Fluorinated Hyperbranched Cyclotriphosphazene Simultaneously Enhances the Safety and Electrochemical Performance of High‐Voltage Lithium‐Ion Batteries

Abstract As high‐energy‐density lithium‐ion batteries (LIBs) are being developed, their thermal stability problems become more apparent. In spite of elaborate precautions, exothermic reactions between electrolytes and electrode materials at elevated temperatures can lead to battery explosion. In this study, we introduce a novel flame‐retardant additive with a fluorinated hyperbranched cyclotriphosphazene structure for high‐voltage LIBs. Along with the effective reduction of flammability, it enhances the electrochemical performance by generating a thermally and electrochemically stable solid electrolyte interphase on both the cathode and the anode, which is rare for conventional additives. In full cells composed of a 5 V‐class spinel cathode and a graphite anode with practical‐level mass loading, this new additive demonstrates significant improvements in discharge capacity retention and coulombic efficiency during cycle testing.