Thermal runaway (TR) of lithium-ion cells results in an enormous release of heat, which can also bring the neighboring cells in the module into a safety-critical temperature range. To avoid TR propagation from cell to cell while simultaneously maximizing energy density on module and pack level, an optimal design of safety measures is necessary. Therefore, a detailed understanding of the TR behavior depending on different conditions is required. This study aims to experimentally investigate the influence of temperature gradients in this context using large-format lithium-ion pouch cells.

First, a standard accelerating rate calorimetry (ARC) test with homogenous heating under quasi-adiabatic conditions was performed. Then, to investigate the influence of temperature gradients on TR, two custom experiments were performed in an autoclave where the cells were locally heated up to TR with a constant and a stepwise heating profile. Thereby, the internal temperatures were measured with built-in sensors that had been installed prior to the experiments.

The results show that with respect to temperature gradients, local temperatures within the electrode-separator stack significantly exceeded corresponding temperatures in the ARC test right before the cell enters TR. Additionally, the speed of in-plane and through-plane TR propagation within the cell was obtained by comparing the internal and external temperature measurements at 12 different sensor positions.

Based on the findings of this study, it can be corroborated that more sophisticated approaches instead of fixed trigger temperatures obtained from ARC measurements are necessary for sufficient TR modeling to optimize packing density within battery systems while maintaining maximum safety.