Enabling Durable, Subzero Li/Na‐Metal Batteries with Scalable, In Situ Ultrathin Polymerized Electrolytes

ABSTRACT Solid polymer electrolytes are promising for high‐energy‐density alkali metal batteries but are hindered by excessive thickness, insufficient interfacial contact, and uncontrolled dendrite growth. Herein, we introduce a scalable in situ polymerization strategy to fabricate ultrathin (∼10 µm) yet robust solid‐like polymer electrolytes (SPE‐Na and SPE‐Li) that simultaneously enable rapid ion transport and intimate interfacial integration. Ion transport is dominated by fast migration through solvent‐rich pathways while the polymer matrix ensures mechanical integrity. To stabilize metal electrodes, potassium perfluorobutane sulfonate is introduced to form SPEs, where K + provides dynamic electrostatic shielding to suppress dendrite formation, while fluorine‐containing species promote LiF/NaF‐rich, inorganic‐reinforced interphases. The optimized SPE‐0.05K‐Na and SPE‐0.02K‐Li showed high of ∼0.5 and ionic conductivities of ∼4.78 × 10 −4 and ∼4.87 × 10 −4 S cm −1 . Consequently, Li||Li and Na||Na symmetric cells achieved stable cycling for 4000 h (0.5 mA cm −2 ) and 1800 h (0.2 mA cm −2 ), respectively. Full cells operated reliably under rigorous conditions, such as elevated mass loadings (e.g., 20.0 mg LFP cm −2 , 20.2 mg LCO cm −2 , 14.3 mg NCM cm −2 , and 7.2 mg NVP cm −2 ), subzero temperatures (−20°C), and Ah‐level pouch cells. This work integrates interface engineering with ultrathin electrolyte design to advance practical, low‐temperature, and high‐energy‐density batteries.