Designing Ionic Liquid Gel to Realize Integrated Modification Constructed High Voltage (4.5 V) and High Temperature (≥100 °C) Resistant Solid‐State Lithium Metal Battery

Abstract Solid‐state batteries have emerged as one of the most promising candidates to meet the demand of high safety and high energy density in energy storage systems. Nevertheless, the internal interfacial issues critically hinder their further development, particularly under high‐temperature and high‐voltage conditions. This study presents an innovative interfacial engineering strategy through the integration of a flexible ionic liquid gel onto both the LiNi _0.8 Co _0.1 Mn _0.1 O ₂ cathode surface and Li _1.5 Al _0.5 Ge _1.5 (PO ₄ ) ₃ solid‐state electrolyte matrix. The approach establishes conformal interfacial contact between components, effectively minimizing interfacial impedance and reducing stress concentration while enabling continuous Li ⁺ migration across multiphase boundaries. Remarkably, the in situ electrochemical conversion of the gel spontaneously constructs the gradient LiF‐rich layers at both the cathode and anode surfaces. This self‐optimized architecture could suppress detrimental interfacial reactions under aggressive 4.5 V polarization and mitigate thermal degradation at elevated temperatures upto 100 °C. Ultimately, the engineered battery achieves exceptional cyclability with 76.5% capacity retention after 100 cycles under extreme operational conditions (4.5 V, 100 °C). More notably, the energy density of the solid‐state pouch batteries can reach 435 Wh kg ^{− 1} . This facile and scalable integrated interfacial optimization engineering represents significant progress in developing high‐voltage and high‐temperature resistant solid‐state lithium metal batteries.