A crown-ether-enabled eutectic electrolyte for ultra-high temperature lithium metal batteries

Lithium metal batteries (LMBs) are generally restricted to operating below 60 °C and not suitable for high-temperature applications due to their thermally susceptible carbonate electrolytes. Deep eutectic electrolytes (DEE) with strong supramolecular interactions are a new emerging type of liquid-state ionic conductors, and here we present a novel DEE—which is composed of 18-crown-6 ether (18C6) and lithium difluoro(oxalate)borate (LiDFOB)—to extend the working temperature up to 150 °C. Via combined physiochemical and theoretical characterizations, it is identified that such high thermotolerance is a result of specific solvation structure of Li(18C6)4DFOB with multiple Li–O interactions. Moreover, our newly developed DEE triggers the formation of a B/F-rich solid electrolyte interphase, contributing to superior thermal stability and enhanced compatibility with Li-metal anodes. Consequently, Li|DEE|LiFePO4 cell exhibits great tolerance to extremely high temperatures with a capacity reversibility of 83 % after 250 cycles at 120 °C. Even at 150 °C, the cell can retain 77 % of the initial capacity (105 mAh g−1) after 100 cycles. To our knowledge, the DEE is the first liquid electrolyte for LMBs working at unprecedented high-temperature. This work opens intriguing perspectives to design novel electrolytes for demanded performance of LMBs across a wide temperature range.