Effective 3D open-channel nanostructures of a MgMn2O4 positive electrode for rechargeable Mg batteries operated at room temperature

Abstract

Room-temperature operations of rechargeable Mg coin-cell batteries have been achieved using a Mg alloy negative electrode and a spinel MgMn₂O₄ (MMO)-positive electrode. The present work focuses on clarifying the effects of the physiochemical properties of the MMO powder including the specific surface area (S_BET) and porosity of the positive electrode on Mg battery performances in practical applications. Finally, optimal specific surface area and porosity parameters were obtained that ensured excellent battery performances as a standard coin-cell at room temperature. A typical MMO powder synthesized using a modified sol–gel method with propylene oxide-driven complex polymerization had a large S_BET > 200 m² g⁻¹, and more than 90% porosity with a triple-tiered 3D open-channel network. Here we evaluated the discharge capacity and the cyclability for room-temperature operation as a function of S_BET in a full cell with a Mg negative electrode as well as half cells with a carbon graphite electrode. Irrespective of whether half cells or a full cell was used, the initial discharge capacity was found to depend linearly on the S_BET of the porous MMO powder in a Mg tetrakis(hexafluoroisopropyl)borate/triglyme electrolyte with a wide potential window, ΔE > 3.6 V. Eventually, the maximum discharge capacity of 220 mA h g⁻¹ was realized at 25 °C in the full cell using the 3D open-channel nanostructure with S_BET = 236 m² g⁻¹. The cyclability in the full cell with the Mg alloy negative electrode, however, degraded with the increasing S_BET, while no cyclability degradation was observed in the half cell with the carbon electrode. A possible mechanism is discussed regarding passivation of the Mg alloy electrode in discharge/charge cycles.