Effect of Iron Substitution in the Electrochemical Performance of Na3V2(PO4)3 As Cathode Material for Na-Ion Batteries

Sodium is an abundant and non-toxic alkali element with outstanding electrochemistry that places sodium batteries as serious competitors of lithium batteries for large-scale stationary energy storage. In fact, a number of recent reports have demonstrated the reliability of sodium batteries to provide reversible energy storage devices [1]. Transition metal phosphates exhibiting a NASICON-type structure offer a rigid framework with interconnected vacant sites facilitating the insertion and fast diffusion of alkaline ions. Particularly, Na₃V₂(PO₄)₃ have demonstrated interesting properties as potential cathode material because of the high operating voltage and good cyclability [2]. The goal of this work is to explore alternative compositions derived from the partial substitution of vanadium by other transition metals. The low cost and environmentally friendliness of iron and the possibility of using ⁵⁷Fe Mössbauer spectroscopy to characterize the influence of the substituting element led to the selection of Na₃V_1.7Fe_0.3(PO₄)₃.

A sample with Na₃V_1.7Fe_0.3(PO₄)₃nominal stoichiometry was synthesized by dissolving stoichiometric amounts of reagents in deionized water. Citric acid was added as a carbon source for improving the conducting properties of the raw material. After solvent evaporation, the precursor was annealed under an inert atmosphere at 750ºC.

XRD pattern of Na₃V_1.7Fe_0.3(PO₄)₃ sample was indexed in the R-3c space group (Fig. 1). The unit cell parameters were a=8.737(1) Å and c= 21.825(2) Å. These values are quite like to those reported for Na₃V₂(PO₄)₃ as expected for the close similarity between the ionic radii of V³⁺ (0.78 Å) and Fe³⁺ (0.785 Å) [3]. ⁵⁷Fe Mössbauer spectrum recorded on Na₃V_1.7Fe_0.3(PO₄)₃ showed a large doublet at 0.45(1) mm/s ascribable to Fe³⁺. Minor presence of divalent irons can be ascribed to the reducing conditions of the thermal treatment.

Two-electrode sodium test cells were subjected to cycling at room temperature and 0.5C rate and revealed a good electrochemical performance. The galvanostatic profile shows a flat plateau at ca. 3.4 V, with an extension that agrees well the expected electron consumption for the transition metals oxidation (Fig. 2). Moreover, a short reversible plateau is also observed at ca. 4.0 V. Previous reports on Li₃V₂(PO₄)₃ have indicated the occurrence of a similar high voltage feature ascribable to the removal of the second lithium atoms and V³⁺ to V⁴⁺ oxidation [4]. However, a similar feature has not been previously reported for Na₃V₂(PO₄)₃from our knowledge. The extended galvanostatic cycling of this composition showed good capacity retention for more than 50 cycles and capacity values (100 mAh/g) close to the theoretical one for the removal of two sodium ions.

References

[1] V. Palomares, M. Casas-Cabanas, E. Castillo-Martínez, M. H. Hanb, T. Rojo, Energy Environ. Sci. 6 (2013) 2312-2337.

[2] M. Pivko, I. Arcon, M. Bele, R. Dominko, M. Gaberscek, J. Power Sources 216 (2012) 145-151.

[3] K. Du, H. Guo, G. Hu, Z. Peng, Y. Cao, J. Power Sources 223 (2013) 284-288.

[4] M.Y. Saidi, J. Barker, H. Huang, J.L. Swoyer1, G. Adamson, J. Power Sources 119–121 (2003) 266–272.