Glass–ceramic Li2S–GeS2–P2S5 electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li4−x Ge1−x P x S4 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 50μAcm−2 between 2.5 and 4.3V (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 129mAhg−1 at the second cycle and considerably improved cycling stability.