Controllable Synthesis And Structure Optimization Of Ni-Co Sulfides And Their Energy Storage Properties

In the context of ecological civilization construction and high-quality economic development,energy storage devices and systems are in urgent need of rapid development in order to meet the growing energy needs of humans.Under this situation,supercapacitors,as representative energy storage devices,have been widely studied and developed rapidly with the continuous development of nanomaterials and the increasing innovation of preparation technology.Transition metal sulfides are considered as the most promising electrode materials for supercapacitors due to large specific surface areas,rich valence states and high electrical conductivity.In this project,nickel-cobalt sulfides were studied by adjusting the proportion and exploring the corresponding functions of Ni²⁺/Co²⁺ions.These technological objectives of nickel-cobalt sulfide materials were gradually realized,such as the design and preparation,morphology optimization,structure regulation,performance improvement and practical expansion.In addition,designing the asymmetric device provides high energy and power density for the supercapacitors based on nickel-cobalt sulfides,which creates a feasible strategy for its practical application.The main research contents and results of this thesis are as follows:(1)Preliminarily prepare the materials and study the energy storage mechanism:Ni Co₂S₄ nanomaterials were synthesized by a two-step hydrothermal method,which shows the morphology of urchin-like microsphere surrounded by nanotubes.The hierarchical structure is conducive to increasing the specific surface area,forming better pore size distribution,and facilitating the full contact between electrolyte and active materials.Whereupon,the enhanced electrochemical performance of Ni Co₂S₄ electrode is obtained due to the rapid electrons transfer and unblocked ion transport.As an electrode material of supercapacitors,the Ni Co₂S₄ material provides a high specific capacity of 1361.6 m F cm^-2 at the current density of 1 m A cm^-2,and exhibits the more excellent electrochemical performance than the Ni Co₂O₄ electrode.It is found that the capacitive contribution based on surface diffusion is the main energy storage mechanism through the sweep voltammetry analysis on its electrochemical behavior,which lays a foundation for the next work of regulating and optimizing the electrochemical properties.(2)Simplify the process flow and analyze the growth mechanism:A mild,simple and efficient solvothermal method was used to prepare petaloid Ni Co₂S₄ nanosheets.By regulating the growth time,we investigated the growth mechanism of petaloid Ni Co₂S₄ nanosheets,which is similar to the"bud"blossoming.The prepared Ni Co₂S₄electrodes can deliver an ultrahigh specific capacitance of 2036.5 F g^-1 at a current density of 1 A g^-1,superior rate capability and remarkable cycle stability(94.3%of capacitance retention after 5000 cycles).The as-fabricated asymmetric supercapacitors based on petaloid Ni Co₂S₄ nanosheets//active carbon electrodes demonstrate a high energy density of 35.6 W h kg^-1 at a power density of 819.5 W kg^-1,with both long-term cycling and good rate stability.Two supercapacitors are connected in series to light up a"heart"array consisting of 10 parallel LEDs,revealing the application prospect of this device.(3)Explore flexible application and study the functions of Ni²⁺:The hollow Ni-Co₉S₈ nanospheres were grown by one-step electrodeposition on the flexible carbon cloth with a specific weight much lower than that of metal.The effect of Ni²⁺on the structure optimization and performance improvement of Ni-Co sulfides was discussed by comparing the presence and absence of Ni²⁺.In addition to ensuring excellent electrochemical performance,the flexibility of Ni-Co₉S₈electrodes were also studied.It is found that the existence of strong adhesion between materials and the current collector ensure the stability of the electrode in the process of bending test.The Ni-Co₉S₈ electrode delivers an ultrahigh areal and volumetric specific capacitance of 5.64F cm^-2 and 171.85 F cm^-3 at 1 m A cm^-2,respectively,superior rate capability and excellent cycling stability.Also,the as-fabricated flexible quasi-solid-state asymmetric supercapacitors based on Ni-Co₉S₈ and active carbon electrodes present high energy density,which also shows a broad application prospect in the field of flexible electronics.(4)Study the functions of Ni²⁺/Co²⁺ions and synthesize the electrodes with stable electrochemical performance and high mass loading:A series of Ni-Co-S materials were synthesized by electrodeposition with double electrode system.By designing preparation solutions with different molar ratios of nickel and cobalt ions,the role of Ni²⁺/CO²⁺ion in controlling the structure of Ni-Co-based electrodes with high mass loading and maintaining stable electrochemical performance was investigated.By optimizing the composition,controlling the morphology and modifying the structure,the Ni-Co-S electrodes with high mass loading and unfading specific capacitance were obtained.The specific capacitance of the optimal Ni-Co-S electrode is stable around640 F g^-1 when the mass loading is gradually increased from 1.63 mg cm^-2 to 8.84 mg cm^-2.The electrode also exhibits excellent rate capability,superior electrical conductivity and outstanding cyclability.The asymmetric supercapacitor assembled based on the optimal Ni-Co-S-10h cathode delivers a high energy density of 33.9 W h kg^-1 at a power density of 839.0 W kg^-1.