Valence state of V combined with D-tartaric acid and heat treatment affect sodium storage performance of NASICON-type multiphase composite

Abstract

The polyanionic compound Na3V2(PO4)3 (NVP) and Na3V2(PO4)2F3 (NVPF) are promising cathode materials for sodium-ion batteries, but their poor electronic conductivity affects the electrochemical performance and the output energy density. The easy interconversion between V4+ and V5+ in the solution appears due to the weak bonds of D-tartaric acid, which lowers the energy barrier for further transition to V3+ and results in an excess V3+ in the product. A small amount of NVPF/C is converted to NVP/C due to the volatility of fluorine after heat treatment. D-tartaric acid as a carbon source forms a 3D network structure after calcination, thresholds the growth of NVP or NVPF and hinders the volume expansion during sodium storage. The initial discharge specific capacity of NVPF/C-650 is 85 mAh g−1 at 5C, and remains 59.2 mAh g−1 after 1000 cycles, which is related to the low charge transfer resistance, rapid Na+ ion diffusion and the pseudocapacitance effect. The binding of V and the carbon source in the solution affects the content of V3+/V4+ content, and the annealing temperature affects the formation and composition of the conducting carbon, both of which influence the electrochemical performance. This result provides a concept for possible screening of the carbon source for sodium storage materials with excellent performance.