A Fluoropolymer‐Based Gel Polymer Electrolyte Crosslinked by NaBH$_4$ for Application in Room‐Temperature Sodium Batteries

Rechargeable sodium‐metal batteries are attractive for large‐scale energy storage due to the high abundance and low cost of sodium. However, their development is hindered by the limited availability of electrolytes that simultaneously provide high ionic conductivity, high Na metal compatibility, and suppressed anion mobility. Here, we report a chemically simple gel polymer electrolyte (GPE) based on a commercially available hydroxyl‐terminated fluoropolymer, in which a borate‐mediated network is generated in situ using sodium borohydride as a low‐cost crosslinking agent. The resulting GPE exhibits excellent Na$^+$ ionic conductivity up to 1.6 × 10$^{−3}$Scm$^{−1}$ at 30 °C and near single‐ion conduction (Na$^+$ ion transference number up to ~0.84). Galvanostatic cycling of a symmetric Na||Na cell using glass fiber‐supported GPE exhibits stable performance over 750 cycles, demonstrating excellent GPE compatibility with Na metal. A coin cell assembled from an Na anode and a Na$_3$V$_2$(PO$_4$)$_3$/C cathode with this GPE exhibits an exceptional cycling performance with over 2000 charge–discharge cycles within the voltage range from 1 to 2 V.