Synthesis and Characterization of Na3V2(PO4)2F3 Based Cathode Material for Sodium Ion Batteries

The Na₃V₂(PO₄)₂F₃ was synthesized using a two-step carbothermal reduction (CTR) process, similar to method proposed by Barker et al. [1] involving the orthorhombic vanadium(III) phosphate, VPO₄ as the reaction intermediate. In the second step, the VPO₄was reacted with a stoichiometric quantity of NaF to yield the Na₃V₂(PO₄)₂F₃ final product.

VPO₄ was prepared by mixing V₂O₅, NH₄H₂PO₄ and D-Glucose, and thoroughly grinding using agate mortar and pestle as described by Lu et al. [2]. The carbon in glucose reduces V⁵⁺ to V³⁺ and the remained carbon improves the electronic conductivity of the cathode material. VPO₄ was analysed by XRD method and besides the main Bragg peaks, characteristic of the VPO₄ phase, appearance of some weak extra peaks was assigned to V₂O₃ and VP traces. The presence of these latter phases is characteristic of an incomplete reduction of V⁵⁺ to V³⁺ despite of the large carbon excess used [3]. Thus, the main difficulty of the above reaction lies in the complete reduction of V⁵⁺ to V³⁺.

The Na₃V₂(PO₄)₂F₃ powder was prepared by mixing the prepared VPO₄ with NaF in a 2:3 molar ratio. The mixture was grinded and mixed in an agate mortar and pelletized, applying 440 MPa pressure. The pelleted sample was then heated under argon in a stationary quartz bed reactor for 6 h at 750 °C.

Na₃V₂(PO₄)₂F₃ was characterized by FIB-SEM combined with energy dispersive X-ray (EDX) method with which ~8.9 wt.% of carbon content (due to the incomplete reduction reaction) was measured. It should be mentioned that the ratio of Na/V, determined by EDX, was found to be ~3/2, providing the evidence that the formula of the F-based material is Na₃V₂(PO₄)₂F₃, also confirmed by XRD. Thus, somewhat different cathode material from the “NaVPO₄F”, reported by Barker et al. [2] has been synthesised and studied in this paper.

The electrochemically active material powders were mixed with graphite and polytetra fluoroethylene in a 90:5:5 weight ratio to form slurry from which the 140 μm thick electrodes were press-rolled and thereafter coated with 2 μm aluminum layer via magnetron sputtering method. The Na₃V₂(PO₄)₂F₃ composite electrodes were investigated by TOF-SIMS method up to m/z = 200.

For non-aqueous measurements, sodium metal was used as both the counter and reference electrode. It can be seen that in 1 M NaPF₆ EC:DMC (1:1) electrolyte all cyclic voltammograms exhibit two main peaks in the cathodic and anodic sweeps, demonstrating that the intercalation/deintercalation of Na⁺ ions is being carried out in two steps: the first step at 3.5/3.7 V and the second step at 4.1/4.3 V vs. Na/Na⁺. Thus, the structural transformation of the composite material takes place under the potential cycling conditions applied. However, at very slow 10 μV s^-1 scanning rate the third very small oxidation peak has been observed.

The discharge capacity of 55 mAh g^-1 for the first two cycles has been calculated. It should be noted that the discharge capacity decreased nearly 50% after the third cycle but stabilized after 10^th cycle at 22 mAh g^-1. Even after 3000 cycles capacity of ~ 20 mAh g^-1 was retained.

For aqueous systems activated carbon was used as the counter and Ag/AgCl as the reference electrode, respectively. In 1 M Na₂SO₄ aqueous electrolyte, the main peak in the cathodic and anodic sweeps can be seen, demonstrating that the intercalation/deintercalation of Na⁺ ions is being carried out at 0.54 V and at 0.48 V vs. Ag/AgCl.

The discharge capacity of 40 mAh g^-1 for the second cycle has been calculated. It should be noted that the discharge capacity decreased nearly 30% after the second cycle, but stabilized after 10^th cycle at 27 mAh g^-1. All electrochemical measurements were carried out at 25 ± 0.5 ºC.

In this work it has been demonstrated, that the so-called „NaVPO₄-like“ Na₃V₂(PO₄)₂F₃ material exhibits promising characteristics as a cathode material for application in both aqueous and non-aqueous sodium ion batteries.

Acknowledgements 

The present study was supported by the Estonian Center of Excellence in Science project 3.2.0101.11-0030, Estonian Energy Technology Program project 3.2.0501.10-0015, Material Technology Program project 3.2.1101.12-0019, Project of European Structure Funds 3.2.0601.11-0001, Estonian target research project IUT20–13, and projects 3.2.0302.10-0169, 3.2.0302.10-0165.

References

1. J. Barker, M.Y. Saidi and J.L. Swoyer, Electrochem. Solid-State Lett., 6, A1 (2003).

2. Y. Lu, S. Zhang, Y. Li, L. Xue, G. Xu and X. Zhang, J. Power Sources, 247, 770 (2014).

3. J. Barker, R.K.B. Gover, P. Burns and A.J. Bryan,  Electrochem Solid-State Lett., 9, A190 (2006).