Outstanding performances of Ni₂CoS₄/expanded graphite with ultrafine Ni₂CoS₄ particles for supercapacitor applications

Nickel sulfides are desirable electrode materials for supercapacitors, while low electronic conductivity and poor cyclic stability restrict their wide applications. Herein, Ni₂CoS₄/expanded graphite (Ni₂CoS₄/EG) composite was prepared in mixed solvents of ethylene glycol and H₂O via a rapid and energy-saving microwave heating method. Scanning transmission electron microscopy image shows that Ni₂CoS₄ particles are ultrafine with an average diameter of 2 nm and uniformly distributed on expanded graphite. The specific capacitance of the Ni₂CoS₄/EG composite can reach up to 2056.8 F g⁻¹ at 5 A g⁻¹ as compared to 1574.4 F g⁻¹ of Ni₃S₄, 229.1 F g⁻¹ of CoS and 1516.6 F g⁻¹ of Ni₂CoS₄; and even at higher current density of 30 A g⁻¹, the specific capacitance can still demonstrates 1923.3 F g⁻¹, thus 92.5% of rate capability can be achieved as the current density increases from 5 to 30 A g⁻¹. Moreover, it exhibits an excellent stability of 94.4% after cycling at current density of 30 A g⁻¹ for 2000 cycles. The composite delivers high initial capacitance, excellent rate capability, and fantastic stability. Furthermore, the fabricated AC//Ni₂CoS₄/EG asymmetric supercapacitor also exhibits a high specific capacitance of 120.3 F g⁻¹ at 0.5 A g⁻¹, an superior cycle life (91% at 5 A g⁻¹ for 5000 cycles), and an extremely high energy density of 52 Wh kg⁻¹ at 477 W kg⁻¹. This work offers a new insight to synthesize ultrafine bimetallic sulfides, and the superior high performances of the Ni₂CoS₄/EG composite can provide practical applications in supercapacitors.