Significance of dynamic thickness changes and compression effects of commercial lithium-ion spring-braced cells

Lithium-ion battery (LIB) cells undergo thickness changes during cycling that can be reversible and irreversible. According to the literature, cell compression can prevent various defects, such as volume-change-induced contact losses. Moreover, densely packed cells represent the battery electric vehicle (BEV) use case, where volumetric energy density can still be optimized. In this study, a uniaxial compression test bench is developed to evaluate the dynamic dilation behavior and stress susceptibility of commercial LIB pouch cells. Using a spring-compression approach, the active material can expand and contract under moderate normal stress variations while keeping the inner cell layers in permanent contact. After applying stress to the cell, the cycling behavior and thickness changes are monitored under the variation of several parameters. A rig-integrated force measurement mat is used to monitor stress distribution, providing additional insight. Our results show that varying the charging rate leads to significant differences in the thickness change of a mildly compressed cell. In this particular case, both reversible and irreversible dilation fractions are present and electro-mechanical effects were identified. As a result, gained insights can serve battery system manufacturers with information to optimize their battery pack regarding operating efficiency, sustainability and mechanical safety.