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 the 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 the proposed conference contribution, a meso-mechanical finite-element model of the rolling process for cylindrical cells will be presented. This process step can be seen as critical since it already introduces mechanical stresses in the jelly roll and influences the quality of the cell. The effects during this step are analysed by means of a simulation study. It will be shown how different process parameters influence the stress-distribution inside the cell. For conventional 18650 cells, the tab introduces an inhomogeneity and influences the shape of the jelly roll. The tool can be used to assess how a variation in tab position and/or thickness affects the rolling process. Further potential applications comprise the variation of electrode coating thickness or the used active material in general.
The simulations are done the following: For each cell component (separator, current collector/active material for anode and cathode) a component model is created. They are parameterised from tensile and compression tests and validated by simulations of the tests itself. The active materials used are a Si-alloy/graphite blend (55% Si-alloy) for the anode, and NMC622 for the cathode. As separator, tri-layer PP/PE/PP membrane from Celgard is used. The material model for separator and active materials (both modelled as solid elements) is a modified honeycomb model allowing to distinguish between parameters in tension and compression. For the separator, measurement data has shown that also anisotropy between machine and transverse direction needs to be considered. The current collectors Al and Cu are simulated as shell elements using a piecewise linear plastic material model.
For the rolling itself, the components are wound around a mandrel, starting with the separator, then adding anode and cathode. Tabs are included for both electrodes (Al for cathode, Ni for anode). Variable parameters for the simulation study are the rolling speed, the counterforce on the electrodes, as well as the position and thickness of the tab. As outcome of the simulation the stress distribution and volume/shape change of the elements in the roll is analysed and presented. By assessing the stress values, a potential material failure can be estimated. The simulations are done with a commercially available explicit FE-solver on a representative cross-section of the jelly roll. An example can be seen in the attachment.