Thickness changes in compressed lithium-ion battery cells during charge and discharge cycles

Lithium-ion battery cells consist of a graphite anode, lithium composite cathodes, and lithium-based electrolytes. During charge and discharge cycles, lithium ions intercalate within the graphite structure, leading to a reversible expansion of the battery cells. This expansion pattern is typically characterized by a notable increase during mid to full charge, while remaining stable between the discharged and mid-charge states. We employed a custom-developed measurement system to assess semi-local thickness changes along the area of the flat battery cell. In our setup, pressure is applied at the center of the cell, allowing the sides of the battery to expand freely. Three strategically positioned sensors on the outer periphery of the battery cell measure the inhomogeneous expansion resulting from the positioning of the electrical contacts. Furthermore, the choice of active materials in the anode and cathode significantly influences the extent of expansion observed in the battery cells. Repetitive expansions can lead to delamination and other degradation processes, necessitating appropriate compression of the battery cells during operation to mitigate the aging. This study aims to highlight the critical importance of monitoring battery cell expansion throughout charge and discharge cycles and to enhance the understanding of the electrochemical reactions that drive this physical behavior.