Effect of Ti doping on the structure and electrochemical properties of Na3V2(PO4)(2)F-3 as anode material for sodium ion batteries

In this article, Co-doped Na3V2- (x) Ti (x) (PO4)(2)F-3(NVTPF x=0, 0.1, 0.5, 0.7) sodium ion battery anode material was prepared by one-step hydrothermal method and it was coated with graphene. The crystal structure and morphology of NVTPF cathode material were characterized by scanning electron microscope and X-ray diffractometer. The results show that the particle diameter of the sample becomes smaller and the shape gradually becomes regular with the increase of Ti doping amount. The graphene coating did not change the crystal structure of NVTPF, but it was helpful to improve its electrochemical performance. The substitution of Ti4+ for V3+ contributes to improve the electronic conductivity and diffusion coefficient of sodium ions, reduce the charge transfer impedance, and accelerate the reaction kinetics of NVTPF electrode, thus significantly improving the cycling stability of NVTPF electrode. Among the four groups of materials, Na(3)v(1.5)Ti(0.5)(PO4)(2)F-3 has the best electrochemical performance. At 1C, the specific capacity of initial discharge is 53.4 mAh/g, and the capacity retention rate of 35 cycles is 72.3%. The graphene coated NVTPF has a discharge capacity of 125.9mAh/g for the first cycle and 44.8mAh/g for 35 cycles at 1C.