Ultrathin Polymer‐Based Composite Electrolyte Compatible with 4.7 V Cathode for High‐Voltage Li‐Metal Batteries

Polymer solid‐state electrolytes that combine high room‐temperature ion transport with high oxidative stability are crucial for practical all‐solid‐state Li‐metal batteries, yet remain challenging due to concurrent bulk oxidation and interfacial failure. Here we engineer a high‐voltage tolerant polymer‐based composite solid‐state electrolyte by integrating a cyclobutene sulfone (SF)‐containing t copolymer with a lithium sulfonate‐functionalized mullite filler. The 15 µm ultrathin membrane delivers a room‐temperature ionic conductivity of 3.18 × 10 −4 S cm −1 , a high Li + transference number of 0.62, and an electrochemical stability window beyond 5.2 V. Spectroscopic and theoretical analyses reveal that the highly polar SF units promote salt dissociation and decouple Li + from deep polymer traps via competitive coordination. This creates a homogenized, moderate‐energy microenvironment that facilitates fast, continuous Li + hopping and in situ builds thin, dense inorganic‐rich SEI/CEI layers. Consequently, Li||Li cells show accelerated interfacial kinetics and stable cycling over 1000 h. Paired with a Li‐rich Mn‐based cathode (Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O 2 , LRMO), the cell delivers 239.8 mAh g −1 within 2.3–4.7 V and retains 61% after 100 cycles. The nonflammable, mechanically resilient electrolyte further enables a pouch cell to power a device even after folding and cutting, highlighting a viable route to high‐voltage, high‐safety solid‐state batteries.