Oxide‐Type Positive Electrode Design Toward High‐Energy Rechargeable Magnesium Batteries

While rechargeable magnesium batteries (RMBs) promise high energy density, their room‐temperature operation is still limited; strong Mg 2+ –O 2− interactions suppress ion diffusion and complicate positive electrode evaluation. This concept review outlines a practical pathway coupling cell design with nanoparticle strategies. First, weakly coordinating‐anion electrolytes—especially Mg[ Z (hfip) 4 ] 2 ( Z = B, Al) with high oxidative stability—provide a fair baseline for rigorously verifying genuine Mg intercalation. On this foundation, the extreme downsizing strategy is summarized. Nanosized MgMn 2 O 4 operates when particle dimensions approach the Mg penetration depth, and composition control further reduces resistance and overpotential, as electronically conductive CuMn 2 O 4 nanospinels deliver higher capacities and rates. For rigid tunnel frameworks, ultrasmall, low‐aspect‐ratio α ‐MnO 2 shortens 1D diffusion paths, increases discharge capacity, and improves retention. Beyond size effects, geometry‐guided design motivates the exploration for host tunnels with the preferred Mg 2+ site; romanechite with asymmetric 3 × 2 channels enables reversible intercalation without phase transition or tunnel collapse. Looking ahead, nanoparticulation remains essential for realizing stable, high‐energy RMB positive electrodes operating at room temperature.