Influence of Fluoroethylene Carbonate on Thermal Safety Properties of Lithium Metal Batteries

In the process of developing rechargeable Li metal batteries to provide commercially available battery systems for high-energy applications, emphasis is typically placed on the design of novel battery materials enabling enhanced performance and energy density. However, safety properties of novel battery materials and their combinations are often insufficiently analyzed and, thus, require detailed investigations. In this context, thermal safety properties determine overall battery safety as abusive conditions are typically associated with heat generation potentially ending in thermal runaway. Within this study, a methodology was applied that combines thermal and pressure analysis with cross-section imaging, enabling detailed investigations and comparison of the thermal behavior of a liquid organic carbonate-based reference electrolyte with and without fluoroethylene carbonate (FEC). Herein, FEC distinctly improved thermal safety properties indicated by an increase in onset temperature of exothermic reactions and a decrease in generated heat. However, temperature-dependent pressure analysis on the individual electrolytes revealed FEC to aggravate gas evolution upon heating, thus, emphasizing the importance of pressure build-up for battery safety. Overall, this study demonstrates the relevance of battery safety assessment both on material and cell level while also considering gas evolution besides thermal indicators like onset temperature and generated heat.