Yuzheng Xie, Ruoqian Lin
Solid polymer electrolytes (SPEs) are promising for sodium–metal anode batteries due to their mechanical strength, which helps suppress dendritic sodium growth. Despite their solid-state nature, SPEs can pose safety risks similar to liquid electrolytes, as many polymer components are flammable, though to a lesser extent. As such, safety cannot be assumed by default for solid polymer electrolytes. In this work, we evaluate the effectiveness of combining two common strategies used to impart flame retardancy to SPEs: (1) incorporating inherently fire-retardant polymers as the electrolyte backbone and (2) introducing flame-retardant additives. However, whether these materials are compatible with sodium metal and can maintain sufficient ionic conductivity when combined remains unresolved. Here, we introduce two halogen-free flame-retardant materials, diethyl vinylphosphonate (DEVP) and triethyl phosphate (TEP), into sodium-ion SPEs. DEVP is polymerized into the SPE backbone, while TEP is added as part of the plasticizer. Our study shows that combining the two strategies significantly improves the self-extinguishing time, i.e., the flame retardancy, compared to single-material approaches. Electrochemically, the dual-strategy SPE exhibits a high ionic conductivity of 1.92 mS cm –1 at 30 °C and stabilizes the sodium–metal anode, attributed to the formation of an SEI rich in inorganic compounds such as NaF and Na x PO y F z, which is revealed by X-ray photoelectron spectroscopy and cryogenic transmission electron microscopy (cryo-TEM). It demonstrates a critical current density of 4 mA cm –2, over 800 h of cycling in Na/SPE/Na symmetric cells, and more than 600 cycles in Na/SPE/Na 3 V 2 (PO 4 ) 3 full cells. These findings demonstrate the potential of developing safer sodium–metal anode battery systems based on SPEs.