Aging of Cations of Ionic Liquids Monitored by Ion Chromatography hyphenated to an Electrospray Ionization Mass Spectrometer

Highlights: • Proposed decomposition mechanisms for the cations. • No influence of the electrolyte salts on the decomposition behavior of the IL cation was observed. • A new method for the identification of decomposition products in ILs. - ABSTRACT: The stability of 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR₁₃TFSI), 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl) imide (PYR₁₃FSI), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR₁₄TFSI) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMIM TFSI) ionic liquids (ILs) at different temperatures (room temperature and 95 °C) was investigated by ion chromatography (IC) hyphenated to a mass spectrometer. The degradation of the cations can be summarized in the following way: The side chain of the main molecule cleaves and reacts subsequently with the main cation molecule to new decomposition products. In addition, the results show that at elevated temperatures more decomposition products are formed. The influence of Li⁺ cation based electrolyte salts, lithium hexafluorophosphate (LiPF₆), lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and lithium perchlorate (LiClO₄), on the decomposition of the ionic liquids was analyzed over a long term stability study at room temperature and 60 °C. It was found out that in the mixtures of ILs and electrolyte salt, the same decomposition products were formed like in the pure ILs at the same temperature. Accordingly, there is no influence of the electrolyte salts on the decomposition behavior of the IL cation. The influence of the counter anion on the decomposition of PYR₁₃⁺ was analyzed as well. It was observed that different decomposition products were detected for PYR₁₃TFSI and PYR₁₃FSI