Conduction Mechanism in 70LiS-30PS Glass by Ab Initio Molecular Dynamics Simulations: Comparison with LiPS Crystal.

The 70LiS-30PS glass is a promising solid-state electrolyte for all solid-state lithium-ion batteries. Nevertheless, understanding the Li conduction mechanism is limited because of the complex amorphous nature of the glass. Herein, we present an ab initio molecular dynamics study of the 70LiS-30PS glass using a long simulation run (800 ps), improving the general understanding of its structure, dynamics, and electronic polarizability by comparing the results to those of the LiPS crystals. The shape difference of the PS between the LiPS crystal and 70LiS-30PS glass is clearly observed in P-S-P bond angle, indicating that the PS units in the 70LiS-30PS glass are relatively free from stress for crystallographic ordering. From the Li trajectories for 800 ps, the diffusion within the limited space in the unit cell is derived as the effective porosity. The lower effective porosity in the 70LiS-30PS glass compared to the LiPS crystal implies that a part of volume in the 70LiS-30PS glass cannot contribute to Li conduction. This reduction is attributed to the rotational motion of PS, which is observed only in the 70LiS-30PS glass. The sulfur polarizability is thoroughly analyzed for isotropic and anisotropic span through the Born effective charge tensor. The uniformly anisotropic polarizability of sulfur in the 70LiS-30PS glass is a characteristic property, which cannot create fast Li conduction paths as that in LiPS crystal.