Conduction Mechanism in 70Li₂S-30P₂S₅ Glass by Ab Initio Molecular Dynamics Simulations: Comparison with Li₇P₃S₁₁...

The 70Li₂S-30P₂S₅ 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 70Li₂S-30P₂S₅ 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 Li₇P₃S₁₁ crystals. The shape difference of the P₂S₇^4- between the Li₇P₃S₁₁ crystal and 70Li₂S-30P₂S₅ glass is clearly observed in P-S-P bond angle, indicating that the P₂S₇^4- units in the 70Li₂S-30P₂S₅ 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 70Li₂S-30P₂S₅ glass compared to the Li₇P₃S₁₁ crystal implies that a part of volume in the 70Li₂S-30P₂S₅ glass cannot contribute to Li⁺ conduction. This reduction is attributed to the rotational motion of PS₄^3-, which is observed only in the 70Li₂S-30P₂S₅ 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 70Li₂S-30P₂S₅ glass is a characteristic property, which cannot create fast Li⁺ conduction paths as that in Li₇P₃S₁₁ crystal.