Uncovering the degradation mechanism induced by ion-diffusion kinetics in large-format lithium-ion pouch cells

Battery electrochemistry in an actual cell is a complicated behavior influenced by the current density, uniformity, and ion-diffusion distance, etc. The anisotropism of the lithiation/delithiation degree is usually inevitable, and even worse, due to a trend of big-size cell design, typically such as 4680 and blade cells, which accelerated a battery failure during repeat lithiation and delithiation of cathodes. Inspire by that, two big-size pouch cells with big sizes, herein, are selected to reveal the ion-diffusion dependency of the cathodes at different locations. Interestingly, we find that the LiCoO₂ pouch cell exhibits ∼5 A h loss after 120 charge–discharge cycles, but a 15 A h loss is verified in a LiNi_xMn_yCo_1−x−yO₂ (NCM) cell. Synchrotron-based imaging analysis indicates that higher ion-diffusion rates in the LiCoO₂ than that in the LiNi_xMn_yCo_1−x−yO₂ is the determined factor for the anisotropic cathode fading, which is responsible for a severe mechanical issue of particle damage, such as cracks and even pulverization, in the cathode materials. Meanwhile, we verify the different locations at the near-tab and bottom of the electrode make it worse due to the ion-diffusion kinetics and temperature, inducing a spatially uneven electrochemistry in the big-size pouch cell. The findings give an in-depth insight into pouch cell failure and make a guideline for high-energy cell design and development.