Thermal stability of ionic liquids for lithium-ion batteries: A review

Conventional carbonate-based organic electrolytes in commercial lithium-ion batteries are considered as responsible for battery safety issues because of their volatile and flammable nature. Ionic liquids, which are nonvolatile, noncombustible with good electrochemical performance and high thermal stability, are regarded as competitive next-generation solvents. Previous studies have focused on the development of ionic liquid systems with better electrochemical performance. However, the thermal stability of ionic liquid electrolytes is seldom discussed. This work comprehensively reviews and summarizes thermal safety studies on ionic liquid electrolytes at three levels: solvents, electrolytes, and batteries. This study compiles thermal stability indicators and values from literature to build a database for further quantitative analysis. According to the database, thermal stability at the electrolyte level does not correlate well with performance at the battery level. Ionic liquids with high thermal stability can generate more heat at the battery level due to their interactions with both the cathode and anode. Through a detailed analysis of reaction sequences at the battery level, this work further proposes the first report on the underlying thermal reaction pathways of ionic liquid-based lithium-ion batteries. Results show that the thermal runaway of TFSI-based ionic liquid lithium-ion batteries is primarily triggered by the Hofmann elimination, along with the involvement of oxygen. Finally, this work outlines potential future developments and directions for creating safer ionic liquids.