An Electrically Conductive CuMn₂O₄ Ultrananospinel Cathode for Room-Temperature Magnesium Rechargeable Batteries

Magnesium rechargeable batteries are potential successors to lithium-ion batteries, owing to their low cost, superior safety, and high volumetric energy density. However, the development of high-energy and high-rate cathode materials remains challenging. Oxide-type cathodes, specifically spinels, have become a focus of attention due to their higher voltage and operation capacity. Nevertheless, previous studies have predominantly centered on high-temperature operations, on account of the sluggish diffusion of Mg ions in solids and low electrical conductivity. In this study, an electrically conductive CuMn₂O₄ ultrasmall (<5 nm) spinel is fabricated using an alcohol reduction process. This “ultrananospinel” shows a semireversible phase transition along with Mg insertion/extraction and a dual-redox system involving copper and manganese ions, exhibiting the high voltage operation (>1.5 V) with a theoretical discharge capacity of 225 mAh g^-1 and high-rate capability compared with other oxide-type cathodes.