Effect of nondegeneracy on Ni3-xCoxS4 for high performance supercapacitor

The micro-nano hierarchical structure of Ni3-xCoxS4 is successfully synthesized by the diffusion control and trapped dopant methods. The Ni3-xCoxS4 hierarchical architectures, which serve as electrodes for supercapacitors, exhibit a high specific capacity of 961.1 F g(-1) at a current density of 1 A g(-1). The Ni3-xCoxS4 hierarchical architectures also display an excellent rate capacity and a high capacitance retention of 91.07% after 5000 cycles at 10 A g(-1). The long-term cycling performance of Ni3-xCoxS4 electrodes can be attributed to eliminating degeneracy for Ni3S4 structure by Co atoms doping, which effectively improves the electrochemical stability of the electrode material. With the increase of Co contents, the electrochemical performance of Ni3-xCoxS4 hierarchical architectures exhibits a parabolic tendency and reaches its maximum at a doping concentration of 0.83%. The Ni3-xCoxS4 (x = 0.83%)//AC asymmetric supercapacitor exhibits a relatively high energy density of 74.32 Wh kg(-1) at a power density of 517.51 W kg(-1), as well as a maximum power density of 14155.71 W kg(-1) at an energy density of 55.05 Wh kg(-1). The excellent electrochemical properties of Ni3-xCoxS4 hierarchical architectures can be attributed to their unique band structures and great conductivities, which allows efficient charge transport and electrolyte diffusion. First-principles calculation is carried out to systematically explain the origin of the specific capacitance of Ni3S4 and Ni3-xCoxS4 hierarchical architectures.