Reaction Product Analyses of the Most Active “Inactive” Material in Lithium-Ion Batteries—The Electrolyte. I: Themal Stress and Marker Molecules

The identification of decomposition products in lithium-ion battery (LIB) electrolytes enables a better understanding of interphase layer compositions at the electrode||electrolyte interfaces and of the effects impairing LIB cell performance. Besides electrochemical cell operation, thermal strain can affect the electrolyte in particular. The distinction between electrochemical and thermal degradation can lead to the identification of “marker molecules” of thermal strain and enhance reverse-engineering capabilities of LIB post-mortem analysis. Structure elucidation, applying the scope of chromatography hyphenated to high-resolution mass spectrometry for optimal certainty, has been neglected in LIB electrolyte investigations in the past. Therefore, this study focused on the structure elucidation of electrolyte decomposition products after thermal treatment (Part 1) and electrochemical operation (Part 2). Furthermore, possible marker molecules, the status quos after cell formation and long-term cycling, as well as the influence of the electrolyte additive vinylene carbonate were addressed. Isolated thermal aging resulted in 206 solvent-based suspected targets. Oligomeric carbonates, ethylene glycols, carbonate–ethylene glycol mixtures, and possible structural isomers were identified and classified via a copolymer scheme. No mixed compounds of electrolyte solvents and conducting salts were identified.