Controllable synthesis of hydrangea-like NixSy hollow microflower all-solid-state asymmetric supercapacitor electrodes with enhanced performance by the synergistic effect of multiphase nickel

Abstract

In this work, through controlling the urea content in the synthesis system, the nucleation rate of Ni_xS_y can be adjusted, and a series of Ni_xS_y with multiphase nickel and various sizes and surface morphologies can be achieved. Among them, the Ni_xS_y sample prepared with a urea dosage of 2 mmol (NSU-2) exhibits a perfect hierarchical hydrangea-like hollow microflower structure, in which the center hollow sphere composed of a NiS/Ni₃S₂ complex has a high work function (Φ_f = 5.18 eV), and the ultra-thin Ni₃S₄ nanosheet petals covering the surface of the NiS/Ni₃S₂ complex have a lower work function (Φ_f = 4.97 eV). The work function difference between Ni₃S₄ and NiS/Ni₃S₂ can lead to an interfacial electric field, which promotes electron transfer in the electrochemical process. Moreover, due to the combination of the high capacity of Ni₃S₄ and the excellent electrical conductivity of Ni₃S₂, NSU-2 displays excellent electrochemical performance. When the current density is 3 A g⁻¹ and 50 A g⁻¹, the specific capacity of NSU-2 reaches 2234.3 F g⁻¹ and 1526.3 F g⁻¹, respectively. And under the conditions of 10 A g⁻¹ for 6000 cycles, its capacitance is still maintained at 87.5%. When assembled as a solid asymmetric supercapacitor, NSU-2 shows a high energy density of 51.4 W h kg⁻¹ at a power density of 800 W kg⁻¹, which still remains at 33.5 W h kg⁻¹ even at a higher power density of 8000 W kg⁻¹. Furthermore, the long-term stability test shows that NSU-2 still achieves a capacitance retention of 78.4% at a current density of 10 A g⁻¹ even after 10 000 cycles, which indicates its excellent stability and strong practical value.