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 ~10−10 cm2 s−1, which was better than ~10−12 cm2 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.