Na3V(PO4)2: a new layered-type cathode material with high water

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Na3V(PO4)2: a new layered-type cathode material with high water stability and power capability for Na-ion batteries Jongsoon Kim, Gabin Yoon, Hyungsub Kim, Young-Uk Park, and Kisuk Kang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00458 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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Chemistry of Materials

Na3V(PO4)2: a new layered-type cathode material with high water stability and power capability for Na-ion batteries Jongsoon Kim*,a,†, Gabin Yoonb,c,d†, Hyungsub Kime, Young-Uk Parkf and Kisuk Kang*,b,c,d. a

Department of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul, Republic of Korea b Department of Materials Science and Engineering and cResearch Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea d Center for Nanoparticle Research at Institute for Basic Science (IBS), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea e Korea Atomic Energy Research Institute (KAERI), Daedeok-daero 989 Beon-Gil, Yuseong-gu, Daejeon, Republic of Korea f LG Chem, Daejeon 34122, Republic of Korea

ABSTRACT: We introduce Na3V(PO4)2 as a new cathode material for Na-ion batteries for the first time. The structure of Na3V(PO4)2 was determined using X-ray diffraction and Rietveld refinement, and its high water stability was clearly demonstrated. The redox potential of Na3V(PO4)2 (~3.5 V vs. Na/Na+) was shown to be sufficiently high to prevent the side reaction with water (Na extraction and water insertion), ensuring its water stability in ambient air. Na3V(PO4)2 also exhibited outstanding power capability, with ~79% of the theoretical capacity being delivered at 15C. First-principles calculation combined with electrochemical experiments linked this high power capability to the low activation barrier (~433 meV) for the well-interconnected two-dimensional Na diffusion pathway. Moreover, outstanding cyclability of Na3V(PO4)2 (~70% retention of the initial capacity after 200 cycles) was achieved at a reasonably fast current rate of 1C.

Introduction The development of energy storage systems (ESSs) is considered a high priority worldwide because of the urgent global environmental problems arising from the exhaustion of fossil fuels. Li-ion batteries (LIBs) have received widespread attention as potential ESSs for various applications such as electric vehicles and mobile devices because of their high energy density, long cycle life, and high power capability.1-12 However, the lack of Li sources and their uneven worldwide distribution limits the extension of LIBs to large-scale ESSs, demanding the development of alternative systems. Recently, Na-ion batteries (NIBs) have emerged as a promising alternative to LIBs because of the essentially unlimited Na sources and their similar reaction chemistry to LIBs.13-16 The most attractive cathode materials for NIBs include layered materials such as Na0.44MnO2, Na0.7CoO2, NaNi0.5Mn0.5O2, and Na0.67Fe0.5Mn0.5O2 because of their high capacity and large interlayer space, which enable facile two-dimensional Na diffusion.13, 17-29 However, despite the substantial merits of these layered materials, their susceptibility to moisture in ambient air prevents their practical application. Na sites with operation voltages below ~2.7 V (vs. Na+/Na) are spontaneously extracted from the structure in ambient air, leading to further side reactions including water insertion into vacant Na sites.2933 This water contamination is considered a major drawback that prevents the commercialization of layered materials as cathode materials for NIBs. Furthermore, the big interests on the aqueous NIB resulting from the high ionic conductivity of aqueous

electrolytes and the mild cell-assembly conditions require the development of novel electrode materials with high water-stability.34-36 Our approach to solve this problem involves the raise of redox potential above the water insertion potential by introducing the inductive effect of P on the layered structure, as reported previously.37-39 Herein, we report Na3V(PO4)2 as a novel layered-type cathode material for NIBs with high redox potential and outstanding electrochemical performance for the first time. It exhibited that at 15C, the discharge capacity of Na3V(PO4)2 was maintained up to ~71 mAh g−1, corresponding to ~79% of the theoretical capacity. Furthermore, the cycle test at 1C revealed the outstanding cyclability of Na3V(PO4)2, with ~70% retention of the first discharge capacity after 200 cycles.

Experimental Section Synthesis of Na3V(PO4)2 Na3V(PO4)2 powder was synthesized using the sodium-phosphate flux. NaH2PO4, Na4P2O7, and V2O3 with molar ratios of 1 : 3.6 : 2.2 were used as precursors. They were thoroughly mixed and ground by planetary ball-milling. After evaporating the acetone, the precursors were then fired at 900 oC under Ar condition for 10 hours. The final product was then waterwashed and dried at 200 oC overnight. Materials characterization

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The structural information of Na3V(PO4)2 was analyzed using an X-ray diffractometer (Rigaku, D/MAX 2500) equipped with Cu Kα radiation (λ = 1.5406 Å). Data were recorded over a 2θ range of 10 to 90o, with a step size of 0.01 o. Each step was exposed for 6s. XRD data were refined by the Rietveld method using Fullprof software. The particle size was investigated by field-emission transmission electron microscopy (FETEM) and field emission scanning electron microscopy (SEM). Electrochemistry Electrochemical tests were performed in a CR2032-type coin cell assembled in an Ar-filled glove-box. The electrode was prepared as follows. First, the powder was mixed with super P by dry ball milling (100 rpm) at a ratio of 80:20 wt %. Then, the total composition of the electrode was adjusted to 72 wt % of the active material, 18wt % super P, and 10 wt % polyvinylidene fluoride (PVdF). The electrode was fabricated by pasting a slurry of the powder mixture in N-methyl-2-pyrrolidone (99.5%, Aldrich) (NMP) onto Al foil using a doctor blade. NMP was evaporated for 2 h in an oven at 120 oC. The loading amount of the electrode was 2ⅹ10-3 g cm-2. The cell was assembled using a Na counter electrode, a separator (GF/F glass fiber), and 1 M solution of NaPF6 in ethyl carbonate/propylene carbonate (EC/PC, 1:1 v/v) in an Ar-filled glove-box. Galvanostatic charge/discharge tests were performed at various C rates (C/5, C/2, 1C, 2C, 5C, 10C and 15C in the 2.0-3.9V window, 1C = 90 mA g-1) for Na3V(PO4)2 using WBCS 3000 (WonA Tech). For the QOCP measurement, the electrochemical cells were rested for 4 hours after each hour-long charging or discharging at the C/20 rate (voltage window: 2.0-3.9 V) at room temperature. 1C corresponds to ~90 mA g-1.

Computational Details Density functional theory (DFT) calculations are performed to gain information on the phase reaction, average Na intercalation voltage and activation barriers for Na diffusion. Projector-augmented wave (PAW)40 pseudopotentials are used as implemented in Vienna Ab initio Simulation Package (VASP).41 Exchange-correlation energies are described with spin-polarized generalized gradient approximation (GGA) proposed by Perdew-Burke-Ernzerhof (PBE).42 A Hubbard U parameter (GGA+U)43 is introduced for the correction of self-interaction error. The Ueff value for vanadium is set to 4.2 eV as reported in the study on NASICON structured Na3V2(PO4)3.44 An appropriate number of k-points and a kinetic energy cut-off of 500 eV are used in all calculations. All structures are optimized until the force in the unit cell converges within 0.02 eV Å -1. Phase stability of Na3-xV(PO4)2 (0 < x < 1) is investigated by generating all configurations of Na in specific Na contents and calculating the formation energies. CASM software45 is used for the generation of all Na / vacancy configurations at each composition, followed by full DFT calculations on maximum 40 configurations with lowest electrostatic energy at each composition to obtain a convex hull of Na3-xV(PO4)2 (0 < x < 1). Nudged elastic band (NEB)46 calculations are carried out in order to determine the activation barrier for Na diffusion in Na3V(PO4)2 structure. A 1 X 3 X 1 supercell is adapted to avoid the interactions between periodic unit cells. Five intermediate images are generated between the initial and final images for Na diffusion pathways. These structures are then calculated by

Figure 1 . (a) Refined XRD patterns of Na3V(PO4)2 (RP = 4.12%, RI = 2.35%, RF = 2.01%, and χ2 = 4.64%) and (b) schematic illustration of Na3V(PO4)2 structure.

NEB algorithm with the fixed lattice parameters and free internal atomic positions.

Results and Discussion Na3V(PO4)2 was synthesized using the sodium phosphate flux; the detailed synthesis process is described in Supporting Information. The X-ray diffraction (XRD) pattern in Figure 1a indicates that Na3V(PO4)2 was successfully synthesized with a monoclinic (C2/c) space group identical to that of Na3Fe(PO4)2.47-48 No contamination or secondary phases except for Na3V(PO4)2 were detected. Rietveld refinement of the XRD pattern yielded lattice parameters of Na3V(PO4)2 of a = 9.09390(8) Å , b = 5.03264(5) Å , and c = 13.86310(13) Å with β = 91.2471(9)°. Other structural information such as the atomic position, B iso and occupancy with the preferred orientation parameter are tabulated in Supporting Table T1. The accuracy of the calculated structural model using Rietveld refinement was confirmed by the small reliability factors (RP = 4.12%, RI = 2.35%, RF = 2.01%, χ2 = 4.64%). The particle sizes and morphologies of the Na3V(PO4)2 powder were examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The particle size was larger than ~1 μm, and the micron-sized particles were single crystalline rather than agglomerates of


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Chemistry of Materials that Na3V(PO4)2 is water stable despite the layered structure. This finding contradicted our expectation because most of the layered materials for NIBs are susceptible to moisture. The water stability of Na3V(PO4)2 was attributed to the high operation voltage above ~2.7 V (vs. Na+/Na), which could inhibit the formation of vacant spaces arising from spontaneous desodiation under ambient air. The structural analyses performed using Rietveld refinement revealed the full occupation of 3 Na ions in the Na3V(PO4)2 structure (Supporting Table T1), indicating that water intercalation into the structure and the subsequent side reactions could not occur.

Figure 2 (a) Formation energy of Na3−xV(PO4)2 (0 < x < 1) and (b) QOCP profile of Na3V(PO4)2 measured at C/20 and calculated average voltage [inset: dQ/dV of Na3V(PO4)2].

nanocrystals, as indicated by the TEM and SEM images and selected area electron diffraction (SAED) analysis in Supporting Figure S1. Figure 1b shows the atomic structure of Na3V(PO4)2 calculated by Rietveld refinement. The Na3V(PO4)2 framework has a layered structure, with each layer consisting of a combination of VO6 octahedra and PO4 tetrahedra. All the VO6 octahedra share corners with tetrahedral PO4 units along the ab plane, and Na occupies two distinguishable interlayer sites: one in the vicinity of the [VO6–PO4]∞ chain and the other in the middle of the wide interlayer space. These structural properties provide a well-interconnected two-dimensional channel for Na diffusion in the Na3V(PO4)2 structure, which can possibly allow the realization of fast Na diffusion kinetics. Compared with other structure of the double phosphate family such as Na3In(PO4)249, Na3Fe(PO4)247, 50, Rb3In(PO4)251 and K3In(PO4)251-52, Na3V(PO4)2 phase shows the structural similarity with Na3Fe(PO4)2 and Na3In(PO4)2 that exhibit more regular octahedra than the others, arising from the size difference of the cations (Na+, K+, Rb+) in the octahedron. Of particular interest is the high water stability of Na3V(PO4)2 compared with that of other layered Na cathode materials.26-32 Supporting Figure S2 presents XRD patterns of the as-synthesized Na3V(PO4)2 and Na3V(PO4)2 exposed to humid air for 10 days. No noticeable difference was detected between the XRD patterns, indicating

The redox potential of Na (de)intercalation in Na3V(PO4)2 was verified through combined experiments and first-principles calculations. The structural information for Na3V(PO4)2 obtained from Rietveld refinement was used to construct initial structures for all the first-principles calculations. Figure 2a presents the convex hull plot of Na3−xV(PO4)2 (0 < 𝑥 < 1) for various Na contents, which was generated based on first-principles calculations. Because no stable intermediate phase was observed, it was predicted that the biphasic reaction of Na3V(PO4)2 and Na2V(PO4)2 could occur during Na (de)intercalation at Na3V(PO4)2. Based on the information on the phase stabilities, the theoretical redox potential of Na3V(PO4)2 was calculated by comparing the formation energies of the two end phases of Na3V(PO4)2 and Na2V(PO4)2. The predicted potential was verified to be ~3.67 V (vs. Na+/Na). These first-principles calculation results correspond well with the experimental data. The quasi-open-circuit potential (QOCP) and dQ/dV profiles in Figure 2b reveal that the equilibrium potential was ~3.5 V (vs. Na+/Na) with a flat voltage profile for all Na contents. Additionally, we tried to verify the possibility of further desodiation by increasing the charge cutoff voltage to 4.2 V (vs. Na+/Na). Supporting Figure S3 shows that although Na3V(PO4)2 was charged to 4.2V (vs. Na+/Na), the re-intercalated amount of Na remains unaffected, indicating that only one Na ion participates in the redox reaction of Na3V(PO4)2. The biphasic reaction identified by the first-principles calculation and QOCP analysis was further investigated using XRD analysis. Full XRD patterns of each Na3−xV(PO4)2 (0 ≤ x ≤ 1) structure are presented in Supporting Figure S4. As observed in Figure 3a, only the signals related to the two end phases of Na3V(PO4)2 and Na2V(PO4)2 were detected in the ex situ XRD patterns of Na3-xV(PO4)2 (0 ≤ x ≤ 1), confirming the biphasic reaction mechanism of Na3V(PO4)2. The common trends of a biphasic reaction were observed, with shrinkage of the (002) peak from the pristine phase and the emergence of a new (002) peak from the charged phase during Na deintercalation. To understand the change in the (002) peak position during charging, we more closely examined the calculated structural information. The pristine Na3V(PO4)2 phase has a total of three Na ions in the formula unit occupying two crystallographically distinguishable Na sites; Figure 1b shows that one Na ion is located in the middle of the interlayer spacing (Na1 site) and the other two are located close to the [VO6–PO4]∞ layer (Na2 site). As observed in Figure 3b, the first-principles calculations indicated that the Na ions in the Na2 sites would be preferentially extracted to form Na2V(PO4)2 upon charging. As half of the Na ions in the Na2 site are extracted upon charging, the attraction force between [VO6–PO4]∞ layers decreases, resulting in a slight elongation along the c-axis, as typically observed in layered materials.17-18, 53 This c lattice parameter increase from



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Figure 3 (a) Ex situ XRD patterns of Na3V(PO4)2, Na2.6V(PO4)2, and Na2V(PO4)2 and (b) calculated structures of end-member phases. As half of the Na ions in Na2 sites (indicated with black circles) are extracted in Na 2V(PO4)2, the distance between VO6 polyhedral along the c-axis increases from 6.98 to 7.25 Å .

Figure 4 (a) Na diffusion pathways of Na3V(PO4)2 and magnified view of repeating unit. (b) Activation barriers for various pathways of Na diffusion in Na3V(PO4)2 structure. The lowest activation barrier of the connected Na diffusion pathways is 433 meV.

~6.98 to ~7.25 Å corresponds to the difference in the (002) peak positions of Na3V(PO4)2 and Na2V(PO4)2. The change of the V oxidation state for charge compensation upon Na (de)intercalation at Na3V(PO4)2 during charge/discharge was identified in Xray absorption near-edge structure (XANES) spectra, as described in Supporting Figure S5a. During Na deintercalation, the V K-edge shifted toward higher energy and the V pre-edge gradually increased, indicating oxidation from V3+ to V4+. This XANES result agrees well with previous XANES results for compounds containing V3+ and V4+ ions.53-55 The oxidation of V3+ to V4+ upon deintercalation was also confirmed by first-

principles calculations, as shown in Supporting Figure S5b. Additionally, extended X-ray absorption fine structure (EXAFS) measurement showed that the local environment of V in Na3V(PO4)2 was affected as the oxidation state of V changes upon desodiation (Supporting Figure S5c). The activation barriers for Na diffusion along the ab plane were calculated to predict the kinetic properties of Na3V(PO4)2. Figure 4a shows the Na diffusion pathways in the Na3V(PO4)2 structure. The Na diffusion channels consist of repeating basic units that are two-dimensionally interconnected with each other. These well-defined two-dimensional Na diffusion pathways


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Chemistry of Materials performances of NASICON-Na3V2(PO4)3 could be greatly improved through the fabrication of nano-architectures.62-63 Thus, it was supposed that the nano-effect enables the highly enhanced electrochemical performances of Na3V(PO4)2. Galvanostatic cycling testing at a rate of 1C revealed the superior cycle stability of Na3V(PO4)2, with ~70% of the first discharge capacity retained after 200 cycles of charge/discharge, as shown in Figure 5b. We suppose that one of the main reasons for the capacity fading is the large structural change during charge/discharge. As shown in Figure 3a, Na3V(PO4)2 undergoes the twophase reaction accompanying the large increase of c lattice parameter upon Na (de)intercalation. Repeating exposure on this harsh condition may damage the electrode, resulting in the decrease of reversible capacity upon cycling. Additionally, we carried out the full-cell test of Na3V(PO4)2 using hard-carbon as the anode electrode (Supporting Figure S7). The hard-carbon at the anode electrode was prepared with the less weight percent compared to that of Na3V(PO4)2, in order to balance the difference of specific capacities between Na3V(PO4)2 and hard-carbon. Our full-cell system showed excellent cycle stability for more than 20 cycles, suggesting that Na3V(PO4)2 could be one of the promising candidates for Na-ion battery cathodes.

Figure 5 (a) Discharge curves of Na3V(PO4)2 as a function of C rate (C/5, C/2, 1C, 2C, 5C, 10C, and 15C) and (b) cyclability of Na3V(PO4)2 over 200 cycles at 1C in a Na cell. [inset: Galvanostatic curves of Na3V(PO4)2 over 200 cycles at 1C in a Na cell].

contribute to the higher power capability of Na3V(PO4)2 than materials with one-dimensional diffusion pathways, where the ionic diffusion is significantly hindered by the presence of a small amount of atomic defects.56-58 The basic unit for the Na diffusion channel includes three migration paths from Na1 to Na2, as shown in Figure 4a. The activation barriers for each diffusion channel are tabulated in Figure 4b. Because Na ions more stably occupy Na1 sites than Na2 sites, migrations from Na1 to Na2 sites are more sluggish than the opposite paths. The calculated activation barrier for Na diffusion in the Na3V(PO4)2 structure was only ~400 meV, which is sufficiently low to ensure the facile migration of Na ions along the ab plane when combined with the well-interconnected two-dimensional pathways. This finding also corresponds well with our experimental results, which demonstrated the excellent power capability of Na3V(PO4)2, as shown in Figure 5a. The discharge profiles of Na3V(PO4)2 were obtained at various current rates (C/5, C/2, 1C, 2C, 5C, 10C, and 15C in the 2.0–3.9 V window, 1C = 90 mA g−1). At C/5, Na3V(PO4)2 delivered a discharge capacity of ~90 mAh g−1, which is very close to its theoretical capacity (~90 mAh g−1). Even at 15C, a discharge capacity of up to ~71 mAh g−1 was maintained, corresponding to ~79% of the theoretical capacity. Compared to this result, the pristine Na3V(PO4)2 without the carbon-coating using planetary ball-miller did not deliver the outstanding power-capability (Supporting Figure S6), which means that improvement of the electronic conductivity of Na3V(PO4)2 is essential for its application as the electrode material for NIB, like other phosphate-based materials.59-61 Furthermore, the recent reports showed that on the electrochemical

In summary, layered-structure Na3V(PO4)2 was prepared for the first time. Unlike most layered cathode materials (NaxMO2) for NIBs, Na3V(PO4)2 exhibits great water stability with a high redox potential of ~3.5 V vs. Na+/Na resulting from the inductive effect of P. The electrochemical properties of Na3V(PO4)2 were extensively analyzed using combined experiments and first-principles calculations. Na3V(PO4)2 was verified to undergo a biphasic reaction during Na (de)intercalation accompanying the V3+/V4+ redox reaction. First-principles calculations further revealed that well-interconnected two-dimensional Na diffusion pathways along with low activation barriers resulted in the excellent power capability of Na3V(PO4)2 (~79% retention of the theoretical capacity). Furthermore, the cycle life of Na3V(PO4)2 was stably retained over 200 cycles (~70% retention of the first capacity). We believe that our approach to design a noble electrode material can provide impressive insights for general readers studying material chemistry as well as the NIB community.

ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.” XRD patterns of fleshly synthesized Na3V(PO4)2, Na3V(PO4)2 exposed to humid air for 10days and the TEM and SEM images of Na3V(PO4)2, full ex-situ XRD patterns of each Na3-xV(PO4)2 (0 ≤ x ≤ 1), V K-edge XANES and spin integration around V of Na3V(PO4) electrochemical profiles with different charge cutoff voltages, power-capability, full-cell tests and structural information of Na3V(PO4)2.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] and [email protected]

Author Contributions †J.K.

and G.Y. contributed equally to this work.

Notes The authors declare no competing financial interest.



ACKNOWLEDGMENT This work was supported by the Research Fund (PNK5600) of Korea Institute of Materials Science (KIMS) and by the National Institute of Supercomputing and Network/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2017-S1-0028)

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61. Aragón, M. J.; Gutiérrez, J.; Klee, R.; Lavela, P.; Alcántara, R.; Tirado, J. L., On the effect of carbon content for achieving a high performing Na3V2(PO4)3/C nanocomposite as cathode for sodium-ion batteries. J. Electroanal. Chem. 2017, 784, 47-54. 62. Wang, Q. Y.; Zhao, B. D.; Zhang, S.; Gao, X. H.; Deng, C., Superior sodium intercalation of honeycombstructured hierarchical porous Na3V2(PO4)3/C microballs prepared by a facile one-pot synthesis. J. Mater. Chem. A 2015, 3, 7732-7740.

63. Klee, R.; Aragon, M. J.; Lavela, P.; Alcantara, R.; Tirado, J. L., Na3V2(PO4)3/C Nanorods with Improved Electrode-Electrolyte Interface As Cathode Material for Sodium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 23151-23159.


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Chemistry of Materials

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Na3V(PO4)2: a new layered-type cathode material with high water

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