Temperature has a large impact on the performance and aging behavior of Li-ion batteries. Therefore, the temperature range for optimal performance and minimum aging of Li-ion batteries should be ensured during operation. The knowledge of the internal temperature field in Li-ion batteries is essential in this respect. Since in-situ measurements of the internal temperature distribution prove to be difficult, thermal models are used for predictions under various conditions. Those conditions can be different C-rates and thermal boundary conditions. Thermal management systems for example, affect the internal temperature distribution and can induce temperature inhomogeneities.
The internal temperature field is also largely affected by the internal thermal transport paths. One important factor to accurately depict these transport paths in a thermal model is the resolution of the geometry. On a cell level, in case of a pouch cell, this would include each layer in a cell stack (anode current collector, anode active material, separator, cathode active material and cathode current collector), as well as the insulating separator layer, the pouch foil and the tabs. Commonly, the cell stack is modeled as a homogeneous block with anisotropic material properties. The importance of a resolved housing is well studied for prismatic cells. The housing is known to be an important heat transport path[2,3] and is therefore usually resolved. The impact of the resolution of the cell stack on the other hand and of its thermal connection to the housing and the tabs is not yet discussed in great detail.
This work aims to achieve clarity about the influence of the level of homogenization of the cell stack on the internal temperature field. Therefore, a new approach of partial homogenization of the cell layers is presented. Consequently, the results of the partial and full homogenization are compared to a fully resolved stack. The latter case is regarded as the most accurate case. The tab and pouch foil as well as the encasing separator layer are always resolved. The dimensions and material properties needed for the thermal model were determined in-house on the investigated pouch cell,[4,5] while the results are transferable to pouch cells with other dimensions and materials.
Keywords: Li-ion battery, Thermal Modeling, Level of Homogenization, Internal Temperature Distribu-tion, Cooling scenarios.
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