Simulation of the Production of Lithium-Ion Cells and Battery Packs

Electrode Production

• Granular Flow Simulation of the Mixture of Electrode Slurries
• Thermal Simulation of Electrode Drying
  
• Mechanical Simulation of Electrode Calendering 
   

Cell and Pack Assembly

• Flow Simulation of the Electrolyte Filling of Cells
  
• Electrochemical Simulation of Cell Formation
  
• Flow Simulation of the Foaming of Battery Modules for Thermal Insulation
  

The first production process is the mixing of the so-called electrode slurries, a mixture of granular starting materials and solvent, for the two electrode layers. The aim is to achieve a homogeneous mixture of the starting materials in the shortest possible time. Our GRAIN software is able to calculate granular mixing processes even taking segregation effects into account for industrial mixers with high rotational speeds and for different materials. In addition, a coupled GRAIN-FLUID simulation allows the interaction of the granular electrode material with the liquid solvent to be investigated.

Electrode drying is a very energy-intensive production step in typical solvent-based production processes. After the electrode slurry has been applied as a layer to the current collector, the liquid solvent must be completely removed by evaporation on a drying section up to 100 m long. Convective drying processes are commonly used for this, but inductive, conductive, laser or infrared drying are also being investigated. The goal of an optimized process is to dry as quickly as possible without negatively affecting the electrode properties (e.g. through cracking, binder migration or loss of adhesion).

We are working on the development of physical models that are able to capture the different process variants and parameters (e.g. drying temperature, belt speed) and allow a statement to be made about the degree of drying, energy input and product quality.

After electrode drying, the so-called calendaring process creates a defined electrode layer thickness and porosity by compression. This results in mechanical stresses and deformations across the thickness of the electrode, which can directly influence the mechanical aging behaviour of the cell over time. Furthermore, the power capabilities of the cell can be negatively affected by excessive calendaring, if it results in blocking of pore spaces and extension of ionic transport paths inside the electrode. 

The structural changes induced by this process (e.g. change in porosity or pore structure) can be analyzed in three-dimensional, microstructure-resolved structural mechanics simulations based on our software FeelMath. In particular, the simulation allows to investigate locally resolved mechanical forces on the material, such that potential mechanical damage during the process can be evaluated. 

A time-consuming production step in cell manufacturing is the wetting process that follows the injection of the liquid electrolyte into the cell housing. The electrolyte – driven by capillary forces and external pressures – must infiltrate the pore structure of the electrodes and the separator as homogeneously as possible. The time required for this process, the influence of external conditions and the homogeneity of the electrolyte distribution can be simulated and analysed with our software FLUID.

The electrolyte filling is immediately followed by the also very time consuming formation process, in which the cell is slowly cycled for the first time to form the passivating SEI (solid electrolyte interphase) layer on the anode. The material-specific formation protocol is decisive for the final cell properties, in particular the service life and capacity. During formation, so-called »formation losses« occur, i.e. a decrease in capacity, which must be taken into account during cell design so that the capacity of the two electrodes is optimally balanced at the end. Based on the electrochemical model in our software BEST we are working on the development of a formation model that makes it possible to evaluate and optimize the influence of process parameters on the duration and quality of formation and to predict formation losses.

Cells in a battery module must be thermally insulated from each other (to protect against rapid thermal runaway – overheating of the cell due to a self-reinforcing heat-producing process) and mechanically fixed. Cylindrical cells are commonly arranged in dense patterns, while the void spaces in-between are filled with foam. With our software FOAM we can simulate the spread and expansion of the foam.  Based on these simulations, the degree of homogeneity and coverage of the foam can be evaluated.