Nonflammable Gel Polymer Electrolyte with Ion-Conductive Polyester Networks for Sodium Metal Cells with Excellent Cycling Stability and Enhanced Safety

Sodium metal batteries have received a considerable amount of attention because of the low cost of Na resources and high theoretical capacity of Na metal. However, liquid electrolytes used in batteries cause safety problems such as fires and explosions under abnormal conditions. The uncontrolled dendritic Na growth in the cell also results in poor cycling stability. Herein, we report nonflammable gel polymer electrolytes (GPEs) synthesized by in situ cross-linking of a gel precursor containing ion-conductive polycaprolactone triacrylate. The GPE exhibits a high ionic conductivity of 6.3 mS cm^–1 because of the Na⁺–carbonyl interactions and high segmental motion of polycaprolactone chains despite its three-dimensional network structure. The ion-conductive polymer networks effectively suppress the growth of Na dendrite by inducing uniform Na deposition on the Na electrode, resulting in improved interfacial characteristics of the Na electrode. The Na/Na₃V₂(PO₄)₃ cell employing GPE delivers high discharge capacities at high C rates and exhibits excellent cycling stability. Additionally, the superior thermal stability of GPE prevents a short circuit of the cell at high temperature, which allows safe operation of the Na/Na₃V₂(PO₄)₃ cells.