Glass–ceramic and glass Li2S–GeSe2–P2S5 electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li4−x Ge1−x P x S2(1+x)Se2(1−x) with/without heat treatment (HT) from x =0.55 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 GeSe2. HT also affected the crystallinity. Incorporation of GeSe2 in Li2S–P2S5 kept high conductivity with a maximum value of 1.4×10−3 Scm−1 at room temperature for x =0.95 in Li4−x Ge1−x P x S2(1+x)Se2(1−x) without HT. All-solid-state LiCoO2/Li cells using Li2S–GeSe2–P2S5 as solid-state electrolytes (SE) 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 extremely high conductivity of the SE, LiCoO2/Li cells showed a large irreversible reaction especially during the first charging cycle. LiCoO2 with SEs heat-treated at elevated temperature exhibited a capacity over 100mAhg−1 at the second cycle and consistently improved cycle retention, which is believed to be due to the better interfacial stability.