Interface-rich core-shell ammonium nickel cobalt phosphate for high-performance aqueous hybrid energy storage device without a depressed power density

Introducing diffusion-controlled battery materials to supercapacitors, can significantly enhance the energy density of supercapacitors, which however encounter depressed power density due to the intrinsic sluggish charge storage kinetics of battery materials. This problem can be efficiently solved by modifying the microstructure, and enable capacitive controlled charge storage mechanism in the battery materials. In this work, a novel interface-rich core-shell structure with (NH₄)(Ni,Co)PO₄·0.67 H₂O nanosheets @ single crystal microplatelets (NH₄)(Ni,Co)PO₄·0.67 H₂O (NCoNiP@NCoNiP) is constructed via a facile two-step hydrothermal method, taking advantage of etching induced Kirkendall effect and Ostwald ripening. This unique structure can enable the extrinsic pseudocapacitance by providing extra charges (e.g. holes, electrons or voids) on the interfaces, and realize synergy and fast charge storage. Specifically, a maximum specific capacity of 190.3 mAh g⁻¹ and ultrahigh rate performance with capacity retention of 96.1% from 1 to 10 A g⁻¹ in a three-electrode test. The kinetic analysis indicates that the electrochemical response of the hybrid battery-supercapacitor storage devices shows obvious characteristic of supercapacitor especially at high scanning rates. Simultaneously, the hybrid battery-supercapacitor devices based on NCoNiP@NCoNiP exhibits a high energy density of 44.5 Wh kg⁻¹ at a power density of 150 W kg⁻¹, which maintains 30 Wh kg⁻¹ at high power density of 7.4 kW kg⁻¹ with capacitance retention 77.5% after 7000 cycles. This work provides a novel strategy for the application of battery materials in high power devices, by enabling the capacitive charge storage mechanism of battery materials through nanostructure engineering.