The Heat-Wait-Seek (HWS) test conducted in Accelerating Rate Calorimeters (ARC) serves as a valuable tool for assessing the safety performance of lithium-ion cells. This test allows the extraction of critical parameters, including important data such as critical temperatures and heat generation, which are key to evaluating the safety characteristics of the cells. The generated heat can be estimated integrating the temperature dependent heat capacity of the cell over the exothermal data in the test. The temperature dependent heat capacity is based on the temperature dependent heat capacities of its components and their gravimetric amount in the examined cell. The exothermic behaviour is influenced by the previous aging conditions, which can induce different reactions of the cell, e.g. due to cyclic aging.

In this study we show how the aging mechanism due to cycling at different temperatures leads to different exothermal behaviour of the aged cells and therefore different critical temperatures and generated heat. Moreover, the influence of additional heating steps, venting of the cell, melting of the separator and the overall accuracy of the measurement are taken into account. In this work the approach is applied to commercial type 21700 cells. The cells were measured in an ES-ARC from Thermal Hazard Technologies, UK. The generated heat was compared for either fresh cells and cells after cyclic aging. The cyclic aging tests were performed at 0 °C, 20 °C and 50 °C until a state of health of 80 % was reached. The three different temperatures studied in the cyclic aging were chosen as 0 °C to cause lithium plating, as 20 °C to cause a mixture between lithium plating and solid-electrolyte-interface (SEI) growth and 50 °C to cause mostly a SEI-growth. The lithium plating and SEI growth have both a high influence on the measured properties in the Heat-Wait-Seek test and therefore on the generated heat. The measured onset temperatures for exothermal behaviour were 81 °C (new cells), 37 °C (cycled at 0 °C), 83 °C (cycled at 20 °C) and 82 °C (cycled at 50 °C). The calculated generated heat using temperature dependent Cp were 6.0 kJ (new), 9.4 kJ (cycled at 0 °C) , 6.5 kJ (cycled at 20 °C) and 6.8 kJ (cycled at 50 °C).

In this study different aging mechanisms and their influence on the critical temperatures and generated heat in the HWS test were thoroughly investigated and discussed. This valuable insight can aid manufacturers in comprehensively understanding the impact of cyclic aging into the safety considerations of their cells, battery packs, and battery management systems.