Electrochemical impedance spectroscopy is a powerful tool to monitor systems in operation. The analysis of impedance data mostly involves pattern recognition and fitting with a simple electric equivalent circuit model. This has become the standard for studying liquid systems, but the analysis of inhomogeneous solid-state systems is more complex. This is due to the microstructure of the sample and the morphology of interfaces, which introduce additional degrees of freedom. As a result, the transport through real structures cannot be adequately described by 1D models. However, this is typically neglected when analyzing the impedance in terms of the transport processes taking place in the system, which in turn can lead to serious misinterpretations.

For example, diffusion processes on the metal anode interface strongly influence the cycling performance of a solid-state battery. This includes charge transfer driven morphological (in)stabilities, e.g., at the interface between lithium and Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO). For a long time, the interface impedance response has been mistakenly attributed to charge transfer reactions, leading to the misconception that an inherently high transfer resistance prevents the realization of the reversible metal anode concept. However, experiments by Krauskopf et al. have shown that the actual cause is the non-ideal physical contact between lithium and LLZO, i.e., the presence of pores at the interface.[1] The charge transfer resistance, on the other hand, is negligible.

Although the current constriction effect is well-known as a DC phenomenon, its AC behavior is not yet fully understood. Thus, we have built on earlier work by Fleig and Maier[2],[3] to understand the various dependencies of the constriction phenomenon, e.g., on the microstructure of LLZO, or the interface morphology. We show that the signature of the current constriction signal in the impedance spectrum is identical to that of ion transport. However, it is not a migration process in the strict sense. It is related to the frequency-dependent change of the electrode area that actively contributes to the transport.[4],[5] It is a geometric effect that is expected at different length scales.

As a result, various processes (e.g., charge transfer reactions, morphological and chemical instabilities, etc.) affect the interface properties of metal anodes. Identifying the dominant interface effect(s) is crucial, as the strategy for improving a solid-state battery depends on the rate-limiting process that needs to be overcome. For this purpose, we have developed a guideline for the interpretation of experimental impedance data of parent metal anodes to increase the reliability of future impedance analyses.[6]