Experimental Analysis of the Mechanical Aging Behavior of Large-Format Lithium-Ion Cells Under Various Compression and Temperature Conditions

Prior works with lithium-ion cells show that volume work in the active material is caused by reversible (de-)intercalation as well as irreversible degradation phenomena during cyclic aging [1, 2]. This is known to cause a change in layer thickness, porosity or both [3] and subsequently a partly irreversible volume change at prismatic full cell level [4]. These effects pose a major challenge during development of high-performance and long-lasting batteries due to the complexity of holistic modelling approaches. This study contributes to covercoming these challenges by experimentally investigating the influence of various compression and temperature levels on the mechanical aging behavior of large-format lithium-ion cells. Cyclic aging tests are conducted on 72 Ah prismatic lithium-ion cells using different mechanical boundary conditions to analyze the cell aging phenomena via their mechanical, electrochemical and thermal characteristics. The tests are designed in such a way that cells are removed sequentially from cycling that results in different states of health (SoHs). This enables subsequentially post-mortem analysis comprising mechanical, optical and electrochemical methods in order to determine degradation modes on electrode level. The test results under non-fixed mechanical boundaries suggest initial compression as a major influence on the maximum cell expansion. Increased ambient temperature accelerates the battery aging and increases the resulting forces under fixed mechanical boundaries. In addition, results of the post-mortem analysis suggest a negative correlation between layer thickness, mass loading of the anode as well as the internal gas pressure with regard to SOH. Lastly, an increase in lithium content on the anode is shown with decreasing SOH, while the content on the cathode decreases. In conclusion, this study provides insights into the mechanical aging behavior of large-format lithium-ion cells operated at various boundary conditions. Moreover, the proposed approach bridges the gap between electrochemical phenomena on electrode level with mechanical effects on full cell level. These results create the basis for multiphysical full cell modelling under different aging conditions, which will be presented in future work. References: [1] Zhu, S., Le Yang, Fan, J., Wen, J., Feng, X., Zhou, P., Xie, F., Zhou, J., and Wang, Y.-N., “In-situ obtained internal strain and pressure of the cylindrical Li-ion battery cell with silicon-graphite negati