Improved Polymer/Inorganic Interfacial Compatibility in Composite Solid-State Electrolytes by Ionic Liquids Toward...

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 (BF₄ ^-, FSI^-, and OAc^-) were systematically investigated. Among them, the BF₄ ^--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|LiFePO₄ 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.