Abstract: The reliability and lifetime of Li-ion cells can be improved by understanding their operational wear. In this work, an experimental characterization of Li-ion cells is carried out, and novel phenomenological models are developed to elucidate the dynamic electrochemical-thermal mechanics of Li-ion cells. Two sources of swelling and reaction force, namely Li-ion intercalation and temperature variations, are identified through experiments. The swelling and force from temperature variation and the state of charge (SOC) shows nonlinear characteristics. To account for these behaviors, the model must include a coefficient of thermal expansion and stiffness at the cell-level, which depend on temperature and the SOC.

Based on the experimental characterization, a 1-D phenomenological multi-physics model is proposed to predict the complex dynamic behavior of Li-ion cells. The model consists of an electro-thermal model, a swelling model, and a force model. The electro-thermal model estimates the SOC and the surface/core temperature with current and ambient temperature profiles. The swelling model predicts the volume change of a cell due to Li-ion intercalation and temperature variation as a function of the SOC and surface/core/ambient temperature. The force model incorporates nonlinear elastic stiffness and separates the overall SOC region into three regions to account for Li-ion intercalation and phase transitions. The force model estimates the reaction force caused by the battery swelling and the preload as a function of the estimated SOC and total swelling. Experimental validation demonstrates that the proposed multi-physics model accurately predicts complex physics behind cells at the wide range of preload and ambient temperature.

A 3-D numerical and phenomenological cell model is developed also to predict strain and stress distribution on the surface of cells. To model the inherently different microphysical swelling processes from Li-ion intercalation and thermal expansion, the model incorporates the nonlinear equivalent modulus of elasticity and the equivalent coefficient of thermal expansion, which are functions of the SOC. Comparison between experiments and model predictions clearly demonstrates that this 3-D model accurately reproduce the swelling shape, suggesting that the model can be useful for cell design, developing new strain/pressure sensors, and optimizing sensor locations.