Development of a validated simulation model for all solid-state batteries

All-solid state batteries offer promising advantages in energy density and safety compared to conventional lithium-ion batteries. However, many variants of this kind of batteries suffer from a low cycling stability. This might originate from mechanical fatigue caused by mechanical stresses in particular of the cathode during charging and discharging. The cathode active materials change their volume during this process and cannot expand or shrink freely in the rigid structure.

We introduce a method of qualifying the stresses occurring in 3D microstructures using histograms of the stress distribution and derived properties. The stress calculations itself were performed in GeoDict/ElastoDict. The effect of the orientation of the cathode active material grains (aligned or random) on the stress distribution was studied and the results showed that aligned microstructures yield much lower stresses (using LiCoO₂ and Li₇La₃Zr₂O₁₂ materials in the mixed cathode and an experimental microstructure). In addition, separator and cathode supported cell designs were analysed as well as different material combinations for the mixed cathode. In a next step, the microstructure was reproduced using GeoDict and the solid volume fraction of the cathode active material, the porosity and the grain sizes of the solid components were varied systematically. We found that the mechanical stresses depend linearly on the solid volume fraction and non-linearly on the porosity, whereas the grain sizes had no influence.