Lithium ion and oxygen ion based electrochemical cells are of outstanding importance in solid state ionics, and particularly lithium ion batteries (LiBs) but also solid oxide cells (SOCc) have gained tremendous relevance for a more sustainable energy economy. Interestingly, the concepts to investigate and interpret materials properties in such Li⁺ or O^2- based devices are often different, despite all the similarities of the fundamental atomistic processes. For example, defect chemical considerations are very common for SOC materials, but rarely used for LiB materials. Chemical storage of a formally neutral species, on the other hand, is key in LiBs but much less applied in SOCs; only very recently an oxygen ion battery (OiB) has been introduced, based on voltage driven oxygen insertion into oxides [1].

In this contribution, the so-called chemical capacitance [2] is shown to be a measurable quantity which is highly useful for interpreting and understanding materials properties in both “worlds” of solid state ionics, in the Li world as well as the oxygen world. First, the meaning of the chemical capacitance in mixed conducting materials is recapitulated, which often gives a direct measure of minority charge carrier concentrations. In the next step, it is demonstrated how the chemical capacitance can be determined by means of impedance measurements and data analysis with transition line models. Then, the dependence of the chemical capacitance on the exact electrode composition (charging state) and its defect chemical interpretation are used to understand measured electrode potentials and thus charge-discharge curves of battery materials in both “worlds”. In the case of LiBs, it is shown that, for example, already the consideration of site restriction in defect chemical models allows us to understand peaks of measured chemical capacitances and thus shapes of charging curves of spinel-type cathodes (e.g. LiMn₂O₄) [3]. Also the relation between the chemical capacitance and the Li chemical diffusion coefficient is discussed. For OiBs, on the other hand, the chemical capacitances are related to Brouwer-diagrams of electrode materials (e.g. (La,Sr)FeO₃, (La,Sr)(Cr,Mn)O₃, SrTiO₃) and subsequently connected to the electrode potentials of these materials, when being used as electrodes in OiBs. Thus, chemical capacitance measurements can help identifying promising novel electrode materials. Finally, it is shown that also oxygen in the gas phase can act as a chemical capacitor which exhibits a peak when increasing the chemical potential due to activity reasons [4].