Cosolvent Engineered Phosphaphenanthrene-Based Self-Extinguishing Electrolyte for Safer Lithium-Ion Batteries

Battery-induced fire accidents are on the rise worldwide. The severity of battery fires has a significant correlation with the combustion characteristics of carbonate electrolytes, which is the main fuel of a battery. Here, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), an environmentally benign, smokeless, and highly efficient flame retardant, is introduced into a carbonate electrolyte for the first time to formulate a self-extinguishing electrolyte. It is shown that 5 wt % DOPO in the carbonate electrolyte is enough to achieve flame retardancy. Moreover, in the thermal runaway test, adding DOPO effectively delays the exothermic peak of graphite (Gr) and reduces the amount of heat. The cosolvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) may act as a solvent shell to weaken the irreversible decomposition of DOPO on Gr at low potentials and improve the interface compatibility, enabling acceptable cycling performance of LiFePO4 (LFP)–Li batteries with a 99.7% capacity retention after 65 cycles. The design strategy for electrolyte formulation provides a promising path to revisit obsolete flame retardants of the past, which gives insights into the development of nonflammable electrolytes.