Superior Rate Capability of Crystalline Nickel–Cobalt Layered Double Hydroxide@Amorphous Nickel–Cobalt Phosphate Core–Shell Heterostructure for Aqueous Supercapacitors and Zn-Based Batteries

Abstract

In general, nickel–cobalt phosphate materials have a poor rate performance due to their structural collapse during a prolonged charging and discharging process. Rational design of three-dimensional core–shell heterostructures can effectively ameliorate the problems of structure collapse and morphology agglomeration of nickel–cobalt phosphate materials, contributing to improving the rate and cycle performance. Herein, a nickel–cobalt phosphate material was grown in situ on NiCo-LDH via a combined solvothermal and template method. The resulting product, denoted as NiCo-LDH@NCP, exhibits a core–shell heterostructured hollow nanosphere morphology. This core–shell structure can not only retain the spherical frame of the NiCo-LDH material, provide a larger growth area of nickel–cobalt phosphate materials, and increase the proximity of the connection between NiCo-LDH and nickel–cobalt phosphate materials, but also shorten the diffusion distance of electrolyte ions, promote the transport rate of ions, and enhance the mechanical support stability of electrode materials. Density functional theory (DFT) calculations reveal that the work function difference between NiCo-LDH and NCP induces significant band bending, leading to the formation of a Mott–Schottky heterointerface. This interfacial engineering results in spontaneous charge redistribution within the NiCo-LDH@NCP composite, generation of a built-in electric field, reduction of reaction energy barriers, and the acceleration of surface redox kinetics. The optimized electrode materials (NiCo-LDH@NCP-2 h) show a high specific capacitance of 1615.6 F g–1 at a current density of 1 A g–1 and retain 81.71% of their initial capacitance after 3000 cycles at 10 A g–1. Besides, the assembled aqueous asymmetric supercapacitor (NiCo-LDH@NCP-2 h//AC) delivers an energy density of 27.56 Wh kg–1 at a power density of 399.42 W kg–1. The NiCo-LDH@NCP-2 h//Zn battery displays a discharge capacity of 158.67 mAh g–1 at 1 A g–1 and an 81.2% capacity retention rate even at 20 A g–1, indicating its ultrahigh rate capability.