Hierarchical nanosheet-based Ni₃S₂ microspheres grown on Ni foam for high-performance all-solid-state asymmetric supercapacitors

The hierarchical nanosheet-based Ni₃S₂ microspheres directly grew on Ni foam using a two-step hydrothermal method. The microsphere with a diameter of ∼1 microns and a rough surface was well connected to each other without any binders to provide a larger specific surface area, shorter ion/electron diffusion paths, richer electroactive sites as a supercapacitor electrode. As a three-electrode supercapacitor, it delivers a high specific capacity of 981.8 F g^-1 at 2 A g^-1, an excellent rate capability of 436.4 F g^-1 at 12 A g^-1, and a good cycling stability of 950.9 F g^-1 with 96.9% retention after 1000 cycles at 2 A g^-1. Furthermore, an asymmetric supercapacitor based on Ni₃S₂-microsphere as a positive electrode and active carbon as a negative electrode shows a high energy density of 29.4 Wh kg^-1 at 324.5 W kg^-1 and a high power density of 3197.6 W kg^-1 at 15.1 Wh kg^-1. This work demonstrates that nanosheet-based Ni₃S₂ microspheres coated Ni foam can be an effective electrode for a real supercapacitor.