MgB₂Se₄Spinels (B = Sc, Y, Er, Tm) as Potential Mg‐Ion Solid Electrolytes – Partial Ionic Conductivity and the Ion Migration Barrier

The magnesium chalcogenide spinel MgSc2Se4 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 Mg2+ migration barrier are yet not well understood. Here, results from a combined experimental and computational study on four MgB2Se4 spinels (B = Sc, Y, Er, Tm) are presented. The room-temperature ionic conductivities (σion = 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 Mg2+ migration barriers Ea(th) and Ea, respectively, further periodic density functional calculations are performed on the MgB2Se4 spinel system, revealing the role of trigonal distortion on the migration path geometry and Ea(th). These findings provide deeper understanding how to reach small Mg2+ migration barriers Ea in the MgB2Se4 spinels.