The Origin of Gaseous Decomposition Products Formed During SEI Formation Analyzed by Isotope Labeling in Lithium-Ion Battery Electrolytes

Abstract

Interphase formation during the first charge and discharge cycle(s) of a battery cell is among the least understood processes in lithium-ion batteries (LIBs). The formation of interphases is a result of electrolyte decomposition and accompanied by gassing. The direct analysis of these interphases is challenging and indirect methods are required to obtain information about this process. For example, indirect, ex situ analyses of gaseous decomposition products can help to draw conclusions about occurring reactions in the cell. In this work, the origin of several permanent gases and hydrocarbons (CO, CH4, C2H4 and C2H6) emerging during the formation of LiNi0.6Mn0.2Co0.2O2 (NMC622)||graphite pouch type LIBs equipped with a gas sampling port was investigated. Isotope labeled electrolytes (13C3-EC, D4-EC) were used to assign formation gas products to the individual constituents of the electrolyte mixture. The effect of common linear carbonates in combination with ethylene carbonate (EC) as well as the effect of vinylene carbonate (VC) as film forming additive was investigated. The obtained mass spectra from gas chromatography mass spectrometry (GC-MS) and the mass shift caused by the labeled electrolyte components, enabled an assignment of individual electrolyte components to the resulting gas products. The aim of this work is to gain more information about interphase reactions and thus to improve understanding of the mechanisms behind formation.