Finding the limits of ionic conductivity
Measuring the ionic conductivity of a solid electrolyte can be as easy as recording an electrochemical impedance spectrum, but what are you actually measuring?
Common solid electrolytes don’t consist of a single crystal, but are “polycrystalline”, meaning that they contain multiple single-crystal grains (the “bulk”). The contact areas between grains are known as “grain boundaries”, and they can have a large impact on the performance of the electrolyte. The ion (and electron) transport through and along grain boundaries can be faster or slower than through the bulk, and so might increase or decrease the total ionic conductivity. In addition, the grain boundaries can serve as pathways for lithium growth through the electrolyte, as discussed in a previous application note.
To improve the conductivity of solid electrolytes, it is necessary to know whether to change the bulk or the grain boundary properties. After all, achieving a higher bulk conductivity through each grain will have little effect if the grain boundaries are already limiting the overall conductivity through the sample.
Micrograph of a polycrystalline metal with evident grain boundaries
© Edward Pleshakov (CC BY 3.0)
Accurate interpretation of impedance spectra
Electrochemical impedance spectroscopy can be an effective tool to analyse the different contributions to electrolyte conductivity. Grain boundaries typically have shorter time constants than bulk processes, which can be used to distinguish the two. Nevertheless, it can be challenging to accurately assign spectrum features since many sample processes as well as measurement artefacts can appear with similar time constants. For example, a high frequency semicircle in a Nyquist plot is often interpreted as a sign of grain boundary resistance, but can also originate from instrument input capacitance, poor electrode contact, or cell geometry artefacts. Furthermore, even a rather feature-less impedance spectrum, like the one for LPSCl shown on the left, can still contain multiple contributions to the conductivity, as we see in this study.
Distinguishing bulk and grain boundary conductivity
A method to separate the bulk and grain boundary conductivity contributions to the overall sample impedance would clearly be useful for the solid electrolyte field. In a recent publication by Yamakov et al., they describe such a method which takes into account the elastic properties of solid electrolytes under pelletizing pressures, and the resulting effect on the conductivity. By constructing the type of plot shown to the right and analysing the slope, an electrolyte can be classified according to whether the bulk or grain boundary conductivity is limiting, or if they are on the same order of magnitude.
In this application note, we have analysed three different solid electrolyte samples according to this method. Using the CompreDrive, the experiment could be completely automated in one procedure, rather than having to construct a separate cell for each pressure data point.
Reproducibility in solid-state research and development
The large difference found between the two samples of LPSCl from different suppliers illustrates the need to carefully characterize the materials used to construct all-solid-state batteries. A single material such as LPSCl can have very different properties depending on particle size distribution, particle shape, surface coating, impurities, etc. Using the same pelletization pressures, the two LPSCl samples had large differences in both porosity and conductivity. Moreover, the grain boundaries were limiting the ionic conductivity in one case, but not the other. In order to achieve reproducible results for all-solid-state batteries, consistency between batches of each component material is crucial, as is accurate control of both the fabrication pressure and measurement pressure.
To cite this application note, please use: “Pacetti et al, rhd instruments GmbH & Co. KG, Application Note: Distinguishing Grain Boundary and Bulk Conductivity in Solid Electrolytes, August 2024,
https://docs.rhd-instruments.de/appnotes/application-note_Bulk_vs_Grain_Boundary_Conductivity.pdf”.