Magnesium batteries have been considered as sustainable energy-storage solutions beyond lithium-ion batteries due to the potential advantages of the Mg metal anode, such as high Mg earth abundance, low costs and high theoretical volumetric energy density.[1] To circumvent the drawbacks of passivation and corrosion of the Mg metal anode, and ensure higher levels of safety, solid-state battery concepts are being pursued. Nevertheless, the high charge density of the Mg²⁺-ion, compared to the Li⁺/Na⁺ counterparts, leads to strong coulombic interactions in the solid host-framework, making the development of suitable Mg-ion conductors quite challenging.

Among the potential candidates for inorganic Mg-ion solid electrolytes, the crystal structure of MgB₂X₄ spinels offers a relatively large distance between the B-cations and the mobile Mg-cation in its trigonal transition state along the tetrahedra-octahedra-tetrahedra Mg-ion migration path.[2] This results in inherently weaker cation-cation repulsion, possibly crucial to enabling multivalent-ion conduction even at ambient temperature. Moreover, computational investigations on the magnesium chalcogenides MgB₂X₄ (B = Sc, Er, Tm, Y; X = S, Se) predict low Mg²⁺ migration barriers (<415 meV), making these materials to be potential candidates as good Mg-ion conductors.[3,4] Especially, the compounds with larger B-ions (Er, Tm, Y) are likely more promising, since the shared trigonal face between tetrahedra and octahedra along the migration path is widened and therefore higher ionic conductivities can be expected. However, with exception of a handful practical work on MgSc₂Se₄,[4-5] no experimental conductivity studies on the other magnesium chalcogenide spinels exist, and an unequivocal electrochemical evidence for the Mg-ion transport in these materials is still missing.[6]

To address this challenge, we successfully synthesized five of the MgB₂X₄ spinels (B = Sc, Er, Tm, Y; X = S, Se) via solid-state reaction. The phase-purity of the prepared compounds was examined by X-ray powder diffraction combined with Rietveld refinements. DC polarization and electrochemical impedance measurements were performed to investigate the electronic partial conductivity. Additionally, we present the first electrochemical evidence for Mg-ion transport in the spinels using two independent methods, namely reversible Mg plating/stripping cycling and electrochemical deposition of Mg metal. To overcome the difficulty of measuring the ionic conductivity caused by the known mixed-conducting character, we inserted two pure ion-conducting interlayers in a symmetrical transference cell. Thus, the electron transport was effectively suppressed, allowing an accurate determination of the room-temperature ionic conductivities (5.5x10^–6-6.5x10^–5 S cm^–1) and the low Mg-ion migration barriers (426-381 meV) of the MgB₂X₄ spinels. These findings demonstrate the potential of the magnesium chalcogenide spinels as a promising class of solid electrolytes and open a new door for exploring additional mixed-conducting Mg-based phases.