Effects of Li+ Solvation Structures on Aluminum Corrosion in Ether-Based Electrolyte Solutions with Lithium Bis(Fluorosulfonyl)imide (LiFSI)

Lithium bis(fluorosulfonyl)imide (LiFSI) is widely used in lithium-metal batteries to form a stable lithium fluoride (LiF)-based solid electrolyte interphase (SEI). However, the FSI⁻ itself fails to create a protective passivation layer on aluminum (Al) current collectors, leading to Al³⁺ dissolution and severe corrosion. While fluorinated ether solvents have shown promise in mitigating Al corrosion, the mechanisms remain unclear. Here, the role of cation solvations and ion pairing structures is shown in corrosion mitigation. 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB), a 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE)/1,2-dimethoxyethane (DME) mixture, and non-fluorinated ethers are evaluated in 1 m LiFSI. FDMB promoted the formation of AlF₃ while preventing corrosion under extreme conditions (e.g., 4.5 V vs Li/Li⁺, 60 °C). Electrochemical and DFT analyses showed that FDMB underwent favorable defluorination in coordination with both Li⁺ and Al³⁺ that arose from the oxidizing Al surface. Meanwhile, the formation of aggregated ion pairs between Li⁺ and FSI⁻ inhibited the generation of soluble Al³⁺ species coordinated with FSI⁻. Modifying FDMB with alkyl chains further enhanced the anti-corrosive effects by reducing the solubility of Al³⁺ species. In contrast, DME/TTE exhibited more Al corrosion, similar to tetraethylene glycol dimethyl ether (TEGDME), due to less favorable defluorination by the limited solvation of Li⁺ and Al³⁺ on TTE.