High lithium ion conducting Li2S–GeS2–P2S5 glass–ceramic solid electrolyte with sulfur additive for all solid-state lithium secondary batteries

Glass–ceramic Li₂S–GeS₂–P₂S₅ electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li_4−xGe_1−xP_xS₄ from x = 0.70 to x = 1.00 were systematically investigated. Structural analysis by X-ray diffraction (XRD) showed gradual increase of the lattice constant followed by significant phase change with increasing GeS₂. All-solid-state LiCoO₂/Li cells were tested by constant-current constant-voltage (CCCV) charge–discharge cycling at a current density of 50 μA cm⁻² between 2.5 and 4.3 V (vs. Li/Li⁺). In spite of the high conductivity of the solid-state electrolyte (SSE), LiCoO₂/Li cells showed a large irreversible reaction especially during the first charging cycle. Limitation of instability of Li₂S–GeS₂–P₂S₅ in contact with Li was solved by using double layer electrolyte configuration: Li/(Li₂S-P₂S₅/Li₂S–GeS₂–P₂S₅)/LiCoO₂. LiCoO₂ with SSEs heat-treated with elemental sulfur at elevated temperature exhibited a discharge capacity of 129 mA h g⁻¹ at the second cycle and considerably improved cycling stability.