A comparative study on diffusion-induced stress and thermal stress during discharge of ternary soft pack lithium-ion battery
This study investigates the effects of diffusion-induced stress and thermal stress on lithium-ion batteries during discharge by establishing an electrochemical-mechanical-thermal coupling model for an 18.5 Ah soft-package NCM111 lithium-ion battery using Comsol Multiphysics 6.0. The analysis encompasses the lithium concentration difference between the centers and surfaces of anode particles, diffusion-induced stress, thermal stress, and expansion behavior at different discharge rates. Diffusion-induced stress is simulated using a one-dimensional electrochemical model and its derivative particle dimension, while thermal stress is addressed through a three-dimensional solid mechanics and heat transfer model. The findings indicate that both diffusion-induced and thermal stresses escalate with an increase in discharge rate, and a lower discharge rate mitigates the stress experienced by the battery. Specifically, diffusion-induced stress in the anode particles correlates with the lithium concentration difference between the centers and surfaces of these particles, intensifying progressively during discharge. In the pre-discharge phase, this concentration difference is higher near the separator than near the collector, with the situation reversing post-discharge. A critical turning point occurs at a depth of discharge (DOD) of 60％-70％, suggesting a similar trend for diffusion-induced stresses. Notably, the diffusion-induced stress in the anode particles reaches the order of MPa, substantially exceeding the thermal stress in the cell, which is on the order of kPa. Furthermore, the maximum thermal stress and displacement in the cell exhibit a linear relationship with the cell's temperature difference, escalating with discharge rate. A distinctive observation is the significant thermal stress at the connection points between the tabs and the cell in soft-pack batteries, in contrast to cylindrical batteries. This comparative analysis of diffusion-induced and thermal stresses during the discharge of ternary soft-pack NCM111 lithium-ion batteries aims to provide theoretical insights for the development and stress monitoring of electrodes and cells.
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