Formation of Micron-Sized Nickel Cobalt Sulfide Solid Spheres with High Tap Density for Enhancing Pseudocapacitive Properties

The synthesis of nickel cobalt sulfide is well-established because of its good electrical conductivity and great structural flexibility with low cost. It is extremely challenging to fabricate a unique structure with high tap density to improve its volumetric energy density and practical application. Here, we report a simple one-step hydrothermal method to synthesize micron-sized Ni-Co mixed sulfide solid spheres (1-2 mu m in diameter). Because of the high tap density of more than 1.0 g cm(-3) and the unique structure, the Ni-Co mixed sulfides demonstrate exceptional energy-storage performance. When acting as an electrode material for supercapacitors, these Ni-Co solid spheres can deliver a specific capacitance of 1492 F g(-1) at a current density of 1.0 A g(-1) and a retention of 76% at 10 A g(-1) after 10 000 cycles. An asymmetric supercapacitor based on these solid spheres exhibits a high energy density of 48.4 W h kg(-1) at a power density of 371.2 W kg(-1) with excellent long-term cycling performance (91% retention of the initial specific capacitance at S A g(-1) after 20 000 cycles). All the experiment results illustrate that the micron-sized solid-sphere Ni-Co mixed sulfides will be a promising electrode material for high-performance supercapacitors.