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Safety is a critical aspect of Li-ion batteries due to the severe consequences associated with thermal runaway (TR) events. TR in Li-ion batteries is characterized by rapid heat release, substantial gas generation, and intense combustion. While battery safety standards primarily aim to prevent TR from happening, specifying extensive testing under various electrical, mechanical, and thermal abuse conditions, there remains a pressing need to understand and manage TR when it does occur. However, the characterization of TR events is not well standardized, leading to employment of diverse test setups and analytical approaches, making it difficult to compare results across studies and establish consensus on key influencing factors and optimal safety strategies. For that reason, this thesis revolves around the characterization of TR scenarios for Li-ion battery cells and how analysis and test methodology can be improved. A new test setup is presented enabling simultaneous gas generation and heat generation measurements in inert atmosphere. This calorimeter was developed also to distinguish between ejected and non-ejected heat during TR, revealing that the fraction of ejected heat increases with state of charge (SOC), while maximum non-ejected heat occurs at lower SOC levels. This insight is critical for understanding thermal propagation risks. Improved analysis of the gas release during TR includes measurement of the gas generation rate, an essential parameter for ventilation design and explosion risk assessments. Further, the impact from SOC, TR triggering methods, ambient atmosphere, and cell size on gas and heat generation, gas composition, temperature development, and mass loss during TR is discussed. An important finding is the significant effect from the TR triggering method, which potentially affects thermal propagation characteristics in battery systems. Cell size and format will impact the gas and heat generation, but the variations are relatively small as compared to the difference between lab-scale cells and commercial industrial-scale cells. Finally, this thesis also evaluates the uncertainties and limitations of using oxygen consumption calorimetry for measuring heat release rate and total heat released in Li-ion battery fires. In that context, it is important to distinguish between cell internal heat generation during TR and external heat generation in a flaming battery fire. The overall results contribute to a deeper understanding of Li-ion battery safety and offer tools for improving test protocols, system design, and mitigation strategies.