Application of Gas Chromatography Hyphenated to Atmospheric Pressure Chemical Ionization-Quadrupole-Time-of-Flight-Mass Spectrometry (GC-APCI-Q-TOF-MS) for Structure Elucidation of Degradation Products Based on the Cation in Pyr₁₄TFSI

Abstract

In this study, the hyphenation of gas chromatography to atmospheric pressure chemical ionization-quadrupole-time-of-flight-mass spectrometry (GC-APCI-Q-TOF-MS) is applied for the investigation of degradation products of ionic liquid (IL) based electrolytes. The advantage of APCI compared to electron ionization (EI) for amine-based analytes due to a higher sensitivity of the molecular ion was demonstrated and the results obtained in this work contribute to a better understanding of the IL aging behavior in regard to their application as green electrolyte for lithium metal batteries (LMBs). Pristine N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) and Pyr14TFSI-based electrolytes with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as conducting Li salt were investigated. For this purpose, ion source optimization was performed for amine-based analytes using N-butyl-N-methylbutan-1-amine (BMBA) as standard compound. Furthermore, a customized water flow was directed into the ion source to inhibit in-source reactions, such as fragmentation or oxidation processes, and therefore to promote the [M+H]+-ion formation. The respective headspace (HS) above the ionic liquids (ILs) and electrolytes was sampled at first for the detection of highly volatile analytes. Structure proposals were provided by matching mass spectra obtained from GC-APCI-Q-TOF-MS/MS and GC-EI-MS measurements. Aliphatic amine-, pyrrolidine- and pyrrole-based aging products were identified as decomposition species of the Pyr14 +-cation, e.g. N-butylpyrrole and N-butyl-N-methylpent-4-en-1-amine. Furthermore, the presence of lithium metal on a copper substrate in the pristine IL led to significantly stronger aging effects. Galvanic corrosion processes at the redox-couple Li and Cu were suggested as possible causes. This phenomenon questions the practicability of using copper current collectors with lithium anodes for IL-based battery cell systems. Additionally, the LiTFSI concentration in the electrolyte showed an impact on aging caused by corrosion.

Galvanic Couples in Ionic Liquid‐Based Electrolyte Systems for Lithium Metal Batteries—An Overlooked Cause of Galvanic Corrosion?

“…The according Hofmann-elimination product of the N 1114 + cation, trimethylamine, was detected at earlier retention times t r due to its high volatility, as depicted in Figure S5 in the Supporting Information. Interestingly, also reaction products from intramolecular Hofmann-eliminations (Ic, IIc, and IIIc), as described for Pyr 14 TFSI in the literature, [29,34] could be detected for Pyr 14 TFSI, but also for Pyr 13 TFSI and Pip 14 TFSI. This type of ring-opening reaction is plausible, as the pyrrolidinium and piperidinium rings of the IL cations presumably are prone to deprotonation in β-position.…”

“…As the peak intensity dependent of the amount of degradation products, it can be concluded that the extent of galvanic corrosion of Li is heavily dependent on the concentration of LiTFSI in the IL-based electrolyte. A correlation between degradation stability and LiTFSI content was reported in the literature only for Pyr 14 TFSI, [34] but the reactivity of Pyr 13 TFSI, Pip 14 TFSI, and N 1114 TFSI appears to follow a similar pattern. During galvanic corrosion, electrons from the Li pass through the metal of higher electrochemical potential and enable reactions at the metal-electrolyte interface.…”

“…[32,33] In our recent study, forced galvanic corrosion was abused to amplify the amount of detectable decomposition products of only one IL (Pyr 14 TFSI), but without further investigations of the underlying corrosion mechanism. [34] Thus, we herein present a systematic study of the occurrence and the extent of galvanic corrosion of Li electrodes in the context of IL-based electrolytes. For this, TFSI − -based ILs with four quaternary ammonium cations of varying ring size and alkyl chain length (Scheme 1), namely N-butyl-N-methylpyrrolidinium (Pyr 14…”

An XPS Study of Electrolytes for Li-Ion Batteries in Full Cell LNMO vs Si/Graphite

“…Another interesting observation is the gradual lowering of the binding energy of the N 1s peak assigned to PYR 13 with increasing cycle number. References − indicate decomposition reactions for PYR 13 that give rise to an organic SEI layer. This observation of the gradual decomposition of PYR 13 may also be linked to the high amount of CB peak seen for cathodes cycled in the co-solvent electrolyte, in a way that the CB peak has overlapping signals originating from PYR 13 decomposition products at the anodes.…”