Influence and correlation of stress factors on the cyclic ageing of NMC622-graphite lithium-ion batteries

Various stress factors are known to affect the performance and ageing of lithium-ion batteries. While the individual impacts of these factors on the electrochemical processes within a battery are well documented, their ranking and the combined effects of multiple stress factors remain less understood. In this study, we investigate the ageing behavior of NMC622|Graphite lithium-ion pouch cells under systematically varied cycling conditions, including charge and discharge current rates, temperatures, and rest times. Over 50 cells, each with a nominal capacity of 1 Ah, were cycled for more than 2000 full cycles. The test conditions were selected to avoid exacerbating specific ageing mechanisms by maintaining all stress factors within the manufacturer's specifications. To quantify and interpret degradation behavior, we apply a combination of analytical and modeling approaches, including equivalent circuit modeling to analyze resistance evolution, cell balancing analysis to observe capacity divergence, and grouping of test conditions to classify cells by identical degradation mode. We observed a strong calendar ageing influence on cyclic ageing and a pronounced temperature dependence of the ageing rate. Although different test conditions led to different evolutions of cell parameters, some variations converged over time, resulting in comparable overall ageing rates. Extended periods at a high state of charge were associated with increased capacity loss per Ah-throughput under the studied conditions. A classification study further revealed that distinct combinations of stress factors can induce similar ageing behavior in the cells, though progressing at different rates.