A Nanoparticle ZnMn<sub>2</sub>O<sub>4</sub>/Graphene Composite Cathode Doubles the Reversible Capacity in an Aqueous Zn‐Ion Battery
Yuto Katsuyama, Chie Ooka, Ruijie Zhu, Reona Iimura, Masaki Matsui, Richard B. Kaner, Itaru Honma, Hiroaki Kobayashi
Abstract
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.