Structure and Ionic Conductivity of Li2S–P2S5 Glass Electrolytes Simulated with First-Principles Molecular Dynamics

Lithium thiophosphate-based materials are attractive as solid electrolytes in all-solidstate lithium batteries because glass or glass-ceramic structures of these materials are associated with very high conductivity. In this work, we modeled lithium thiophosphates with amorphous structures and investigated Li mobilities by using molecular dynamics calculations based on density functional theory (DFT-MD). The structures of xLi2S–(100− x)P2S5 (x=67, 70, 75, and 80) were created by randomly identifying appropriate compositions of Li, PS4 , P 4 2 2S7 , and S − and then annealing them with DFT-MD calculations. Calculated relative stabilities of the amorphous structures with x=67, 70, and 75 to crystals with the same compositions were 0.04, 0.12, and 0.16 kJ/g, respectively. The implication is that these amorphous structures are metastable. There was good agreement between calculated and experimental structure factors determined from X-ray scattering. The differences between the structure factors of amorphous structures were small, except for the first sharp diffraction peak, which was affected by the environment between Li and S atoms. Li diffusion coefficients obtained from DFT-MD calculations at various temperatures for picosecond simulation times were on the order of 10−3–10−5 Å2/ps. Ionic conductivities evaluated by the Nernst–Einstein relationship at 298.15K were on the order of 10−5 S/cm. The ionic conductivity of the amorphous structure with x=75 was the highest among the amorphous structures because there was a balance between the number density and diffusibility of Li. The simulations also suggested that isolated S atoms suppress Li migration.