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 mu m. 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 m(2) 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 (similar to 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 ovMnO(2)@ 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.