A Simple and Scalable LiF‐Reinforced PEO Gel Electrolyte for Durable Lithium Metal Batteries

Solid‐state polymer lithium metal batteries offer a pathway to significantly higher energy density and intrinsic safety compared with conventional liquid electrolyte systems. However, their practical deployment is limited by low ionic conductivity, interfacial stability, and mechanical robustness of polymer electrolytes. Here, we report a scalable, solution‐cast poly(ethylene oxide) (PEO)–based composite gel polymer electrolyte (CGPE) containing lithium fluoride (LiF) ceramic fillers. The optimized formulation with 20 wt% LiF (CGPE20) provides multiple synergistic benefits: The rigid LiF framework reinforces the mechanical stiffness of the membrane and effectively suppresses lithium dendrite growth; the polymer–ceramic interfaces create continuous Li+ transport pathways, enabling a high ionic conductivity of 2.07 mS cm−1 at 60°C; and the presence of TEGDME enhances interfacial wettability and polymer chain mobility, expanding the electrochemical stability window from 3.9 to 4.1 V. CGPE20 enables symmetric Li‖Li cells to cycle stably for over 1400h at 0.1 mA cm−2 with low overpotential (∼50 mV) and Li‖LFP full cells to retain 75% of their initial capacity after 300 cycles at 0.5 C, demonstrating both long‐term stability and high‐rate performance. The combination of optimized composition, enhanced ionic conductivity, mechanical reinforcement, and scalable fabrication positions CGPE20 as a promising candidate for practical, high‐performance solid‐state lithium metal batteries, providing a well‐balanced solution to the challenges that have limited polymer‐based solid electrolytes.