Abstract
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 xLi2S-(100 - x)P2S5 (x = 67, 70, 75, and 80) were created by randomly identifying appropriate compositions of Li+, PS43-, P2S74-, and S2- 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+ diffusion coefficients obtained from DFT-MD calculations at various temperatures for picosecond simulation times were on the order of 10-3 - 10-5 Angstrom2/ps. Ionic conductivities evaluated by the Nernst-Einstein relationship at 298.15 K 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.
Highlights
The possibility of producing all-solid-state lithium-ion batteries (LIBs) has attracted much attention because the replacement of an organic liquid electrolyte with an inorganic solid electrolyte (SE) would simplify battery design, increase energy density, and make batteries safer and more durable
It is noteworthy that no decomposition or segregation of the units was observed in almost all trajectories, whereas rearrangement of the units, such as 2P2S7 → PS4 + P3S10, occurred for only one amorphous structure model with x = 67
We investigated the pair correlation function (PCF) between Li and S and that between S and S belongs to the other PS34− units
Summary
The possibility of producing all-solid-state lithium-ion batteries (LIBs) has attracted much attention because the replacement of an organic liquid electrolyte with an inorganic solid electrolyte (SE) would simplify battery design, increase energy density, and make batteries safer and more durable. Realization of such batteries is critical for practical applications such as electric vehicles and plugin hybrid electric vehicles. Sulfide SEs with the same level of conductivity as conventional liquid electrolytes have been discovered. The ionic conductivity of a Li10GeP2S12 crystal, in particular, is 1.2 × 10−2 S/cm (Kamaya et al, 2011). The conductivities of some glasses, including 30Li2S–26B2S3–44LiI (Wada et al, 1983), 50Li2S–17P2S5–33LiBH4 (Yamauchi et al, 2013), and 63Li2S–36SiS2–1Li3PO4 (Aotani et al, 1994), have been reported to be as high as 1.5–1.7 × 10−3 S/cm
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