High lithium ion conducting Li 2S–GeS 2–P 2S 5 glass–ceramic solid electrolyte with sulfur additive for all solid-state lithium secondary batteries

Abstract

Glass–ceramic Li 2S–GeS 2–P 2S 5 electrolytes were prepared by a single step ball milling (SSBM) process. Various compositions of Li 4− x Ge 1− x P x S 4 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 GeS 2. All-solid-state LiCoO 2/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), LiCoO 2/Li cells showed a large irreversible reaction especially during the first charging cycle. Limitation of instability of Li 2S–GeS 2–P 2S 5 in contact with Li was solved by using double layer electrolyte configuration: Li/(Li 2S-P 2S 5/Li 2S–GeS 2–P 2S 5)/LiCoO 2. LiCoO 2 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.