showed a negligibly low specific capacity. This can probably be explained by the results of the LSV at room-temperature which confirm that the spinels are not redox active between at least -1.1 V and 3.7 V and perhaps even beyond, in contrast to the report by Wang et al . In addition, we found that all MgB2Se4 spinels studied in this work enable reversible Mg plating and stripping at 60 °C in a symmetric Mg|UiO66-MgIL|MgB2Se4|UiO66-MgIL|Mg cell. As shown in Figure , the cells were initially activated by 50 cycles of alternating plating and stripping with a current density of ±1.57 µA cm -2 and a dwell time of 30 min per step (plated charge amount of 0.785 µAh cm -2 , corresponding to 2 nm of Mg per cycle assuming homogeneous plating/stripping). Afterwards, the step time was increased to 10 h and further 20 cycles were recorded (15.7 µAh cm -2 , corresponding to 41 nm of Mg). During the cycling process stable Mg plating and stripping with a symmetrical overpotential of about ±0.4 V was demonstrated for the cells containing the spinels with a phase purity ≥95% (MgSc2Se4, MgTm2Se4, MgEr2Se4). The performance is better and more symmetrical than for the reference cell itself (±0.5 V), possibly due to the separation of the ionogel layer in the cell by a homogenous solid-state spinel pellet. Nevertheless, note that the overpotential generally exceeds the estimated IR drops of 0.62-2.58 mV across the spinels, calculated from their ionic conductivities at 60 °C, indicating interface-dominating resistances inside the symmetrical cells. MgY2Se4 and e) without spinel pellet by applying a current density of ±1.57 µA cm -2 , which corresponds to a plated charge amount of ±0.785 µAh cm -2 and 1.57 µAh cm -2 , respectively.