Facile design of asymmetric flame-retardant gel polymer electrolyte with excellent interfacial stability for sodium metal batteries

Gel polymer electrolytes (GPEs) are considered one of the most forward-looking materials for next-generation high-energy-density sodium metal batteries (SMBs). However, the inherent flammability of GPEs and the unexpected growth of Na dendrites caused by the poor mechanical properties and inferior interfacial stability severely restrain their commercial application. Herein, an asymmetric flame-retardant gel polymer electrolyte (A-FRGPE) with excellent interfacial stability is constructed by in-situ encapsulating of non-flammable phosphate within porous g-C3N4 coated asymmetric glass-fiber matrix, where plenty of sodiophilic active sites and well-defined nanochannels are formed at Na anode side, and thus enhance interfacial compatibility with Na metal anode through facilitating the interfacial kinetic process and regulating the uniform Na+ deposition. Based on the unique structure and composition, the A-FRGPE synchronously possesses high room-temperature ionic conductivity (2.0 mS cm−1), considerable Na+ transference number (0.68), widen electrochemical window (5.63 V vs. Na+/Na), as well as enhanced mechanical properties, which help to boost interfacial stability at anode side and effectively suppress Na dendrites. Specifically, the Na/A-FRGPE/Na symmetric cells exhibit significantly improved cycling stability over 950 h at 0.1 mA cm−2. Furthermore, the resultant solid-state SMBs coupled with Na3V2(PO4)3 cathode remarkable capacity retention of 96.1% over 1100 cycles with extremely high Coulombic efficiency (99.99%) even at high current density of 1 C. The new design strategy in this work creates enormous routes and opportunities to develop advanced GPEs for solid-state alkali metal batteries.