Simulation of the electrode rolling process for cylindrical 18650 lithium-ion cells using the finite-element method

Summary:

Li-ion cells are the main technology for powering electric vehicles and power electronics nowadays. The number of produced cells is steadily increasing, and optimisation of current cell production is necessary to make the processes more effective. To fulfil this task, a detailed understanding of these production processes is vital. Digital twins and simulations can be useful tools on this regard to shed light on effects which occur during cell production and are difficult to measure.
In this poster, first simulation results of a meso-mechanical model of the rolling process for cylindrical cells are presented. This process step can be seen as critical since it already introduces mechanical stresses in the jelly roll and influences the cell quality. The goal of these simulations is to make the stress distribution within the roll visible for different process parameters in order to assess their influence.
As first step, for each cell component (separator, current collectors, active materials) a component model is created. Parameters are taken from mechanical tests and validation is done by simulations of the tests itself. The active materials used for these specific simulations are a Si-alloy/graphite composite for the anode, and NMC622 for the cathode. The separator is a tri-layer PP/PE/PP membrane. Separator and active materials are modelled as solid elements. The material model used allows to distinguish between tensile and compressive load. For the separator, orthotropy is considered in addition. The current collectors Al and Cu are modelled as shell elements using an elasto-plastic material model.
For the rolling simulations itself, all components are rolled around a mandrel, starting with the separator, then adding the coated anode and cathode. Tabs are included for both electrodes with two different thickness values. The results are compared to a configuration without tabs. Results show that the tab is a stress hot spot and introduces inhomogeneity into the jelly roll. The configuration without tab shows a lower overall stress level. In the simulations, a counterforce is applied to the components to keep them under tension. This value has been varied and its influence on the separator stresses is shown. In addition, the final jelly roll is compared to a CT-image of this cell chemistry.
The simulations are done with a commercially available FE-solver on a representative cross-section of the jelly roll. Results are shown for one specific cell format and chemistry, but the model is more generally applicable to other formats and chemistries as well. The information gained by the simulations might support cell manufacturers in better understanding and optimising the cell production process.