Molecular design of methylated fluorinated-ether electrolytes for high-voltage lithium-metal batteries

Fluorinated-ethers are recognized as promising electrolyte solvents for high-voltage lithium metal batteries (HV-LMBs) due to their exceptional oxidation resistance and high reductive stability on anode. However, their oxidative and chemical stability are strongly influenced by the fluorination degree and α-hydrogen reactivity, which can easily be attacked by active oxygen species. Herein, a series of methyl-modified fluorinated-ethers on the acetal structure motif were designed and synthesized, and the fluorinated-ether featuring simultaneous trifluoromethyl and methyl substitution delivered outstanding electrochemical performances. With the designed fluorinated-ether of 1,1,1-trifluoro-2-(1-(2,2,2-trifluoroethoxy) ethoxy) ethane (TFEE), the electrolyte enabled highly reversible Li deposition/stripping, achieving an average Coulombic efficiency of 98.5% over 455 cycles. Li||LiNi_0.8Co_0.1Mn_0.1O₂ cells maintained 79.2% capacity retention after 410 cycles under 4.5 V, exceeding those containing non-methylated or monofluorinated ether analogs. Li||LiNi_0.8Co_0.1Mn_0.1O₂ pouch cell with 1 Ah theoretical capacity could stably cycle for 80 cycles with 79.6% capacity retention. This molecular design strategy provides new insights for developing advanced high-voltage electrolytes for next-generation LMBs.