High lithium ion conducting Li₂S-GeS₂-P ₂S₅ 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-P ₂S₅ in contact with Li was solved by using double layer electrolyte configuration: Li/(Li₂S-P₂S ₅/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.