Preparation of Li 2S–GeSe 2–P 2S 5 electrolytes by a single step ball milling for all-solid-state lithium secondary batteries

Abstract

Glass–ceramic and glass Li 2S–GeSe 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 2(1+ x) Se 2(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 GeSe 2. HT also affected the crystallinity. Incorporation of GeSe 2 in Li 2S–P 2S 5 kept high conductivity with a maximum value of 1.4 × 10 −3 S cm −1 at room temperature for x = 0.95 in Li 4− x Ge 1− x P x S 2(1+ x) Se 2(1− x) without HT. All-solid-state LiCoO 2/Li cells using Li 2S–GeSe 2–P 2S 5 as solid-state electrolytes (SE) 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 extremely high conductivity of the SE, LiCoO 2/Li cells showed a large irreversible reaction especially during the first charging cycle. LiCoO 2 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.