Quantifying the Mixing Quality of Composite Cathodes and Its Implications for Reproducibility and Comparability of Solid-State Battery Cells

Solid-state batteries (SSBs) are heavily investigated in academia and industry driven by their potential to deliver high energy and power density as well as enhanced safety compared to conventional, liquid electrolyte based, lithium-ion batteries (LIBs). A significant number of investigations into SSBs are conducted on model-type pelletized press cells. These facilitate a comprehensive examination of materials and interfaces with a relatively straightforward process due to their reduced complexity in comparison to pouch cells. However, a recent wide interlaboratory round robin study [1] demonstrated considerable discrepancies among different research groups regarding the assembly and electrochemical performance of their cells. Potential sources of error were identified, but the underlying reasons for the significant performance variations remain to be fully elucidated.
In light of these findings, we undertook a critical assessment of our cell construction methodology with regard to reproducibility, conducting an intralaboratory investigation. We worked on a representative, commercially available, state-of-the-art material system comprising a composite cathode with single-crystalline Ni-rich NCM (LiNi_0.82Mn_0.7Ni_0.11O₂) as the active material, Li₆PS₅Cl as catholyte and vapor-grown carbon fibers as the conductive additive, a Li₆PS₅Cl sulfide solid electrolyte separator and an indium-lithium foil as the anode.
Our findings demonstrate that despite identical assembly parameters within a standard operation procedure, notable discrepancies in the specific capacities of nominally identical cells still persist. It was revealed that these variations could be unambiguously traced to the microstructure of the composite cathode, which depended exclusively on the cathode composite mixing process. The use of conventional hand mortaring to mix cathode composites resulted in unsatisfactorily large variations, whereas the reproducibility of the cell performance could be significantly enhanced through the utilization of machine-made cathode composites. As a result, it is evident that a reproducible mixing process can be challenging even on a small scale and is dependent on the materials and compositions utilized.
In order to quantify the quality of the mixing process in terms of the electrochemically active mass, we discuss the open-circuit potential relaxation method, which is an in-situ electrochemical method. We show that this procedure can be readily incorporated into any electrochemical testing program and discuss how it enhances the reliability of the data and avoids misinterpretation. The method is applicable to a wide range of studies and is also valid for pouch cells, enabling a better discrimination between static and kinetic microstructural capacity limitations.
Overall, our results stress the importance of the composite preparation for the subsequent electrochemical analysis and highlight the necessity for a reproducible composite mixing procedure. This will not only enhance the quality of laboratory-scale studies but also potentially facilitate the transition to large-scale mixing and processing of SSBs.

[1] Puls, S., Nazmutdinova, E., Kalyk, F. et al. Benchmarking the reproducibility of all-solid-state battery cell performance. Nat Energy (2024)