Mechanochemical Synthesis and Characterization of Metastable Hexagonal Li₄SnS₄ Solid Electrolyte

A new crystalline lithium-ion conducting material, Li₄SnS₄ with an ortho-composition, was prepared by a mechanochemical technique and subsequent heat treatment. Synchrotron X-ray powder diffraction was used to analyze the crystal structure, revealing a space group of P6₃/ mmc and cell parameters of a = 4.01254(4) Å and c = 6.39076(8) Å. Analysis of a heat-treated hexagonal Li₄SnS₄ sample revealed that both lithium and tin occupied either of two adjacent tetrahedral sites, resulting in fractional occupation of the tetrahedral site (Li, 0.375; Sn, 0.125). The heat-treated hexagonal Li₄SnS₄ had an ionic conductivity of 1.1 × 10^-4 S cm^-1 at room temperature and a conduction activation energy of 32 kJ mol^-1. Moreover, the heat-treated Li₄SnS₄ exhibited a higher chemical stability in air than the Li₃PS₄ glass-ceramic.