Preparation Of Sulfides Based On Core-Shell Structure And Their Applications In Supercapacitors

Supercapacitors have a lot of advantages, such as high specific capacitance, long cycle life, short charging time, environment friendly and so on. Therefore, supercapacitors have attracted wide attention. However the lower energy density becomes a main technical obstacle. According to the formula（E=1/2CV2）, the work voltage and capacitance of supercapacitors are particularly important.In this paper, a series of electrode materials with core-shell structure were prepared by the combination of sulfides and core-shell structure. Then the asymmetric supercapacitor（ASC） was designed by using studied materials as the positive electrode and active carbon as negative electrode.Ni₃S₂ nano-triangular pyramid（NTP） arrays on Ni foam were fabricated though a hydrothermal process. Then the Ni₃S₂ NTP was enwrapped with curl CoS nanosheets by electro-depositon. Thus, Ni₃S₂@CoS core-shell NTP arrays were successfully synthesized. The structure and morphology of the material were characterized by X-ray diffraction（XRD）, scanning electron microscope（SEM）, transmission electron microscope（TEM） and X-ray photoelectron spectroscopy（XPS）. The electrochemical performances of Ni₃S₂ and Ni₃S₂@CoS electrodes were tested in a three-electrode system. The obtained Ni₃S₂@CoS electrode displayed an area specific capacitance of 4.89 F cm-2 at 4 mA cm-2, much higher than bare Ni₃S₂ electrode（1.97 F cm-2）. The superior performances of Ni₃S₂@CoS electrode originated from two aspects. On one hand, both Ni₃S₂ and CoS were representative pseudocapacitive materials. The combination of two materials had a synergistic effect. On the other hand, the structure of Ni₃S₂@CoS core-shell NTP arrays could accelerate diffusion of electrolyte ions and provide a valid path for the charge transfer.The Ni₃S₂ and Ni₃S₂@CoS were used as positive electrode and activated carbon was used as negative electrode to assemble ASC devices. The Ni₃S₂@CoS ASC device showed an area specific capacitance of 0.68 F cm-2 at 1.43 mA cm-2. Meanwhile, the asymmetric supercapacitor demonstrated an outstanding energy density of 28.24 Wh kg-1 at a power density of 0.14 k W kg-1. Additionally, the Ni₃S₂@CoS ASC device exhibited a stable cycle life（98.83% retained after 2000 cycles）.NiCo₂S₄ nanotube arrays on Ni foam were prepared by anion exchange. Then curl Co（OH）₂ nanosheets were coated on the surface of NiCo₂S₄ nanotubes by electro-deposition. Thus, NiCo₂S₄@Co（OH）₂ core-sheel nanotube arrays were successfully synthesized. The structure and morphology of the material were characterized by XRD, SEN and TEM. The electrochemical performances of NiCo₂S₄ and NiCo₂S₄@Co（OH）₂ electrodes were tested a three-electrode system. The NiCo₂S₄@Co（OH）₂ electrode showed a relatively high area capacitance of 9.6 F cm-2 at 2 mA cm-2（almost twice as high as that of bare NiCo₂S₄ electrode）, even at 32 mA cm-2, the area capacitance was maintained at 6.4 F cm-2. The excellent performances of NiCo₂S₄@Co（OH）₂ electrode were mainly due to three reasons. First, NiCo₂S₄ nanotubes possessed a hollow array structure, which was conducive to the diffusion of electrolyte ions. Secondly, the NiCo₂S₄ also had good conductivity. In addition, NiCo₂S₄@Co（OH）₂ nanotube arrays firmly attached to the Ni foam, so that it wasn’t easy to fall off which was conducive to the cycle stability.The NiCo₂S₄ and NiCo₂S₄@Co（OH）₂ were used as positive electrode and activated carbon was used as negative electrode to assemble ASC devices. The mass specific capacitance of NiCo₂S₄@Co（OH）₂ ASC device ranged from 100.94 F g-1 to 73.75 F g-1 with the current density increasing from 0.5 A g-1 to 4 A g-1. It was worth noting that NiCo₂S₄@Co（OH）₂ ASC device exhibited an outstanding energy density of 35.89 Wh kg-1 at a power density of 0.4 kW kg-1。Meanwhile, NiCo₂S₄@Co（OH）₂ ASC device exhibited excellent cycling performances（70.10% capacitance retention after 5000 cycles）. Therefore the studied materials of NiCo₂S₄@Co（OH）₂ endowed with a great potential in the application of asymmetric supercapacitor.