Is the Cation Innocent? An Analytical Approach on the Cationic Decomposition Behavior of N-Butyl-N-methylpyrrolidinium Bis(trifluoromethanesulfonyl)imide in Contact with Lithium Metal

The stability of the ionic liquid (IL) N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) against lithium metal at room temperature was investigated by gas chromatography/mass spectrometry (GC/MS), solid phase microextraction gas chromatography/mass spectrometry (SPME-GC/MS), and gas chromatography/high-resolution mass spectrometry (GC/HR-MS). The focus of this work is the degradation behavior and mechanism of the Pyr14+ cation in the presence of lithium metal as the anion participation in the formation of the solid electrolyte interphase (SEI) in IL-based systems has been, and continues to be, well described. N-Butyl-N-methyl-N-but-3-eneamine could be identified as a decomposition product for the first time. The existence of this compound was validated by chemical ionization (CI) experiments as well as with high-resolution results and the comparison with N,N-dibutyl-N-methylamine (DBMA) as a reference substance. Additionally, N-methylpyrrolidine (NMP) was identified as well as an analogue of N,N-dibutyl-N-methylamine, whose molecular structure could not be conclusively determined. The special feature of this finding is the terminal vinyl group of the N-butyl-N-methyl-N-but-3-eneamine which could generally enable polymerization reactions. It suggests that the Pyr14+ cation is not “innocent” and can participate in the formation of an organic layer at the surface of lithium metal. Therefore, this work contributes to a better understanding of the aging behavior of Pyr14TFSI or pyrrolidinium-based ILs in general.