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.

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A stable α-MnO2 nanowire@NiCo2O4 nanosheet core–shell heterostructure and a 3D-nanocage N-doped porous carbon nanosheet with high electrical conductivity are synthesized by a two-solution phase reaction and a facile one-step self-template technique, respectively. The unique α-MnO2@NiCo2O4 heterostructure is characterized by a stable nanostructure, fast electron transport, and numerous ion diffusion channels. The electrode exhibits a high specific capacitance of 1101 F g–1, and a cycling stability of 95.8% after 10 000 cycles. Moreover, by introducing N atoms which is favorable for rate performance, the 3D porous carbon offers a large surface area, a proper pore structure and especially high electron conductivity. The specific capacitance of the 3D N-doped porous nanocage carbon electrode reaches 100 F g–1 at a current densities as high as 100 A g–1. The all-solid-state symmetric supercapacitor with excellent electrochemical properties is fabricated using the α-MnO2@NiCo2O4 core–shell heterostructure as positive electrode, a 3D N-doped porous nanocage carbon as negative electrode, and a PAAK/KOH gel as solid-state electrolyte. The supercapacitor demonstrates an expanded working potential of 1.7 V, a maximum energy density of 46.2 Wh kg–1, a maximum power density of 15.3 kW kg–1, and good capacitance retention of 90% after 2000 cycles.