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.

Because of the depletion of fossil fuels and increasing demand for sustainable energy, lthium-ion batteries (LIBs) have been considered one of the promising power sources for electric vehicles and other energy storage systems. However, increasing market demand for LIBs and limited lithium resources have caused a significant rise in cost of lithium sources. Thus, sodium-ion batteries (SIBs) have been considered a promising alternative to LIBs due to the natural abundance and low cost of sodium resources. However, like LIBs, conventional liquid electrolytes containing organic solvents are highly volatile and flammable, which causes safety problems such as fires and explosions. In this respect, ionic liquids (ILs) can be promising solvents for SIB electrolytes because of their attractive properties such as non-flammability, low volatility and exceptional electrochemical stability. Quasi-solid-state electrolytes (QSEs) have also received considerable attention to overcome the safety issues by avoiding leakage of organic solvents. Furthermore, they possess high ionic conductivity and superior flexibility. In this study, we report an IL-based QSEs with high ionic conductivity (1.5 mS cm-1) and non-flammability. The QSEs were prepared by in-situ thermal curing of boron-centered cross-linkers (BCs) in the IL-based electrolyte. Because the BC has anion-trapping boron moieties, it can promote the ionic mobility of Na+ in the QSE. As a result, the sodium cell employing the QSE exhibited stable cycling performance and enhanced safety.