Thermal events remain a safety challenge for lithium-ion batteries due to self-reinforcing exothermic reactions that occur at elevated temperatures. Higher states of charge have been shown to exacerbate the onset and severity of these events. This has been attributed to the release of lattice oxygen in cells containing Ni-rich layered oxide electrodes. The degradation reactions on the electrode/electrolyte interface triggered by this oxygen remain insufficiently understood. In this study, we investigate high-temperature degradation pathways of ethylene carbonate (EC)-based electrolytes in contact with Ni-rich positive electrode active materials up to 130 ◦C. By combining in-situ high-temperature online electrochemical mass spectrometry with post-mortem analyses, we identify and validate key degradation intermediates and products. Two distinct EC oxidation pathways are revealed: one activated at high voltages, and one initiated by trace water impurities. Theoretical calculations reveal the reactions are thermodynamically favorable and quantify their heat release. Both pathways produce significant heat and lead to gassing of CO$_2$ and H$_2$. These findings suggest a significant contribution of EC to thermal gas evolution and exothermicity under abuse conditions, thereby establishing a mechanistic link between electrolyte chemistry and thermal events. This integrated experimental-computational approach provides critical insights to guide improved electrolyte formulations and predictive thermal models.

Data provided from experimental results presented in the publication and supporting information of HT-OEMS, XRD and OEMS analysis.