The Electrochemical Society hosted “How Cathode Microstructure Impacts Solid State Batteries,” a live webinar by Philip Minnmann (Justus-Liebig-Universität Gießen and hte GmbH) and Johannes Schubert (Justus-Liebig-Universität Gießen), on March 26, 2025. A live Question and Answer session followed. Answers to some of the questions not addressed during the broadcast follow.
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Johannes Schubert
Philip Minnmann
Q&A
With the impedance data, how can you tell if your results are not heavily influenced by the location of the pores, e.g., the pores are in the center of the electrolyte versus at the interface between the electrolyte and active material?
Very good question. The location of the pore will definitely impact the impedance data. Pores inside the SE will only impact the ionic conductivity, while pores at the CAM/SE interface will impact the charge transfer and the polarization of the interface. This can be considered in the TLM as this will also fit the contributions from the interface.
Does your group use carbon additives in cathode composites to increase the electron percolation?
Yes for cell testing of materials and in slurry-based processes. Small amounts of carbon additives can improve electronic percolation especially if polymer binders are being used.
On slide 68 of the presentation, the theoretical versus experimental might be due to misclassified pores. What is the reason for it? Is it because of “see through” pores that are identified as material? How could this be improved in segmentation?
Yes, the segmentation is definitely one of the reasons, either because nano-pores cannot be resolved properly, or because even in the secondary electron images, “see-through” pores are falsely classified. Here machine learning can help to identify such pores, as simple grey scale values seem to fail.
While the LPSCl-Li metal stability window is not large, in theory they should be stable enough to run eis and dc polarization so long as the voltage of cell does not go outside the stability window (1.7V-2.1V iirc). Do you think it would thus be possible to run these tests without having to make Liin?
Difficult to judge. Even without applying a potential, there is already a degradation at the Li-LPSCl interface. If one can make sure that the interface is reasonably stable (i.e., through waiting for an extended time period for the degradation to slow down) it is possible to use Li. There is still the risk of dendrites though, so in this case, applying a current amplitude instead of a voltage amplitude would be necessary.
How long is it recommended to wait to achieve DC polarization stationary state conditions?
That depends a bit on the system. In our experience, electronic conductivity is quite fast in steady state (i.e., 30 minutes), while ionic conductivity may take six hours to reach that point.
At what size of electrolyte material do we get excessive diminishing returns on electronic conductivity?
Assuming this question relates to the composite and not the SE itself, that question is not that easy to answer, since this also relates to the preparation method and the size of the CAM. In principle, even with very small SE particles, the electronic conductivity in cathodes containing > 50 vol% CAM should stay quite high, since during densification, the harder CAM will deform the softer SE and will still achieve contact in that case.
How do you know that the improvement with the use of small SE particles isn’t due to enhanced ionic surface conductivity?
That is a very good question. If this would be the case, then an improvement of ionic conductivity could also be observed in pure solid electrolyte samples. However, the opposite is the case. With smaller SE particles, a higher ionic resistance can be observed, due to, i.e., particle-particle contact resistance.
How would you know if your ionic conductivity is higher or electronic especially when you are working with composite membrane fabrication?
Use selectively blocking electrodes for these as well. If you block one charge carrier, you can determine the partial conductivity of your composite.
Is there a sensitivity to the non-spherical shape of the particles?
Probably yes. Anisotropy will likely play a role in composite manufacturing.
Referring to slide 66 of the presentation: How do you consider the resolution to determine the interface area? I refer to the issue that a low resolution in FIB-SEM of, e.g., a spherical particle, highly overestimates the particles surface, as the pixels/voxels cannot build a sphere. This for sure depends on the particle size. I like to mention that I recently read about a phasefield approach, that allows a more accurate surface detection by smoothing the sharp pixels (S. Daubner and B. Nestler, Journal of The Electrochemical Society 171 (12), 120514).
This is a very good comment. To reduce the error, we used the marching cubes algorithm to generate a surface mesh which consists of triangular faces and allows a more realistic depiction of curved surfaces. In addition, the resolution was considerably lower than the particle size. Therefore, we don’t expect a significant overestimation.
Which model can be used to examine the influence of cathode-active materials with different pore diameters in their structure on electrochemical performance?
Ideally, you have a model system that allows you to investigate the interface between electrolyte and CAM, since this will change with the pore diameter.
I want to know about the fabrication of cathode composite. I am currently using agate mortar and pestle. How can I know if my cathode composite has been prepared correctly and how much pressure (0: very low, 10: very high) should I put while mixing?
Ideally, a ball mill will be used. If using a mortar and pestle, I recommend to start with 0 and to slowly increase up to 8-9 over a time of 15 minutes.
Does the cathode microstructure affect power density or energy density or both in solid state batteries?
Both will be affected. Percolation affects energy density and tortuosity affects power density.
How do you control precise volume for a specific CAM:SE volume ratio?
By using constant weight ratios. Pores will change the total volume of CAM and SE but not their ratio.
What do you think the most promising manufacturing method is to minimize porosities in composite cathodes?
Probably warm-isostatic pressing or a multi-step calandering process.
If possible, what are techniques to generate ordered paths to reduce tortuosity for ion/charge transport, for a constant porosity?
Good question. In principle laser structuring or using a template of SE or CAM matrix would be possible. The question is whether they are economical.
How high should the pressure be for the Ni-rich cathode with LPSCl electrolyte in a PEEK cell assembly device? Sometimes too much pressure decreases the capacity output. What would you suggest about the mixing of the cathode composite material, particularly using the sulfide SE?
For mixing, I would go for low intensity ball milling. Regarding the pressure, it is hard to answer. Typically, the range of 250-400 MPa should work well.
If microstructure tunes the performance/properties of solid state batteries, how can we modify the microstructure of the same materials without any additive?
Excellent question. Especially for solid state electrolytes, the morphology can help, i.e., by using plate-like or rod-shaped materials. If they can be processed, i.e., by precipitation, needs to be investigated.
What type of FIB system was used for analyzing your solid state battery sample? Specifically, was it a plasma FIB (e.g., Xe-based) or a Ga-based FIB? Also, did you perform the FIB-SEM slice-and-view analysis at cryogenic temperatures?
We used a Xe-plasma FIB. We have also tested milling under cryogenic temperatures but found it to be not necessary. The details are specified in our publication.
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