Meaning
Structural degradation occurs in battery cells due to the repeated bulk volume changes of the electrodes during charge and discharge cycles. This macro swelling fatigue is particularly evident in high capacity anodes like silicon which expand significantly upon lithiation. The physical stress weakens the cell casing and the internal bonds between the active material and the current collector.
Over time, these mechanical movements lead to cracks in the electrode coating.
Degradation Pathway
Compression of the separator reduces its porosity and impairs the flow of ions through the electrolyte. When macro swelling fatigue reaches an advanced stage, the increased mechanical resistance raises the internal temperature of the cell. This creates a feedback loop where heat accelerates chemical decomposition and further swelling.
Mechanical Constraint
Module designs include expansion gaps or compliant materials to accommodate these movements without damaging the surrounding structure. If the design fails to account for macro swelling fatigue, the resulting pressure can rupture the seals of the battery. This risk is managed by selecting materials with lower volume expansion or by applying precise external clamping forces.
Testing Metric
Laser displacement sensors measure the change in cell thickness over thousands of cycles to quantify the rate of expansion. Engineers use macro swelling fatigue data to predict the end of life for a specific cell chemistry under different load profiles. A stable thickness indicates a robust electrode structure while rapid growth signals imminent mechanical failure.
