Li-ion batteries (LIBs) offer diverse benefits for sustainable energy storage; they have a high storage capacity for their size, preserve good charge/discharge cycling stability over long periods, and have minimal charge loss during storage. However, like any energy storage technology, LIBs are not impeccable. Over time, battery deterioration can lead to safety and performance issues that shouldn’t be underestimated. One of the main issues is the gas generation during cell cycling, resulting in cell bulging in pouch cells, high internal pressure in cells with rigid metal housing, electrode delamination, and de-contacting of active materials. Identification of degradation products is the first step towards safer, longer-lasting, higher performing and more easily recyclable LIBs. This knowledge will help to understand the mechanistic pathways of degradation and how they can be inhibited. On the other hand, lithium-based batteries have the potential to undergo thermal runaway, during which mixtures of hazardous gases are. In the current study, a series of cylindrical lithium-ion cells was exposed to abuse conditions (thermal) to investigate the composition of the released gas mixture. Mass spectrometry has been used to detect the evolved gases in the swollen cells (off-line analysis) and during cycling of lab-made cells (on-line analysis). Recently, we have introduced a new coupling of the mass spectrometry with the accelerating rate calorimetry (ARC) to monitor the emitted gases during the venting and thermal runaway of different LIB types