Design and synthesis of K-doped tremella-like δ-MnO2 for high-performance supercapacitor

The development of K-doped δ-MnO2 is an effective strategy to achieve a significant improvement in capacitance for MnO2-based supercapacitors. In this paper, we prepared a series of K-doped δ-MnO2 through the redox reaction between KMnO4 and MnCl2·4H2O with the assistance of KOH and acetone. The morphology and K content can be easily controlled by adjusting the amount of MnCl2·4H2O. The optimal sample not only has a tremella-like structure assembled from leaf-like nanosheets but also has the highest K content. This fascinating structure has following advantages: 1) The two-dimensional nanosheets can provide a large surface (specific surface area of 92.7 m2 g−1) for ion adsorption and ion diffusion; 2) The three-dimensional structure can avoid the accumulation of nanosheets, and its robust structure is difficult to be destroyed, which is conducive to improving cycling stability. Combining the advantages of the presence of large amounts of K+, i.e., the significant increase in electrical conductivity and ion intercalation sites, the sample exhibited superior electrochemical performance, such as the specific capacitance of 726.0 and 283.7 F g−1 at 1.0 A g−1 in KOH electrolyte and Na2SO4 electrolyte, respectively. Furthermore, the assembled K-doped δ-MnO2//activated carbon asymmetric supercapacitor with KOH electrolyte displayed an energy density of 24.38 Wh kg−1, whereas that with Na2SO4 electrolyte displayed a high energy density of 45.64 Wh kg−1.