(Invited) Model and Knowledge Based Design of Lib Electrode Production Processes
Authors: Kwade Arno , Haselrieder Wolfgang , Mayer Julian , Sangros Clara 
Published: February 2020

Besides the active and passive materials and their compositions, the processing of these materials to form the electrodes of lithium-ion batteries is very crucial for the resulting cell performance. Due to the long process chain and high number of process parameters, a try and error procedure should be absolutely avoided. On one side models and subsequent simulation methods which are usually well known from other process technology fields like for pharmaceutics, ceramics and minerals. On the other side from systematic investigations of the different process steps for electrode production, so-called process-structure-property-relations are already derived showing the influence of different process parameters on the particle and electrode structure and the effect of these structures on the resulting cell performance.

In the presentation, along the process chain from dry mixing to calendering important interrelations and models for knowledge based process design will be explained. Among others in dry and wet mixing processes stress intensity and stress frequency determine mainly the carbon black structure described for example by the inner porosity which can be well correlated with the cell performance especially at higher discharge/charge rates. For example, the viscosity and shear rates in a wet dispersing process determine the shear stress distribution and especially the maximum stress intensity, which is decisive for the degree of carbon black dispersion even for very long mixing times. The shear rate and shear stress distributions in planetary mixers and extruders can be well determined by CFD simulations. The results will show that extrusion is a very effective process to produce electrode suspensions and has a high potential for directly forming electrodes from very high concentrated electrode pastes.

During drying the wet electrode coatings should be dried as fast as possible with a homogeneous structure. Especially the migration of binder and carbon black to the electrode surface has to be avoided which depends strongly on mass loading and drying temperature. Moreover, for thick electrodes the structure has to be designed in a way that fast ion diffusion is possible.

An often-underestimated process is the calendering of the electrodes to the final density and porosity. The electrodes density and porosity change with increasing line load according to an exponential function and reach almost constant values at very high line loads. The parameters of the exponential function can be predicted as functions of the mass loading, the temperature and the material composition. Beside the simple models for the integrated properties density and porosity the change of the micro structure and with that of the pore size distribution during calendering can be predicted by so-called discrete element simulations. This simulation method is also able to predict changes in the mechanical behavior of the electrodes due to change of particle sizes, mass loading and binder content.

Overall, it can be concluded that a lot of systematic process studies and several models and simulation tools are available today which enable a knowledge and model based design of the process chain based on a low number of experiments. Moreover, also models and simulation methods exist to evaluate the cell performance based on predicted electrode structure parameters.