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 Li4−xGe1−xPxS4 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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► High conductivity over 1 × 10−3 S cm−1 for solid electrolyte using SSBM method. ► Solid electrolyte stability against LiCoO2 improved with elemental sulfur addition. ► Performance of all-solid-state batteries improved with double layer electrolyte.