Comparative Performance Evaluation of Flame Retardant Additives for Lithium Ion Batteries - I. Safety, Chemical and Electrochemical Stabilities

Within this 1st part of a comparative study, flame retardant electrolyte additives (FRs), as candidates for lithium ion battery electrolytes, from four different phosphorous-containing molecule classes, are investigated. The five FRs (tris(2,2,2-trifluoroethyl)phosphate (TFP), tris(2,2,2-trifluoroethyl)phosphite (TTFPi), bis(2,2,2 trifluoroethyl)methylphosphonate (TFMP), (ethoxy)pentafluorocyclotriphosphazene (PFPN) and (phenoxy)pentafluoro-cyclotriphosphazene (FPPN)) are investigated in a comparative manner to conclude structure-property relationships according to their self-extinguishing time (SET), onset temperature of the thermal runaway, chemical and electrochemical stability. SET experiments using standard electrolyte (1 M LiPF6 in ethylene carbonate : dimethyl carbonate 1 : 1 wt%) confirm high reproducibility of the used SET device. The results reveal a strong dependency of the ignition time on the SET. Therefore it is suggested, to investigate the SET for various ignition times. All FR additives remain chemically stable for weeks after added to the standard electrolyte during storage (approved by nuclear magnetic resonance). Cyclophosphazenes show superior results concerning their first time of inflammation and thermal electrolyte stability (approved by adiabatic reaction calorimetry). All additives show partial electrolyte decomposition only during the first charge in voltammetric experiments.