Materials with a layered structure have attracted tremendous attention because of their unique properties. The ultrathin nanosheet structure can result in extremely rapid intercalation/de-intercalation of Na ions in the charge–discharge progress. Herein, we report a manganese oxide with pre-intercalated K and Na ions and having flower-like ultrathin layered structure, which was synthesized by a facile but efficient hydrothermal method under mild condition. The pre-intercalation of Na and K ions facilitates the access of electrolyte ions and shortens the ion diffusion pathways. The layered manganese oxide shows ultrahigh specific capacity when it is used as cathode material for sodium-ion batteries. It also exhibits excellent stability and reversibility. It was found that the amount of intercalated Na ions is approximately 71% of the total charge. The prominent electrochemical performance of the manganese oxide demonstrates the importance of design and synthesis of pre-intercalated ultrathin layered materials.

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.

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