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

In this work, through controlling the urea content in the synthesis system, the nucleation rate of NixSy can be adjusted, and a series of NixSy with multiphase nickel and various sizes and surface morphologies can be achieved. Among them, the NixSy 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 (Φf = 5.18 eV), and the ultra-thin Ni3S4 nanosheet petals covering the surface of the NiS/Ni3S2 complex have a lower work function (Φf = 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.