Although the spinels show reversible Mg plating/stripping cycling and an electrochemical stability till 3.7 V to be suitable in combination with the most transition metal chalcogenide cathodes, their room-temperature ionic conductivity (Figure ) still does not reach the order of mS cm -1 . However, it should be noted that during impedance fitting of the SS|UiO66-MgIL|MgB2Se4|UiO66-MgIL|SS cells, the contribution of impurities and interfacial resistances could not be extracted from the ionic resistances of the spinels. Accordingly, the actual ionic conductivity could be higher. Moreover, the spinel's potential is far from being exhausted, as some examples of other solid electrolytes have shown that the ionic conductivity can be increased by compositional tuning. Another criterion for the application of the spinels as a solid electrolyte remains their electronic conductivity. The electron transference numbers [tel(MgSc2Se4) = 8x10 -2 %, tel(MgTm2Se4) = 5x10 -1 %, tel(MgEr2Se4) = 6%, tel(MgY2Se4) = 19%] are more than two orders of magnitude higher than the 10 - -10 -4 % of typical solid electrolytes, that increases the risk of self-discharge of a battery. Here, impurities of binary compounds, such as BSe and B2Se3 (B = Sc, Y, Er, Tm), may play a role for the electronic transport in these materials, as we found that the electronic conductivity increases with the binary phases in the investigated spinels. Thus, there is still the potential to make the spinels applicable by minimizing the electronic conductivity due to optimization of the synthesis and the purity of precursors. Additionally, compositional tuning or the development of electron-blocking interlayers and coatings may be further options.