Exploring the correlation between ultrasound time of flight and the state of health of cells

Electric vehicles (EVs) have become a popular mode of transportation, with their performance depending on the aging of the Li -ion batteries used to power them. However, it can be challenging and time-consuming to determine the capacity retention of a battery in service. Moreover, the state of health (SoH) of retired batteries is critical for their second -life applications. A rapid and reliable testing method for SoH determination is desired. Ultrasonic testing techniques are promising due to their efficient, portable, and non-destructive features. In this study, we demonstrate that the time of flight (ToF) of through -thickness transmitted ultrasonic waves increases with the degradation of the capacity of an LFP prismatic cell. We explain this correlation through numerical simulation, which describes wave propagation in porous media. We propose that the reduction of binder stiffness can be a primary cause of the change in ultrasound ToF during battery aging. This work brings new insights into ultrasonic SoH estimation techniques.