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||LiFePO4 can maintain ∼100% capacity after 3500 cycles at 5.0 C.