Co‐sustained Release Strategy of Nonflammable Gel Polymer Electrolytes Enables Long‐Life Sodium Metal Batteries

Sodium metal batteries (SMBs) with gel polymer electrolytes (GPEs) are considered promising candidates for high energy‐density batteries due to their high theoretical capacity and cost effectiveness. However, the intrinsic flammability of GPE poses challenges for their widespread application. Inspired by the concept of the capsules, a triethyl phosphate (TEP)‐based GPE with co‐sustained‐release effect has been designed. This structure features an insoluble ethoxylated trimethylopropane triacrylate (ETPTA) polymer matrix combined with carbonate co‐solvents, effectively reducing the corrosion of TEP on sodium metal anodes while maintaining a continuous flame‐retardant effect. In this distinctive structure, the abundant carbonyl group on ETPTA promotes the uniform migration of sodium ions, while the carbonate co‐solvent facilitates the formation of a NaF‐rich solid electrolyte interphase (SEI) layer, effectively suppressing the growth of sodium dendrite. Low content of TEP guide the preferential orientation of the (100) crystal plane of sodium anode, enhancing the long‐term cycling stability. Na/GPE/Na cells can achieve stable cycling for over 1600 hours, and the NVP/GPE/Na full cells exhibit 86.6% capacity retention after 4000 cycles. Moreover, the engineered GPE enables the operation of a 4.5 V high‐voltage cathode for 500 cycles. This strategy paves a new way for designing high‐safety GPE tailored for high‐performance SMBs.