Engineering Li + Kinetic‐Buffering and Anion‐Anchoring Interphase via One‐Step Sequential In Situ Polymerization for Stable Solid Li Metal Batteries

The pursuit of high‐energy solid‐state Li metal batteries is hindered by unregulated Li + deposition kinetics at the electrolyte/anode interface. Current research generally focuses on enhancing bulk ionic conductivity, often overlooking the critical need to engineer interfacial Li+ kinetics. Here, we report a kinetically modulated bilayer solid polymer electrolyte constructed by one‐step sequential in situ polymerization. Distinct kinetics reactions consisting of fast radical polymerization followed by slow cationic ring‐opening is exploited to spontaneously engineer a thin, fluorine‐rich PHFBMA interphase between a poly(1,3,5‐trioxane) matrix and Li anode. This interphase functions as a Li + kinetic‐buffering and anion‐anchoring zone, which not only enhances interfacial chemical/mechanical stability, but also synergistically immobilizes FSI − anions and Li + , creating a localized high Li + concentration. The resulting architecture establishes a guided ion‐transport pathway, where Li + flux is first “accelerated” across the interphase of bilayer polymer electrolyte (facilitated by Li + immobilization), and then “buffered” and homogenized before deposition onto the Li metal surface (ensured by localized high Li + concentration). Consequently, Li||Li symmetric cells demonstrate stable cycling over 1500 h and Li||LiFePO 4 can maintain ∼100% capacity after 3500 cycles at 5.0 C.