First-Principles Investigations on Sodium Superionic Conductor Na₁₁Sn₂PS₁₂

Sodium superionic conductors are key components of solid-state sodium ion batteries, which are regarded as promising alternative energy storage options for large-scale application. Recently, a new crystalline sodium superionic conductor Na11Sn2PS12 was reported with a remarkably high ionic conductivity over 1 mS/cm at room temperature. Herein, we report the comprehensive first-principles investigations on this new sodium superionic conductor. Our ab initio molecular dynamics simulations confirm the intrinsically fast and isotropic diffusion of sodium ions in Na11Sn2PS12 involving all the sodium sites. From a series of first-principles calculations, we propose a sodium diffusion mechanism and discuss the effects of various defects or substitutions on the diffusion kinetics, which may aid in further development of this class of materials. Moreover, we argue that the inherent vacant sites (Wyckoff position 8b), whose presence has been claimed to be critical for the fast sodium diffusion in this material, are less likely to contribute to the sodium diffusion. Finally, the thermodynamic stability and chemical compatibility of Na11Sn2PS12 are comparatively explored. Our theoretical study provides a more comprehensive understanding of Na11Sn2PS12-type conductors as well as helpful guidance on their optimal design for application in solid-state batteries.