Solvothermal synthesis of hierarchical α-NiS particles as battery-type electrode materials for hybrid supercapacitors

Abstract To meet the increasing demand for electrochemical energy conversion and storage applications, the development of hierarchical materials with high electrochemical performance is surely desired. Herein, we report the synthesis of a hierarchical cabbage-like α-NiS material via a two-step process: solvothermal treatment followed by an annealing procedure. As the time of solvothermal treatment increased, the surface of the α-NiS material became increasingly rough until the structure finally collapsed. The optimum electrochemical performance of the α-NiS material resulted from 9 h of solvothermal treatment followed by sintering at 400 °C in argon. The primary structure of the material showed a solid spherical core with worm-like strips covering its surface. Nanometre-sized α-NiS particles collected in the grooves of the outside shell, which greatly increased the surface area. The subsequent sintering process improved the structural stability and electrochemical performance of the hierarchical material. This material exhibited outstanding electrochemical properties: 235.88 mAh g−1 at a current density of 1 A g−1; 143.10 mAh g−1 at 10 A g−1, and 87.1% retention after 2000 cycles at 5 A g−1. In addition, a hybrid supercapacitor was fabricated using a nickel sulfide cathode and reduced graphene oxide (rGO) anode, delivering an energy density of 48.67 Wh kg−1 at a power density of 800 W kg−1.