Lithium ion batteries use highly flammable liquid organic electrolytes. Solutions include flame retardants. Current state-of-the-art lithium ion batteries use LiPF6 as electrolyte salt in an organic carbonate solvent, a system which is flammable and thermally unstable. Flame retardants are included into the electrolyte to improve fire resistance and thermal stability. This paper details different FR solutions. FRs used can be based on phosphorus, fluorine or both. Phosphorus-based FRs used include alkyl phosphate esters (e.g. trimethyl phosphate TM), triethyl phosphate TEP, dimethyl methyl phosphonate DMMP), phenyl phosphate esters (e.g. triphenyl phosphates TPPs, diphenyl octyl phosphates DPOF, and diphenyl methyl phosphates CDP), phosphorus-nitrogen compounds (e.g. phosphazenes). The PFRs have the disadvantage of requiring significant loadings which reduce electrical performance. Fluorine FRs also deteriorate electrical performance somewhat and pose problems by generating toxic HF gas in fires or decomposing to toxic organic fluoride compounds. Composite FRs including both F and P can offer improved performance. Other possible future solutions to reduce lithium ion electrolyte fire risk include solid electrolytes, high concentrated liquid electrolytes or ionic liquids (organic molten salts).
“Nonflammable Liquid Electrolytes for Safe Lithium Batteries”, X. Mu et al., Small Struct. 2023, 2300179 https://doi.org/10.1002/sstr.202300179