Operando GC/MS for the investigation of different decomposition pathways during solid electrolyte interphase (SEI) formation with SEI forming additives

The interphases on the electrodes that form from electrolyte decomposition play a crucial role in the battery performance. However, with a thickness of only a few nanometres the formed passivation layers present a challenge to explore. In this work, we implemented a new approach by combining operando GC/MS gas analysis supported by surface sensitive XPS to investigate the electrolyte decomposition during SEI formation and deepen the understanding of the decomposition reactions of vinylene carbonate (VC) and fluoroethylene carbonate (FEC). Electrolyte degradation results in the formation of 29 different gas species. Adding additives resulted in (i) reduced gas formation and (ii) a clear shift from hydrocarbons to carbon oxides. Carbon dioxide was found to be indicative of the formation of poly-VC which was supported by findings from post-mortem XPS. An increased amount of fluorine was found in the interphases of cells using FEC as electrolyte additive. Acetaldehyde was identified as gaseous component to differentiate the decomposition of VC and FEC. For the decomposition of the base electrolyte components ethylene carbonate (EC) and diethyl carbonate (DEC), methane, ethane, ethene and butane were found to be indicative, and a scavenging effect of VC for alkoxides was confirmed by monitoring ethanol.