Phase Transfer-Mediated Degradation of Ether-Based Localized High-Concentration Electrolytes in Alkali Metal Batteries

Localized high-concentration electrolytes (LHCEs) have attracted interest in alkali metal batteries due to the advantages of forming stable solid-electrolyte interphases (SEIs) on anodes and good chemical/electrochemical stability. Herein, a new degradation mechanism is revealed for ether-based LHCEs that questions their compatibility with alkali metal anodes (Li, Na, and K). Specifically, the ether solvent reacts with alkali metals to generate solvated electrons (e(s)(-)) that attack hydrofluoroether co-solvents to form a series of byproducts. The ether solvent essentially acts as a phase-transfer reagent that continuously transfers electrons from solid-phase metals into the solution phase, thus inhibiting the formation of stable SEI and leading to continuous alkali metal corrosion. Switching to an ester-based solvating solvent or intercalation anodes such as graphite or molybdenum disulfide has been shown to avoid such a degradation mechanism due to the absence of e(s)(-).