Controllable Synthesis Of Sulfide Composites And The Study Of Supercapacitive Performance

As a new generation of energy storage device,supercapacitors have attracted intense attention in society due to their prominent advantages of high power density,long cycle life,fast recharge ability.However,low specific capacity and low energy density become a technical bottleneck that can not be ignored,which greatly limited the application in the field of energy storage.According to the energy density formula?E=1/2QV?,we can see that the specific capacity and the working voltage are the determinants of the energy density of supercapacitor.Hence,the sulfides with high specific capacity and good conductivity were selected as the research object in this paper.We synthesized the sulfides and their composites with different morphology successfully.Then the sulfide composites and the prepared three-dimensional graphene gels were used as positive and negative electrode,respectively.Eventually,we designed and assembled an asymmetric supercapacitor?ASC?.An unique NiCo₂S₄ nanotube arrays was synthesized in situ on Ni foam by a facile anion exchange method.Subsequenty,appropriate mass Ni₃S₂ nanosheets were uniformly coated on the NiCo₂S₄ skeleton via electro-deposition process and obained NiCo₂S₄@Ni₃S₂ core-shell nanotube arrays composites.The morphology and structure of the electrode materials were characterized by scanning electron microscopy?SEM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?and the electrochemical behaviors of NiCo₂S₄ and NiCo₂S₄@Ni₃S₂ electrodes were tested in a three-electrode system.The results show that the area specific capacity of NiCo₂S₄@Ni₃S₂ is 4.25 C cm-2 and increased 62.21% than pure NiCo₂S₄ electrode?2.62 C cm-2?at a current density of 4 mA cm-2.Even at a current density maximum of 40 mA cm-2,the area specific capacity of NiCo₂S₄@Ni₃S₂ composites was maintained at 3.12 C cm-2.The outstanding electrochemical properties of NiCo₂S₄@Ni₃S₂ composites are attributed to the following points: 1 ? NiCo₂S₄ nanotubes hollow structure and superior conductivity contribution;2?the core-shell structure is favorable to enhance the mechanical stability of the electrode material and cycle stability;3?NiCo₂S₄ and Ni₃S₂ are excellent Faraday electrode materials,which generate heterogeneous synergies effect.The NiCo₂S₄ and NiCo₂S₄@Ni₃S₂ were used as cathode material and the three-dimensional graphene gel?rGO gel?were used as anode material to construct an ASC devices.The electrochemical test results show that the NiCo₂S₄@Ni₃S₂//rGO gel ASC has achieved superior electrochemical performance,which displayed a mass specific capacity of 163.15 C g^-1 at a current density of 0.5 A g^-1 and an energy density of 32.75 Wh kg^-1 at a power density of 0.36 kW kg^-1.In addition,the cycle life test showed that the NiCo₂S₄@Ni₃S₂//rGO gel ASC had a remarkable cycling stability(at a current density of 2 A g^-1,77.50% capacity retention after 5000 cycles of charge and discharge).It can be seen that the NiCo₂S₄@Ni₃S₂//rGO gel ASC has a superior application prospect in the field of energy storage.The NiCo₂S₄ microspheres with controllable morphology and uniform particle size were synthesized by one-step solvothermal reaction.The morphology and structure of the obtained materials were characterized by SEM,EDS and XRD.Also the electrochemical property of NiCo₂S₄ electrode was tested in the three-electrode system.The results displayed that the mass specific capacity of NiCo₂S₄ electrode achieves 599.4 C g^-1 at current density of 1 A g^-1.Even at a current density maximum of 20 A g^-1,its mass specific capacity still maintained 298 C g^-1.The advantage of NiCo₂S₄ microspheres electrode in electrochemical performance is mainly due to the following factors: On the one hand,NiCo₂S₄ is a typical Faraday material,which not only has rich and reversible redox reaction,but also has good electrical conductivity.On the other hand,NiCo₂S₄ microspheres have cross-linking nanosheets,which greatly promote the diffusion of electrolyte ions,while provide an effective path to the charge transfer to.The NiCo₂S₄ microspheres and the three-dimensional graphene gel were used as positive and negative electrode of ASC devices,and then assembled it.The test results showed that the mass specific capacity of NiCo₂S₄//rGO gel ASC ranged from 171.2 C g^-1 to 117.6 C g^-1 with the increasing current density.It was worth noting that NiCo₂S₄//rGO gel ASC exhibited an prominent energy density of 36.72 Wh kg^-1 at a power density of 0.39 kW kg^-1.Even at a high power density of 3.10 kW kg^-1,its energy density is still up to 25.33 Wh kg^-1.Furthermore,the cycle life test showed that the NiCo₂S₄//rGO gel ASC had a specific capacity retention of 77.85% after 5000 cycles at a high current density of 4 A g^-1,which displayed excellent cycle life.NiCo₂S₄/rGO composite gel was synthesized by simple one-step solvothermal reaction.NiCo₂S₄ nanoparticles were evenly embedded on the surface of three-dimensional graphene sheets.The structure and morphology of the synthesized composites were characterized by XRD,Raman,SEM and TEM.A series of electrochemical performance tests were performed on electrochemical workstations and LAND battery systems.The results showed that the mass specific capacity of NiCo₂S₄/rGO composite gel achieves 714 C g^-1 at a current density of 1 A g^-1.Even at the current density maximum of 20 A g^-1,its specific capacity still maintained 81.23%(580 C g^-1),more higher than the pure NiCo₂S₄,which reflected a outstanding rate capability.Moreover,the specific capacity of NiCo₂S₄/rGO composite gel still remained 90.21% of the initial capacity value at a high current density of 10 A g^-1 after 5000 cycles,suggesting prominent cycle stability.The superior electrochemical performance results from the contribution of the large surface area and the high conductivity of the three-dimensional porous structure rGO,which was not only increases the active site of the NiCo₂S₄ active material Faradic reaction,but also provides fast and efficient charge transfer and ion diffusion rate.