To measure the ionic conductivity, a home-made battery cell casing was used, as introduced in an earlier work. Different amounts (160, 220 and 280 mg) of MgB2Se4 powder were filled in the PEEK housing of the cell with a diameter of 10 mm. Then, the powders were pressed between two stainless-steel stamps (SS) at 3 t for 3 min to create a pellet inside the housing. Two purely Mg-ion conducting interlayer were added by pressing 40 mg of an ionogel SE, denoted as UiO66-MgIL, on both sides of the MgB2Se4 pellet under previous conditions. The ionogel SE consisted out of a Mg-ion conducting metal-organic framework (MOF) structure UiO-66 impregnated in a mass ratio of 1:1.25 with a 1M Mg(TFSI)2 ionic liquid based on [EMIM][TFSI], as described in an earlier report. A constant pressure was applied by the means of the screw of aluminum framework around the homemade cell casing with 10 Nm torque. The impedance spectra of the SS|UiO66-MgIL|MgB2Se4|UiO66-MgIL|SS cells were recorded between 0 °C and 60 °C in a frequency range from 3 MHz to 100 mHz with an AC amplitude of 10 mV. After evaluation of the ionic resistances of the MgB2Se4 spinels, the ionic conductivities were calculated according to Equation 1. Then, the Mg-ion migration barriers (Ea) were obtained by rearrangement of the Arrhenius equation (Equation ):