Improved Polymer/Inorganic Interfacial Compatibility in Composite Solid‐State Electrolytes by Ionic Liquids Toward Wide‐Temperature Lithium Metal Batteries

Poor polymer/inorganic interfacial compatibility remains a central obstacle to achieving fast Li + transport and stable interphase chemistry in composite solid‐state electrolytes (CSEs) for lithium metal batteries. Herein, we establish an anion‐mediated interfacial regulation strategy by exploiting the preferential adsorption of ionic liquids on inorganic fillers to tune the interfacial energetics of polyethylene oxide (PEO)‐based CSEs. In a model PEO/neuron‐like silicone nanofilament‐grafted montmorillonite (SNFs@MMT) system, three imidazolium‐based ionic liquids with different anion chemistries (BF 4 − , FSI − , and OAc − ) were systematically investigated. Among them, the BF 4 − ‐based IL exhibits the strongest interfacial affinity for SNFs@MMT, improves polymer/filler compatibility, and reduces energetic discontinuities across heterogeneous interfaces. As a result, Li + transport barriers are lowered, and a fluorine‐enriched interfacial microenvironment is established, favoring the formation of a uniform LiF‐rich interphase. The optimized CSE exhibits an ionic conductivity of 6.2 × 10 −4 S cm −1 and a Li + transference number of 0.56 at 30°C. Correspondingly, Li symmetric cells and Li|LiFePO 4 full cells show stable long‐term cycling from 0 to 60°C, and flexible pouch cells remain operational under mechanical deformation. These findings identify anion‐directed interfacial regulation as a viable design principle for wide‐temperature CSEs.