Dual-anion ionic liquid electrolytes: a strategy for achieving high stability and conductivity in lithium metal batteries†

Ionic liquid electrolytes (ILEs) provide promising thermal and electrochemical stability characteristics for safer lithium metal batteries (LMBs). However, their development faces challenges due to their low ionic conductivity and poor wettability on separators. In this study, we introduce a dual-anion locally concentrated ionic-liquid electrolyte (D-LCILE), designed with a diluent and two distinct anions to significantly improve the ionic conductivity and wettability. These improvements were confirmed through electrochemical impedance spectroscopy (EIS) measurements on stainless steel symmetric cells, contact angle tests, and rate capability assessments on a 300 μm thick lithium metal half-cell. Notably, the dual-anion design enhances the interfacial stability, as density functional theory (DFT) calculations revealed a more stable solvation shell structure, further supported by molecular dynamics (MD) simulations. Additionally, scanning electron microscopy (SEM) experiments confirmed the deposition of a thin and, dense lithium layer, while X-ray photoelectron spectroscopy (XPS) depth profile analysis showed a stable solid electrolyte interphase (SEI) with increased LiF content. Performance tests on a 20 μm-thick Li‖LiFePO₄ full cell revealed an average Coulombic efficiency exceeding 99.90% and capacity retention >99.93% after 200 cycles at 1C, making D-LCILE a highly promising candidate for next-generation, high-performance LMBs.