Improved Thermal Stability of Oxysulfide Glassy Solid-State Electrolytes

Abstract

In this study, the crystallization kinetics of (oxy)sulfide 70Li2S·(30-x)P2S5·xP2O5 (x = 0, 2, 5) solid-state electrolytes are reported. It was found that 5 mol% P2O5 glass co-former slowed the crystallization rate of the Li7P3S11−x/4Ox/4 ceramic phase by a factor of 10. After 10 min at 230 °C, a 70Li2S·30P2S5 sulfide glass was 92% devitrified whereas a 70Li2S·25P2S5·5P2O5 oxysulfide glass was only 8% devitrified. The improved thermal stability of oxysulfide glasses was then utilized to demonstrate the fabrication of a standalone, reinforced SSE separator by hot pressing. More importantly, it was recognized that the microstructure of 70Li2S·25P2S5·5P2O5 oxysulfide SSE separators could be modified by hot pressing without changing ionic conductivity. This result was achieved because the precipitation of a superionically conductive Li7P3S11−x/4Ox/4 ceramic phase was limited. A study was then conducted to determine what effect microstructure has on the susceptibility of SSE separators to shorting by lithium metal penetration. Hot-pressed separators were found to be more susceptible to shorting than cold-pressed separators. X-ray Computer Tomography (XCT) of post-mortem samples showed that hot-pressed samples failed by transverse microcrack pathways, which underscores the importance of low defect density in dense SSE separators.