To be or not to be. Is MgSc${}_2$Se${}_4$ a Mg-ion solid electrolyte?

Magnesium batteries offer promising potential as next-generation sustainable energy-storage solutions due to the high theoretical capacity of the magnesium metal anode. Facilitating dendrite-free operation of metal anodes necessitates the development of solid electrolytes with high magnesium-ion conductivity. While the chalcogenide spinel MgSc${}_2$Se${}_4$ is predicted to exhibit high magnesium ion mobility, unequivocal experimental evidence for magnesium ion conduction beyond short-range motion is still missing. This study confirms magnesium-ion transport in MgSc${}_2$Se${}_4$ through two independent electrochemical methods: electrochemical deposition of magnesium metal and reversible magnesium plating/stripping cycling. To overcome the difficulty of measuring the ionic conductivity of the mixed conducting MgSc${}_2$Se${}_4$ spinel, a pure ion conducting interlayer is employed in a symmetric transference cell. This approach effectively suppresses the electron transport, allowing accurate characterization of the ionic conductivity. The experimental results confirm a low migration barrier of (386 ± 24) meV for magnesium ion transport in MgSc${}_2$Se${}_4$ and demonstrate one of the best performances at room temperature among the reported inorganic magnesium solid electrolytes. The findings open a new door for exploring additional mixed magnesium ion conductors and highlight the potential of magnesium chalcogenide spinels as a promising class of magnesium solid electrolytes. (© 2023 The Authors. Advanced Energy Materials published by Wiley‐VCH GmbH)