Sol–gel preparation and electrochemical properties of Na3V2(PO4)2F3/C composite cathode material for lithium ion batteries

A Na3V2(PO4)2F3 cathode material for lithium ion batteries was prepared by using a sol–gel method to prepare precursor, and then heat treated the precursor at 650 °C based on TG analysis. Transmission electron microscopy showed that the material had a small particle size about 20 nm. The Na3V2(PO4)2F3 particles were embedded in a network of residual carbon. The content of residual carbon was determined to be 8.5 wt.%. This residual carbon enhanced the electronic conductivity of the material, which was measured to be 1.0 × 10−3 S cm−1. The electrochemical insertion mechanism of Na3V2(PO4)2F3 gradually shifted from a predominant Na+ insertion to a Li+ insertion in the initial 10 cycles. The material showed a reversible discharge capacity of 117 mAh g−1, with quite good capacity retention. The good electrochemical performance of Na3V2(PO4)2F3 was attributed to its NASICON-type structure, which provided a large ion diffusion coefficient about 7.2 × 10−10 cm2 s−1. The good electrochemical performance of Na3V2(PO4)2F3 provides an example to use sodium-based materials directly as cathode materials for lithium ion batteries.