Role of Halogen Doping on Ionic Diffusion in the Grain Boundary Structure of Cubic Na3PS4: Ab Initio Molecular Dynamic Study

Using halogen doping to introduce Na vacancies is an effective way to improve the ionic conductivity of Na3PS4. The halogen dopants are thermodynamically favored to segregate at the grain boundaries, but the grain boundaries in a polycrystalline solid-state electrolyte are widely known to substantially reduce the ionic diffusion. The physical mechanism behind such effect and the role of the halogen dopants on the ionic diffusion are still not clear. In this study, density functional theory (DFT)-based MD simulations were applied to study the role of halogen doping on the ionic diffusion of Sigma 3 (112) and Sigma 5 (013) grain boundary structures of cubic Na3PS4. The results show that halogen doping has a different influence on the ionic diffusion of Sigma 3 (112) and Sigma 5 (013) structures, and it is governed by both the thermodynamic stability of the vacancy and the atomic environment around sodium ion hopping. This research will contribute to the future design of high-performance polycrystalline solid-state electrolytes by GB engineering.