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.