In situ measurement and modeling of internal thermal runaway propagation within lithium-ion cells under local overheating conditions

Understanding internal processes before and during thermal runaway (TR) is essential for designing safe battery systems. This study aims to experimentally investigate and model the influence of temperature gradients on the initiation of TR and its propagation within lithium-ion cells. Local overheating abuse tests in an autoclave are performed with constant and stepwise heating profiles to apply extreme thermal inhomogeneities to the cells. The internal TR propagation is measured in situ with built-in thermocouples at different positions within the electrode-separator stack and on the surface of the cells. Results show the influence of the layered structure on in-plane and through-plane TR propagation. Further, the stepwise overheating procedure discloses the considerable influence of temperature gradients and local decrease in reactivity due to the consumption of reactants with a measured local temperature of 347 °C inside the cell before TR. Additionally, a simplified TR model with two Arrhenius equations is parameterized using accelerating rate calorimetry (ARC) and validated with local overheating experiments. In this regard, the influence of mass loss is also examined. The results demonstrate that the simplified approach is sufficient for considering thermal inhomogeneities and can significantly improve simulations compared to utilizing fixed trigger temperatures obtained directly from ARC experiments.