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

Abstract

• Description of a new low-temperature synthesis of fluorophosphates. • Titanium doping improves storage capacities of fluorophosphates as positive electrodes do ion-sodium batteries. • Niobium doping increase capacity retention of fluorophosphates as positive electrodes do ion-sodium 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 Ti 4+ (NVTPF) and Nb 5+ (NVNPF) via an original low-temperature synthesis. It was observed that Ti 4+ doping facilitated access to electrode sites and provided higher capacities at high current densities while Nb 5+ 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 cm 2 s −1 , which was better than ~10 −12 cm 2 s −1 of NVPF. Additionally, electrochemical impedance spectroscopy and ex situ X-ray diffraction measurements confirmed that Ti 4+ 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.