The decomposition of state‐of‐the‐art lithium ion battery (LIB) electrolytes leads to a highly complex mixture during battery cell operation. Furthermore, thermal strain by e.g., fast charging can initiate the degradation and generate various compounds. The correlation of electrolyte decomposition products and LIB performance fading over life‐time is mainly unknown. The thermal and electrochemical degradation in electrolytes comprising 1 m LiPF6 dissolved in 13C3‐labeled ethylene carbonate (EC) and unlabeled diethyl carbonate is investigated and the corresponding reaction pathways are postulated. Furthermore, a fragmentation mechanism assumption for oligomeric compounds is depicted. Soluble decomposition products classes are examined and evaluated with liquid chromatography‐high resolution mass spectrometry. This study proposes a formation scheme for oligo phosphates as well as contradictory findings regarding phosphate‐carbonates, disproving monoglycolate methyl/ethyl carbonate as the central reactive species.

Sufficient interphase formation during the first cycles is crucial for the long‐term performance of lithium ion batteries. During the first cycles, electrolyte salt and solvent molecule decomposition caused by electrochemical instabilities leads to a wide range of decomposition species contributing to the formation of performance‐beneficial interphases at the electrodes as well as performance‐impairing side reactions. Due to structural similarities of carbonate educts, elucidation of underlying reaction pathways is complex. In this work, isotope‐labeled ethylene carbonate (13C3−EC) is applied to differentiate contributions of cyclic and linear carbonates to occurring interphase and decomposition reactions. Thereby, carbon atom tracing of electrolyte soluble species is performed by means of gas chromatography‐mass spectrometry (GC−MS). Reaction pathways are postulated based on identified molecular origins. Among others, a new DMC‐EC cross reactivity via lithium ethylene monocarbonate is considered. Moreover, radical reactions resulting in C−C bond formation are investigated and radical polymerizations of stoichiometric equivalents of ethene formed in situ during EC reduction are proposed. Finally, alkyl carbonates with C>4 alkyl chains are identified considering underlying reactivities. Discussed electrolyte decomposition reactions extend the understanding of EC‐based solid electrolyte interphase (SEI) formation emphasizing possible radical reactivities and solvent molecule contributions.

Electrolyte decomposition of lithium-ion battery as a consequence of thermal stress was investigated in Part 1 of this two-part study. The focus of Part 2 is on the influence of the battery cell operation conditions on the electrolyte during cell formation and long-term cycling. Especially, the reactivity of the negative electrode surface and the varied properties of the formed solid electrolyte interphase via vinylene carbonate addition, changing the picture of decomposition products, were addressed. With the help of liquid chromatography hyphenated to high-resolution mass spectrometryand fragmentation capabilities, structure elucidation was performed with optimal certainty. This Part 2 confirmed, summarized, and extended previous findings to 140 different carbonate, oligo phosphate, and mixed phosphate−carbonate species in the state-of-the-art electrolytes after moderate cycling conditions and contributes to a targeted investigation of LIB electrolyte aging processes. Furthermore, thermal and electrochemical aging phenomena were discussed and thermal stress-marker molecules that eased reversed-engineering were postulated.