Solvothermal sulfurization in a deep eutectic solvent: a novel route to synthesize Co-doped Ni3S2 nanosheets supported on Ni foam as active materials for ultrahigh-performance pseudocapacitors

Solvothermal processing is one of the most promising methods for materials synthesis. The main problems plaguing the current processes are harsh conditions, complex synthesis steps, and long reaction time. Herein, 3D-interconnected and porous Co-doped Ni3S2 nanosheets supported on nickel foam (NF) are rationally designed and fabricated by a facile and novel deep eutectic solvent (DES) derived solvothermal sulfurization process. Interestingly, the sulfurization process triggers a structural rearrangement on the surface, leading to the formation of an ultrathin nanosheet-packed Co-Ni3S2/NF 3D-network, and regulates the Co component in the host electrode with a modified electronic structure and improved electrical conductivity. Benefiting from the 3D-networked self-supported nanosheet array architecture and compositional features, the Co-Ni3S2/NF electrode exhibits highly attractive electrochemical performance with ultrahigh capacitance (23 491 mF cm(-2) at 5 mA cm(-2)) and excellent cycling stability (capacity retention of 90.0% after 3000 cycles at 10 mA cm(-2)). The asymmetric supercapacitor with Co-Ni3S2/NF and active carbon (AC)/NF as the positive and the negative electrode, respectively, can achieve a high energy density of 247.1 W h kg(-1) at 3.99 kW kg(-1) and excellent specific capacitance (695 F g(-1) at 5 A g(-1)) and electrochemical cycling stability (89.6% retention after 14 000 cycles). This work offers a facile environmentally benign route for the massive synthesis of self-standing binary metal sulfide nanoarrays that are promising for practical applications with control over the size, shape, composition, and homogeneity in high-performance energy storage devices.