Fluorine-rich deep eutectic electrolytes enabling robust interphases and nonflammability of high-voltage lithium metal batteries

High-voltage Li metal batteries (LMBs) are characterized by superior energy density compared to state-of-the-art Li-ion batteries, but it is an essential challenge to develop electrolytes that can be stably and safely cycled with both reactive Li metal anodes and high-voltage cathodes. Herein, we report a nonflammable deep eutectic electrolyte (DEE) consisting of 3-cyano-6-trifluoromethylpyridine (CTFP) and lithium bis(trifluoromethane)sulfonimide. Systematic investigations verify that the dual-site coordination with Li⁺ by pyridine-nitrogen and cyanide-nitrogen in CTFP induces a deep-eutectic effect, yielding room-temperature liquid electrolytes with a high ionic conductivity (1.36 × 10⁻⁴ S cm⁻¹) and Li⁺ migration number (0.81). A series of derivative DEEs are studied to demonstrate that -CF₃ groups in CTFP account for not only the lowered highest occupied molecular orbital (HOMO) energy level and therefore improved oxidation potential (∼4.84 V vs. Li/Li⁺), but also robust LiF-rich interphases on both anode and cathode sides. Finally, the optimized DEE exhibits compatibility with various LMB cathodes (LiFePO₄, LiMn₂O₄, and LiNi_0.8Co_0.1Mn_0.1O₂), among which the 4.5 V-class LiNi_0.8Co_0.1Mn_0.1O₂ LMB achieves an initial capacity of 205.5 mA h g⁻¹ at 0.1C and stable cycling over 200 cycles at 0.5C. Our findings provide a practical electrolyte and insightful general principles for electrolyte designs of LMBs. © 2024 The Royal Society of Chemistry.