a great deal of research effort has been devoted on high energy density supercapacitors. According to the equation of energy density E = 1/2 CV 2 , the energy density (E) of supercapacitors can be enhanced by increasing either voltage window (V) or specific capacitance (C). [5] For high specific capacitance, one of the research efforts should concentrate on using transition metal oxides (TMO) (e.g., MnO 2 , RuO 2 ) as electrode materials. [6,7] They can provide great specific capacitance due to the pseudocapacitive characteristic. [8,9] Among TMO materials, MnO 2 is one of the most potential materials for supercapacitors due to its high theoretical specific capacitance, environmental friendliness, and the high practical voltage window (about 1 V). [2] However, it has suffered from intrinsically low conductivity and specific surface area, which severely restrict its further development in practical application of supercapacitors. In this regard, integrating nanostructured MnO 2 and conductive carbon materials to fabricate novel hybrid nanostructures is a plausible solution to overcome this obstacle. [4] Further, some research efforts have been accordingly performed to synthesize hybrid nanostructures electroactive materials for constructing supercapacitors with considerable performance. Some researchers accordingly synthesized highperformance nanostructured MnO 2 -carbon materials electrodes, whose specific capacitance is close to theoretical value. However, the undesirable contact resistances that produced by the weak and noncoherent TMO/conductor interfaces lead to sluggish kinetics for charge transport, which requires further improvement. [10,11] In order to further improve the energy density, some researchers have concentrated on enlarging the voltage window of supercapacitors. The applications of aqueous electrolytes have been limited by their theoretical voltage window (≈1.23 V) and the practical voltage window is mostly lower than about 1 V for supercapacitors. [12] To extend the voltage window, various techniques have been applied, such as dual shuttle-ion electrolytes, [13,14] pH adjustment of electrolytes, [15] and concentrated electrolytes. [16][17][18] All of these methods have complex processing technologies and sacrifice capacitance, so being difficult for practical applications. Recently, Zhu and co-workers

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To realize high-performance and long life span supercapacitors, highly electrochemically active materials and rational design of structure are highly desirable.

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Carbon nanomaterials have been widely used to enhance the performance of MnO 2 -based supercapacitors. However, it still remains a challenge to directly fabricate high combining strength, mesostructured and high-performance MnO 2 /carbon nanotube (CNT)-nanostructured composite electrodes with a little weight percentage of carbon materials. Here, we report a novel mesostructured composite of the CNT-on-MnO 2 nanosheet with a high MnO 2 percentage, which consists of vertically aligned MnO 2 nanosheets with nanopores and in situ formed oriented CNTs on MnO 2 nanosheets (tube-on-sheet). The optimized CNTs/MnO 2 possesses favorable features, namely, vertically aligned nanosheets to shorted ion diffusion path, a hierarchical porous structure for increased specific surface areas and active sites, and in situ formed CNTs for enhanced conductivity and robust structural stability. It is found that the unique tube-on-sheet CNTs/MnO 2 nanocomposites with the high MnO 2 percentage (>90 wt %) exhibit a high specific capacity of 1131 F g −1 based on total electrodes and 1229 F g −1 based on MnO 2 at a current density of 1 A g −1 , high rate capability, and ultrastable cycling life (94.4%@10 000 cycles). This electrode design strategy in this paper demonstrates a new way for high-performance electrodes for supercapacitors with high active material percentage.