Ultrathin inverse opal manganese dioxide nanosheets of a few layers as high-performance cathodes for aqueous zinc-ion batteries

Abstract

Zinc ion batteries (ZIBs) have high potential to be used as future energy storage devices as they have higher safety, lower cost of production and high availability of material. However, the performance of current ZIB is currently inadequate to cope with the increasing energy consumption due to the lack of suitable cathode materials. In this experiment, a solution template method was used to synthesize an ultrathin inverse opal manganese dioxide nanosheets that consist of a few layers. The experiment was carried out at mild temperature. Thickness of 1 nm could be obtained and no aggregation was observed which indicates that the nanosheets synthesized were separated well. Excellent performance was achieved when a combination of both the inverse opal structure and the nanosheets were used as cathode material for ZIBs. When cycled for 100 times using a current density of 300 mAg-1, specific capacity of 262.9 mAg-1 was retained. This indicates capacity retention was 95.6 % which is very high. When cycled for 5000 times using a higher current density at 2000 mAg-1, a high specific discharge capacity of 121 mAg-1 could be obtained. During charging and discharging, zinc sulfate hydroxide hydrate was formed on the cathode and this formation process is reversible. At the same time, the layered birnessite in-plane crystal structure can be preserved. Both of these are indicated by the selected-area electron diffraction patterns (SAED), Ex-situ X-ray diffraction patterns (XRD) and high-resolution transmission electron microscopy (HRTEM) that was carried out. This MnO2 structure has high potential to be used in a cathode for long-term aqueous zinc-ion batteries with high capacity and high rate capability.