Variation in capacity aging trend of lithium-ion cells regarding sudden death spread
Oliver Makan, Matthias Gossen, Kai-Peter Birke
Abstract
Battery packs consist of multiple single cells, which are connected in series. During service life the performance parameters of each cell behave individually under constant environmental conditions causing significant differences. These differences are attributed to variances in process and product parameters within the production line of Li-ion cells and directly affect the performance of battery packs, in which the cell with the steepest aging curve, referred as the worst-case cell, defines the overall performance of the battery. As a result of accelerated aging tests, with a focus on enhancing the effect of lithium plating, we show, that not only the cells have individual aging behavior through their life cycle, but also quantify noticeably growing capacity spread in the cell compound towards end of life and show the impact on the use case of battery systems during lifetime. A mathematical model is introduced to fit & analyze the degradation curve of 50 tested lithium-ion cells. This novel model allows, compared to normally used regression curves, to gain information about how certain aging mechanisms affect the capacity degradation & quantifies the capacity loss related to these. As a result, a comparison of the effect of these aging mechanisms between different cell chemistries is possible. Additionally, the quality of cell production lines and the impact on the applications can be evaluated by using the model. Furthermore, the visualization of characteristic areas within the life cycle of a battery cell is possible & gives useful information regarding the decrease of the impact of certain aging mechanism during the development phase or suitable SoH strategies during life cycle.