Microstructural parameters governing the mechanical stress and conductivity of all-solid-state lithium-ion-battery cathodes

All-solid-state lithium ion batteries are considered a promising future battery concept due to their high safety and energy density. However, they might suffer from mechanical fatigue upon cycling, caused by mechanical stresses due to the volume changes of the electrode active materials constrained by solid electrolyte. Based on a reconstruction of actual microstructure of a mixed cathode (LiCoO2/Li7La3Zr2O12: LCO/LLZO) in a lab sized cell and using computer aided material design, we calculated the thermal stresses after manufacturing and the electrochemical stresses during cycling. This approach allows us intensively study the impact of microstructural parameters (grain size, solid volume fraction and relative density) on the mechanical stress distribution and conductivities, even for cases not easily manufactural in the lab. We found that the mechanical stresses and conductivities linearly depend on the solid volume fraction of LCO and are correlated to the relative density, whereas the grain sizes influenced neither the mechanical stresses nor the conductivities. We introduced a new factor Kn as a ratio of the relative interface area between solid phases and the volume fraction of the solid phase (n = LCO or LLZO) which represents the governing factor for the stresses. On the other hand, the volume fraction of LCO and LLZO are the governing factors of their electronic and ionic conductivities. This allows for a sound forecast and determination of an optimal cathode microstructure for maximum cell performance.

Automated summary

All-solid-state lithium ion batteries have high safety and energy density, but may suffer from mechanical fatigue caused by volume changes of the electrode materials. Through computer aided material design and reconstruction of microstructures, we studied the impact of microstructural parameters on mechanical stress distribution and conductivities. We found that mechanical stresses depend on the solid volume fraction, while conductivities depend on the volume fraction of specific materials. This allows for accurate prediction and determination of optimal cathode microstructure for maximum cell performance.