Facile synthesis of mesoporous Ni x Co 9- x S 8 hollow spheres for high-performance supercapacitors and aqueous Ni/Co-Zn batteries.

Porous micro/nanostructure electrode materials have always contributed to outstanding electrochemical energy storage performances. Co 9 S 8 is an ideal model electrode material with high theoretical specific capacity due to its intrinsic two crystallographic sites of cobalt ions. In order to improve the conductivity and specific capacitance of Co 9 S 8 , nickel ions were introduced to tune the electronic structure of Co 9 S 8 . The morphology design of the mesoporous hollow sphere structure guarantees cycle stability and ion diffusion. In this work, Ni x Co 9- x S 8 mesoporous hollow spheres were synthesized via a facile partial ion-exchange of Co 9 S 8 mesoporous hollow spheres without using a template, boosting the capacitance to 1300 F g -1 at the current density of 1 A g -1 . Compared with the pure Co 9 S 8 and Ni-Co 9 S 8 -30%, Ni-Co 9 S 8 -60% exhibited the best supercapacitor performance, which was ascribed to the maximum Ni ion doping with morphology and structure retention, enhanced conductivity and stabilization of Co 3+ in the structure. Therefore, Ni/Co-Zn batteries were fabricated by using a Zn plate as the anode and Ni-Co 9 S 8 -60% as the cathode, which deliver a high energy density of 256.5 W h kg -1 at the power density of 1.69 kW kg -1 . Furthermore, the Ni/Co-Zn batteries exhibit a stable cycling after 3000 repeated cycles with capacitance retention of 69% at 4 A g -1 . This encouranging result might provide a new perspective to optimize Co 9 S 8 -based electrodes with superior supercapacitor and Ni/Co-Zn battery performances.