Flammability of Li-Ion Battery Electrolytes: Flash Point and Self-Extinguishing Time Measurements

The flammability of electrolytes is an important aspect of the thermal safety behavior of Li-ion batteries. Flash points (FPs) and self-extinguishing times (SETs) of 25 solvents (including carbonates, ethers, esters, lactones, dinitriles, a sulfone, and others), 3 solvent mixtures, and 15 electrolytes are presented. The FPs have been measured according to the Abel and Pensky-Martens closed-cup methods using 12 mL of sample and electric ignition. The SETs have been determined with the pure liquids, without any carrier substrates. A correlation of the FPs with the SETs, as well as with vapor pressures and boiling points is attempted. Furthermore, the effect of the addition of two non-flammable solvents [1-butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)imide and methyl nonafluorobutyl ether] and a flame-retardant additive (trimethyl phosphate) on both the SET and the FP of a carbonate-based electrolyte is discussed. As an alternative to their experimental determination, the FPs of the pure solvents have been calculated from other physical and chemical properties of the substance, and the FPs of the solvent mixtures and electrolytes from the FPs of their flammable constituents. Some of the models predict the FPs with an accuracy of ± 10°C, which may be sufficient to estimate the flammability hazards for many applications.