Solid-state lithium batteries: Safety and prospects

Solid-state lithium batteries are flourishing due to their excellent potential energy density. Substantial efforts have been made to improve their electrochemical performance by increasing the conductivity of solid-state electrolytes (SEs) and designing a compatible battery configuration. The safety of a solid lithium battery has generally been taken for granted due to the nonflammability and strength of SEs. However, recent results have shown the release of dangerous gases and intense heat due to the formation of lithium dendrites, indicating the safety of solid-state lithium batteries may have been overestimated. In this review, we introduce a safety evaluation methodology, then focus on the garnet Li7La3Zr2O12 (LLZO) and sulfide-based SEs, summarizing their structure, conductivity, compatibility with a lithium metal anode, electrochemical/chemical stability, and mechanical/thermal stability, which correlate closely with battery safety. We also evaluate the safety of all-solid-state lithium batteries, then conclude by discussing future avenues for improving the safety of SE-based batteries.