Flame‐Retardant, Self‐Purging, High‐Voltage Electrolyte for Safe and Long‐Cycling Sodium Metal Batteries

Abstract Sodium metal batteries (SMBs) remain greatly challenging in safety and stability. Herein, a flame‐retardant s designed, self‐purging high‐voltage electrolyte is designed to stabilize SMBs with the use of ethoxy (pentafluoro) cyclotriphosphazene (PFPN) as the electrolyte additive. PFPN can participate in the shell structure of solvation through stronger van der Waals force to form Na 3 N, NaF‐rich solid/cathode electrolyte interphase (SEI/CEI) with electronic insulation and fast ion transport. Moreover, harmful impurity (PF 5 ) also can be scavenged by van der Waals force to avoid HF production, which helps to stabilize the electrode interface. Additionally, combustion radicals (H, HO) can be cleared by van der Waals force between radical (RPO) formed by breaking with PFPN and combustion radicals for flame‐retardation purpose. As expected, the high‐voltage Na||Na 3 V 2 (PO 4 ) 2 O 2 F battery with modified electrolyte can deliver reservation of 92.4%, CE of 99.71% after 2000 cycles, and simultaneously possess excellent high‐rate and fast charging/slow discharging performance.