Asymmetric Ionic Liquid Modulated Anion-Reinforced Electric Double Layer for Advanced Durable Lithium Batteries

The electric double layer (EDL) governs local electrolyte enrichment and reduction pathways, thereby directing the nucleation and evolution of solid electrolyte interphase (SEI). However, electrolyte design is still largely guided by bulk solvation descriptors. Here, an asymmetric room temperature phosphonium ionic liquid, (2-methoxyethoxy)methyl phosphonium hexafluorophosphate (PMEP), is designed to promote an anion-reinforced EDL. Molecular asymmetry lowers the melting point of PMEP and promotes PF₆ ^- participation in Li⁺-centered solvation structures. Molecular dynamics (MD) simulations and density functional theory (DFT) calculations suggest that PF₆ ^- can participate in Li⁺-centered interfacial solvation clusters under selected charge states, which contributes to the formation of an SEI containing both organic reduction products and inorganic species such as LiF and Li₂O. This organic/inorganic SEI structure lowers interfacial impedance and the apparent activation barrier for Li⁺ transfer, enabling more uniform lithium deposition and a mechanically robust interface. Li|LiFePO₄ batteries with an areal loading of 11.3 mg cm^-2 deliver 94.9% capacity retention after 600 cycles. The fabricated 1.6 Ah Graphite|LiFePO₄ cylindrical cell operates stably for over 500 cycles with a Coulombic efficiency above 99.8%. This work demonstrates a shift in electrolyte design from bulk formulations toward interfacial solvation structure engineering for next generation batteries.