The risk of thermal runaway is, for Li-ion batteries, a critical issue for large-scale applications. This compels manufacturers to find suitable materials or additives, which are able to minimize the heat generation and thereby mitigate safety-related risks. In an attempt to get more insight and understand the exothermic processes that take place at the negative electrode/electrolyte interface, we implemented GC/MS analytical technique to detect volatile compounds. Based on a mechanistic study, we propose a general electrolyte degradation scheme in the 100–250°C temperature range, involving electrochemically driven carbonates reduction followed by chemical reactions. The mechanisms for decomposition deduced from these analyses shed new light on the processes involved in the formation of the precipitated (SEI layer) and soluble molecules upon cell formation cycles and ageing.