Fluorinated ether-based co-solvent electrolytes for lithium-metal batteries: High ionic conductivity and suppressed dissolution of fragmented anions

Advanced electrolyte solutions for lithium (Li) metal batteries have been developed to improve their compara-bility with Li electrodes and enhance high oxidative stability. While the solvation structures of Li+ are critical for determining the quality of the solid electrolyte interphase (SEI) layer and cyclability, there is limited under-standing of these correlations for co-solvent systems and long-term cycles. Herein, we investigate the role of fluorinated ether-based co-solvents consisting of 2,2,3,3-tetrafluoro-1,4-dimethoxylbutane (FDMB) and 1,2-diethoxyethane (DEE) with 1 M lithium bis(trifluoromethanesulfonyl)imide (LiFSI). FDMB exhibits high oxida-tive stability and low flammability, and the addition of DEE improves ionic conductivity by sharing Li+ coor-dination with FDMB. SEI formation is derived from FSI-. Notably, increasing the DEE volume ratio promotes lithium fluoride (LiF) deposition and dissolution of fragmented FSI- into the electrolyte solution. FDMB inhibits such dissolution and enhances the formation of lithium nitride (Li3N) and lithium oxide (Li2O) in the SEI. Thus, the DEE/FDMB co-solvents complemented the weaknesses of the two solvents and offer the low SEI resistance. It explains optimal cycling performance for Li|Li and Li|LiFePO4 cells with 1:6 DEE/FDMB with 1 M LiFSI compared to that of the single DEE and FDMB.