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

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.