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

Abstract

While rechargeable magnesium batteries (RMBs) promise high energy density, their room-temperature operation is still limited; strong Mg²⁺–O²⁻ 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 MgMn₂O₄ operates when particle dimensions approach the Mg penetration depth, and composition control further reduces resistance and overpotential, as electronically conductive CuMn₂O₄ nanospinels deliver higher capacities and rates. For rigid tunnel frameworks, ultrasmall, low-aspect-ratio α-MnO₂ 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 Mg²⁺ 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.