| Summary: | Lithium thiophosphate-based materials are attractive as solid electrolytes in all-solid-state 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 xLi<sub>2</sub>S-(100 - x)P<sub>2</sub>S<sub>5</sub> (x = 67, 70, 75, and 80) were created by randomly identifying appropriate compositions of Li<sup>+</sup>, PS<sub>4</sub><sup>3-</sup>, P<sub>2</sub>S<sub>7</sub><sup>4-</sup>, and S<sup>2-</sup> and then annealing them with DFT-MD calculations. Calculated relative stabilities of the amorphous structures with x = 67, 70, and 75 relative 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<sup>+</sup> diffusion coefficients obtained from DFT-MD calculations at various temperatures for picosecond simulation times were on the order of 10<sup>-3</sup> - 10<sup>-5</sup> Angstrom<sup>2</sup>/ps. Ionic conductivities evaluated by the Nernst-Einstein relationship at 298.15 K were on the order of 10<sup>-5</sup> 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<sup>+</sup>. The simulations also suggested that isolated S atoms suppress Li<sup>+</sup> migration.
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