Synergistic Anion Confinement in a Poly(Ionic Liquid)/MOF Composite Electrolyte Decouples Ionic Conductivity and Mechanical Strength for High‐Performance Solid‐State Lithium Metal Batteries

ABSTRACT Poly(ionic liquid)s (PILs) are promising solid electrolyte for solid‐state lithium metal batteries, but are fundamentally constrained by the trade‐off between ionic conductivity and mechanical strength. To overcome this issue, we design a novel composite Poly(ionic liquid)s based solid electrolyte (CPLE) through the synergistic integration of a main‐chain imidazolium‐based PIL backbone and a metal‐organic framework (MOF‐5). The main‐chain PIL architecture intrinsically mitigate strong intermolecular ionic interactions, while MOF‐5 framework acts as a multi‐functional filler that strongly immobilizes TFSI − anions via Lewis acid‐base interactions. This dual‐anchoring strategy effectively promotes Li salt dissociation, provides additional ion transport pathways, and remarkably enhance the Li + transference number. Consequently, the CPLE achieves a high room‐temperature ionic conductivity of 1.4 mS cm − 1 , an exceptional Li + transference number of 0.7, and a widened electrochemical stability window up to 4.6 V. The strong interfacial interaction between the PIL and MOF‐5 also reinforces mechanical strength (1.9 MPa tensile strength). This unique synergy enables outstanding interfacial stability: Li||Li symmetric cells cycle stably for over 2000 h at 0.1 mA cm − 2 , and Li||LiFePO 4 full cells retain 80% capacity after 700 cycles at 1 C. Crucially, the CPLE is compatible with high‐voltage LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathodes.