Comparative Study on Cyclic Phosphazene-Based Flame Retardants for Ether Electrolytes in Li-S Batteries: Flame Retardancy and Performance Analysis

This study presents a systematic investigation of three cyclic phosphazene-based flame retardant additives, hexachlorocyclotriphosphazene (HCCP), hexafluorocyclotriphosphazene (HFPN), and ethoxy-pentafluorocyclotriphosphazene (EtPFPN), for ether-based electrolytes in lithium-sulfur batteries. Through comprehensive analysis of molecular orbital calculations, self-extinguishing time measurements, and electrochemical characterization, we reveal distinct structure-dependent flame retardancy mechanisms and their impact on battery performance. HCCP demonstrates superior flame suppression through P-Cl bond decomposition but exhibits electrochemical instability above 15 wt%. HFPN functions primarily as a co-solvent, achieving significant flame retardancy above 20 wt% while maintaining stable electrochemical performance. EtPFPN emerges as the most promising candidate, effectively integrating into the electrolyte solvation structure to provide optimal flame retardancy, though requiring higher concentrations for complete suppression. Notably, synergistic effects between LiNO3 and the flame-retardant additives enhance both safety and electrochemical stability. These findings provide critical insights for the molecular design of flame-retardant electrolytes in high-performance Li-S batteries, demonstrating the importance of balancing safety enhancement with electrochemical stability through strategic additive selection and concentration optimization.