Ingenious construction of hierarchical spherical nanostructures by in-situ confining Ni-Co-Mn hydroxide nanosheets inside/outside hollow carbon nanospheres for high-performance hybrid supercapacitors

Although the hollow nanostructures as supercapacitor electrodes possess great advantages, the huge interior void seriously hindered the enhancement of volumetric power and energy densities. In this work, a hierarchical spherical nanostructure has been successfully designed and constructed by confining Ni-Co-Mn hydroxide nanosheets inside/outside hollow carbon nanospheres. The hollow carbon nanospheres show unique spatial confinement effect and surface-confined effect, which can well control the size of the Ni-Co-Mn hydroxide nanosheets about 340 nm encapsulated inside hollow carbon nanospheres as well as the thickness of Ni-Co-Mn hydroxide nanosheets (similar to 13 nm) wrapped outside hollow carbon nanospheres. This ingenious hierarchical spherical nanostructure significantly enhanced the electrical conductivity and structural stability of Ni-Co-Mn hydroxide nanosheets as well as the packing density of electrode materials. Benefiting from the structural and compositional features, the as-obtained electrode achieves a high volumetric capacity (1455.2 C cm(-3)), favorable rate performance, and long cycle life. Impressively, the assembled hybrid supercapacitor device shows a high specific energy of 44.9 Wh kg(-1) at the 793.5 W kg(-1) and a high capacitance retention ratio of 91.8% after 10,000 cycles.