Hierarchical NiCo2O4 Nanosheets@halloysite Nanotubes with Ultrahigh Capacitance and Long Cycle Stability As Electrochemical Pseudocapacitor Materials

One-dimensional hierarchical nanostructure of NiCo2O4 nanosheets@halloysite nanotubes was successfully prepared through a facile coprecipitation method followed by a thermal annealing treatment. The microstructure and chemical composition of NiCo2O4 nanosheets@halloysite nanotubes are investigated by SEM, TEM, HRTEM, XRD, and XPS. The specific capacitance of the unique NiCo2O4 nanosheets@ halloysite nanotubes is 1728 F g(-1) at the end of 8600 cycles when the charge-discharge current density is 10 A g(-1) leading to only 5.26% capacity loss. Broadly, the as-obtained NiCo2O4 nanosheets@halloysite nanotubes reveal ultrahigh capacitance and remarkable cycling stability in virtue of the ultrathin and hierarchical nanosheets and intense cation/anion exchange performance of halloysite.