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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

A High Voltage Cathode Material for Sodium Batteries: Na3V(PO4)2 Vadim M. Kovrugin,†,‡ Rénald David,†,‡ Jean-Noël Chotard,†,‡ Nadir Recham,†,‡ and Christian Masquelier*,†,‡ †

Laboratoire de Réactivité et de Chimie des Solides (LRCS), CNRS UMR 7314, Université de Picardie Jules Verne, 80039 Amiens Cedex, France ‡ RS2E, Réseau Français sur le Stockage Electrochimique de l’Energie, FR CNRS 3459, 80039 Amiens Cedex, France

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S Supporting Information *

ABSTRACT: A novel layered Na3V(PO4)2 compound was synthesized and studied as a positive electrode material for Na-ion batteries for the first time. The as-prepared material exhibits two relatively high voltage plateaus at around 3.6 and 4.0 V vs Na+/Na. Operando X-ray diffraction investigation provides insight into the mechanisms of structural transformations upon cycling.

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INTRODUCTION

and demonstrate its electrochemical behavior accompanied by a structural transformation during cycling.

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Nowadays, energy conversion and storage applications suffer from limited power capacities, lower-than-desired rates of charge/discharge processes, limitations of life cycles, high cost, etc. One strategy to overcome these issues is the rational design of novel materials with crystal structures suitable for Na+ diffusion. Na-ion batteries have been proposed as candidates for energy storage as a result of heightened interest in renewable cheaper energy sources.1,2 However, Na-ion batteries are initially expected to have a lower operating voltage and heavier weight than present lithium ion batteries. One option to avoid these drawbacks is the investigation of the specific role of transition metal cations in polyanionic structures.3,4 For example, the wide range of oxidation states (from +2 to +5) enables vanadium to form various polyhedra (octahedra, pyramids, and tetrahedra), which occur in numerous V-based crystal structures, where the types of polyhedral linkages (corner-, edge-, face-sharing, or weak van der Waals interactions) play a fundamental structural role.5−7 Among many possible polyanionic candidates, very recent studies of Na-based vanadate phosphates revealed a promising perspective of this chemical system for the exploration of new materials for Na-ion batteries.8,9 At present, there are only five reported phases in the Na2O−V2O3−P2O5 system (Figure 1). Most of them were extensively studied in terms of their electrochemical properties and showed good performance for electrochemical operation.10−14 In order to continue further exploration of the system under investigation, we report on a new Na3V3+(PO4)2 composition offering a possibility to transfer more than one electron per V ion and a high theoretical capacity of 173 mAh/g (for two e− transfer per V). Herein, we present its layered aphthitalite-like crystal structure © XXXX American Chemical Society

EXPERIMENTAL SECTION

Synthesis. The synthesis of Na3V(PO4)2 was performed in several steps, where noncommercial VPO4 was used as an intermediate precursor. The synthesis of VPO4 is described in the Supporting Information. A mixture of 2.167 mmol of as-prepared VPO4 and 2.333 mmol of commercial Na3PO4 (96% Aldrich) was mechanically ballmilled in a stainless-steel jar at a rotation speed of 200 rpm for 15 h using a Retsch Miller PM100 in air. It resulted in the mixture of VPO4, Na3PO4,15 and a small amount of impurities identified as Na2HPO416 by powder X-ray diffraction (PXRD). The presence of H leading to the formation of Na2HPO4 may result from moisture during the long-time milling. At the next step, the ball-milled powder of light brown color was pressed into a pellet of 13 mm in diameter and annealed for 1 h in a furnace heated to 850 °C at a rate of 7 °C/ min in an intensively flowing argon atmosphere. The resulting solid products contained Na3V(PO4)2 with a small amount of Na3PO4 as an impurity. Finally, the melted pellet obtained at the previous step was reground in an agate mortar and washed with distilled water for 2 h under magnetic stirring at room temperature in order to dissolve the admixture of soluble Na3PO4 phase. The resulting crystalline product was filtered through a filter paper and dried. The PXRD pattern of the pure polycrystalline sample of the new Na3V(PO4)2 composition is given in Figure S1b. Note also that the new V3+-based compound demonstrates a good stability in humid air. For the preparation of single crystals of Na3V(PO4)2, we did not press the ball-milled powder into a pellet. In this case, the resulting solid products after annealing at 850 °C were a mixture of single crystals of Na3V(PO4)2 and Na3V2(PO4)317 phases. Powder X-ray Diffraction. PXRD routine analyses of the samples were performed at room temperature in the 2θ range of 10−50° with Received: February 13, 2018

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DOI: 10.1021/acs.inorgchem.8b00401 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Ternary diagram of reported compounds in the Na2O−V2O3−P2O5 system. The theoretical capacity is given for one electron transfer per V ion. a scan step width of 0.02° and a scan rate of 1.0 s/step using a laboratory D8 Advance Bruker diffractometer (CuKα radiation, λ = 1.5418 Å). High-quality XRD pattern used for the Rietveld refinement was recorded in the 2θ range of 10−90° with a step size of 0.01° and a scan rate of 6.0 s/step. Operando XRD patterns were recorded upon electrochemical cycling in the 2θ range of 10−38° with a step size of 0.029° and a scan rate of 3.7 s/step, so that each XRD scan corresponded to 1 h of the electrochemical reaction. The JANA2006 crystallographic system18 was used to perform the profile refinements of the PXRD patterns in a profile matching mode. The Rietveld refinement of the crystal structure from high quality laboratory XRD diffraction data was carried out using the single crystal structural model in the JANA2006. The background was fitted using Chebyshev polynomial function, and the peak shapes were described by a Pseudo-Voigt function. The atomic positions, as well as the isotropic atomic displacements for all atoms, were also refined. The refined unit-cell parameters in the space group C2/c are a = 9.0959(1) Å, b = 5.0343(1) Å, c = 13.8630(1) Å, β = 91.247(1)°, and V = 634.67(1) Å3. The final observed, calculated, and difference powder XRD patterns resulting from the Rietveld refinement of the crystal structure of new Na3V(PO4)2 phase based on powder XRD data are plotted in Figure 2. Single Crystal X-ray Diffraction. A single crystal of Na3V(PO4)2 suitable for XRD data collection was examined under an optical microscope and mounted on a polymer microloop for single crystal Xray diffraction studies. More than a half of the Ewald sphere was collected with frame widths of 0.5° in ω, and a 45 s count time for each frame using a Bruker D8 Venture diffractometer equipped with a 2D CMOS detector (PHOTON 100) and a microfocus X-ray tube operated with MoKα radiation (λ = 0.71073 Å) at 50 kV and 1 mA. The diffraction data were integrated and corrected for absorption using a multiscan type model using the APEX and SADABS Bruker programs. The lattice parameters of Na3V(PO4)2 (C2/c, a = 9.1043(13) Å, b = 5.0371(8) Å, c = 13.851(2) Å, β = 91.258(7)°, V = 635.06(17) Å3, Z = 2) were determined and refined by leastsquares techniques on the basis of 1811 reflections with 2θ in the range between 5.8 and 55.80°. The crystal structure of Na3V(PO4)2 was solved in the C2/c space group by direct methods and refined to R1 = 0.044 (wR2 = 0.083) for 584 reflections with |F0| ≥ 4σF by using

Figure 2. Results of the Rietveld refinement of the crystal structure of new Na3V(PO4)2 based on laboratory powder XRD data. the SHELXL-2013 program implemented in the WinGX program package.19 The final structural model includes coordinates and anisotropic displacement parameters for all atoms. Data collection refinement parameters, crystallographic information, and fractional atomic coordinates are provided in Tables 1 and 2. The results of the bond-valence sum (BVS) calculations and selected bond distances are listed in Tables S1 and S2, respectively. All empirical parameters required for the bond-valence analysis were taken from Brese and O’Keeffe, 1991.20 CSD-433789 (ICSD, FIZ Karlsruhe) and CCDC1586953 (CSD, Cambridge) contain the supplementary crystallographic information for Na3V(PO4)2. Electrochemical Measurements. The electrochemical characterization of the sample was done after carbon coating. A mixture of 100 mg of Na3V(PO4)2 and 25 mg of sucrose (C12H22O11, Acros Organics) was ball-milled in a 50 mL stainless steel jar using a Spex Miller 8000M for 10 min in argon atmosphere. Then the mixture was heated to 700 °C at a rate of 2 °C/min and calcined for 10 min in an intensively flowing argon atmosphere. The final amount of carbon in the as-prepared composite was about 4.37 massive %. B


Article

Inorganic Chemistry Table 1. Crystallographic Information for New Na3V(PO4)2 empirical formula Mr crystal system, space group a, b, c [Å] β [°] V [Å3] Z ρcalc [g cm−3] μ [mm−1] reflections collected independent reflections, Rint R1 [F2 > 2σ(I)], wR2 [F2 > 2σ(I)] R1 [all data], wR2 [all data] goodness-of-fit largest diff. peak and hole [e Å−3]

Na3V(PO4)2 619.70 monoclinic, C2/c 9.1043(13), 5.0371(8), 13.851(2) 91.258(7) 635.06(17) 2 3.241 2.29 9310 842, 0.108 0.044, 0.089 0.083, 0.103 1.06 0.89, −0.58

Electrochemical tests were performed in Swagelok-type cells assembled in an argon-filled glovebox. Sodium metal was used as a negative electrode. About 8 mg of the composite was studied as the positive electrode separated from the sodium metal by two sheets of Whatman GF/D borosilicate glass fiber soaked in a 1 M molar solution of NaPF6 in EC/DMC (1:1 weight ratio) with 3 massive % of fluoroethylene carbonate (FEC) as additive. The latter additive demonstrates a positive effect on the efficiency of Na-based cells.21 Galvanostatic tests were conducted with an Apple Mac Pile II potentiostat. The cells were also cycled by using the galvanostatic intermittent titration technique (GITT) to obtain a charge/discharge curves closer to thermodynamic equilibrium. For the GITT protocol, a constant current of C/20 was applied for 30 min, and then, it was interrupted for 1 h to achieve the open circuit condition. This process was repeated until the electrode potential reached the cutoff voltage. For the Operando measurements, about 19 mg of the as-prepared Ccoated sample was loaded into the in situ cell equipped with a beryllium window protected by a thin aluminum foil (3 μm) to prevent a possible oxidation of the Be window at high voltages.22

Figure 3. General view of the crystal structure of Na3V(PO4)2 (a); yavapaiite-type layer (b); coordination environment of Na cations (c).

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RESULTS AND DISCUSSION Crystal Structure Analysis. The new Na3V(PO4)2 phase crystallizes in the monoclinic space group C2/c (Figure 3a). It belongs to the aphthitalite-like structural family of compounds. In contrast to the title composition, the crystal structure of aphthitalite (K3Na(SO4)2; synonym: glaserite) is trigonal, space group P3̅m1.23 Its general structural formula can be written as A(2)[12]A(1)2[10]M[6](T[4]O4)2, where S6+, Na+, and K+ cations occupy T[4], M[6], and A(1)[10] and A(2)[12] sites, respectively. The atomic coordinates for new Na3V(PO4)2 are listed in Table 2. In the crystal structure of Na3V(PO4)2, a unique tetrahedral T[4] site is filled by phosphorus with an average P−O bond length of 1.538 Å. One crystallographically independent V3+ cation has a quite regular octahedral coordination (1.987−

2.049 Å) typical for V3+-based compounds,5 and it occupies the M[6] position. Each VO6 octahedron shares all its O vertices with six PO4 tetrahedra, while phosphate groups are tridentate and share three O corners with adjacent octahedra, thus forming yavapaiite-type24 layered blocks (Figure 3b). Nonshared vertices of PO4 are oriented either “up” or “down” relative to the plane of the blocks. Yavapaiite-type layered units are quite common for inorganic compounds; their geometrical distortions have been recently considered in detail.25,26 As a result of the layered topology of the oxygen polyhedra around vanadium and phosphorus, a high operating voltage vs Na+/Na may be expected to be achieved owing to the inductive effect, which has been reported in other phosphate-based electrode materials.27,28

Table 2. Fractional Atomic Coordinates and Equivalent Displacement Parameters (Å2) for New Na3V(PO4)2

V1 P1 Na1 Na2 O1 O2 O3 O4

Wyck.

x

y

z

Ueq

4a 8e 4e 8f 8f 8f 8f 8f

0 0.16948(10) 0 0.16833(17) 0.1646(3) 0.1100(3) 0.0792(3) 0.3310(3)

0 0.52185(18) 0.0482(5) 0.5415(3) 0.3908(5) 0.3293(5) 0.7849(5) 0.5959(5)

0 0.38703(7) 1/4 0.13306(12) 0.28961(19) 0.4628(2) 0.38686(18) 0.4137(2)

0.0048(3) 0.0049(3) 0.0210(6) 0.0147(4) 0.0146(7) 0.0102(6) 0.0085(6) 0.0110(6)

C


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Inorganic Chemistry

Figure 4. Sodium diffusion pathways in the structure of Na3V(PO4)2 defined from BVEL with an activation energy of 1.6 eV (a), 3.0 eV (b), 4.5 eV (c), and 5.5 eV (d).

migration pathways compared with DFT simulations as demonstrated for example in ref 34. Despite the same long Na···Na distance (5.037 Å) between two Na1 or two Na2 sites, the calculations of the energy landscape in the structure also suggest a higher mobility of Na+ of the Na1 site. Taking into account its lower coordination number (6 and 8 for Na1 and Na2, respectively) and lower bond valence sums (0.98 and 1.13 valence units for Na1 and Na2, respectively), it might be therefore assumed that Na+ are progressively extracted from the Na1 site at first. The crystal structure of new Na3V(PO4)2 is isotypical with the reported one of Na3Fe(PO4)2 determined from powder XRD data.35,36 Several related monoclinic aphthitalite-like phases were also found in the literature. This structural type has been shown to be highly flexible and amenable to various substitutions on all possible structural sites to form a variety of similar compounds. Aside from the two Na3M(PO4)2 (M3+ = V, Fe) phosphate compositions, the monoclinic aphthitalitelike crystal structures may be partitioned into three other groups. Consequently, there are also three molybdates with vacancies at the A(1) site, six vanadates with larger cations (Sr2+, Ba2+, Ag+) at the A(2) site, and six phases, where Na+ ions occupy the octahedral M site in the yavapaiite-type layered block. The main crystallographical parameters of these compounds are given in Table S3. Electrochemistry. As recently reported,37,38 vanadium provides the exciting opportunity of exchanging more than one electron per transition metal, thus leading to high theoretical energy densities. The presence of three Na+ ions in the crystal structure of Na3V3+(PO4)2 may also lead to the possible activation of the V4+/V5+ redox couple giving a theoretical capacity of 173 mAh/g, one of the highest values in the Na−V−P−O chemical system. Hence, in order to have a full picture of the electrochemical activity of the new material, galvanostatic Na+ extraction/insertion tests were performed in different voltage ranges, i.e., in 2.4−5.0 V, 2.4−4.3 V, and 2.4− 3.8 V vs Na+/Na in Swagelok-type cells. The resulting data of Na+ (de)intercalation from the structure of Na3V(PO4)2 are plotted in Figures 5 and S2. The electrochemical extraction of Na+ from Na3V(PO4)2 occurs through two voltage-composition “plateaus” (Figure 5). A long and rather flat voltage profile at ∼3.56 V vs Na+/Na and

Contrary to the rather regular coordination environment of the A(1)[10] and A(2)[12] sites in the trigonal structure of aphthitalite, two crystallographically inequivalent Na+ in Na3V(PO4)2 show lower coordination numbers of 6 and 8, and occupy the irregular A(1)[4+2] (Na1, 4e Wyckoff position) and A(2)[6+2] (Na2, 8f Wyckoff position) sites, respectively (Figure 3c). The average Na−O bond lengths are 2.511 and 2.576 Å for Na1 and Na2, respectively. Despite the layered nature of Na3V(PO4)2, sodium ions in both A(1) and A(2) sites are located at rather long distances of 5.037 Å from each other. It may be considered as a limiting factor for the mobility of Na+ within this structure. Indeed, the bond valence energy landscape (BVEL) defined using the BondStr software (FullProf suite29) also demonstrates that sodium diffusion is limited inside the structure of Na3V(PO4)2 (non-interconnected diffusion path) considering a percolation energy of 1.6 eV reported for the movement of Na+ in polyanionic compounds.30−32 As illustrated in Figure 4, a value of 4.5 eV is necessary to make possible sodium diffusion along [001]. From a high energy value of 5.5 eV only, a sodium diffusion becomes possible along [100] and, thus, in the (001) plane of the interlayer space. Note, however, that a lithiated analogue of the title compound may have a higher ionic conductivity, as recently found by a similar observation for tavorite-like NaVPO4F and LiVPO4F phases.31 Note that the electrochemical reaction still occurred in spite of non-interconnected pathways for Na migration at low percolation energies estimated by the BVEL analysis. This apparent contradiction is explained by the fact the generated BVEL provides a semiquantitative information about migration pathways deduced from the static structural model only (cf. DFT calculations) considering the energies that would be required to move the ions without taking into account relaxations of the atomic sites in the structure.33 Therefore, it is reasonable to assume that we should consider higher energy values than 1.6 eV for Na+ migration in the structure of Na3V(PO4)2. This is in agreement with the BVEL approach developed by Adams and Rao: “the resulting energies should be scaled by a factor that depends on how rigid the network is”.33 Nevertheless, the BVEL approach still can indicate a general trend of ion diffusion and predict the correct ion D


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Inorganic Chemistry

Figure 5. First charge−discharge curves for Na3V(PO4)2 cycled at C/ 20 rate in ranges 2.4−3.8 V (red) and 2.4−4.3 V (blue) vs Na+/Na.

another shorter process centered at ∼4.02 V vs Na+/Na. A deeper insight into the sodium extraction/insertion processes was provided by a galvanostatic intermittent titration technique (GITT) experiment (C/20 for 30 min, relaxations of 1 h), which confirms the existence of two voltage regions during the extraction of Na+ ions (Figure S3). Operating on the V3+/V4+ redox couple during the first process ∼3.6 V vs Na+/Na, ∼0.9 Na+ can be extracted, and thus, the new cathode material delivers about 79 mAh/g at C/ 20. Our attempts to achieve the theoretical value and extract 1 Na+ between 2.4 and 3.8 V vs Na+/Na failed in many electrochemical tests. The oxidation of V4+ to V5+ toward the final theoretical NaV5+(PO4)2 composition may lead to an increase of the theoretical capacity value up to 173.0 mAh/g. Indeed, as seen in Figure 5, we found substantial electrochemical activity at around 4.0 V for the V4+/V5+ redox couple: only ∼0.6 Na+ were additionally extracted, thus increasing the experimental charge capacity up to 128.2 mAh/g (∼74% of the theoretical value). Five first cycles in the two voltage ranges are shown in Figure 6a,b. The Na+ extraction corresponding to the first or the second redox couple has an impact on the cycling performance. In the 2.4−4.3 V range, a stable cycling becomes possible after the fourth cycle with a reversible discharge capacity of ∼50 mAh/g. Experiments with the cutoff voltage limited to 3.8 V vs Na+/Na demonstrated a more stable cycle performance, and a higher reversible discharge capacity of ∼59 mAh/g can be obtained after the second cycle. The overall electrochemical cycling performance of Na3V(PO4)2 for the two redox couples involved in the electrochemical process is shown in Figure 6c. After a first discharge capacity of only ∼61 mAh/g, the assembled cells showed a moderate cyclability with ∼74% retention of the discharge capacity at C/20 over 50 cycles. The morphology and the size of Na3V(PO4)2 were analyzed by scanning electron microscopy (SEM, FEI Quanta 200F field-emission scanning electron microscope). As demonstrated in Figure 7a, the pristine material is characterized by relatively large platy particles up to 9 μm with sharp edges. The effect of C-coating procedure of Na3V(PO4)2 appears to be a decrease in size of particles with the average value being ∼3 μm (Figure 7b). This leads to an overall increase of the surface area and may positively contribute to the sodium accessibility and interfacial contact between the electrode and electrolyte.

Figure 6. Five first charge−discharge curves for Na3V(PO4)2 cycled at C/20 rate in the ranges 2.4−4.3 V (a) and 2.4−3.8 V (b) vs Na+/Na; long-term cycling performance (c).

In Operando XRD Characterization. To get more information about the mechanisms of Na+ extraction from Na3V(PO4)2 and verify the structural reversibility, Operando XRD measurements were performed during the full charge/ discharge cycle in the ranges of 2.4−4.3 V and 2.4−3.8 V vs Na+/Na (Figures 8 and S4). As demonstrated in Figure 8, Na3V(PO4)2 undergoes a series of biphasic reactions upon Na+ extraction, with formation of new phases. Indeed, the intensities of the initial diffraction peaks begin to decrease as soon as the first 3.6 V plateau is reached, while new ones of another phase concurrently grow (patterns #0 to 17). The same occurs for the 4.0 V region upon charge (patterns #17 to 27). For this region, note also that there are only small changes in powder XRD patterns compared with the extracted sodium contents, which can indicate the small structural change and probable electrolyte decomposition. In both examined voltage ranges, we observe a similar complicated material’s evolution: globally, the reactions appear as not complete, and we evidence a rather strong irreversibility E


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Inorganic Chemistry

peaks is indeed different in the 2.4−4.3 V and 2.4−3.8 V ranges. It suggests the different structural change of the charged material at 4.3 and 3.8 V upon discharge. Consequently, the structures obtained at the of end discharge in the two voltage ranges are distinct as can be seen from the patterns #40 and #28. Our attempts to determine the lattice parameters and possible structural model of the new phases formed after the full charge faced a serious challenge that arises from nonsufficient quality of collected laboratory X-ray powder data, and of probable overlapping of several diffraction peaks of the phases. The indexation of new peaks of the second phase using Treor90 and Dicvol06 programs failed (Figure S5). The new peaks were also considered as possible sub- (Figure S6) or superlattice (Figure S7) reflections. However, neither these symmetry transformations nor the depopulation of any Na sites could give a reliable theoretical structural model of the new phase. Simulated XRD patterns based on reported structural information on a number of other monoclinic, orthorhombic, and hexagonal aphthitalite-like compounds did not fit well with the observed reflections of the new phase. Note also the triclinic structure of Na2.88Fe(PO4)2,39 which was used as one of the basic structural models for simulations as well (Figure S8b). One of the possible causes of the structural transformation in Na3V3+(PO4)2 that occurred during the electrochemical Na+ extraction process is various coordination environments of vanadium cations in different oxidation states. From this point of view, it should be taken into account that V4+ ions are usually characterized by the presence of one short VO vanadyl bond and thus adopt a distorted octahedral coordination.5 Accordingly, in the new crystal structure of theoretical Na2V4+(PO4)2 composition, V4+ cations will take place in irregular octahedral positions. However, we suppose that such kind of distortion would not be possible without a fundamental structural transformation in the yavapaiite-type layers in the structure of the pristine Na3V3+(PO4)2 phase. This is due to high topological symmetry of the layers, where V3+

Figure 7. SEM images of pristine Na3V(PO4)2 (a) and C-coated Na3V(PO4)2 samples (b).

of the overall electrochemical reaction. Non-monotonous changes of the peak intensities also suggest some sort of biphasic reactions during discharging and eventually irreversible structural changes. The structure of the pristine material is never recovered after a full electrochemical cycle, whatever the upper cutoff voltage. It is noteworthy that the discharge curves in the 2.4−4.3 V and 2.4−3.8 V vs Na+/Na voltage ranges have different shapes (Figures 5 and 6). As shown in Figure 9 by enlarged 2θ regions of the collected Operando XRD data, the evolution of some

Figure 8. In-situ XRD patterns recorded Operando for Na3V(PO4)2 positive electrode during the one full cycle at C/20 in the 2.4−4.3 V vs Na+/Na voltage range. F


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Inorganic Chemistry

Figure 9. Enlarged views of 2θ regions of the in situ XRD patterns recorded Operando for Na3V(PO4)2 upon discharging at C/20 in the 2.4−4.3 V (a) and 2.4−3.8 V (b) vs Na+/Na voltage ranges. Different evolution of the changes of the XRD peaks are marked.

The results of the Operando X-ray diffraction measurements revealed an irreversible structural transformation upon charge/ discharge processes through a biphasic mechanism with formation of another undetermined phase. The investigation of the second phase is quite a challenging task and is expected to be reported in our next works using a high-resolution synchrotron radiation. Nevertheless, the structural type of new Na3V(PO4)2 amenable to various ion substitutions is shown to be promising for the design of novel, high-energy density materials.

cations are located almost nearly equidistant from six O on inversion centers in the C2/c space group (4a Wyckoff position). This kind of structural rearrangement may also lead to formation of diphosphate groups in the theoretical crystal structure of the new phase. In the literature, there are two reported Na2(V4+O)(P2O7) polymorphs,40−42 but none of them could be assigned with the new peaks appeared upon cycling (Figure S8c,d).

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CONCLUSION In summary, we synthesized and structurally characterized a novel, aphthitalite-like phase of Na3V(PO4)2 composition by using a several-step, solid-state method for the first time. Electrochemical tests demonstrated the original properties of Na3V(PO4)2 delivering about ∼90% (∼79 mAh/g) of the theoretical capacity operating on the V3+/V4+ redox couple, and a probable activation of the V4+/V5+ redox couple in the high voltage region with increasing of the charge capacity up to ∼128 mAh/g at the end of the first charge. However, due to a limited Na+ diffusion in Na3V(PO4)2 upon electrochemical desodiation, the pristine crystal structure of the material is not recovered after a full cycle. The observed reversible capacity is only ∼29% (∼50 mAh/g) of the theoretical value for the transfer of two electrons per V ion. Nevertheless, the new material has an excellent theoretical energy density of about ∼622 Wh/kg (173 mAh/g at an average charge/discharge potential of 3.6 V for two e− transfer per V). This value is higher than the ones experimentally observed for Na3V2(PO4)2 (∼401 Wh/kg, for one e − transfer per V) 10 and Na3V2(PO4)2F3 (∼506 Wh/kg, for one e− transfer per V),8 two NASICON-type compositions with the highest theoretical energy densities in the series of V-based polyanionic cathodes for Na batteries. Although the observed electrode performance of new Na3V(PO4)2 is far from the theoretical value at the present stage, it opens a perspective for further optimization of the electrochemical behavior of the novel layered material.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00401. Synthesis details, structural tables, electrochemical plots, Operando in situ XRD patterns, simulated XRD patterns (PDF) Accession Codes

CCDC 1586953 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vadim M. Kovrugin: 0000-0002-9010-625X Jean-Noël Chotard: 0000-0002-9867-7954 Christian Masquelier: 0000-0001-7289-1015 Notes

The authors declare no competing financial interest. G


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Inorganic Chemistry

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ACKNOWLEDGMENTS This work was carried out thanks to a postdoctoral funding provided by the Conseil Régional de Picardie and the University of Picardie Jules Verne within the framework of the NAIADES project. We also thank the anonymous referees for their constructive comments and useful suggestions.

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