Fluorinated-ethers are recognized as promising electrolyte solvents for high-voltage lithium metal batteries (HV-LMBs) due to its 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 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_0.1 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_0.1 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.