Preparation And Lithium Storage Performances Of MOF-Derived Cobalt-Based Metal Sulfides/Graphene Composites

Negative electrode material is one of the important components of lithium ion batteries,but the specific capacity of commercial graphite anode material is low,which is difficult to meet the needs of future development.Cobalt sulfide is a kind of material with high theoretical specific capacity,but its conductivity is poor and its volume changes greatly during electrochemical reaction.Metal-organic framework（MOF）can be used as a precursor to prepare cobalt-based sulfides to effectively solve the above problems.In this thesis,cobalt-based sulfide derived from MOF was studied,and three kinds of composite materials were prepared to explore their lithium storage properties when used as anode materials for lithium ion batteries.The main contents of this thesis are as follows:The mixture of N,N-dimethylformamide and deionized water was used as solvent to prepare ZIF-67,and then the CoS₂-nitrogen-doped carbon/graphene（CoS₂-NC/r GO）composite was prepared by gas phase vulcanization method after electrostatic assembly with graphene oxide（GO）.The effect of graphene content on lithium storage performance of CoS₂-NC/r GO composite materials is explored.The results show that when the content of graphene is 20%,the lithium storage performance of the composite is the best.The specific discharge capacity of the material is 1137 and 586 m Ah g^-1 at the current density of 0.1 and 5 A g^-1,and 1272 m Ah g^-1 after 400 cycles at the current density of 0.5 A g^-1.In order to further improve the electrochemical activity,Ni element was doped into CoS₂material.The nickel-doped CoS₂/graphene(Co_0.95Ni_0.05S₂/r GO)composite was prepared by liquid-phase synthesis and gas-phase vulcanization method.The effects of Ni doping ratio and graphene content on lithium storage performance of the Co_0.95Ni_0.05S₂/r GO composite were investigated.The results show that when the doping ratio of Ni is 5%and the content of graphene is 20%,the lithium storage performance of the composite material is the best.The discharge capacity of the material is 1213 m Ah g^-1 and 632 m Ah g^-1 at current density of 0.1and 5 A g^-1,and 1007 m Ah g^-1 after 500 cycles at current density of 0.5 A g^-1.In order to further improve the cyclic stability of CoS₂ material,the zinc-cobalt bimetallic organic framework was prepared.Subsequently,zinc-cobalt sulfide/graphene（Zn-Co-S/r GO）composite was prepared by liquid phase synthesis and hydrothermal vulcanization.The effect of zinc to cobalt ratio and graphene content on lithium storage performance of the Zn-Co-S/r GO composite were investigated.The results show that when the ratio of zinc to cobalt is 1:2 and the content of graphene is 20%,the lithium storage performance of the composite material is the best.The specific discharge capacity of the material is 1152 and 655 m Ah g^-1 at the current density of 0.1 and 10 A g^-1,and 908 m Ah g^-1 after 550 cycles at the current density of 0.5 A g^-1.