Yolk–shell-structured MnO2 microspheres with oxygen vacancies for high-performance supercapacitors

Abstract

Yolk–shell-structured MnO₂ microspheres with oxygen vacancies (ov-MnO₂@MnO₂) were successfully constructed by a facile three-step method. Morphological observations showed that the as-obtained ov-MnO₂@MnO₂ microspheres possessed distinctive yolk@void@shell configurations with an average diameter of 1.13 μm. Both the shell and yolk were assembled by a large amount of homogeneous MnO₂ nanoparticles with an average diameter of 12 nm. The yolk–shell-structured ov-MnO₂@MnO₂ microsphere electrode exhibited a large specific surface area (259.83 m² g⁻¹) and good conductivity, thus it achieved high specific capacitance (452.4 F g⁻¹ at 1 A g⁻¹ and 316.1 F g⁻¹ at 50 A g⁻¹), excellent cycling stability (10 000 cycles) and superior rate capability (∼79.2% and 69.9% of the initial capacity at 20 A g⁻¹ and 50 A g⁻¹, respectively). It is noted that the asymmetric supercapacitor (ASC) composed of yolk–shell-structured ov-MnO₂@MnO₂ microspheres (as the positive electrode) and commercial activated carbon (as the negative electrode) can deliver a high energy density of 40.2 W h kg⁻¹ and a maximum power density of 22.28 kW kg⁻¹. The superior electrochemical performance of ov-MnO₂@MnO₂ is mainly ascribed to the unique yolk@void@shell nanostructure, the presence of oxygen vacancies in the crystal lattice and the synergistic effect of the individual components of the hybrid.