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Insight into the Electrochemical Sodium Insertion of Vanadium Superstoichiometric NASICON Phosphate María J. Aragón, Pedro Lavela,* Gregorio F. Ortiz, Ricardo Alcántara, and José L. Tirado Departamento de Química Inorgánica e Ingeniería Química, Instituto Universitario de Investigación en Química Fina y Nanoquímica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie, E-14071 Córdoba, Spain

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S Supporting Information *

ABSTRACT: A slight deviation of the stoichiometry has been introduced in Na3−3xV2+x(PO4)3 (0 ≤ x ≤ 0.1) samples to determine the effect on the structural and electrochemical behavior as a positive electrode in sodium-ion batteries. X-ray diffraction and XPS results provide evidence for the flexibility of the NASICON framework to allow a limited vanadium superstoichiometry. In particular, the Na2.94V2.02(PO4)3 formula reveals the best electrochemical performance at the highest rate (40C) and capacity retention upon long cycling. It is attributed to the excellent kinetic response and interphase chemical stability upon cycling. The electrochemical performance of this vanadium superstoichiometric sample in a full sodium-ion cell is also described.

1. INTRODUCTION Sodium-ion batteries are being envisaged as serious competitors of Li-ion analogues as electrochemical storage devices in specific applications. Particularly, the former ones can be used in large stationary batteries, in which the volumetric energy density specifications are not so constraining. The high abundance, low cost, and environmental friendliness of sodium compounds may entail economic benefits that counterbalance minor disadvantages as there is a slightly lower redox potential for a larger ionic radius as compared to that of lithium.1−6 The research on new electrode materials for Na-ion batteries is being promoted because the reaction mechanism is only slightly different from that of Li-ion batteries. This facile implementation is boosting the interest in this technology.7−10 In the case of cathode materials, the open framework, offered by polyanionic compounds with a NASICON structure, has demonstrated its usefulness for the diffusion of sodium ions.11,12 Among them, Na3V2(PO4)3 is recently drawing much attention. The V4+/V3+ redox couple allows the exchange of two electrons per formula unit at an operating voltage close to 3.6 V, leading to sodium batteries with high energy densities. Otherwise, the diffusion path provided by concatenated 8-fold sites ensures a proper ionic conductivity.13−15 Notwithstanding, the presence of PO4 tetrahedra, linking ribbons of subsequent VO6 and NaO6 polyhedra along the c direction, avoids an infinite concatenation of metal and oxygen atoms unlike that for metal oxides. This leads to poor electronic conductivity because the electron delocalization is hindered. Several successful solutions have been proposed to promote their electronic conduction such as carbon coating,16,17 vanadium substitution,18,19 and new particle morphology.20,21 Alterna© 2017 American Chemical Society

tively, new strategies rely on the induction of a mild superstoichiometry in the original compound to slightly modify the parameters of the host framework. It has been successfully implemented in related polyanionic compounds for Li-ion batteries. These works provided evidence that both changes in the lattice parameters and the appearance of favorable impurities were crucial to explain the improved electrochemical performance.22−25 Concerning Na-ion batteries, only a few examples of nonstoichiometry have been proposed for pyrophosphate26,27 and sulfate28 polyanionic compounds. To the best of our knowledge, an approach based on transition metal superstoichiometry has not yet been evaluated in sodium based NASICON structures. For this purpose, an adept synthetic route in conjunction with detailed X-ray diffraction, electron microscopy, and X-ray photoemission spectroscopy studies was performed to unveil their structural and textural properties, while galvanic tests will elucidate the electrochemical behavior.

2. EXPERIMENTAL SECTION Na3−3xV2+x(PO4)3 (0 ≤ x ≤ 0.1) samples were prepared by a single and easily scalable sol−gel procedure. The highest value of x was set after checking preliminary results demonstrating that larger distortions of the stoichiometry would lead to samples with poor electrochemical properties. Citric acid (Aldrich, 99%), in a 3:2 citric to vanadium ratio, was used as a reagent favoring metal chelation. Several molar ratios of NH4VO3 (Panreac, 98%), NaH2PO4 (Aldrich, 98−102%), and NH4H2PO4 (Panreac, 98%) were employed to yield the eventual sample superstoichiometry. In order to ensure a homogeneous Received: July 20, 2017 Published: September 13, 2017 11845

DOI: 10.1021/acs.inorgchem.7b01846 Inorg. Chem. 2017, 56, 11845−11853

Article

Inorganic Chemistry precursor, ammonium metavanadate and citric acid were first dissolved in deionized water. Then, an aqueous solution containing the previously dissolved phosphate salts was poured dropwise. This solution was gently stirred for 1 h, and water was then removed in a rotary evaporator at 70 °C and 200 mbar. The so-obtained gels were dried at 120 °C, and the greenish solid product was ground in a planetary ball miller for 5 min at 300 rev/min before thermal annealing at 800 °C for 8 h (heating ramp: 5 °C min−1) under an Ar stream (80 mL/min). The final product is characterized by a black color resulting from the presence of a residual carbon phase yielded by the thermal decomposition of citric acid in the inert atmosphere. This favors the electronic conductivity of the samples. The carbon content was determined in an Elemental CHNS Eurovector EA 3000 analyzer. These samples will be named NVP-x (0 ≤ x ≤ 0.1), with x being the subscript value in the Na3−3xV2+x(PO4)3 general formula. X-ray powder diffraction (XRD) patterns were recorded on a BrukerD8 Discover A25 diffractometer using filtered Cu Kα radiation within a 2θ angle range between 10° and 80° and a scan rate of 0.025°/s. The structural determination of lattice parameters and the crystallographic occupancies of sodium and vanadium were calculated with TOPAS software. The structural scheme was drawn with Vesta 3 software.29 Transmission electron microscopy (TEM) images were acquired in a JEOL 1400 microscope. Field-emission scanning electron micrographs (FE-SEM) were recorded in a JSM-7800F Prime microscope equipped with an EDX analyzer. The chemical state of the elements was analyzed in an X-ray photoelectron spectrometer (XPS) (SPECS Phobios 150 MCD) furnished with a Mg Kα source. The samples were supported on an aluminum disk and subjected to high vacuum overnight (4 × 10−9 mbar). The binding energy scale was referred to the C 1s line of the adventitious carbon (284.6 eV). Electrochemical characterization of electrode materials was carried out in sodium half-cells. Working electrodes were manufactured by mixing the active material (80%), carbon black (10%), and PVDF (polyvinylidene fluoride) (10%) in N-methyl-2-pyrrolidone. This paste was spread onto 9 mm aluminum disks and vacuum-dried at 120 °C for 2 h. The electrode mass loading was ca. 5 mg cm−2. The counter electrode was a 9 mm sodium disk. The cell assembly consisting of both electrodes separated by glass fiber disks (GF/A-Whatman) soaked in 1 M NaPF6 (ethylene carbonate/diethyl carbonate) (EC/ DEC) (v/v = 1/1) (5 wt % fluoroethylene carbonate (FEC)) was mounted in a Swagelok-type two-electrode cell in an argon filled glovebox under controlled O2 and H2O traces. Galvanostatic cycling was tested in a VMP multichannel system setting a potential window between 2.0 and 4.3 V and several discharge and charge rates ranging from C/2 to 40C. Electrochemical impedance spectra (EIS) were scanned on a SP-150 Biologic equipment to determine the cell impedance. For this purpose, Swagelok type three-electrode cells, with sodium disks as counter and reference electrodes, were subjected to a few cycles. After voltage relaxation pursuing a quasiequilibrium state, the impedance spectra were measured by perturbing the open circuit voltage with an ac signal of 5 mV from 100 kHz to 0.001 mHz. A full sodium-ion cell was assembled in a Swagelok-type three electrode cell furnished with metallic sodium as a pseudoreference electrode and a selected sample as a working electrode. As a counterelectrode, hard carbon obtained by sucrose carbonization at 1200 °C for 6 h was employed. This electrode was composed by hard carbon (90%), carbon black (5%), and PVDF (5%). The electrolyte solution was also 1 M NaPF6 (EC:DEC) (v/v = 1/1) (5 wt % FEC). Capacity values are referred to the active mass of the positive electrode. Thus, an excess of the anode was allowed to ensure that the cell was controlled by the positive electrode. In addition, the initial irreversibility during the first half cycle was circumvented by sequentially discharging the negative and charging the positive electrodes in half-cell configuration at 2C.

first evaluated by X-ray diffractometry. The patterns, displayed in Figure 1, show narrow reflections characteristic of highly

Figure 1. X-ray diffraction patterns of off-stoichiometric Na3−3xV2+x(PO4)3 (0 ≤ x ≤ 0.1) samples. Experimental pattern (blue). Calculated pattern (red) and differential curve (gray) are included for each sample. DIF patterns for Na3V2(PO4)3 indexed in the C2/c space group are also included (green).

crystalline samples. All these peaks were indexed in the C2/c space group17,20,31 of the monoclinic system, and no additional peak associated with the presence of impurities was detected. The amorphous character of the in situ generated carbon phase prevents a proper structural determination by this technique. In addition, the chemical analysis provided evidence for a significant contribution of this carbonaceous phase to the electrode materials, as indicated in Table 1. These patterns were refined by the Rietveld method to calculate cell parameters, atomic coordinates, and occupancies. The slight deviation of the stoichiometry involved a cell volume expansion for the entire range in Na3−3xV2+x(PO4)3 (Table 1). This trend was also reported for the Li3−3xV2+x(PO4)3 series.32 These results point to a good flexibility of the NASICON framework when the Na/V ratio is slightly changed. In addition, this cell expansion may favor sodium diffusion through the framework and provide a high rate during the cell charge and discharge.33 In order to have a more precise insight into the structural modification induced by the superstoichiometry, the Rietveld refinement of these patterns was performed. The full occupancy of the vanadium position prevented the allocation of the transition metal excess at these same crystallographic sites. Thus, a first approach consisted of allowing for the occupancy of Na1 positions, which could be vacant after introducing the sodium defect for charge compensation. The results yielded large numerical inaccuracies and fitting errors, so that it was discarded (not shown). Alternatively, the occupancy of Na2 sites was proposed. Table 2 shows the atomic coordinates and occupancies for NVP-0.02 sample while assuming this approach. As can be seen, the atomic positions of V2 sites in which the vanadium excess is allocated are close to those of the Na2 sites. Moreover, the occupancy of these sites is quite similar to that expected from nominal stoichiometry. Table S1 (Supporting Information) shows the Rietveld treatment for the remaining samples confirming this behavior. Rietveld refinement also allowed the calculation of angles and bond lengths for sodium sites in NVP-0 and NVP-0.02. Figure 2 depicts the

3. RESULTS AND DISCUSSION The effect of the superstoichiometry induced by the incorporation of a vanadium excess and a concomitant sodium defect on the structural properties of Na3V2(PO4)3 has been 11846


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Inorganic Chemistry Table 1. Carbon Content and Unit Cell Parameters in the C2/c Space Group for the Na3−3xV2+x(PO4)3 Samples x

0

0.02

0.05

0.08

0.1

a/Å b/Å c/Å β/deg V/Å3 Na/V (Rietveld) Na/V (EDX) carbon/% wt

15.093(2) 8.717(1) 8.821(1) 124.59(1) 955.5(3) 1.5 1.42 12.1

15.106(2) 8.717(1) 8.830(1) 124.56(1) 957.7(2) 1.46 1.4 10.4

15.108(2) 8.721(1) 8.839(1) 124.65(1) 958.0(2) 1.4 1.32 11.3

15.111(2) 8.725(1) 8.848(1) 124.72(1) 958.8(3) 1.34 1.27 13.7

15.111(2) 8.728(2) 8.860(2) 124.71(1) 960.3(6) 1.28 1.27 13.6

Table 2. Rietveld Refined Parameters for the Superstoichiometric Na3−3xV2+x(PO4)3 Phases (x = 0.02)a x = 0.02

Wyckoff site

x

y

z

occ

Na1 Na2 V1 V2 P1 P2 O1 O2 O3 O4 O5 O6

4d 8f 8f 8f 8f 4e 8f 8f 8f 8f 8f 8f

0.25 0.3175(25) 0.1018(10) 0.35(16) 0.3503(18) 0 0.0792(31) 0.1241(35) 0.2545(33) 0.3776(32) 0.4109(30) 0.5588(28)

0.25 0.4162(36) 0.2611(19) 0.40(26) 0.1220(25) 0.0390(31) 0.1227(47) 0.4202(39) 0.1698(42) 0.1645(52) 0.4739(40) 0.1764(41)

0.5 0.2839(49) 0.05878(99) 0.27(33) 0.2613(33) 0.25 0.25 0.2001(46) 0.2156(50) 0.0537(41) 0.0825(45) 0.1052(58)

1 0.97 1 0.008 1 1 1 1 1 1 1 1

a Atomic fractional coordinates (x, y, and z) and atoms occupancy (occ) values are written. Fitting parameters as R-expected (Rexp), R-weighted pattern (Rwp), and R-Bragg (RBragg) are indicated as follows: Rexp, 10.10; Rwp, 13.95; RBragg, 8.268.

To gather information about the chemical state of the probe elements at the surface, XPS spectroscopy at the V 2p, O 1s, and Na 1s core levels was performed (Figure 3). The V 2p spectra are characterized by two broadened and asymmetric bands which can be decomposed in two different components at 516.6 ± 0.2 eV and 517.2 ± 0.2 eV for V 2p3/2 and 522.6 ± 0.1 eV and 524.5 ± 0.3 eV for V 2p1/2. The nominal stoichiometry of these samples was chosen so that the excess of vanadium would be compensated by a deficiency of sodium. Thus, the presence of oxidized vanadium would not be expected. We are tempted to ascribe these profiles to the presence of trivalent vanadium, as commonly reported elsewhere.34,35 Thus, we explain the shoulder at high binding energies to vanadium located in a different local environment, in good agreement with the Rietveld results. The relative contribution of this new site for vanadium progressively increases with x, following the same trend as that for the cell volume and vanadium occupancy in Na2 sites (Figure 4a). The O 1s core level spectra of Na3+3xV2−x(PO4)3 also revealed changes in the chemical state of this element. Asymmetric profiles clearly revealed the presence of two overlapped signals at 531.1 ± 0.1 and 533.0 ± 0.2 eV (Figure 3). These components can be interpreted in terms of the different ionicity existing in Na−O−P and V−O−P bonds. The progressive increase of its relative contribution with x would agree with the increase in V stoichiometry along the series (Figure 4b). Notwithstanding, these values notably differ from the x values. Bearing in mind the enhanced attenuation of the XPS signal from deeper layers, we must conclude that the superstoichiometric effect mainly manifests close to the particle surface. Thus, it would affect the electrode−electrolyte interface and significantly affect the accessibility of sodium ions to the NASICON framework. XPS spectra at the Na 1s core level

Figure 2. Angle and bond lengths, calculated from Rietveld refinement, for Na1 and Na2 sites in (a) NVP-0 and (b) NVP-0.02.

coordination polyhedra for both Na sites in which larger Na−O distances can be detected for NVP-0.02 in good agreement with larger cell volumes. This fact should favorably contribute to sodium diffusion. Also, less distorted octahedra were observed for the superstoichiometric sample, which may lead to a high structural stability (Figure 2b). 11847


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particles. This morphology is commonly observed when these materials are prepared by the sol−gel method. TEM images allowed a close inspection of this morphology (Figure 5c,d). The inorganic phosphate is characterized by large micrometric and sharp-edged particles. Small and light flakelike particles are observed to coat Na3V2(PO4)3. Although the thickness of the carbon coating can be highly variable, this presence ensures a good electrical connectivity among the Na3V2(PO4)3 particles and may contribute favorably to preserve the solid interface from an undesirable corrosive reaction during cycling.17 The sol−gel route used for the preparation of precursors ensured a homogeneous dispersion of elements in the final product, as evidenced by the EDX analysis (Figures S1−S3, Supporting Information). This technique also allowed the quantitative determination of the Na/V ratio. These values are written in Table 1 and provided evidence of a good agreement with those derived from the theoretical stoichiometry and atom occupancies. Galvanostatic cycling at different C rates was performed in sodium half-cells (Figure 6a). The voltage response exhibited extended plateaus at ca. 3.41 and 3.34 V during charging and discharging, respectively. These plateaus were ascribed to the reversible insertion of sodium ions into the NASICON framework by a biphasic mechanism, while V(III) is reversible oxidized to V(IV).13,14,36 The occurrence of a short plateau at the end of discharge with a slightly lower voltage is a feature commonly attributed to an increase in polarization of the metallic sodium counter electrode rather than changes in the potential of the working electrode.37 Further experiments in a full cell, in which a carbonaceous material will be employed as a counter electrode, will provide evidence for the disappearance of this effect. The common behavior in all cases was a capacity decrease and charge/discharge polarization due to the fast kinetics imposed on increasing rates. These experimental conditions limit the ability of sodium ions to diffuse through the host framework. The rate capability can be inferred from Figure 6b, in which cycling at a variable C rate was performed. Slight deviations of the stoichiometry until x = 0.05 did not involve large changes in the capacity; the sodium half-cells were subjected to the lowest rate of C/2. Thus, NVP-0 delivered 108 mA h g−1, after 5 cycles at C/2, while NVP-0.02 and NVP-0.05, respectively, performed with 103 and 106 mA h g−1. In contrast, samples with x ≥ 0.08 suffered an enhanced capacity decrease delivering values below 90 mA h g−1 at the same low rate. The increase of the current

Figure 3. X-ray photoelectron spectra at the V 2p, O 1s, and Na 1s core levels for Na3−3xV2+x(PO4)3 (0 ≤ x ≤ 0.1) samples.

were also recorded (Figure 3). A unique signal at 1071.4 ± 0.1 eV was detected and assigned to sodium atoms in the NASICON structure. In a previous report about superstoichiometric Na3V2(PO4)3 with sodium excess, we detected a second signal at high binding energies ascribable to Na4P2O7 impurities.30 The absence of a similar shoulder would confirm the high purity of the samples obtained here. SEM images in Figure 5a,b show micrographs of selected samples to unveil a similar texture in all cases. Micrometric sharp-edge particles with an irregular shape reveal a rough surface coming from the presence of nanometric coating

Figure 4. Relative contribution of deconvoluted components calculated for the (a) V 2p and (b) O 1s core level spectra of the studies samples. 11848


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Figure 5. Electron microscopy images of selected samples. (a, b) SEM micrographs of NVP-0.02 and NVP-0.05 samples. (c, d) TEM micrographs of NVP-0.05 and NVP-0.1 samples.

Figure 6. (a) Galvanostatic charge and discharge curves at rates from C/2 to 40C of sodium half-cells assembled with Na3−3xV2+x(PO4)3 samples (0 ≤ x ≤ 0.1). (b) Rate capability of sodium half-cells assembled with Na3−3xV2+x(PO4)3 samples as working electrodes and cycled to increasing C rates from C/2 to 40C and then C/2. (c) Extended galvanostatic cycling at 2C of sodium half-cells assembled with Na3−3xV2+x(PO4)3 samples (0 ≤ x ≤ 0.1).

passing through the cell provided evidence of the benefits of the superstoichiometry in Na3−3xV2+x(PO4)3 samples. From C/2 to 40C, NVP-0 showed a significant loss of capacity, recording only 41 mA h g−1 at 40C. This involves capacity retention of only 38%. This value is even lower than that of NVP-0.08,

despite its low initial capacity. However, the recovery of capacity evidenced by NVP-0 after returning to C/2 at the end of the cycling experiment was good (106 mA h g−1, 98%), indicating that the low capacity recorded at 40C cannot be ascribed to cell failure. NVP-0.02 performed an attractive rate 11849


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Inorganic Chemistry

Figure 7. (a) Nyquist plots of Na3−3xV2+x(PO4)3 (x = 0, 0.02, and 0.1) samples recorded after a few cycles at 2C. (b) Equivalent circuits used for the spectra fitting of NVP-0 and NVP-0.02 (top) and NVP-0.1 (bottom). (c) Apparent diffusion coefficients, calculated from the Warburg element of the impedance spectra, for NVP-0 and NVP-0.02 after a number of cycles at 2C.

capability at 40C. A capacity value of 64 mA h g−1 was recorded, involving a retention of 62% of the capacity recorded at C/2. The poor electrochemical behavior determined for the NVP-0.1 sample leads us to restrict the superstoichiomeric range to this upper value. Electrochemical impedance spectroscopy was performed with the aim of determining the resistance at the interphase between the working electrode and the electrolyte as being responsible for the kinetic response of the working electrodes under operation at increasing C rates. The plotting of these data as Nyquist diagrams (Figure 7a) allows inferring easily the different components of the cell resistance by fitting the spectrum to an equivalent circuit (Figure 7b). Rel refers to the ohmic drop at the electrolyte. Rsl and Rct correspond to the resistance values at the surface layer on the electrode and charge-transfer reaction, respectively.30 The former Rel value is negligible as compared to the latter ones and hence will not be considered. Eventually, the CPE element describes the capacitive behavior at the interphases, while W is the Warburg element determining the diffusion impedance. The plots recorded for NVP-0 and NVP-0.02 at the open circuit voltage (OCV) of the freshly assembled cell and after a number of cycles are displayed in Figure 7a. All of them show a common profile consisting of two depressed and overlapped semicircles at high and medium frequencies. They are commonly attributed to the ion migration through the surface layer (sl) and the charge-transfer process (ct) when Na+ ions are transferred from the electrolyte toward the bulk of the particle.38 For NVP-0.1, an inductive loop appears at low frequencies resulting from the diffusion of the alkali ions in a core−shell-like biphasic mechanism. The inductive loop is a consequence of the coupling of relaxations at both phases in the high frequency

region of diffusion impedance according to the theoretical calculation.39 Thus, a phase transition resistance (Rpt) parallel to inductance L was included in the equivalent circuit employed for the fitting of the spectra recorded for NVP-0.1 (Figure 7b). The resistance values reveal that the charge-transfer resistance (Rct) is crucial to determine the overall impedance of the electrode (Table 3). These values were lower for NVP0.2 than for the other samples, and tend to decrease on cycling indicating that the flexible layout of the NASICON structure adapted on cycling facilitates the charge transfer at the Table 3. Resistance Values, Warburg Impedance Coefficients (σW), and Diffusion Coefficients (D) Calculated from the Impedance Spectra of Na3−3xV2+x(PO4)3 (x = 0, 0.02, and 0.1) Samples after Several Cycles at 2C sample NVP-0

NVP-0.02

NVP-0.1

11850

nth cycle

Rel/Ohm g

OCV 5th 10th 15th 20th OCV 5th 10th 15th 20th OCV 5th 10th 15th 20th

0.003 0.005 0.005 0.005 0.001 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.005

Rsl/Ohm g Rct/Ohm g 0.074 0.040 0.028 0.024 0.012 0.049 0.050 0.024 0.037 0.041 0.021 0.013 0.011 0.010 0.004

0.996 0.694 0.589 0.489 0.526 0.505 0.443 0.382 0.317 0.302 0.918 0.601 0.511 0.433 0.420

Rpt/Ohm g

0.113 0.141 0.116 0.111 0.128


Article

Inorganic Chemistry interphase. The Rsl values were 1 order lower than Rct, while Rpt values in NVP-0.1 cannot be neglected when considering the overall impedance. The results provide evidence for the good kinetic response of NVP-0.02 which is reflected in the excellent behavior of this sample at the highest C rate. The apparent diffusion coefficients were also calculated for NVP-0 and NVP-0.02 by evaluating the Warburg impedance coefficients (σW). For this purpose, the real part of the impedance, Z′, is plotted versus the reciprocal root square at low angular frequencies (Figure S4, Supporting Information). Warburg coefficients are calculated from the slope of the straight line Z′ = R el + R sl + R ct + σ Wω−1/2

and applied to eq 2: D=

(1)

40

2 1 ⎛ RT ⎞ ⎟ ⎜ 2 2 ⎝ AF σ WC ⎠

Figure 8. X-ray photoelectron spectra at the P 2p, F 1s, and Na 1s core levels for NVP-0 and NVP-0.02 electrodes after 25 cycles at 2C. (2)

Here, D is the apparent diffusion coefficient, T is the absolute temperature, R is the gas constant, A is the electrode geometrical area, F is Faraday’s constant, and C is the molar concentration of Na+ ions according to eq 3: C=

(3 − x) × Z NA × V

eV. A minor signal at ca. 684 eV is commonly attributed to NaF arising from FEC decomposition.45 Eventually, a small component appears at ca. 609.5 eV; the assignments to NaxPOyFz and NaxPOyFz can be again carried out due to the similarities with lithium analogues.46 As compared to spectra for pristine materials, Na 1s core level spectra reveal a new signal slightly shifted to high binding energy (1074.0 ± 0.2 eV), attributed to NaF.47 As a general trend, the contributions of the side products yielded upon electrolyte degradations are more significant for NVP-0 than for NVP-0.02 which agrees with the better capacity retention observed for the latter sample. To corroborate that the good electrochemical behavior of superstoichiometric Na3V2(PO4)3 in the half-cell configuration can be applied to a full cell assembly, NVP-0.02 and a sucrose based hard carbon were chosen as positive and negative electrodes, respectively. An excess of negative electrode was employed, while the current density and gravimetric capacity were referred to the positive electrode mass. The usual irreversibility of the hard carbon during the first discharge was circumvented by subjecting both cycles to a previous half cycle versus a sodium metallic counter electrode and hence starting the full cell cycling experiment from a charged state. Figure 9 shows the charge and discharge curve recorded at 2C for the full cell and each electrode versus a sodium metallic

(3)

where x is the stoichiometric coefficient, Z is the number of unit formula per cell, NA is the Avogadro number, and V is the cell volume. These coefficients are plotted in Figure 7c and are close to those reported elsewhere for Na3V2(PO4)3 systems employing this same technique.41,42 This plot confirms the improved kinetic properties induced by the slight modification of the superstoichiometry in Na2.94V2.02(PO4)3 as compared to NVP-0 for the entire range of measured cycles. The extended galvanostatic cycling was performed at an intermediate 2C rate for all the studied samples. Figure 6c provides information about the capacity retention for a large number of cycles. This plot confirms the poor electrochemical behavior of NVP-0.1 evidenced by a pronounced capacity fading during the first cycles, leading to retention of only 70% after 150 cycles. In contrast, more stoichiometric samples (0.02 ≤ x ≤ 0.08) performed values as high as 85% after the same number of cycles. These values overperformed those recorded for the stoichiometric sample (NVP-0), with significantly lower (79%) retention. This trend was maintained after 300 cycles. Postmortem analysis of cycled electrodes by XPS threw some light on the chemical changes occurring at the interface after 25 cycles. Figure 8 shows the P 2p, F 1s, and Na 1s core level spectra of cycled electrodes. P 2p spectra feature a band at 133.7 ± 0.1 eV attributed to phosphorus in the Na3V2(PO4)3 structure. Also, an asymmetric band at higher binding energies was decomposed into two signals at 137.0 ± 0.3 and 139.0 ± 0.3 eV. This feature is not present in pristine materials and hence must be attributed to electrolyte decomposition products. The literature about the XPS analysis of NaPF6 decomposition is rather scarce, but the similarities with LiPF6 prompt us to ascribe these signals to the formation of NaxPOyFz and NaxPOyFz decomposition products, respectively.43,44 Concerning F 1s spectra, a broad band at ca. 679 eV was assigned to an Auger signal coming from the iron containing sample holder. The most important contribution is due to the presence of PVDF binder as a band at ca. 687.8

Figure 9. Charge and discharge curves of a hard carbon//NVP-0.02 full sodium-ion cell cycled 2C. Inset: Plot of charge and discharge capacity and Coulombic efficiency. 11851


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Inorganic Chemistry pseudoreference. The equation describing the electrochemical reaction undergoing cycling can be written as follows: NaV2 + x(PO4 )3 + (2 − x)NaC20 ⇌ Na3 − xV2 + x(PO4 )3 + C20

■

(4)

AUTHOR INFORMATION

Corresponding Author

The full cell featured an average voltage of 3.2 V and delivered a first discharge capacity of 98.1 mA h g−1 at 2C, which involves an energy density value of 314 W h kg−1 after 150 cycles; the discharge capacity was preserved at 82 mA h g−1, involving a capacity retention that was as high as 84%. This performace is quite competitive with those previously reported for the related full cells assembled with Na3V2(PO4)3. Thus, Ren et al. reported 60 mA h g−1 after 100 cycles at C/2 for a similar hard carbon based full cell.41 Other examples of full cells substitute hard carbon by alternative anodes such as NaTi2(PO4)321 or Sb/C,48 but the increase of the anode operating voltage penalizes the energy density which can be delivered by these systems, despite their good cyclability. The excellent performance of our full cell led us to consider this superstoichiometric Na2.94V2.02(PO4)3 compound as a promising positive electrode for sodium-ion cells.

*Phone: 34 957 218 663. E-mail: [email protected]. ORCID

Pedro Lavela: 0000-0002-5182-3440 José L. Tirado: 0000-0002-8317-2726 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Ministerio de Ciencia, Industria e Innovación (MINECO), and ERDF funds (MAT2014-56470R), and Junta de Andaluciá (FQM288) for financial support. We also thank SCAI (UCO Central Service for Research Support).

4. CONCLUSIONS Superstoichiometric Na3−3xV2+x(PO4)3 (0 ≤ x ≤ 0.1) samples were synthesized by the sol−gel method to unveil the effect of vanadium excess on the NASICON structure and morphological properties, and their consequences on the electrochemical behavior as positive electrode in sodium-ion batteries. XRD patterns showed an increase in cell volume for the whole stoichiometric range in good agreement with the incorporation of superestoichiometric vanadium. Rietveld refinement revealed that the additional transition metal ions are hosted in vacant Na2 sites. XPS spectra at V 2p and O 1s core levels revealed the appearance of new signals ascribable to the allocation of excess vanadium in Na2 sites. Although the electron microscopy showed similar textural features for all samples, EDX analysis provided evidence for the homogeneous dispersion of elements and Na/V ratios in good agreement with the nominal stoichiometries. Electrochemical tests showed improved rate capabilities at the highest rate (40C) and capacity retention upon prolonged cycling for samples with 0.02 ≤ x ≤ 0.08. This fact was correlated with the good kinetic response, correlated to a low charge-transfer resistance at the electrode interphase and a high sodium diffusion coefficient for NVP-0.02. In addition, XPS spectra of cycled electrodes revealed that the deposition of side products, hindering sodium migration at the electrode interphase, was less significant for the superstoichiometric NVP-0.02 sample. A full cell assembled with this sample as a positive electrode featured an energy density as high as 314 W h kg−1 after 150 cycles, which encourages us to consider the reported material as an outstanding cathode for sodium-ion batteries.

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and maps for elements Na, V, P, and O; and plots of the real impedance versus the reciprocal frequency for the calculation of apparent diffusion coefficients (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01846. Tables of Rietveld refined parameters for the superstoichiometric Na3−3xV2+x(PO4)3 phases; EDX spectra 11852


Article

Inorganic Chemistry

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