Hierarchical walnut-like Ni0.5Co0.5O hollow nanospheres comprising ultra-thin nanosheets for advanced energy storage devices

Abstract

Hierarchical walnut-like Ni_0.5Co_0.5O hollow nanospheres (NCO-HNS) were successfully synthesized via a facile and effectual hard-template method. Layered Ni_0.5Co_0.5O nanosheets stacked by several ultra-thin layers with the thickness of 2–3 nanometers were self-assembled into the hollow nanospheres. This unique hierarchical architecture, including micro-, meso-, and macropores, could provide a large specific surface area (123.7 m² g⁻¹) and efficient channel for the diffusion of ions and electrons, as well as the penetration of electrolyte. Significantly, the as-prepared NCO-HNS exhibit an excellent specific capacity of 221.9 mA h g⁻¹ at the current density of 1 A g⁻¹, a remarkable rate capability (172.8 mA h g⁻¹ at 20 A g⁻¹) and capacity retention (99.4% after 3000 cycles). Moreover, the NCO-HNS was then successfully fabricated into hybrid devices with active carbon. These devices deliver a maximum energy density of 38.3 W h kg⁻¹ (31.8 W h L⁻¹) at the power density of 743.5 W kg⁻¹. Note that because of the morphology and hierarchical architecture, the energy density of the device could still maintain 19.3 W h kg⁻¹ (16.0 W h L⁻¹) even at an ultra-high power density of 7604.9 W kg⁻¹. These hierarchical walnut-like Ni_0.5Co_0.5O hollow nanospheres comprising layered nanosheets may have potential as battery-type electrode materials for advanced energy storage devices.