Conduction Mechanism of Li10GeP2S12-type Lithium Superionic Conductors in a Li–Sn–Si–P–S System

Crystal structures of Li10GeP2S12 (LGPS)-type Li10+δ[SnySi1–y]1+δP2−δS12 (Li4–x[SnySi1–y]1–xPxS4) solid electrolytes were analyzed by Rietveld refinement using neutron diffraction data. Maximum entropy method analysis was performed to visualize the distribution of lithium along the c-axis via the Li1–Li3 sites, which indicated one-dimensional (1D) lithium diffusion for all the examined compositions. The Li10.35Sn0.27Si1.08P1.65S12 (Li3.45Sn0.09Si0.36P0.55S4) (δ = 0.35, x = 0.55, y = 0.2) system, which had the highest ionic conductivity in Li–Sn–Si–P–S system, exhibited an additional lithium diffusion pathway in the ab-plane through the Li1 and Li4 sites. High ionic conductivity (>10 mS cm–1) was achieved in the Sn–Si derivatives owing to the formation of three-dimensional (3D) ion diffusion channels. Comparison of the conductivity and related crystal structural parameters revealed the requirements for fast lithium diffusion along the c-axis and 3D lithium diffusion in the LGPS-type crystal structure. Large atomic displacement of the Li1 site, a large S3–S3 distance, a large bottleneck size, and small differences in Li1–Li4 distances are important for 1D and 3D lithium diffusion, respectively.