Impact of the Level of Homogenization in 3D Thermal Simulation on the Internal Temperature Distribution of Li-Ion Battery Cells

Temperature is an important factor for an optimal battery performance. To gain knowledge about the internal temperature distribution in a battery, many thermal simulation studies are performed. Among other factors, they differ in the level of homogenization (LoH) of the geometry, which directly influences the computing time. However, the effects of different LoH, in particular of the cell layers, on the modeling and prediction quality of the temperature field are scarcely investigated. This work discusses the effect of different LoH of the cell stack on a numerical 3D thermal battery model for different thermal management strategies. A new approach of reducing the number of cell layers of the pouch cell geometry while keeping their volumetric proportions constant is proposed. It is clearly shown that the LoH has a large impact on the thermal transport paths, especially through the current collectors and tabs, and therefore on the predicted internal temperature distribution. In addition, the effect of the LoH differs for different thermal management strategies, because they affect the heat transport paths as well

Effective Thermal Conductivity of Lithium‐Ion Battery Electrodes in Dependence on the Degree of Calendering

The thermal conductivity represents a key parameter for the consideration of temperature control and thermal inhomogeneities in batteries. A high-effective thermal conductivity will entail lower temperature gradients and thus a more homogeneous temperature distribution, which is considered beneficial for a longer lifetime of battery cells. Herein, the impact of the microstructure within the porous electrode coating obtained by different compression rates and its thermal contact to the current collector is investigated as both factors significantly determine the overall conduction through the electrode. The effective thermal conductivity of two graphite anodes and two lithium nickel manganese cobalt oxide cathodes is evaluated at different compression rates. It is found that the thermal conductivity does not have a monotone dependence on the porosity with changing compression rates. The results show a strong correlation with the adhesion strength, thus a significant impact of the thermal contact resistance between the coating and current collector is assumed

Thermal Transients to Accelerate Cyclic Aging of Lithium‐Ion Batteries

Cyclic aging tests of lithium-ion batteries are very time-consuming. Therefore, it is necessary to reduce the testing time by tightening the testing conditions. However, the acceleration with this approach is limited without altering the aging mechanisms. In this paper, we investigate whether and how thermal transients accelerate the aging. The tests are performed on NMC/graphite pouch cells by applying temperatures in a range of 5 °C to 45 °C to the cell surface. The results show, that an accelerated capacity loss can be achieved in comparison to the reference cell at a steady-state temperature of 25 °C. However, capacity difference analysis (CDA) prognoses a covering layer for the transient cells, which is confirmed upon post-mortem analysis. We suspect the origin to lie in the dynamics of temperature fields and current distribution during temperature changes when charging. More specifically, areas of higher temperature in the cell lead to high local current densities and plating. Subsequently, high temperatures promote the reaction of the plated lithium with electrolyte. The results show that thermal transients are a critical condition for lifetime and safety and should be treated with caution as they can occur during real life operation