1 µm‐Thick Robust Gel Polymer Electrolyte with Excellent Interfacial Stability for High‐Performance Li Metal Batteries

Abstract Gel polymer electrolytes (GPEs) hold great promise for lithium (Li) metal batteries (LMBs). Nevertheless, a critical challenge lies in reducing the thickness of GPEs while maintaining their mechanical integrity to achieve high‐energy‐density LMBs. Additionally, protecting the Li metal anode via electrolyte engineering in GPEs remains demanding. Herein, an innovative ultrathin (1 µm‐thick) yet robust GPE developed using an in situ curing technique, featuring a nanofibrous, exceptionally strong polyethylene separator is presented. The unique microstructure, interfacial conformability, and ultrahigh mechanical robustness of the ultrathin polyethylene separator are thoroughly verified. Enhanced ionic association within the GPE is achieved due to the strong affinity of electrolyte solvent with the fluorinated polymer network, as confirmed by large‐scale molecular dynamics simulations. The optimized solvation structure with high contact ion pairs and aggregate fractions contributes to forming an anion‐derived inorganic‐rich solid electrolyte interphase (SEI), thereby protecting the lithium anode. Benefiting from the ultrahigh robustness of GPE and the excellent interfacial stability, the Li metal full cell with a high mass loading LiNi_0.8Co_0.1Mn_0.1O₂ cathode (≈17.3 mg cm⁻²) and thin Li foil anode (50 µm) demonstrates 91% capacity retention after 200 cycles. This design demonstrates a feasible approach toward the practical quasi‐solid‐state LMBs.