Electrolyte thermal stability is critical for both battery safety and extending battery life. Battery fires are often the result of thermal breakdown of the liquid electrolyte leading to a flammable mixture of volatile products. Even in the absence of fire, electrolyte decomposition leads to battery swelling, harmful precipitates, reduced ion conductivities, and undesirable battery chemistry.

Current methods of determining electrolyte stability using ex-situ techniques, such as thermal gravimetric analysis or differential scanning calorimetry, can over predict thermal stability, do not address materials compatibility within batteries, and cannot identify chemical decomposition mechanisms. Infrared optical detection of thermal stability offers an in-situ approach providing: speciation, identification of chemical bonds involved in decomposition, real-time detection, and chemical kinetics.

In this work, the thermal stability of the room temperature ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO₄]) is investigated using infrared (IR) spectroscopy. Quantitative IR absorption spectral data are measured for the heated ionic liquid. Spectra have been collected between 25°C and 100°C using a heated optical cell. Multiple samples and cell path lengths were used to determine quantitative values of the absorption coefficient. These results were compared to previous computational models of the ion pair to identify each absorption band with corresponding chemical bonds. The quantitative spectra were then used to measure the rate of thermal decomposition at elevated temperatures revealing the onset of breakdown above 75°C.