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

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, ZnMn2O4 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 (approximate to 1.4 V vs Zn/Zn2+) than V2O5-based cathodes (approximate to 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 ZnMn2O4 spinel (approximate to 224 mAh g(-1)), impairing its attractive features. In this study, the two-electron reaction is successfully enabled by synthesizing ultrasmall ZnMn2O4 spinel nanoparticles (approximate to 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.