reported and possess the advantage to act as a buffer to relieve the strains upon Na+ insertion and results in improved cycle performance.50 Ex-situ XANES was measured at the points denoted on the dis/charge profile as shown in Figure 7d and the resulting XANES spectra along with V4+ and V5+ standards are shown in Figure 7e. The pristine sample shows relatively high pre-edge peak which is caused due to tetrahedral symmetry as already discussed in XRD results. When the electrode is discharged to point 2, there is not much shift in both pre-edge and main peaks. However, discharging to points 3 and 4, the spectra show a substantial shift to lower energy, indicating a reduction in the oxidation state from V5+ to V4+. Charge process (points 5 and 6)


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during the first cycle shows oxidation of vanadium, however, the spectrum at point 6 is at lower energy than the pristine sample (point 1) which might be related to the partial irreversibility or structural changes in the material. Fully charged spectrum during 2nd cycle (point 7) is found overlapped the fully charged spectrum (point 6) which suggests the reversibility of the material in subsequent cycles during Na+ insertion/extraction. Overall, our results suggest that electrochemical reaction in NaVO3 anode takes place between V4+ ↔ V5+ oxidation states with high reversibility after 1st cycle. EXAFS was performed to get insights of the local structure. Fig. 7f shows the EXAFS spectra of pristine, fully discharged and recharged electrodes. The dominant peak at ~1.2 nm corresponds to the nearest V-O interaction and the next peaks in a range of 2.9-3.4 nm belong to the V-Na interaction for the pristine NaVO3 spectrum. The inset of the Fig. 7f shows the bond distance between V-O and V-Na interaction. The different bond lengths of V-O interaction in tetrahedral coordination suggest the distorted symmetry which was also observed in the form of high pre-edge peak in XANES spectra. The existence of V-O interaction peak in EXAFS spectra suggests the preservation of original structure of NaVO3. However, the slight shift and difference in the intensity of the V-O interaction peaks in the fully discharged and recharged spectra occurred which is due to the Na+ de/intercalation.



Figure 7. Ex-situ TEM of (a) pristine, (b) fully discharged, and (c) fully charged NaVO3 electrode. (d) Charge-discharge profile of NaVO3 electrode where marked points indicate where XANES spectra are measured and (e) the resulting XANES spectra along with VO2 and V2O5 standards. (f) EXAFS spectra of pristine (1), fully discharged (4), fully recharged (6), and fully recharged during 2nd cycle (7). The inset shows the local structure coordination with the nearest neighboring atoms.

CONCLUSION In summary, single-phase layered NaVO3 has been obtained with solid state method along (220) orientation. NaVO3 electrode shows promising electrochemical properties where it delivered a high specific capacity of 196 mAh g-1 at the 1st cycle and 125 mAh g-1 in the 80th cycle with a


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high coulombic efficiency of >99%. The kinetic study was systematically conducted using different techniques such as CV, GITT, and EIS to calculate the sodium diffusivity. GITT is applied to discharge/charge process and DNa is calculated at each pulse where it shows large variation in the values. DNa is calculated to be 1.15715 × 10-13 cm2 s-1 at the start and 2.02546 × 10-17 cm2 s-1 at the end of discharge process while its value is recorded as 5.66377 × 10-14 cm2 s-1 at the beginning and 2.25694 × 10-16 cm2 s-1 at the end of charge process. Sodium ions face higher polarization towards the end of each process. The reaction mechanism is investigated which suggests that NaVO3 transformed to the amorphous-like structure as confirmed by ex-situ TEM but retained high reversibility after 2nd cycle as demonstrated by XANES analysis.

ACKNOWLEDGMENT: This research was supported by the KIST Institutional Program (Project No. 2E28142 & 2V05940). This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2016R1A2B4014397).

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