Design And Preparation Of Cobalt-Based Selenide/Carbon Composites And Their Lithium/Sodium Storage Performances

With the the rapid development of portable electronic products and electric vehicles,people have an increasingly strong pursuit of advanced rechargeable energy storage materials/devices with high efficiency and durability.Lithium(sodium)ion battery as a new generation of energy storage devices has great potential in the above field.It is of great significance to explore the anode materials with high specific capacity and stability.In recent years,metal selenides based on electrochemical conversion reactions have attracted much attention because of their high reversible capacity.However,there are still some main problems,such as poor cycle performance and limited rate performance due to the large volume change and low electrical conductivity in the cycle process.Therefore,the design of metal selenide and carbon composites is an effective strategy to solve the above problems.In this thesis,a series of cobalt-based selenides and their composites were prepared by exploring different synthesis methods,and the lithium/sodium storage properties of these materials were studied.It is mainly summarized into the following four parts:1.By adjusting the concentration of KOH,ZIF-67 with different morphology can be synthesized efficiently and macroscopically in aqueous solvents.In theory,the competitive coordination between ligand and KOH metal sites is used to regulate the morphology of ZIF-67.With the increase of KOH concentration,ZIF-67 gradually changes from two-dimensional leaf-like structure to three-dimensional polyhedral structure.Then,using the ZIF-67polyhedron as the precursor,two kinds of cobalt selenide/carbon composites with different crystal phases were obtained by two different selenization methods.Co_0.85Se/C exhibits a specific capacity of 765 mAh g^-1 after 200 cycles at 0.1 A g^-1,while Co Se₂/C has a low specific capacity but good cycling stability,which provides a reference case for the design of high-performance lithium storage anode.2.By controlling KOH concentration,the bimetallic MOFs were successfully synthesized in aqueous solvents.Specifically,FeCo-MOF,ZnCo-MOF and NiCo-MOF were prepared in one step by a green and efficient method for the first time.Furthermore,three kinds of cobalt-based bimetallic selenide/carbon composites were successfully obtained by carbonization and selenization.Among them,the Fe-Co-Se/NC can maintain a high discharge specific capacity of 1325 mAh g^-1 after 100 cycles in the long cycle test of 0.1 A g^-1.Even under a high current density of 1 A g^-1,it can still maintain a specific capacity as high as 1247mAh g^-1 after 550 cycles.At the same time,Fe-Co-Se/NC shows excellent rate performance(700 mAh g^-1 at 2.0 A g^-1;393 mAh g^-1 at 5.0 A g^-1).This could be due to the uniform composition of bimetal selenides and nitrogen-doped carbon,which can possess abundant active sites,promote the electrochemical reaction kinetics,and inhibit the volume expansion.In addition,the unique three-dimensional porous structure not only provides structural stability but also facilitates the full contact between the electrode material and the electrolyte.3.Cobalt-nickel selenides and 3D hierarchical porous carbon(butterfly wing derived)composites were successfully synthesized via hydrothermal self-assembly,high temperature oxidation and ion-exchange selenization.By adjusting the concentration of reactants and excipients in the first step,Zephyranthes-like,flake-like and two-dimensional needle-shaped Co₂NiSe₄were grown on 3D porous carbon network structure,respectively.The results show that the Zephyranthes-like selenides and carbon composite(Z-Co₂NiSe₄/BWC)has higher sodium storage properties as anode electrodes.At the current density of 0.1 A g^-1,it shows a first discharge specific capacity of 568 mAh g^-1.After 100 cycles,the specific capacity retention rate is 80.1%of that of the second lap,and the single cycle attenuation rate is 0.2%.At the same time,it exhibits high average reversible capacity of 524,467,426,376 and 318mAh g^-1 at 0.05,0.1,0.3,0.5 and 0.8 A g^-1,respectively.When the current density returned to0.1 A g^-1,the reversible capacity can be restored to 405 mAh g^-1.This is due to the fact that the strategy effectively combines the advantages of multi-component effect and structure,which can increase the exposure of redox active centers,accelerate the kinetics of electrochemical reaction,and stabilize the electrode structure during charge and discharge.4.Using 3D hierarchical porous carbon skeleton derived from butterfly wing as conductive substrate,cobalt-nickel selenide and carbon composites were obtained by microwave method,salt etching and selenization at different temperatures.The results show that the composites prepared by selenization at 160?(Ni-Co-Se/BWC-160)exhibits the best sodium storage performance.It shows a first specific capacity of 703 mAh g^-1 at 0.1 A g^-1 and remains at 403 mAh g^-1 after 100 cycles,with a high coulombic efficiency of 99.5%.At the same time,it possesses excellent rate performance,and the reversible capacity are 500,417,360,310,247 and 211 mAh g^-1 at 0.05,0.1,0.3,0.5,0.8 and 1 A g^-1,respectively.When the current density is restored to 0.1 A g^-1,the reversible capacity recovery rate is close to 90%.The excellent sodium storage performance of Ni-Co-Se/BWC-160 is attributed to the uniform dispersion and firm growth of nano-scale cobalt nickel selenide on porous carbon skeleton,which not only provides abundant redox sites for electrochemical reaction,but also makes the ion/electron transfer and diffusion between cobalt nickel selenide and carbon substrate more efficient,and effectively accelerates the kinetics of electrochemical reaction.