Effect of Sulfur\nVacancies of CoNi<sub>2</sub>S<sub>4</sub> on Its Electrochemical\nPerformance in Hybrid Supercapacitors

Ternary cobalt nickel sulfides are considered promising\nelectrode\nmaterials due to their unique physical properties. However, its capacitive\nperformance is still limited by the insufficient material utilization\nefficiency. Here, we design and fabricate CoNi<sub>2</sub>S<sub>4</sub> with nanorods and hairy-petal-like nanosheets on nickel foam (NF)\nas an excellent self-standing electrode for a hybrid supercapacitor\n(HSC). The CoNi<sub>2</sub>S<sub>4</sub> electrode material was synthesized\non the NF substrate by cobalt organic framework (Co-MOF) conversion\nand introducing sulfur ion and nickel ion exchange. The CoNi<sub>2</sub>S<sub>4</sub> electrode material with sulfur vacancies was controlled\nby regulating the reduction time, and then electrochemical analysis\nand comparison were performed. The results demonstrate that the synergistic\neffect of the MOF-derived CoNi<sub>2</sub>S<sub>4</sub> skeleton and\nsulfur vacancies can significantly improve the electrochemical activity\nof nickel cobalt sulfide. The CoNi<sub>2</sub>S<sub>4</sub> electrode\nexhibits a superior high specific capacitance of 5.24 F/cm<sup>2</sup> at a current density of 3 mA/cm<sup>2</sup>. Furthermore, the assembled\nCoNi<sub>2</sub>S<sub>4</sub>-60//AC HSC displays a high energy density\nof 59.41 Wh/kg and a power density of 999.98 W/kg. Even after 10,000\ncontinuous charge–discharge cycles, its initial capacitance\nwas retained at 89.24%. These results demonstrate the feasibility\nand practicality of CoNi<sub>2</sub>S<sub>4</sub>-60 as an electrode\nmaterial, showcasing its potential for real-world applications.