Optimizing the electrons/ions diffusion kinetics in δ-MnO2 for realizing an ultra-high rate-capability supercapacitor

Abstract

δ-MnO2 nanosheets are promising cathode candidates for supercapacitors because of their cost-effective, environmental protection, and high theoretical capacity. Unfortunately, the commercial application of δ-MnO2 is hindered by the challenging issues of sluggish diffusion kinetic and poor rate-capability. It is urgent to optimize its kinetics behavior to improve the ion diffusion ability and electronic conductivity. Herein, one-step and one-phase synthesis of few-layer defective δ-MnO2 nanosheets (named δ-MnO2-CTAB) has been developed via a redox reaction between cetyltrimethylammonium bromide (CTAB) and KMnO4, and the kinetics storage mechanisms are investigated by DFT calculations. The migration barrier energies of Na in the surface or interlayer of δ-MnO2 (001) are reduced significantly with the reduction of layer-number (0.32 eV of 3 layers and 0.04 eV of 2 layers), which is conducive to the Na ion diffusion. Moreover, the generation of defects increases the density of states at the Fermi level of δ-MnO2, indicating the improvement of electronic conductivity. Therefore, when the current density is increased 50-fold (1 A g−1 to 50 A g−1), the rate-capability of δ-MnO2-CTAB is as high as 77%, exhibiting ultra-high rate-capability. This work unveils the kinetics storage mechanisms in few-layer δ-MnO2 with defects nanosheets for ultra-high rate-capability supercapacitors.