Flame-retardant electrolyte enables sodium metal batteries to limit dendrites and stabilize cycling

While sodium metal batteries (SMBs) have received much notice because of their low cost and abundant resources, they also suffer from an uncontrollable growth of dendritic crystals during the charging/discharging processes. This paper reports using Hexafluorocyclotriphosphazene (HFPN) as a flame-retardant additive and film-forming additive for SMBs mixed with conventional NaPF6 in a carbonate-based electrolyte. (1) the self-extinguishing time of the electrolyte was tested and the flame retardant mechanism of the flame retardant additive was explained; (2) the growth of sodium dendrites in the symmetric cell during constant current charging and discharging was observed using in situ optical microscopy; (3) the stable formation of HFPN with solid electrolyte interface (SEI) was illustrated by Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) characterization, and the excellent cycling performance of the sodium metal all-cell with Na3V2(PO4)3 as the positive electrode was demonstrated.