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

Abstract

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, Si_0.5Ge_0.5, 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 Si_0.5Ge_0.5, 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.