Abstract: Hybrid battery cells combining liquid electrolytes (LEs) with inorganic solid electrolyte (SE) separators or different SEs and polymer electrolytes (PEs), respectively, are developed to solve the issues of single-electrolyte cells. Among the issues that can be solved are detrimental shuttle effects, decomposition reactions between the electrolyte and the electrodes, and dendrite propagation. However, the introduction of new interfaces by contacting different ionic conductors leads to other problems, which cannot be neglected before commercialization is possible. The interfaces between the different types of ionic conductors (LE/SE and PE/SE) often result in significant charge-transfer resistances, which increase the internal resistance considerably. This review highlights studies evaluating the interfacial resistances and activation barriers in such systems to present an overview of the issues still hampering hybrid battery systems. The interfaces between different SEs in hybrid all-solid-state batteries (SSBs) are considered as well. In addition, a short summary of physicochemical models describing heteroionic interfaces—interfaces between two different ion conductors—is given in an attempt to explain high interface resistances. In doing so, we hope to inspire future work on the crucial topic of interface optimization toward better SSBs. Graphic Abstract: [Figure not available: see fulltext.].
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