Novel Co-doped nickel hydroxyfluorides with rapid electron transfer for high-performance supercapacitors

Abstract

Transition metal electrode materials are promising for applications in high-energy and power-density rechargeable energy devices. However, their poor intrinsic conductivities and limited redox kinetics hinder the rapid energy supply at high current densities. Herein, a series of Co-doped nickel hydroxyfluorides (Co-Ni(OH)F) was constructed as new electrode materials to maximize the charge storage ability at different current densities. The introduction of Co did not only induce abundant defects for regulating the electronic structure but also fine-tuned the surface morphology and shortened the ion/electron diffusion distance. The coexistence of multiple oxidation states of Ni and Co species provided abundant redox processes and effectively enhanced the electrochemical activities. The relationship between Co2+ doping and the increase in conductivity of the system was further confirmed by the calculated density of states (DOS). The optimized Co-Ni(OH)F (Co=20%) electrode material exhibited an ultrahigh specific capacity of 3380.2 F g−1 at a current density of 1 A g−1. Even at an enhanced current density of 20 A g−1, the optimized electrode showed a superior capacity retention rate of 78.4%. After 5000 cycles at a high current density of 30 A g−1, the electrode retained 76.3% of its original capacity. Furthermore, the Co-Ni(OH)F (Co=20%)//Bi2O3 asymmetric device delivered an ultrahigh density of 139.8 Wh kg−1 at a power density of 800 W kg−1, suggesting superior energy density than previously reported systems. Overall, new insights into the rational design of novel high-capacity and high-rate electrode materials were provided, promising for the construction of advanced energy devices.