Temperature and Lithium Concentration Gradient Caused Inhomogeneous Plating in Large-format Lithium-ion Cells

Although lithium plating is one of the most-investigated degradation mechanisms occurring in lithium-ion batteries, many degradation studies still view inhomogeneous lithium plating as an artifact. It is often caused by mechanical stress, gas formation and encapsulation, or temperature and current gradients on account of the cell size. In this work, large-format lithium-ion cells are cycled at conditions harmless on the material level, but cause heavy lithium plating in the center of the commercial cell. Failure analysis requires visual inspection and further material evaluation to identify the reason for malfunction. A combination of temperature gradient and current density gradient evoke a lithium concentration gradient, which equalizes by a lateral lithium-ion flow. The inhomogeneous distribution of lithium again distorts the operating potential window, resulting in local violation of the critical lithium plating condition. Thus, our study shows that inhomogeneity in a large-format lithium-ion cell has to be taken into account when designing the operating window. In addition, accelerated testing of large-format lithium-ion cells raises degradation mechanisms, which do not appear during regular usage.