Y-Doped Na3V2(PO4)2F3 compounds for sodium ion battery cathodes: electrochemical performance and analysis of kinetic properties

Abstract

To improve the intrinsic electronic conductivity and Na ion mobility of Na₃V₂(PO₄)₂F₃ (NVPF), Y(yttrium) atoms are introduced into the NVPF/C complex as a partial substitute for V(vanadium) through a sol–gel method. The effects of Y substitution on the crystal structure, morphology, electrochemical performance and kinetic properties of NVPF were investigated. Based on the battery performance comparison of the Na₃V_2−xY_x(PO₄)₃/C (x = 0, 0.05, 0.1 and 0.2) samples, Na₃V_1.9Y_0.1(PO₄)₃/C showed the best electrochemical performance and cycling stability. At a low rate of 0.5C, the 5 mol% Y-doped sample delivered a discharge capacity of 121.3 mA h g⁻¹, which was very close to the theoretical specific capacity. And even at a high rate of 50C, the discharge capacity achieved was higher than 80 mA h g⁻¹. After 200 cycles, the capacity retention of Na₃V_1.9Y_0.1(PO₄)₃/C could still remain as high as 93.46% at 1C. From the morphology determination and analysis of kinetic properties, it was confirmed that the excellent electrochemical performance of Na₃V_1.9Y_0.1(PO₄)₃/C was mainly due to the enhanced intrinsic electronic conductivity and Na ion mobility caused by introducing a moderate amount of Y to replace the V sites in the NVPF crystal structure. In order to get a better understanding of the relationship between the kinetic properties and the electrochemical performance in a sodium ion battery, a mass and electron transfer process model has been proposed for the first time in the present research.