Asymmetric Fluorinated Ether Enables Balanced Solvation and Robust Interphases in Lithium Metal Batteries

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Abstract
Ether-based electrolytes are widely investigated for lithium metal batteries (LMBs) owing to their favorable compatibility with metallic lithium. However, conventional linear glymes strongly solvate Li+, leading to sluggish desolvation, while their limited oxidative stability restricts high-voltage operation. Here, we report an asymmetric, partially fluorinated ether, diethylene glycol 2,2,2-trifluoroethyl ethyl ether (F3DEGDEE), as a single solvent. Raman spectroscopy, molecular dynamics simulations, and density functional theory calculations reveal that asymmetric fluorination moderately weakens Li+–solvent coordination and promotes FSI− participation in the primary solvation sheath, while avoiding excessive aggregation and viscosity observed in the symmetrically fluorinated analogue. This balanced solvation environment preserves sufficient ionic conductivity, facilitates Li+ desolvation, and enhances oxidative stability. Preferential anion decomposition forms an inorganic-rich SEI with low and stable transport barriers, enabling uniform and reversible Li plating/stripping. The optimized 2 M LiFSI/F3DEGDEE electrolyte achieves 99.02% Coulombic efficiency over 500 cycles in Li||Cu cells and stable Li||Li cycling for >2000 h. Li||LFP cells retain 81.8% capacity after 2000 cycles at 5 C, while Li||NMC811 cells operated at 4.4 V retain 74% after 200 cycles at 1 C. These results highlight asymmetric partial fluorination as an effective strategy to synergize ion transport, desolvation kinetics, and oxidative stability for high-performance LMBs.

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