Impact of microstructure variations on the effective thermal conductivity of porous electrodes and cell stacks

Summary:

The performance and lifetime of Lithium-ion batteries (LIB) are strongly influenced by the temperature distribution within the cells, as electrochemical reactions, transport properties, and aging effects are temperature-dependent. An optimization of LIB systems therefore also requires a profound understanding of the thermal cell behavior as well as efficient tools to describe these phenomena. However, thermal analysis and numerical simulations of the inner cell temperature can only be as accurate as the underlying data of thermal transport properties. For thermal modelling the relevant properties are density, specific heat capacity and thermal conductivity.
This contribution presents an analytical and numerical model for predicting the effective thermal conductivity of porous electrode coatings, electrode stacks and cell stacks as a function of microstructure parameters [1, 2]. Both models take into account the layer thicknesses, morphological parameters and thermal bulk materials of the constitutive components. Morphological parameters considered in both models are the porosity of the electrode coating, particle size distribution, particle shape, particle contact areas and the binder-carbon-black distribution [1, 2]. These parameters are directly included into the equations of the analytical model. In the numerical model, on the other hand, the mentioned parameters are considered via a detailed generic approach for the reconstruction of the microstructure, in which the differential transport equations are solved. The results of both approaches are compared with each other and with experimental data. The surrounding tool of the numerical model automatically generates electrode geometries and meshes according to the specified microstructure parameters.

The analytical model is an extension of the Zehner-Bauer-Schlünder model, well known in process technology for macroscopic packed beds, which has been extended in order to be applicable to porous electrodes with their special properties. For validation, a methodology based on laser flash analysis is used to determine the experimental data of the effective thermal conductivity of the electrodes.

[1] D. Oehler, P. Seegert und T. Wetzel. Energy Technol. 2020. 202000574.
[2] D. Oehler, J. Bender, P. Seegert und T. Wetzel. Energy Technol. 2020. 2000722.