Yolk–shell-structured MnO ₂ microspheres with oxygen vacancies for high-performance supercapacitors

Abstract

An electrode consisting of yolk–shell-structured MnO ₂ microspheres with oxygen vacancies exhibits high specific capacitance, excellent cycling stability (10 000 cycles) and superior rate capability.

Abstract

Yolk–shell-structured MnO ₂ microspheres with oxygen vacancies (ov-MnO ₂@MnO ₂) were successfully constructed by a facile three-step method. Morphological observations showed that the as-obtained ov-MnO ₂@MnO ₂ 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 MnO ₂ nanoparticles with an average diameter of 12 nm. The yolk–shell-structured ov-MnO ₂@MnO ₂ 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 (∼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-MnO ₂@MnO ₂ 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-MnO ₂@MnO ₂ 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.