Titanium- and niobium-doped fluorophosphates as positive electrodes for sodium-ion batteries

Sodium fluorophosphates are among the most promising materials for positive electrodes in sodium-ion batteries; however, their low electronic conductivity, kinetics limitations, and structural instability prevent them from reaching their full potential. We obtained sodium vanadium fluorophosphates (NVPF) doped with Ti4+ (NVTPF) and Nb5+ (NVNPF) via an original low-temperature synthesis. It was observed that Ti4+ doping facilitated access to electrode sites and provided higher capacities at high current densities while Nb5+ provided a 91.7% capacity retention from the 20th to 200th cycle, which was over the 80.1% capacity retention of NVPF. Moreover, both NVTPF and NVNPF provided diffusion coefficients in the range of similar to 10(-10) cm(2) s(-1), which was better than similar to 10(-12) cm(2) s(-1) of NVPF. Additionally, electrochemical impedance spectroscopy and ex situ X-ray diffraction measurements confirmed that Ti4+ enhanced the electrode kinetics and stabilized its structure through sodiation/desodiation reactions. The results presented in this paper might provide insights and new directions to enhance the electrochemical storage properties of fluorophosphates.

Automated summary

The study improved the performance of sodium vanadium fluorophosphates by doping with Ti4+ and Nb5+, enhancing capacity and stability of the electrodes, as well as increasing diffusion coefficients. This provides new insights and directions for enhancing the electrochemical storage properties of fluorophosphates.