Matrix-Assisted Laser Desorption Ionization Mass Spectrometry: An Efficient Approach to Monitor Lithium-Ion Battery Electrolyte Degradation while Preserving Analyte Integrity

One of the major challenges in understanding aging mechanisms of Li-ion batteries (LIB)s is the characterization of the electrode-electrolyte interfaces, solid electrolyte interphase, cathode-electrolyte interphase, and electrolyte composition after charge-discharge cycles. Several analytical methods have been used to understand these processes and, more particularly, high-resolution mass spectrometry has been used for an unambiguous molecular characterization. One of the issues encountered in the analysis of LIB electrolytes or solid interfaces is the instability of the molecules of interest, which are sensitive to degradation in the presence of moisture. Consequently, samples are generally handled under a controlled atmosphere in a glovebox, but degradation can occur during the transfer to the mass spectrometer. In this work, matrix assisted laser desorption ionization high resolution mass spectrometry (MALDI-HRMS) was evaluated for the analysis of electrolytes containing lithium hexafluorophosphate (LiPF₆), lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluoro(oxalato)borate (LiDFOB), and lithium difluorophosphate (LiPO₂F₂) in ethylene carbonate/ethyl methyl carbonate (EC/EMC) solvents. MALDI-HRMS allowed rigorous monitoring of electrolyte degradation after air exposure. This air aging was performed on electrolytes in two physical states: liquid state versus solid state. It was shown that in the solid state there was no degradation of samples over 24 h, whereas in solution degradation occurred in less than 30 min. This result demonstrates the molecular integrity conservation of solid-state electrolytes during the transfer to the mass spectrometer and the relevance of using MALDI-HRMS for the study of LIB electrolyte degradation.