As interlayer, the Mg-ion ionogel electrolyte UiO66-MgIL (σel = 1.7x10 -10 S cm -1 ) was used, consisting of the metal-organic framework (MOF) structure UiO-66 impregnated with an 1M Mg(TFSI)2-[EMIM][TFSI] ionic liquid. Figure shows the exemplary Nyquist plots based on room-temperature EIS measurements of the SS|UiO66-MgIL|MgB2Se4|UiO66-MgIL|SS cells with MgY2Se4 and without spinel pellet (reference cell). The Nyquist plot of the SS|UiO66-MgIL|SS reference cell exhibits only a tail, relating to an imperfect capacitor, as the expected semicircle at higher frequencies cannot be resolved due to the upper limit of the frequency range (1 MHz) of the used equipment. For this reason, a simplified equivalent circuit without constant phase element (CPE) in parallel to the resistance was chosen (see Figure ) to determine the ionic resistance R1ion of the UiO66-MgIL. Applying Equation , for the UiO66-MgIL-2 batch, used in this work, a room-temperature ionic conductivity σion(UiO66-MgIL-2) = 2.8x10 -4 S cm -1 is evaluated, slightly higher than for the UiO66-MgIL-1 batch (σion = 9.5•10 -5 S cm -1 ) taken for the investigation of MgSc2Se4. This small difference can probably be attributed to small, hardly avoidable differences in the MOF pore size or conductive salt (Mg[TFSI]2) concentration. Figure shows the equivalent circuit used for fitting the impedance spectra of all sandwich-type cells, where three contributions are connected in parallel: the ionic charge transport in the spinel R2ion, the electronic charge transport in the spinel R2el including the electron blockage CPE2e1 at the UiO66-MgIL interlayer, and the geometrical capacitance CPE2geo. This part of the model is a modification of the typical circuit used for describing the impedance of a MIEC between ion-blocking electrodes. The previous circuit elements in parallel, R1ion and CPE1ion, represent the UiO66-MgIL interlayers, while CPE3e2 represents the ion-blocking stainless-steel electrodes. Experimental data for MgSc2Se4 reproduced under terms of the CC BY-NC-ND 4.0 license. Copyright © 2023, C. Glaser et al., Adv. Energy. Matter. 2023, published by Wiley-VCH GmbH.