Thickness change investigation of lithium-ion pouch cells using measurements and modeling

Summary:

During cycling and aging, volume changes occur in the active materials of a lithium-ion battery cell. These volume changes lead to a formation of stresses inside the cell, which may compromise battery lifetime. Therefore, the simulation of the mechanical behavior of lithium-ion cells is an important instrument in the early stages of battery cell design.
In this work, the expansion of a single anode and a single cathode electrode are measured with the help of a dilatometer, which detects the thickness change perpendicular to the coating’s surface on the electrode level when cycled against a lithium metal counter electrode. An electrochemical model is implemented in COMSOL Multiphysics and parameterized for the a non-commercial NMC622/graphite pouch cell, produced by Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW) in Ulm, within the scope of the BMBF-project HighSafe (grant number: 03XP0138B). The electrochemical model is validated against voltage characteristics for different C-rates from pouch cell measurements and coupled with a mechanical model. The dilation of the electrode materials is derived using below-mentioned equations, where ?? is the active material volume fraction of the electrode and Δ??(??̅) is the volume change in particles. Δ?? is influenced by the average lithium-ion concentration ??̅ within the active material particle. ??,0 is the initial thickness of the electrode, Δ?? is the vertical thickness change of the electrode and Δℎ is the proportional thickness change of the electrode. Δℎ=Δ??/??,0=??∙Δ??(??̅)
Finally, the electrochemical-mechanical model is validated using a custom-built 2-D laser scanning test bench, which measures the thickness change of the modeled pouch cell for different C-rates during the cycling time. The simulated thickness change of the non-commercial pouch cell is in good agreement with the experiments. Furthermore, the aim is to be able to apply the simulation also for higher C-rates and scaling up the model for thicker multi-layer pouch cells in order to analyze the mechanical limitations of various cell formats.