Mechanical impedance as a tool for electromechanical investigation and equivalent modeling of lithium-ion batteries

Mechanical stress in constrained lithium-ion cells impacts both performance and battery lifetime. Among cell swelling during charging and due to aging, the mechanical properties of the cell stack itself influences stress development. However, the compressive mechanical properties of lithium-ion cells are difficult to measure due to their mechanically complex structure with liquid (electrolyte) and solid components (electrodes and separator). To create ideal stress conditions, it is crucial to estimate the stress development in cell operation. A novel methodology is presented in this work that allows for the in-situ determination of compressive mechanical properties from lithium-ion cells during cell operation. A non-destructive dynamic mechanical analysis is performed to investigate the viscoelastic mechanical behavior of cell stacks. This methodology is developed and demonstrated using large-format cells from automotive applications. Stiffening of the investigated cells at high a state-of-charge and after the first 100 cycles is observed. Electromechanical analysis reveals structural changes within the anode. The newly presented mechanical cell impedance enables the parameterization of simple and accurate mechanical equivalent circuit models. This allows the widely used equivalent circuit modeling for electrical simulation to be extended for mechanical modeling to model state-dependent stress increase in constrained cells.