Construction of a novel hierarchical structured NH4-Co-Ni phosphate toward an ultrastable aqueous hybrid capacitor

Abstract

Aqueous hybrid capacitors (HCs) suffer from sacrificed power density and long cycle life due to the insufficient electric conductivity and poor chemical stability of the battery-type electrode material. Herein, we report a novel NH₄-Co-Ni phosphate with a stable hierarchical structure combining ultrathin nanopieces and single crystal microplatelets in one system, which allows for a synergistic integration of two microstructures with different length scales and different energy storage mechanisms. The microplatelets with a stable single crystal structure store charge through the intercalation of hydroxyl ions, while the ultrathin nanopieces store charge through surface redox reaction providing enhanced specific capacitance. Furthermore, the large single crystal can bridge the small nanopieces forming continuous electronic conduction paths as well as ionic conduction channels, and facilitate both electron and ion transportation in the hierarchical structure. The HC cell based on the as prepared material and a 3D hierarchical porous carbon delivers a high energy density of 29.6 Wh kg⁻¹ at a high power density of 11 kW kg⁻¹. Particularly, an ultralong cycle life along with 93.5% capacitance retention after 10 000 charge–discharge cycles is achieved, which is outstanding among the state-of-the-art aqueous HC cells.