Degraded Li-ion cells may go quiet: investigating the path dependence in ultrasound response of aged commercial pouch cells

Ultrasound scanning has emerged as an effective method for non-destructive health evaluation of Li-ion batteries. As battery ageing shows significant path-dependence, this paper presents an experimental investigation to explore the effect of distinct degradation paths on the ultrasound response. Two types of pouch cells were aged using three different degradation trajectories with ultrasound scans conducted periodically during ageing. For both cells, ultrasound signal features varied among the three degradation paths, thus clearly indicating a path-dependence. The ultrasound data also showed conflicting trends when comparing the signals between the two cell types. Crucially, drastic signal loss was observed for cells of either type subjected to a full-cycling condition. This loss of signal was observed for minor capacity loss of <2% and replicated defective cell behaviour, despite the cells being operational. X-ray computed tomography (XCT) scans revealed greater inter-layer separation in the cells subjected to the full-cycling condition, providing a possible structural explanation for the substantial signal attenuation. The observed loss and non-trivial evolution of ultrasound signal resulted in poor correlation between ultrasound features and capacity loss. Thus, the presented work implies that ultrasound response of Li-ion batteries could be unresponsive to battery degradation under certain conditions and requires detailed investigation for its implementation as a battery health evaluation technique.