MgB ₂ Se ₄ Spinels (B = Sc, Y, Er, Tm) as Potential Mg‐Ion Solid Electrolytes – Partial Ionic Conductivity and the Ion Migration Barrier

Abstract The magnesium chalcogenide spinel MgSc ₂ Se ₄ with high Mg‐ion room‐temperature conductivity has recently attracted interest as solid electrolyte for magnesium ion batteries. Its ionic/electronic mixed‐conducting nature and the influence of the spinel composition on the conductivity and Mg ²⁺ migration barrier are yet not well understood. Here, results from a combined experimental and computational study on four MgB ₂ Se ₄ spinels (B = Sc, Y, Er, Tm) are presented. The room‐temperature ionic conductivities ( σ _ion = 2 × 10 ⁻⁵ –7 × 10 ^–5 S cm ⁻¹ ) of the spinels are accurately measured, as electron transport is effectively suppressed by purely Mg‐ion conducting electrode interlayers. Using the same approach, reversible Mg plating/stripping as well as good electrochemical stability are achieved. Driven by the good accordance of the computationally and experimentally obtained Mg ²⁺ migration barriers E _a (th) and E _a , respectively, further periodic density functional calculations are performed on the MgB ₂ Se ₄ spinel system, revealing the role of trigonal distortion on the migration path geometry and E _a (th). These findings provide deeper understanding how to reach small Mg ²⁺ migration barriers E _a in the MgB ₂ Se ₄ spinels.