To be or not to be - Is MgSc2Se4 a Mg-Ion Solid Electrolyte?

Abstract

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 MgSc2Se4 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 MgSc2Se4 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 MgSc2Se4 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 MgSc2Se4 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.