The Multi-Structure NiCo2S4 Prepared by Solvothermal Method for Supercapacitor Accompanied with Positron Annihilation Study

Fluffy spherical structure NiCo2S4 decorated with cubic phase hexagonal nanosheets was successfully synthesized through a two-step solvothermal method for energy storage. The electrochemical measurements reveal its good reversibility together with its high rate ability (1354 F g−1 at 0.5 A g−1 and 1128 F g−1 at 10 A g−1). Positron annihilation techniques (positron annihilation lifetime spectra and Doppler broadening spectra) were conducted to investigate the defects and the electronic structure of the two-step synthesized NiCo2S4. The results showed high S/W ratio, which is consistent with the electrochemical results. An asymmetric supercapacitor (ASC) was assembled in a button cell using the two-step synthesized NiCo2S4 as the positive electrode material and commercially activated carbon as the negative electrode material. The ASC presents a high energy density of 30.8 W kg−1 at a power density of 352.2 W kg−1 and 15.5 W kg−1 at a power density of 6860 W kg−1, with excellent cycling stability (89.3% capacitance retention after 3000 cycles) and nearly 100% Coulombic efficiency.