A Dense Fluorinated Composite Polymer Electrolyte Featuring Fluoride‐Rich Interface for Long‐Life Lithium Batteries

ABSTRACT Solid polymer electrolytes (SPEs) are promising for solid‐state lithium (Li) metal batteries due to their enhanced safety and high energy density. However, the inherent trade‐off between ionic conductivity and mechanical strength severely limits their practical application. Herein, we report a dense fluorinated composite polymer electrolyte (d‐FCPE) that simultaneously addresses the challenges of mechanical robustness, ionic conductivity, and interfacial stability in Li metal batteries. The dense architecture endows the d‐FCPE with favorable mechanical strength, elongation, and adhesion, ensuring superior processability and intimate interfacial contact. Concurrently, the fluorinated composite polymer suppresses electrolyte crystallinity and regulates the ion solvation structure, thereby achieving a high ionic conductivity of 8.6 × 10 ⁻⁴ S cm ⁻¹ at room temperature. Crucially, the integrated dense structure promotes the in‐situ formation of a robust, LiF‐enriched solid electrolyte interphase, which effectively suppresses lithium dendrite growth and enhances electrochemical cycling stability. Consequently, the Li|d‐FCPE|Li symmetric cell exhibits stable cycling for over 4500 h at 0.5 mA cm ⁻² . The batteries using LiNi _0.5 Co _0.2 Mn _0.3 O ₂ cathode and d‐FCPE deliver a long cycle life, maintaining capacity retention of 83.2% (0.5C) and 84.5% (2C) after 1000 cycles. This work highlights the pivotal role of fluorination‐assisted densification in developing high‐performance SPEs for advanced solid‐state energy storage.