In-situ evolution of CoS/C hollow nanocubes from metal-organic frameworks for sodium-ion hybrid capacitors

Alkaline ion hybrid capacitors bridge the performance gaps between batteries and supercapacitors by combining their merits in terms of energy and power densities. However, the poorly matched electrochemical kinetics between anodes and cathodes leads to reduced energy density and inadequate material utilization. Herein, a hybrid composite has been synthesized by the treatment of a cobalt-based metal–organic-framework (ZIF-67 nanocube), involving the chemical etching-coordination reaction and subsequent carbonization together with sulfidation treatment. The as-prepared material with a hollow structure is composed of nanosized CoS nanoparticles uniformly encapsulated in the mesoporous shells of carbon hollow nanocubes (CoS/C HC). The unique architecture endows CoS/C HC with abundant electrochemical active interfacial sites, good mechanical robustness, and effectively alleviated volume variation. CoS/C HC as electrode material exhibits distinguished sodium storage capabilities relating to specific capacity and prolonged cycling durability at high current densities. Moreover, a sodium-ion hybrid capacitor assembled by using CoS/C HC as anode and activated carbon as cathode delivers an energy density of 199 Wh kg−1 and a power density of 7113 W kg−1. The novel electrode material makes the hybrid capacitors more commercially possible by increasing the utilization of active material and improving structural stability.