Local Acidity as a Corrosiveness Descriptor of Fluorinated Ether Solvents in Lithium-Metal Batteries

Electrolyte engineering based on solvent fluorination has proven effective for stabilizing lithium (Li) metal batteries by weakening the Li⁺-solvent interaction. However, the influence of fluorination on the chemical reactivity of an electrolyte toward Li metal remains poorly understood. Here, the effects of fluorination are systematically investigated by increasing the degree of fluorination (from 2 to 5 substituents) of the terminal alkyl moieties of the 1-ethoxy-2-methoxyethane (EME) and 1-(2-methoxyethoxy)propane (MEP) backbones (denoted F2EME, F4MEP, and F5MEP). Fluorination-induced corrosion deviates from that suggested by conventionally calculated lowest unoccupied molecular orbital (LUMO)-based rationales; instead, it is quantitatively described by the local pK_a as a site-specific thermodynamic descriptor of the fluorination-induced electronic effect. Among the three derivatives, F4MEP is identified as the optimal solvent; it is sufficiently fluorinated to promote the formation of an anion-rich solvation structure for an inorganic-rich, protective Li metal interphase, yet it is not severely corrosive as it is not over-fluorinated. Consequently, an F4MEP-based 20 µm-Li||LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) full-cell retains 80% of its capacity over 368 cycles. This study establishes the local pK_a as a descriptor for evaluating the solvent corrosiveness and highlights the necessity of optimizing the degree of fluorination for high-performance Li metal battery electrolytes.