Synthesis and electrochemical performances of Na3V2(PO4)2F3/C composites as cathode materials for sodium ion batteries

Na3V2(PO4)2F3 (NVPF) with NASCION (Na superionic conductor) is recognized as a potential cathode material owing to its high theoretical capacity. However, the electronic conductivity of NVPF is much lower than its ionic conductivity, which seriously affects the properties of this material. The carbon layer can be used as the conductive medium to enhance the conductivity of NVPF. In this study, we propose a single-step solid-state reaction method based on mechanical activation with pitch as the carbon source to synthesize NVPF/C composites. The crystallographic structure and morphology of all as-prepared samples were investigated by XRD, Raman spectroscopy, BET measurement, thermal analysis, SEM and TEM. Furthermore, the electrochemical performance and kinetic properties were analyzed by CV, galvanostatic charge-discharge and EIS tests. These tests outcomes demonstrated that the NVPF/C-2 composite with a carbon content of 12.14 wt% showed an excellent rate performance and cycle stability. It presented reversible capacities of 103 and 95 mA h g-1 at 0.2 and 10C, respectively, and an outstanding retention of 91.9% after 500 cycles at 5C. These excellent properties of the NVPF/C-2 composite are attributed to its high ion diffusion coefficient and small charge transfer impedance.