Coupling Anion‐Capturer with Polymer Chains in Fireproof Gel Polymer Electrolyte Enables Dendrite‐Free Sodium Metal Batteries

Abstract Sodium metal batteries (SMBs) using gel polymer electrolytes (GPEs) with high theoretical capacity and low production cost are regarded as a promising candidate for high energy‐density batteries. However, the inherent flammability of GPEs and uncontrolled Na dendrite caused by inferior mechanical properties and interfacial stability hinder their practical applications. Herein, an anion‐trapping fireproof composite gel electrolyte (AT‐FCGE) is designed through a chemical grafting–coupling strategy, where functionalized boron nitride nanosheets (M‐BNNs) used as both nanosized crosslinker and anion capturer are coupled with poly(ethylene glycol)diacrylate in poly(vinylidene fluoride‐co‐hexafluoropropylene) matrix, to expedite Na + transport and suppress dendrite growth. Experimental and calculation studies suggest that the anion‐trapping effect of M‐BNNs with abundant Lewis‐acid sites can promote the dissociation of salts, thus remarkably improving the ionic conductivity and Na + transference number. Meanwhile, the formation of highly crosslinked semi‐interpenetrating network can effectively in situ encapsulate non‐flammable phosphate without sacrificing the mechanical properties. Consequently, the resulting AT‐FCGE shows significantly enhanced Na + conductivity, mechanical properties, and excellent interfacial stability. The AT‐FCGE enables a long‐cycle stability dendrite‐free Na/Na symmetric cell, and prominent electrochemical performance is demonstrated in solid‐state SMBs. The approach provides a broader promise for the great potential of fire‐retardant gel electrolytes in high‐performance SMBs and the beyond.