Experimental Investigation of the Effect of Flame-Retardant Additive Dimethyl Methylphosphonate on Combustion Behaviors of Electrolytes

Adding flame-retardant additives to electrolytes can significantly enhance the safety of lithium-ion batteries. To clarify the effects of flame-retardant additive dimethyl methylphosphonate (DMMP) on electrolyte flammability under practical battery fire conditions, experimental studies are conducted on an electrolyte pool fire setup. It is observed that the flame of carbonate solvent is blue, while the flames of electrolyte and electrolyte with DMMP addition are yellow, due to the formation of phosphorus-containing particles in the flame. With 30 wt% DMMP addition, the combustion duration, combustion mass ratio, and flame height decrease significantly by 40%. The electrolyte achieves non-flammability when the additive fraction increases to 40%. It is observed that with DMMP addition the charred layer forms on the surface of electrolyte liquid, and slows down the heat and mass transfer between the gas and electrolyte liquid. This is the flame-retardant mechanism of DMMP in the condensed phase. The flame spectrum results show that with LiPF₆ and DMMP addition the OH emission intensities are weakened dramatically. This is because LiPF₆ and DMMP decompose to the radicals containing phosphorus, which can scavenge the vital radicals (H and OH), and then suppress the combustion chain branching reactions. This is the flame-retardant mechanism of LiPF₆ and DMMP in the gas phase.