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 μm 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% retentio...