Controllable Morphology of Sea-Urchin-like Nickel–Cobalt Carbonate Hydroxide as a Supercapacitor Electrode with Battery-like Behavior

[Image: see text] Nickel–cobalt carbonate hydroxide with a three-dimensional (3D) sea-urchin-like structure was successfully developed by the hydrothermal process. The obtained structure enables the enhancement of charge/ion diffusion for the high-performance supercapacitor electrodes. The mole ratio of nickel to cobalt plays a vital role in the densely packed sea-urchin-like structure formation and electrochemical properties. At optimized nickel/cobalt mole ratio (1:2), the highest specific capacitance of 950.2 F g(–1) at 1 A g(–1) and the excellent cycling stability of 178.3% after 3000 charging/discharging cycles at 40 mV s(–1) are achieved. This nickel–cobalt carbonate hydroxide electrode yields an energy density in the range of 42.9–15.8 Wh kg(–1), with power density in the range of 285.0–2849.9 W kg(–1). The charge/discharge mechanism at the atomic level as monitored by time-resolved X-ray absorption spectroscopy (TR-XAS) indicates that the high capacitance behavior in a nickel–cobalt carbonate hydroxide is mainly dominated by cobalt carbonate hydroxide.