Data for the publication "To be or not to be – Is MgSc2Se4 a Mg-Ion Solid Electrolyte?"

<strong>Description Datasets</strong> Datasets are obtained from X-ray diffraction (XRD), Rietveld analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), nuclear magnetic resonance (MAS NMR) spectroscopy, absorption and photoluminescence spectroscopy, electrochemical impedance spectroscopy (EIS), chronoamperometry (CA) and chronopotentiometry (CP). <strong>Abstract</strong> 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 (SEs) 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 (<em>E</em>_a) 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.