Substitutional Design of Nanosized α-MnO ₂ Cathode for Rechargeable Magnesium Battery

Rechargeable magnesium batteries (RMBs) are promising next-generation batteries, yet realizing high energy density calls for high-voltage oxide cathodes. α-MnO2 is an attractive oxide host because it offers high theoretical capacity and high Mn3+/Mn4+ redox potential together with a rigid tunnel framework that can reversibly accommodate multivalent ions. In practice, however, Mg2+ transport in α-MnO2 is sluggish; strong Coulombic interactions between Mg2+ and O2− kinetically trap Mg2+ and cause a large polarization, limiting the reversible capacity. Here, the substitution chemistry directly governs oxygen-vacancy formation, crystallinity, and ultimately Mg2+ migration in nanosized α-MnO2. Using an alcohol reflux process, nanosized Al-, Ga-, In-, Ti-, and Cr-substituted α-MnO2 cathodes, which modulate the structure and electronic states and thereby alter the electrochemical kinetics, are synthesized. Among them, Ti-substituted α-MnO2 delivers faster Mg transport, reduced overpotential, and improved capacity retention at room temperature. Density functional theory calculations further indicate a reduced Mg migration barrier near Ti sites, consistent with the observed kinetic enhancement. The appropriate heteroatom substitution is a key design principle for multivalent ion cathodes, where effective dopants enable defect control and locally reduce migration barriers.