With the growing demand on portable and resource-saving energy storage systems, multivalent-ion-based batteries with potentially higher energy densities and lower cost than the present-day lithium-ion batteries have attracted significant interest. Among these alternative next-generation batteries, rechargeable magnesium batteries (RMB) stand out due to the excellent characteristics of the Mg anode. Magnesium has a much lower standard reduction potential (-2.37 V vs SHE) than Zn and Al metals (-0.76 V and -1.66 V) and a volumetric capacity of 3833 mAh cm -3 which is almost twice that of Ca and Li and three times as much as Na. Furthermore, magnesium is highly abundant in the earth's crust (with 2.1% around 10 4 times more often than lithium) and suspected to be less prone to dendrite formation as anode, compared to lithium. To circumvent the drawbacks of liquid-based cell systems such as the passivation and corrosion of the Mg electrode, and ensure a lower susceptibility to dendrite formation as well as higher levels of safety, solid-state battery concepts are pursued. Here, one of the main challenges is to develop a solid electrolyte (SE) that has a sufficient Mg-ion room-temperature conductivity (σion >4-5 mS cm -1 ). The high charge density of Mg 2+ , compared to the Li + /Na + alkali counterparts, results in a much stronger Coulomb interaction in the solid hostframework, leading to typically sluggish Mg-ion mobility. For this reason, the design of the crystal structure is fundamental as several structural factors can mitigate the strong Coulomb interactions experienced by the Mg 2+ ion. In general, materials owning interconnected 3D pathways, such as NASICON and spinel compounds, provide better platforms to support long-range ion diffusion, while 2D and 1D pathways are more easily blocked by intrinsic antisite or extrinsic point defects. Compared to NASICON-structured compounds and other crystal structures, the spinel structure MgB2X4 (B = trivalent cation, X = chalcogenide) offers -apart from the 3D paths -a relatively large distance between the mobile Mg-cation in its transition state and the neighboring B-cations. This results in inherently weaker cation-cation repulsion in the activated state, possibly crucial to enabling multivalent-ion conduction even at ambient temperature. In parallel, previous studies have shown that fast ion motion can be achieved when the energetically stable site of the mobile ion is less favored, in terms of coordination, than the activated site. In the normal spinel structure, the Mg-ions, which typically strongly prefer the octahedral coordination environment, reside in tetrahedral sites (tet). During migration, the Mg-ions hop across an empty octahedral activated site (oct) to reach the next tetrahedral site which is expected to lead to a flattening of the energy profile along the tet-oct-tet migration path according to the Brønsted-Evans-Polanyi principle. Motivated by the features of the spinel structure, Canepa et al. studied a series of d 0 -metalbased MgB2X4 spinels (B = Sc, Y and In, X = S, Se, Te) by first principles-based nudged elastic band (NEB) calculations and predicted low migration barriers of Ea(th) = 0.36-0.42 eV in agreement with our own corresponding computational studies. Canepa et al. successfully synthesized MgSc2Se4 and determined a Mg 2+ migration barrier of Ea(NMR) = 0.37±0.09 eV by means of Mg static variable-temperature spin-lattice relaxation nuclear magnetic resonance (SLR NMR) measurements, consistent with their computed data. As a first approach to determine the ionic conductivity of MgSc2Se4, they performed electrochemical impedance spectroscopy (EIS) with ion-blocking electrodes and interpreted the spectra according to the conventional model for mixed conductors described by Jamnik and Maier. Later on, in our previous work, we critically discussed the shortcomings of this approach in case of MgSc2Se4 and presented an alternative way, allowing accurate and unequivocal characterization of the ionic conductivity. Specifically, a new kind of symmetrical transference cell is used, containing a pure Mg-ion conducting interlayer between the ion-blocking electrodes and the spinel pellet, to effectively suppress the electron transport. As a result, the partial ionic conductivity (σion = 2.4-5.5x10 -5 S cm -1 at roomtemperature) was well determined from high-quality impedance spectra using a modified equivalent circuit, describing the impedance of a mixed ion-electron conductor (MIEC) between charge carrier specific blocking electrodes and interlayers (adapted from previous works ). Furthermore, this approach, including electron-blocking interlayers, may also be an option to prevent self-discharge of a charged full cell due to the reported electronic conductivity (σel = ~10 -8 S cm -1 at room temperature) of the MgSc2Se4 SE. This selfdischarge is proposed to arise from increased carrier concentration caused by point defects, and/or the presence of electron conducting secondary phases. The latter seems to contribute to the electronic conductivity in any case, as Kundu et al. demonstrated that electronic transport occurs via a Berthelot-type mechanism of metal-type inclusions (e.g. Sc/ScSe), while Wang et al.'s attempts of compositional tuning to avoid point defects were rather unsuccessful. Recently, Koettgen et al. used density functional theory to calculate the Mg 2+ migration barriers and the stability of several MgB2X4 (B = lanthanide, X = S, Se) spinels. In their work, they demonstrated that the balance between a low Ea(th) and a stable spinel phase (with respect to competing AB2X4-type phases) is primarily given for the selenide spinels with the ionic radius r(Ln) of the lanthanides from Lu to Er [r(Lu-Er) = 0.861-0.890 Å; Ea(th, Lu-Er) = 0.369-0.358 eV]. These predicted stabilities agree well with experimental results (except for Ho). Thus, this group of lanthanide-based selenide spinels emerges as attractive candidate for Mg-ion solid electrolytes, not least due to the predicted slightly lower migration barriers compared to the prototype MgSc2Se4. We assumed that the electrochemical and theoretical study of this series of spinels will provide a deeper understanding of their transport properties. To this end, we report here the successful synthesis of four MgB2Se4 spinels (B = Sc, Er, Tm, Y) together with their structural characterization by X-ray diffraction and 25 Mg magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Furthermore, the partial conductivities as well as the Mg 2+ migration barriers of the spinels with mixed electronic/ionic nature are evaluated from electrochemical impedance and polarization measurements using charge carrier specific blocking electrodes and interlayers. Due to the good agreement of our experimental and computational predicted migration barriers, we extended our theoretical study based on periodic density functional theory (DFT) calculations to provide a deeper understanding on the structural details defining the migration barrier.