Oxygen vacancies enhancing hierarchical NiCo₂S₄@MnO₂ electrode for flexible asymmetric supercapacitors

The limited energy density of supercapacitors hampers their widespread application in electronic devices. Metal oxides, employed as electrode materials, suffer from low conductivity and stability, prompting extensive research in recent years to enhance their electrochemical properties. Among these efforts, the construction of core-shell heterostructures and the utilization of oxygen vacancy (V_O) engineering have emerged as pivotal strategies for improving material stability and ion diffusion rates. Herein, core-shell composites comprising NiCo₂S₄ nanospheres and MnO₂ nanosheets are grown in situ on carbon cloth (CC), forming nanoflower clusters while introducing V_O defects through a chemical reduction method. Density functional theory (DFT) results proves that the existence of V_O effectively enhances electronic and structural properties of MnO₂, thereby enhancing capacitive properties. The electrochemical test results show that NiCo₂S₄@MnO₂-V₃ exhibits excellent 1376 F g^-1 mass capacitance and 2.06 F cm^-2 area capacitance at 1 A g^-1. Moreover, NiCo₂S₄@MnO₂-V₃//activated carbon (AC) asymmetric supercapacitor (ASC) can achieve an energy density of 39.7 Wh kg^-1 at a power density of 775 W kg^-1, and maintains 15.5 Wh kg^-1 even at 7749.77 W kg^-1. Capacitance retention is 73.1 % after 10,000 cycles at 5 A g^-1, and coulombic efficiency reaches 100 %, demonstrating satisfactory cycle stability. In addition, the device's excellent flexibility offers broad application prospects in wearable electronic applications.