Composite electrolytes attract significant attention in solid-state battery research due to their potential to combine the advantages of both inorganic and polymer electrolytes. Ion transport in these electrolytes is frequently investigated using electrochemical impedance spectroscopy, where each impedance contribution is typically assigned to a charge migration and polarization process related to a specific phase or interface present within the composite. Here, we provide fundamental insights into the interrelation between impedance spectra and charge transport using electric network simulations. We demonstrate that for 3D composite electrolytes, the assignment of impedance contributions to individual phases or interfaces is often physically not justified. The spatial distribution of the alternating current in these materials depends on excitation frequency, mesostructure, and the electric and dielectric properties of all constituents and their interfaces. This leads to Maxwell–Wagner-type impedance contributions that have no simple correspondence to direct current conduction mechanisms, i.e., leads to impedance contributions that depend on the electric and dielectric properties of multiple phases. We discuss interrelations with classical equations of effective medium theory, e.g., the Maxwell–Wagner model, the impact of particle size, and practical implications for the impedance analysis of ceramic–polymer composite electrolytes as one important model case. Among others, the widely studied LLZO–PEO system is considered as a specific example. The conclusions drawn are general and extend to other heterogeneous systems like composite cathodes.
Kremer et al. (Wed,) studied this question.