Mechanical Frequency Response Analysis of Lithium-Ion Batteries to Disclose Operational Parameters

During the charge and discharge process, lithium-ion batteries change their mechanical properties due to internal structural changes caused by intercalation and de-intercalation of the ions in the anode and cathode. Furthermore, the behavior changes over the lifetime of the battery due to several degradation mechanisms. The mechanical properties of the cell hold valuable information for monitoring these changes and additionally provide data for mechanical construction and further optimization of battery systems. Hence, in this manuscript, the mechanical frequency response function is investigated as a non-destructive method to determine parameters such as stiffness and damping of pouch cells and their correlation with the state of charge (SOC), the state of health (SOH), and the temperature of the cell. Using a mechanical shaker and an impedance head, it is shown that low amplitude forces of only a few Newton and a low frequency region of several hundred Hertz already suffice to show differences in the state of charge and state of health as well as in mechanical properties and the dependencies on temperature. Also the limitations of the method are shown, as the frequency response is not distinct for each parameter and thus, at the moment, does not allow absolute determination of a single value without prior system knowledge.