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Based on the comparisons of current electrochemical energy storage systems (as shown in Table S1, Supporting Information), electrochemical supercapacitors have higher theoretical specific capacity due to the redox reaction at the interface of electrode material compared with traditional electrostatic capacitors, meanwhile it still can maintain high power density. Therefore, electrochemical supercapacitors (ECs) as alternative energy devices have the ability to bridge the energy-power gap between traditional electrostatic capacitors and Li-ion batteries, and attracted much attention in recent years due to their large current charge/discharge capability, high power density and outstanding cycle stability. [5,6] These merits are essential to accessory power sources, for instance, portable electronic devices, new energy vehicles, emergency power supply and military field and so on. Importantly, the new generation of energy storage devices not only need high power and light weight, but also need high energy density, environmental protection and sustainable development. [7,8] Up to now, although the commercial Li-ion batteries exhibit high energy density (e.g., LiFePO 4 , LNCMO), their cycle and power characteristics are not as good as those of supercapacitors (Table S2, Supporting Information). However, supercapacitors are not able to store the same amount of charge as Li-ion batteries do, which is usually 3-30 times lower. Thus, a major obstacle is how to increase the energy density of supercapacitors without sacrificing power density and cyclability. On the basis of the equation of energy density E = 1/2 CV 2 , the energy density (E) can be improved by increasing voltage window (V) or specific capacitance (C). The working voltage has proven to be mainly dependent on the electrolyte stability. [9][10][11][12] Although organic electrolytes have enhanced the operating voltage of supercapacitors, the high cost and toxicity limit their practical application. Nevertheless, water-based electrolytes pull down the energy density of supercapacitors because of the lower decomposition voltage of water (≈1.2 V). [13,14] In contrast, the all-solid-state asymmetric supercapacitors can extend working voltage windows through restraining the oxygen evolution A stable MnO x @C@MnO x core-shell heterostructure consisting of vertical MnO x nanosheets grown evenly on the surface of the MnO x @carbon nanowires are obtained by simple liquid phase method combined with thermal treatment. The hierarchical MnO x @C@MnO x heterostructure electrode possesses a high specific capacitance of 350 F g −1 and an excellent cycle performance owing to the existence of the pore structure among the ultrasmall MnO x nanoparticles and the rapid transmission of electrons between the active material and carbon coating layer. Particularly, according to the in situ Raman spectra analysis, no characteristic peaks corresponding to MnOOH are found during charging/discharging, indicating that pseudocapacitive behavior of the MnO x electrode have no relevance to the ...

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A stable MnO2 nanowires@NiCo-layered double hydroxide (LDH) nanosheet core–shell heterostructure is prepared via a simple liquid-phase reaction method, in which the NiCo-LDH nanosheets grow uniformly on the surface of ultralong MnO2 nanowires with a stable tunnel structure. Electrochemical studies indicate that the core–shell heterostructure electrode has high specific capacitances of 708 and 630 C g–1 at 1 A g–1 and 10 A g–1, respectively, and exhibits a capacitance retention of 82.3% after 2000 cycles. According to in situ Raman spectral analysis, the NiCo-LDH material in the core–shell structure electrode reveals a very slow transition from α to β phase in the cycle process compared to the pure NiCo-LDH electrode, which can be attributed to the stable core–shell heterostructure buffering the collapse of the layered NiCo-LDH nanosheets and slowing down the irreversible phase transition during the charging–discharging process by the synergistic effect between one-dimensional nanowires and two-dimensional nanosheets. Moreover, the assembled asymmetric supercapacitor using the core–shell electrode displays a high energy density of 31.9 Wh kg–1 at 1 A g–1, a high power density of 7644.9 W kg–1 at 10 A g–1, and an acceptable capacitance retention of 72.4% after 10 000 cycles, indicating the potential practical application.