Origins and Influences of Voids in Composite Cathodes for All-Solid-State Batteries
Authors: Bielefeld Anja , Minnmann Philip , Schubert Johannes , Janek Jürgen 
Published: September 2023

With the exit from fossil-fuel energy becoming more and more urgent, advanced batteries are highly sought after and all-solid-state batteries (ASSBs) are seen as a promising technology potentially enabling lithium metal or reservoir-free anode designs which promote the energy density.

Despite high research interest in the field, ASSBs still perform below expectations, the main challenges being elevated interfacial resistances, microstructure effects and processing, particularly at large scale. While the build-up of high resistances due to void formation upon lithium stripping at the anode side is increasingly understood, the void formation and its evolution in the composite cathode as well as the effects on the cell performance are widely unknown.

Why consider voids in the cathode at all?

In contrast to conventional, liquid electrolytes, solid ion conductors (such as thiophosphates) do not easily infiltrate voids and cracks in the cathode1. They possess a proper particle morphology and, depending on the solid electrolyte particle size distribution, its suitability to the active material particle sizes and morphologies the electrode structure that forms upon manufacturing can be quite different and influences the ion and electron transport as well as the charge transfer capabilities2-5. Experimentally determined void fractions in the composite typically range from 3 to 40 vol%6,7 and are therefore far away from being negligible.

Apart from the void formation at manufacturing, scanning electron microscope (SEM) cross sections of cycled composites have also shown that the volume changes of LiNixCoyMn(1-x-y)O2 active material can lead to contact loss at the interface of the solid electrolyte and the active material8 which reinforces the importance of understanding the origins and the influences of voids on the cell performance.

In this talk, we will discuss these, mainly from the modeling perspective including percolation network models4, flux-based simulations of the effective conductivity5 and electrochemical charge simulations based on the finite-element method9. Additionally, we use focused ion beam(FIB)-SEM tomography for the reconstruction of cathodes with different microstructures obtained by changing the particle size of the solid electrolyte. We investigate the influence of the solid electrolyte particle size distribution on the void space fraction and distribution and provide guidelines for further cathode optimization.

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Figure 1