Improved electrochemical performance of high voltage cathode Na3V2(PO4)2F3 for Na-ion batteries through potassium doping

Abstract

Potassium-doped Na3V2(PO4)2F3@CNT(NVPF@CNT) is employed as a promising cathode for sodium-ion batteries via a simple sol-gel method in order to improve the intrinsic electronic conductivity and ion diffusion rate. The effects of K substitution on the crystal structure and electrochemical performance of NVPF are discussed. It is found that by introducing a moderate amount of K to replace the Na sites in the NVPF crystal structure, the ion diffusion path is effectively broadened, so the electrochemical performance is greatly improved. Excellent cyclic performance with a high specific capacity of 120 mAh g−1 is achieved at a low rate of 1C. After 1600 cycles at a discharge rate of 10C, the discharge capacity can still achieve values higher than 90 mAh g−1. Even at a high rate of 50C, the capacity retention ofNKVPF@CNTcould still remain as high as 90% after nearly 6000 cycles. In order to obtain a better understanding of the relationship between the ion doping and kinetic properties, a Rietveld refinement analysis and Randles-Sevcik equation-based theory are proposed in this research. This is the first time that potassium ion substitution has been used to improve the performance of NVPF and is proved to be an effective way to modify the lattice structure. Such work aids in the progression of sodium-based batteries.