Synthesis and Structural Characterization of Lithium Ionic Conductors Li₂MS₃ (M = Si, Si0.5Ge0.5, Ge) and Li₁₆Ge₅S₁₈

To develop inorganic Li conductors for all-solid-state Li-ion batteries, the present study clarified temperature-dependent structural changes in the Li2MS3 (M = Si, Si0.5Ge0.5, Ge) phases, where new crystal structures favoring ionic conduction are potentially formed. Structural analysis was performed on powder X-ray diffraction data obtained at high temperatures. At 600 °C, tetragonal phases were formed in all three cases. Meanwhile, structural changes on cooling from 600 °C were different according to the M species, leading to various crystal systems at room temperature (monoclinic for M = Si, orthorhombic for Si0.5Ge0.5, and hexagonal for Ge). Their ionic conductivities at 25 °C were 2.3 × 10–5, 1.7 × 10–7, and 8.6 × 10–9 S cm–1, respectively, indicating that the conductivity depends significantly on the framework of the crystal structure. These phase identification results were further utilized to search the Li2S–GeS2 pseudobinary system, revealing that the structure of Li16Ge5S18 could not be assigned to any polymorphs identified for Li2MS3. This new crystalline phase showed an ionic conductivity of 7.2 × 10–5 S cm–1 at room temperature, which is 4 orders of magnitude higher than that of hexagonal Li2GeS3. This study highlights a drastic increase in the lithium conductivity achieved by controlling the crystal structure of the Li2MS3-related materials.