Structures and conductivities of stable and metastable Li5GaS4 solid electrolytes

Understanding the differences in the structures and defects in the stable crystalline phase and metastable phase is important for increasing the ionic conductivities of a solid electrolyte. The metastable phase often has higher conductivity than the stable phase. In this study, metastable lithium thiogallate, Li₅GaS₄, was synthesized via mechanochemistry and stable Li₅GaS₄ was obtained by heating the metastable phase. The metastable Li₅GaS₄ sample was found to have an antifluorite-type crystal structure with cationic disorder, while the stable phase was found to have a monoclinic crystal structure, similar to that of another solid electrolyte, Li₅AlS₄. In both the structures, the Ga³⁺ cations were surrounded by four S²⁻ anions in tetrahedral coordination. The conductivity of the metastable phase was determined to be 2.1 × 10⁻⁵ S cm⁻¹ at 25 °C, which is 1000 times greater than that of the monoclinic phase. The high conductivity of the metastable phase was achieved owing to cation disorder in the crystal structure.

A metastable antifluorite-type Li₅GaS₄ electrolyte prepared by a mechanochemical process exhibits the highest conductivity of 2.1 × 10⁻⁵ S cm⁻¹ at 25 °C, which is three orders of magnitude higher than that of the heated Li₅GaS₄ samples with the stable phase.