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 comparability 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 understanding 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 oxidative stability and low flammability, and the addition of DEE improves ionic conductivity by sharing Li+ coordination 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.