Construction of Hierarchical α‑MnO₂ Nanowires@Ultrathin δ‑MnO₂ Nanosheets Core–Shell Nanostructure with Excellent Cycling Stability for High-Power Asymmetric Supercapacitor Electrodes

Poor electrical conductivity and mechanical instability are two major obstacles to realizing high performance of MnO₂ as pseudocapacitor material. The construction of unique hierarchical core–shell nanostructures, therefore, plays an important role in the efficient enhancement of the rate capacity and the stability of this material. We herein report the fabrication of a hierarchical α-MnO₂ nanowires@ultrathin δ-MnO₂ nanosheets core–shell nanostructure by adopting a facile and practical solution-phase technique. The novel hierarchical nanostructures are composed of ultrathin δ-MnO₂ nanosheets with a few atomic layers growing well on the surface of the ultralong α-MnO₂ nanowires. The first specific capacitance of hierarchical core–shell nanostructure reached 153.8 F g^–1 at the discharge current density of as high as 20 A g^–1, and the cycling stability is retained at 98.1% after 10 000 charge–discharge cycles, higher than those in the literature. The excellent rate capacity and stability of the hierarchical core–shell nanostructures can be attributed to the structural features of the two MnO₂ crystals, in which a 1D α-MnO₂ nanowire core provides a stable structural backbone and the ultrathin 2D δ-MnO₂ nanosheet shell creates more reactive active sites. The synergistic effects of different dimensions also contribute to the superior rate capability.