Room Temperature Operation of Magnesium Rechargeable Batteries with a Hydrothermally Treated ZnMnO<sub>3</sub> Defect Spinel Cathode

The current worldwide energy problems resulting from global environmental issues require highly efficient, environmentally benign energy storage technologies. Among various innovative battery types, rechargeable batteries based on magnesium (Mg) metal anodes represent an attractive option because of their large volumetric/gravimetric capacities and the cost-effectiveness of the base Mg metal. To realize practical Mg batteries, intense efforts have been focused on the development of cathode and electrolyte materials. Defect spinel oxides represent an emerging class of cathode active materials. The representative compound ZnMnO3 has sufficient capacities for superlong cycles at elevated temperatures, owing to the suppression of the undesired spinel-rocksalt phase transition. To further enhance the electrochemical performance of ZnMnO3, hydrothermal treatment was applied to synthesize fine ZnMnO3 nanoparticles. By controlling the pH of precursor solutions, treatment temperature, and reaction duration, fine ZnMnO3 nanoparticles with a remarkably large surface area can be obtained. Our ZnMnO3, prepared using hydrothermal treatment, was able to deliver larger capacities compared with those obtained using the typical coprecipitation method. Furthermore, the hydrothermally treated ZnMnO3 allowed stable battery cycling even at 30 °C; this was achieved by combining the highly efficient, electrochemically stable electrolyte and the ZnMnO3 nanoparticles with the shortened diffusion path.