Controllable Design And Electrochemical Application Of Cobalt-based Sulfides Hybrid Capacitors Electrode Composites

To meet the rapidly growing energy demand and reduce dependence on non-renewable fossil fuels,environmentally friendly energy storage and conversion technologies are essential.In recent years,renewable energy technologies have made significant progress in securing the energy future of humanity.Among them,solar energy and wind energy stand out,but they are intermittent and cannot be controlled when needed.Therefore,it is inevitable to develop efficient energy storage equipments.Supercapacitors and lithium ion capacitors have become promising energy storage devices in new energy vehicles and mobile electronic devices due to their unique advantages.The performance of supercapacitors and lithium ion capacitors is related to the electrode material systems and their energy storage characteristics.Because of their high electrochemical activity,cobalt-based sulfides can be used as electrode materials for supercapacitors and lithium ion capacitors through reasonable design of their compositions and micro/nano structure.In this dissertation,a series of cobalt-based sulfide composites with unique nanostructure were designed.The morphology of the obtained materials was discussed by means of characterization.Moreover,the relationships between the structure and electrochemical properties for composite materials were deeply analyzed.The main research contents are as follows:（1）Construction of hollow-concave CoMo S_x for hybrid supercapacitorsHollow-concave CoMo S_x nanoboxes were prepared by calcination and hydrothermal method.Abundant mesoporous structure and hollow concave configuration assembled with nanoparticles provided larger surface area and more reactive sites for electrochemical reaction at the electrode/electrolyte interface,and promoted rapid diffusion of electrolyte ions.The electrochemical results showed that the specific capacity of CoMo S_x electrode was 784 C g^-1 at a current density of 1 A g^-1,which was obviously better than that of CoMo O₄(73 C g^-1).A hybrid supercapacitor was constructed with CoMo S_x and AC as the positive and negative electrode,respectively.The CoMo S_x//AC device presented the maximum energy density of 36.9Wh kg^-1 at a power density of 798.8 W kg^-1.（2）Construction of CoS_x with hollow double-shell for hybrid supercapacitorsHollow double-shell CoS_x was prepared by solvothermal,calcination and anion exchange method.The assembly of subunits with different dimensions furnished CoS_xlarger specific surface area,suitable mesopores and good structural stability.The unique double-shell structure enabled the electrolyte to be confined between the shells,thus providing higher driving force for the redox reaction.Due to its structural advantages,this CoS_x electrode showed improved battery-type electrochemical performance.The specific capacity of CoS_x was 622 C g^-1 at a current density of 2 A g^-1.A hybrid supercapacitor was assembled with CoS_x and AC as the positive and negative electrode,respectively.The maximum energy density of 38.2 Wh kg^-1 was obtained when a power density was 794.7 W kg^-1.（3）Construction of hollow hierarchical Ni Co-LDH@（Ni Co）_xS_y for hybrid supercapacitorsCo-glycerate was synthesized by hydrothermal method.With the addition of nickel salt,Ni Co-LDH nanosheets were prepared by in-situ reaction on the surface of Co-glycerate with kirkendall effect.Ni Co-LDH@（Ni Co）_xS_y with heterogeneous interface was synthesized by surface vulcanization strategy,which maintained the hollow structure of Ni Co-LDH.The electrochemical results showed that the introduction of sulfur significantly improved the electrical conductivity of the composites,and the synergistic effect of Ni Co-LDH and（Ni Co）_xS_y increased the specific capacity of the composites.At a current density of 1 A g^-1,the specific capacity of Ni Co-LDH was 424C g^-1 and that of Ni Co-LDH@（Ni Co）_xS_y was 586 C g^-1.A hybrid supercapacitor was assembled with Ni Co-LDH@（Ni Co）_xS_y and AC as the positive and negative electrode,respectively.The maximum energy density of the device could reach 24.2 Wh kg^-1 at a power density of 790.9 W kg^-1.（4）Construction of r GO/CoS for lithium ion capacitorsr GO/CoS composites were synthesized by coordination reaction and hydrothermal method.r GO was loaded with CoS nanoparticles derived from ZIF-67.The mechanism of enhanced lithium storage capacity by the coupling of r GO and cobalt sulfide was clarified:the introduction of r GO with excellent electrical conductivity could not only accelerate electron transfer,but also inhibit the agglomeration of CoS.Meanwhile,CoS alleviated the stacking of r GO.The electrochemical results showed that the reversible capacity of r GO（20）/CoS was 556 m Ah g^-1 after 65 charge and discharge cycles when the current density was 0.2 A g^-1.The r GO（20）/CoS//AC device was assembled by using r GO（20）/CoS and AC as the negative and positive electrode,respectively.At a energy density of 37.2 Wh kg^-1,the power density could reach 98.6 W kg^-1.