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

Abstract

High-voltage Li metal batteries (LMBs) are characterized by the superior energy density over state-of-the-art Li-ion batteries, but it is an essential challenge to develop electrolytes which can be stably and safely cycled with both reactive Li metal anodes and high-voltage cathodes. Herein, we report a nonflammable deep eutectic electrolytes (DEEs) 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 the deep-eutectic effect, yielding room-temperature liquid electrolytes with 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 accounts 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 at both anode and cathode sides. Finally, the optimized DEE exhibit compatibility with various LMB cathodes (LiFePO4, LiMn2O4, and LiNi0.8Co0.1Mn0.1O2), among which the 4.5 V-class LiNi0.8Co0.1Mn0.1O2 LMB achieves an initial capacity of 205.5 mAh g−1 at 0.1 C and stable cycling over 200 cycles at 0.5 C. Our findings provide a practical electrolyte and insightful general principles for electrolyte designs of LMBs.