Anion trapping-coupling strategy driven asymmetric nonflammable gel electrolyte for high performance sodium batteries

Gel polymer electrolytes, promising electrolyte candidates for advanced sodium metal batteries (SMBs), suffer from the great challenges of combustion risk and inferior interfacial stability caused by poor mechanical properties and low Na+ selectivity. Herein, we proposed a rational anion trapping-coupling strategy to build a mechanically robust asymmetric nonflammable composite gel electrolyte (AS-NFCGE) by introducing in-situ coupled boron nitride nanosheets (BNNs) on glass fiber matrix embedded with synchronous polymerized flame-retardant polymer. Theoretical calculations, finite-element simulations, and experimental tests prove that the high coupling BNNs with anion trapping effect constructed asymmetric structure can significantly accelerate selective Na+ migration kinetics, enhance mechanical properties of AS-NFCGE and endow a similar single-ion conducting behavior to homogenize ion-distribution at the anode side, thus improving interfacial stability and suppressing dendrite growth to achieve ultra-stable SMBs. Specifically, the AS-NFGE delivers high ionic conductivity and Na+ transference number (0.73) together with excellent flam-retardant. Consequently, the dendrite-free Na/Na symmetric cell exhibits considerable cycling life for over 3000 h at 0.1 mA cm−2 with low overvoltage. Applications of the AS-NFCGE in SMBs further demonstrate the excellent electrochemical performances, and no thermal runaway and no electrolyte leakage occur for pouch cells using AS-NFCGE. This strategy provides new insight into the development of safe and high-energy SMBs and the beyond.