Structures and conductivities of stable and metastable Li 5 GaS 4 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, Li 5 GaS 4 , was synthesized via mechanochemistry and stable Li 5 GaS 4 was obtained by heating the metastable phase. The metastable Li 5 GaS 4 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, Li 5 AlS 4 . In both the structures, the Ga 3+ cations were surrounded by four S 2- 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.