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

Optimized Production of Lithium Batteries

Large battery factories are being built in many places in Europe to meet the demand for cells. As this production is very space-, energy- and time-intensive, it is important to design the production processes as efficiently as possible without negatively affecting the product properties of the battery cells. We provide support here with physical simulation models for specific process steps.
 

Simulations for Battery and Cell Production

The production of cells and batteries is a chain of many complex individual processes. The main cell production processes can be divided into electrode production (mixing, coating, drying, calendering) and subsequent cell assembly (separating, stacking/wrapping, packaging, electrolyte filling, forming). Depending on the cell format (pouch, cylindrical, prismatic), the finished cells are then typically assembled into modules (this includes electrical contacting, thermal insulation, mechanical bracing). Several modules are then combined into a pack. While the assembly of the battery pack is mainly mechanical or electrical in nature, complex physical processes are involved in cell production.

Describing these production processes using simulations requires the adaptation and expansion of simulation techniques and has only been carried out for a few years in funded research clusters (e.g. Project »Cell-Fi« – electrolyte filling of battery cells)and EU projects (e.g. project  »DEFACTO« – New methods in the development and production of battery cells)also developed with the participation of our institute. Our department »Flow and Material Simulation« has gathered know-how in many key areas of industry applications, which now provides a strong foundation for analysing and optimizing cell production.

Overview of the Processes

We support various battery production processes with our software tools.
© Fraunhofer ITWM
We support various battery production processes with our software tools. The processes are shown in chronological order: Mixing of the electrode slurries, calendering of the electrodes, electrode drying, electrolyte filling of the assembled cells, cell forming and foaming of battery modules.

Granular Flow Simulation of the Mixture of Electrode Slurries

Example simulation of a granular mixing process within an EIRICH mixer. ©Fraunhofer ITWM

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.

Mechanical Simulation of Electrode Calendering

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. 

Simulation of the mechanical stress and deformation of a microstructure during calendering with FeelMath.
© Fraunhofer ITWM
Mechanical stress and deformation of a microstructure during calendering calculated with FeelMath.

Thermal Simulation of Electrode Drying

Example of a thermal simulation capturing solvent evaporation in the electrode based on the applied infrared radiation.
© Fraunhofer ITWM
Example of a thermal simulation capturing solvent evaporation in the electrode based on the applied infrared radiation.

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.

Flow Simulation of the Electrolyte Filling of Cells

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.

Privacy warning

With the click on the play button an external video from www.youtube.com is loaded and started. Your data is possible transferred and stored to third party. Do not start the video if you disagree. Find more about the youtube privacy statement under the following link: https://policies.google.com/privacy

Temporal evolution of local and global electrolyte saturation during the wetting process within a Li-ion battery, simulated with FLUID. ©Fraunhofer ITWM

Electrochemical Simulation of Cell Formation

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.

The Formation Cycles
© Fraunhofer ITWM
The Formation Cycles

Flow Simulation of the Foaming of Battery Modules for Thermal Insulation

Privacy warning

With the click on the play button an external video from www.youtube.com is loaded and started. Your data is possible transferred and stored to third party. Do not start the video if you disagree. Find more about the youtube privacy statement under the following link: https://policies.google.com/privacy

We use FOAM to simulate the expansion of foam in order to thermally insulate cells in a battery module from each other. ©Fraunhofer ITWM

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.

Our Suitable Software Tools at a Glance

 

BEST – Battery and Electrochemistry Simulation Tool

BEST is a software environment for the physics-based, three-dimensional simulation of lithium-ion batteries.

 

FLUID – Simulation Software for Complex Fluids

FLUID is a software for simulations with highly complex, non-Newtonian multiphase fluid phenomena.

 

 

GRAIN – Granular and Bulk Flow Simulation

The simulation environment and infrastructure was developed for the development of products and processes with granular materials and bulk solids.

 

FOAM – Simulation of PU Foams

Our FOAM solver simulates the expansion process of PU foams and offers the possibility to calculate the foam formation and resulting foam density in closed molds.

 

FeelMath – Analysis Tool for Elastic Microstructures

The analysis tool deals with elastic microstructures that are specified by 3D images or virtual material structures.