High Room-Temperature Magnesium Ion Conductivity in Spinel-Type MgYb₂Se₄ Solid Electrolyte

We present three magnesium selenide spinels, MgSc0.4Y0.4Er0.4Tm0.4Yb0.4Se4, Mg0.75Sc2Se3.5Br0.5 and MgYb2Se4, as potential solid electrolytes (SE) for magnesium batteries. In particular, the latter spinel exhibits a room-temperature ionic conductivity exceeding 10–4 S cm–1 and a low Mg2+ migration barrier of 364 meV. The high ionic mobility is attributed to the low magnesium insertion energy revealed by density functional theory (DFT), which indicates a weak binding interaction between magnesium ions and the host lattice. Furthermore, like the two multicomponent spinels, MgYb2Se4 exhibits lower electronic conductivity compared to previously studied MgB2Se4 spinels (B = Sc, Y, Er, Tm) and a good electrochemical stability, making it a strong candidate for Mg2+ SE applications.