Glass-ceramic and glass Li2S-GeSe2-P2S5 electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li4-xGe1-xPxS2(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 x 10(-3) S cm(-1) at room temperature for x = 0.95 in Li4-xGe1-xPxS2(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 50 mu A cm(-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 100 mAh g(-1) at the second cycle and consistently improved cycle retention, which is believed to be due to the better interfacial stability. (C) 2010 Elsevier B.V. All rights reserved.