Diffusion Mechanism of the Sodium-Ion Solid Electrolyte Na3PS4 and Potential Improvements of Halogen Doping

Density functional theory (DFT) molecular dynamics (MD)-simulations were performed on cubic and tetragonal Na3PS4. The MD simulations show that the Na-conductivity based on the predicted self-diffusion is high in both the cubic and tetragonal phases. Higher Na-ion conductivity in Na3PS4 can be obtained by introducing Naion vacancies. Just 2% vacancies result in a conductivity of 0.2 S/cm, which is an order of magnitude larger than the calculated conductivity of the stoichiometric compound. MD simulations of halogen-doped cubic Na3PS4 suggest a practical route to introduce vacancies, where Br-doping is predicted to result in the highest bulk conductivity. Detailed investigation of the Na-ion transitions during the MD simulation reveals the role of vacancies and phonons in the diffusion mechanism. Furthermore, the orders of magnitude difference between the MD simulations and experiments suggest that macroscopic conductivity can be significantly increased by reducing the grain boundary resistance.