Modeling Approaches for the Description of the Carbon Black Particle Size in Batch and Continuous Dispersion Processes for Lithium-Ion Battery Slurries
Authors: Mayer Julian Kristoffer , Kwade Arno 
Published: July 2022

Energy storage is a key technology for alternative power trains like electric and hybrid electric vehicles. Lithium-ion batteries (LIB) are widely used for this purpose due to their high energy density and elaborated developmental state. Also, the increasing usage of electrified transportation leads to ever-increasing demands on LIBs in terms of fast charging ability and power density. The fragmentation of carbon black (CB) aggregates and agglomerates, respectively, has high impact on the resulting microstructure and mechanical integrity as well as conductivity of electrodes and furthermore, the electrochemical performance of LIBs [1]. To achieve optimized cell performances and to reduce the process times or increase throughputs while conserving the slurry quality, a method to predict the CB particle sizes based on simulations and experiments has been developed.

This work demonstrates how the dispersion process can be modelled, thus laying the foundation for an optimized microstructure of the electrode right at the beginning of the process chain. For this purpose, a high-intensity batch process in a planetary mixer is investigated and modelled for the description of the CB fragmentation, and the approach is then applied to a continuous twin-screw extruder. Through the use of computational fluid dynamics, theoretical equations and considerations, and experimentally obtained data, the models are parametrized and can be used to predict the result of the dispersion process in form of the CB particle size. The critical values here are the shear rate in the laminar flow and the viscosity of the slurry, which determine the transferred energy onto the particles during the process. By appropriately modelling these characteristic values, it is possible to transfer the models to machines of different types and designs.

Furthermore, it is shown how the predicted particle size of CB affects the microstructure of the electrodes. For this purpose, an innovative structural parameter is calculated from mercury intrusion measurements of manufactured electrodes, which expresses the internal porosity of the CB particles, or of the conductive microstructure, respectively. Based on the modelled CB particle size, this parameter can already be used to estimate the developing microstructure of the electrodes depending on the selected process parameters during dispersion. Such an approach is the basis for further work in order to holistically depict and digitalize the entire process chain of the LIB production and its counteracting influences.

The experimental process data (power uptake of dispersion machines, viscosities, CB particle sizes), electrode structures, electrode properties (adhesive tensile strengths, spec. resistances) and cell performance data are shown to illustrate the approaches and the value of the created models. All LIB cathodes investigated consist of industrially relevant formulations (> 95 wt% active material NMC622, < 2.5 wt% CB).

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[1] Mayer, J. , Almar, L., Asylbekov, E., Haselrieder, W., Kwade, A., Weber, A. and Nirschl, H., Influence of the Carbon Black Dispersing Process on the Microstructure and Performance of Li‐ion Battery Cathodes, Energy Technol. (2019)