Enhancement of the capacitive performance of Ni2CoS4 by incorporation of graphitized carbon dots

Abstract

Nickel cobalt sulfide is a promising electrode material for high-performance energy storage devices but it still suffers from inferior electrical conductivity. Here highly graphitized carbon dots (GCDs, diameter is 10–20 nm) were synthesized by propane pyrolysis via the chemical vapor deposition method. The GCDs are applied to improve the electrical conductivity of Ni₂CoS₄ and also make Ni₂CoS₄ nanoparticles smaller, and then a Ni₂CoS₄/GCD composite was prepared through microwave assisted heating followed by solvothermal methods. The Ni₂CoS₄/GCD composite exhibits an extremely high specific capacitance of 2450 F g⁻¹ at a current density of 5 A g⁻¹, an outstanding rate performance of 90% when the current density increases from 5 A g⁻¹ to 30 A g⁻¹, and an exceptional cycling stability of 96.3% at a current density of 30 A g⁻¹ after 2000 cycles. Furthermore, an asymmetric supercapacitor assembled using the Ni₂CoS₄/GCD cathode and the activated carbon anode provides a promising energy density of 57.3 W h kg⁻¹ at a power density of 491.0 W kg⁻¹. Meanwhile, after 8000 cycles at a current density of 10 A g⁻¹, 90% of its initial specific capacitance can be maintained and excellent stability is achieved. The ultra-high specific capacitance, excellent ratio capability, and outstanding cycling stability of the Ni₂CoS₄/GCD composite are due to the high intrinsic theory capacitance of Ni₂CoS₄ as well as the unique properties of small particles and excellent conductivity of GCDs. Therefore, this composite delivers great prospects for applications in the energy storage field.