Construction of hollow-sphere CuNi2S4 with optimized structure and boosting conductivity for hybrid supercapacitor

Transition bimetallic sulfides with optimal composition and structure are promising battery-type materials for hybrid supercapacitors. The structure and conductivity of bimetallic sulfides are improved compared to monometallic sulfides. Herein, bimetallic spinel sulfide CuNi2S4 is first synthesized via a two-step method of chemical deposition and subsequent solvothermal method. The molar ratio of the Ni:Cu elements, chemical deposition time, and solvothermal reaction temperature are critical factors for forming single-phase CuNi2S4. The formation of hollow-sphere morphology of CuNi2S4 can be ascribed to the Kirkendall effect. CuNi2S4 electrode performs ultrahigh capacity of 458.9 mAh g−1 at 1 A g−1 and remains 84.6% and 77.7% capacity after 5000 and 10,000 cycles with coulombic efficiency of >99% at 4 A g−1. Density Functional Theory (DFT) calculation results demonstrate that the electronic conductivity of CuNi2S4 is higher than Ni3S4. The favorable electrochemical performance of CuNi2S4-2 electrode can be ascribed to the hollow structure with a large active surface and decent conductivity. The synergistic effect between Ni and Cu elements also further enhances the capability of CuNi2S4. The hybrid supercapacitor with high mass loading of 59 mg cm−2 using CuNi2S4-2 as cathode and active carbon as anode obtains an energy density of 87.0 Wh kg−1 with a power density of 1048.8 W kg−1 and cyclic retention of 82.3% after 10,000 cycles. In combination, the superior electrochemical performance of spinel CuNi2S4 makes it a promising candidate for hybrid supercapacitors.