Impact of State of Health (SOH) on the Thermal Safety of Lithium Ion Cells for Long 1st Life and 2nd Life Applications

Lithium ion battery (LIB) safety incidents can be a threat for people and the environment. Since today, only safety tests on fresh cells are decisive for safety level determination, the effect of long-term operation on their safety characteristics needs to be addressed. A large loss of lithium over long periods of time could, for example, result in reduced structural and thermal stability of the cathode. LIBs are normally used until they reach an end-of-life criterion of typically 70%–80% state of health (SOH). However, they can be reused in second-life applications such as stationary (“grid”) energy storage, afterwards. To ensure safety during long first life and second-life, in this study the influence of aging was investigated over a higher cycle number and a longer time period than ever before. 5 Ah LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NMC622)||graphite (G) pouch cells were aged at 20 °C between 530 and 3,806 cycles (151–615 d of continuous cycling). SOHs between 91% and 63% were obtained. After aging, the thermal properties of the cells were investigated by heat-wait-search experiments under adiabatic conditions using an accelerating rate calorimeter. The cells showed almost exclusively improvements in their safety characteristics, the thermal runaway even tended to be shifted to higher temperatures.