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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The synthesis of nickel cobalt sulfide is wellestablished because of its good electrical conductivity and great structural flexibility with low cost. It is extremely challenging to fabricate a unique structure with high tap density to improve its volumetric energy density and practical application. Here, we report a simple one-step hydrothermal method to synthesize micron-sized Ni−Co mixed sulfide solid spheres (1−2 μm in diameter). Because of the high tap density of more than 1.0 g cm −3 and the unique structure, the Ni−Co mixed sulfides demonstrate exceptional energy-storage performance. When acting as an electrode material for supercapacitors, these Ni−Co solid spheres can deliver a specific capacitance of 1492 F g −1 at a current density of 1.0 A g −1 , and a retention of 76% at 10 A g −1 after 10 000 cycles. An asymmetric supercapacitor based on these solid spheres exhibits a high energy density of 48.4 W h kg −1 at a power density of 371.2 W kg −1 with excellent long-term cycling performance (91% retention of the initial specific capacitance at 5 A g −1 after 20 000 cycles). All the experiment results illustrate that the micron-sized solid-sphere Ni−Co mixed sulfides will be a promising electrode material for high-performance supercapacitors.

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