Thermal Behavior of Ionic Liquids Containing the FSI Anion and the Li+ Cation

Ionic liquids (ILs) containing the bis(fluorosulfonyl)imide ([(FSO2)2N]−, "FSI") anion have been attracting growing interest as electrolytes in lithium (ion) batteries because in many instances they offer the best liquid properties. At the same time, though, there have been reports that some examples can undergo highly exothermic reactions under certain conditions. As this behavior could potentially rule out their use in batteries, we present here a study of the thermal stability of a series of lithium battery IL electrolytes that contain the FSI anion. When the ionic liquid N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI) is mixed with either LiFSI or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), an exothermic reaction occurs at ∼120 °C. This reaction is related to the copresence of the Li+ and FSI ions and involves a trace amount of water. The exothermic decomposition is also observed when wet LiFSI is heated. The exothermic enthalpy increases with water content (tested up to 6 wt %) and can be as high as 200 J g−1. Infrared and Raman spectra show that the Li+ cations and the FSI anions form a relatively strong association in ionic liquid media with the result that mixtures of FSI IL and LiTFSI also give rise to the exotherm. Electrochemical investigations reveal that the decomposition of FSI effectively lowers the concentration of lithium ions available. Mass spectrometric analysis strongly suggests that degradation involves reaction to form oligomers of an FSI-derived base unit under dry condition but produces smaller fragments if water is involved. Overall, the results raise concern over the safety of FSI salts as lithium battery electrolytes and at least establish a case for further investigation.