The electrolyte in Li-ion batteries is inherently thermodynamically instable; this leads to formation of the solid-electrolyte interphase and capacity loss. Exposing batteries to high temperatures above ca. 60°C accelerates interphase growth, but also leads to its dissolution and renewed formation. If the related exothermic heat is not sufficiently fast removed, this leads to self-heating and a thermal runaway of the cell. This talk uses modelling to give a deep insight into the processes and properties causing self-heating and thermal runaway of Li-ion batteries. A complex interaction of exothermic and endothermic reactions is revealed, and the effects of evaporative cooling [1], conditions during battery manufacturing and battery age. [2] Gas analysis with online electrochemical mass spectrometry aids in identifying the network and further sensitivities. [3] REFERENCES [1] Baakes F. et al., Unveiling the interaction of reactions and phase transition during thermal abuse of Li-ion batteries, J. Power Sources, 522, 230881, 2022. [2] Baakes, F. et al., Impact of electrolyte impurities and SEI composition on battery safety, Chem. Sci, 14, 13783, 2023 [3] Bläubaum, L. et al., Impact of Lithium-Ion Battery Separators on Gas Evolution during Temperature Abuse, Batter. Supercaps, 7, e20230053, 2024.