High temperature in situ gas analysis for identifying degradation mechanisms of lithium-ion batteries

The primary safety concern associated with lithium-ion batteries is the risk of thermal runaway. The components of the cells can react under heat release when exposed to external or internal heat sources, potentially leading to large-scale fires and explosions. This process is initiated by the decomposition and/or reformation of the Solid Electrolyte Interphase (SEI) and electrolyte; the precise underlying reaction network remains unclear due to insufficient availability of in situ chemical analysis methods during thermal abuse. Herein, we present a method based on high-temperature feasible online electrochemical mass spectrometry that is used to investigate these mechanisms and propose a reaction network of SEI formation and degradation. For a graphite/NMC cell with ethylene carbonate/dimethyl carbonate/LiPF₆ electrolyte, added vinylene carbonate concentration and formation current are shown to impact the composition of the SEI both before and during the thermal stress test up to 132 °C. Higher amounts of the additive vinylene carbonate suppress the evolution of C₂H₄ during thermal abuse, suggesting a reduced presence of the organic SEI component lithium ethylene dicarbonate. Our results indicate that the conductive salt decomposition is amplified by the amount of lithium carbonate and reduced by lithium ethylene glycol. This connects the presence of certain SEI compounds directly to the formation of hazardous species. The work highlights the importance of identifying the underlying degradation pathways and for the understanding of the processes that give rise to thermal runaway.