A Nanoparticle ZnMn₂O₄/Graphene Composite Cathode Doubles the Reversible Capacity in an Aqueous Zn‐Ion Battery

Abstract Zinc‐ion batteries (ZIBs) are promising grid‐scale energy storage devices owing to their low cost, high energy/power densities, high safety, benign environmental impact, etc. Among various cathode materials, ZnMn 2 O 4 spinel has attracted attention because of its high theoretical capacity (448 mAh g −1 ) associated with the two‐electron redox reaction of Mn ions (2+/4+), a higher voltage (≈1.4 V vs Zn/Zn 2+ ) than V 2 O 5 ‐based cathodes (≈1.0 V), and better cyclability among manganese oxide‐based cathodes. However, so far only the one‐electron reaction of Mn ions is used with ZnMn 2 O 4 spinel (≈224 mAh g −1 ), impairing its attractive features. In this study, the two‐electron reaction is successfully enabled by synthesizing ultrasmall ZnMn 2 O 4 spinel nanoparticles (≈5 nm) composited with graphene (US‐ZMO/G) via a rapid room‐temperature alcohol reduction process, achieving the reversible capacity of 445 mAh g −1 at the second cycle. As far as it is known, the US‐ZMO/G composite achieves the highest gravimetric energy/power densities among cathodes for ZIBs. The combination of high capacity and high voltage enables an outstanding energy density approaching that of lithium‐ion batteries.