The introduction of oxygen vacancy defects in Al-doped transition metal silicates derived from fly ash for high-performance aqueous potassium ion capacitor

Abstract

Potassium ion capacitors with both battery and capacity properties are treated as potential energy storage devices. Nevertheless, the large radius of K+ causes problems of kinetic slowness and volume expansion for electrode materials. Transition metal silicates are considered as promising electrode materials because of their high theoretical capacity, environmental friendliness, and abundant sources. Herein, nickel silicate is innovatively prepared from fly ash. Notably, fly ash contains Al2O3 besides SiO2. It is found that Al plays a significant role in maintaining structural stability. Moreover, oxygen vacancies are introduced in nickel silicate, which remarkably increase the specific capacity via exposing more active sites and enhancing the adsorption of potassium ions. Furthermore, the morphology transformation of nickel silicate prepared at different temperatures and the migration process of Al at different stages have been studied deeply. The electrode with oxygen vacancies and Al-doped has the highest specific capacity (263.95 mAh g−1 at 0.5 A g−1) and long cycle life (87.94% capacity retention after 10000 cycles). The assembled device combines the characteristics of a supercapacitor and a metal ion battery with an outstanding energy density of 105.00 Wh kg−1 at a power density of 2.52 kW kg−1. This work not only obtains a potassium ion capacitor with high energy density and excellent stability but also offers a reference for the green utilization of fly ash.