Dual‐Conduction Polymer Electrolyte and Stable Interphase Engineering for Room‐/Subzero‐Temperature, Long‐Cycling All‐Solid‐State Sodium Batteries

All-solid-state sodium batteries (ASSSBs) are promising due to their exceptional energy density, safety, and abundant sodium resources. Poly(ethylene oxide) (PEO) electrolyte is extensively investigated for ASSSBs, but its practical application is limited by low ionic conductivity and interfacial instability at room/sub‑zero temperatures. Here, a dual-strategy solid-state electrolyte design (entirely liquid-free) is presented in which succinonitrile serves as a plasticizer that synergizes with PEO, creating dual-ion conduction pathways that achieve an impressive ionic conductivity of 2.75 × 10^-4 S cm^-1 at room temperature. Concurrently, sodium difluoro(oxalate)borate undergoes in situ reactions with sodium metal anode, synergizing with the artificially formed NaF interphase layer to facilitate the successful formation of a stable, inorganic salt-rich solid electrolyte interphase. This mechanism effectively suppresses undesirable side reactions between the polymer and sodium metal anode. Consequently, Na@NaF||Na@NaF symmetric cells exhibit outstanding cycling stability for over 1,500 hours at 0.1 mA cm^-2 under room temperature. Full cells based on Na₃V₂(PO₄)₃||Na@NaF retain 91.2% of their initial capacity after 1,000 cycles at 2C. Notably, the ASSSBs deliver a discharge capacity of 88.2 mAh g^-1 even at -5 °C, highlighting their suitability for low-temperature applications. This work establishes an electrolyte-interface collaborative design paradigm for high-performance ASSSBs under wide-temperature operating conditions.