Resolving Ionic Liquid Electrolyte-Mediated Microscopic Electrified Interface for Stable Lithium Metal Anode.

Ionic liquids (ILs), renowned for their exceptional safety features, have become promising candidates for developing next-generation electrolyte systems in high-safety, high-energy-density lithium metal batteries (LMBs). However, the ability to precisely control the electric double layer (EDL) structure at the interface of IL-based electrolytes, based on ion geometry and short-range interactions, remains theoretically limited, which severely restricts the efficient and targeted selection of optimal IL structures. Guided by the Kornyshev model, this work demonstrates that IL electrolytes with smaller organic cations exhibit a higher packing parameter in the EDL, leading to denser ion packing and increased interfacial charge density. This ion-enriched EDL structure shows enhanced differential capacitance and significantly elevates the interfacial concentration of both Li+ and FSI- species. As a result, it enables rapid replenishment of Li+ ions during deposition and promotes the formation of a stable, anion-derived solid electrolyte interphase (SEI). Based on this mechanism, we designed a novel fluoropropyl pyrrolidinium-based IL electrolyte. The assembled LMBs using this electrolyte achieved a capacity of 4.5 Ah and an energy density of 505 Wh kg-1, exhibiting stable cycling performance and passing a rigorous nail penetration safety test. This study establishes a crucial link between the microscopic structure of the IL electrolyte-mediated electrified interface and macroscopic battery performance, offering new insights and direction for designing electrolytes for next-generation high-safety LMBs.