Photopolymerized Gel Electrolyte with Unprecedented Room‐Temperature Ionic Conductivity for High‐Energy‐Density Solid‐State Sodium Metal Batteries

Solid‐state sodium metal batteries (SMBs) are highly promising rechargeable batteries owing to the abundance and cost effectiveness of sodium. However, the low room‐temperature ionic conductivity and narrow voltage window of solid‐state electrolytes seriously inhibit the development of SMBs. Herein, an ethoxylated trimethylolpropane triacrylate based quasisolid‐state electrolyte (ETPTA–NaClO4–QSSE) is developed by photopolymerization for high‐energy‐density solid‐state SMBs. The ETPTA–NaClO4–QSSE exhibits remarkable room‐temperature ionic conductivity of 1.2 mS cm−1, a wide electrochemical window of >4.7 V versus Na+/Na, and excellent flexibility. Owing to outstanding interfacial compatibility between this electrolyte and the electrode, Na metal symmetrical batteries show ultralong cyclability with 1000 h at 0.1 mA cm−2, and ultralow overpotential of 355 mV at 1 mA cm−2, indicative of significant suppression of the Na dendrite growth. Notably, Na3V2(PO4)3 (NVP) full batteries (NVP||ETPTA–NaClO4–QSSE||Na) display unprecedented rate capability, with a recorded capacity of 55 mAh g−1 at 15 C, higher than any achieved so far in solid‐state SMBs, and long‐term cycling stability at 5 C, offering a capacity retention of 97% after 1000 cycles. Furthermore, NVP||ETPTA–NaClO4–QSSE||Na pouch cells represent excellent flexibility and exceptional safety, demonstrative of wide applicability. Therefore, this work will open new opportunities to develop room‐temperature high‐energy‐density solid‐state SMBs. A photopolymerized quasisolid‐state electrolyte with unprecedented room‐temperature ionic conductivity of 1.2 mS cm−1 and a wide electrochemical window of > 4.7 V versus Na+/Na is developed for high‐energy‐density solid‐state sodium metal batteries, showing unprecedented rate performance and long‐term cycle stability, remarkable flexibility, and safety.