In Situ Preparation Of Metal Sulfides And Their Composites For Supercapacitors

Supercapacitors is a kind of electrochemical energy storage device with high power density and long cycle life.Developing electrode materials with high specific capacitance have been become the key work to improve the low energy density of supercapacitors.Transition metal sulfides have been widely studied for their high theoretical capacitances.In this thesis,metal sulfides and their composites prepared by using complexes of potassium ethyl xanthate and metal salts as precursors.The main research contents are as follows:(1)Nickel cobalt sulfide(Co Ni₂S₄)nanoparticles are prone to agglomerate due to their high surface energy.The agglomerated Co Ni₂S₄ nanoparticles deliver low specific surface area and limited ion transport path,reducing the electrochemical performance.Graphene oxide(GO)is a kind of two-dimensional carbon materials with large specific surface area,which can be used as the substrate to carrier uniformly distributed Co Ni₂S₄ nanoparticles.The reaction conditions of the common methods to prepare Co Ni₂S₄ nanoparticles/GO composites,such as hydrothermal method,solvothermal method,are relatively harsh and complex.Preparing such materials via mild conditions and simple process can effectively save the reaction cost and improve the yield.Therefore,in this thesis,the potassium ethyl xanthate and metal salts were used as precursors and GO was used as substrate.The metal sulfides and reduced graphene oxide(r GO)composites were prepared by one-step pyrolysis method.In the three-electrode test,the prepared Co Ni₂S₄/r GO composite showed a high specific capacitance of 1526 F g^–1 at 2 A g^–1.Moreover,an asymmetric supercapacitor(ASC)was prepared by using a Co Ni₂S₄/r GO as a positive electrode and an AC as a negative electrode.The ASC possesses a high energy density of 54.8Wh kg^–1 at the power density of 798 W kg^–1,as well as excellent electrochemical stability.In addition,Cd S/r GO,Cu_1.8S/r GO,In₂S₃/r GO,Fe S/r GO,Mn S/r GO and Zn S/r GO composites were prepared via this general method.(2)The powder active materials need to be coated on the metal foil by means of insulating binder,which weakens the conductivity of the electrode and reduce the specific capacitance.The self-supporting Co Ni₂S₄ electrode can effectively improve the specific capacitance and rate performance.In addition,another feasible way to improve the intrinsic conductivity and promote the charge storage dynamics of pseducapacitors is to introduce vacancies in the electroactive materials,because vacancies can optimize the structure of electrode materials.The methods of vacancy introduction,such as electrochemical oxidation and heteroatom doping,are very harsh.Therefore,it is still a challenge to simply prepare self-supporting Co Ni₂S₄electrode with a large number of vacancies.In this thesis,partially amorphous Co Ni₂S₄ nanosheets(PA-Co Ni₂S₄/NF)were successfully synthesized on the nickel foam(NF)substrate by a simple solvothermal method following KBH₄ treatment.The nanosheet structure optimizes the ion transport path,and the amorphous crystal endows the material with a large number of sulfur vacancies,which can effectively improve the conductivity and specific capacitance.Owing to these advantages,the as-prepared materials possess excellent electrochemical properties.Compared with Co Ni₂S₄ grown on NF without KBH₄ treatment(Co Ni₂S₄/NF),PA-Co Ni₂S₄/NF has higher capacitance and conductivity,especially the material treated for 6h(PA-Co Ni₂S₄/NF-6h)has the highest capacitance(3305 F g^-1 at current density of 2 A g^-1).In addition,the ASC with PA-Co Ni₂S₄/NF-6h as positive electrode and AC as negative electrode has a high energy density of 43.38 Wh kg^-1at a power density of800.43 W kg^-1,as well as good long-term cycle stability.