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 (~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.