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

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 P2S74- between the Li7P3S11 crystal and 70Li2S-30P2S5 glass is clearly observed in P-S-P bond angle, indicating that the P2S74- 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 PS43-, 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.