Fabrication Of Metal Sulfide/Graphene Composites Derived From MOFs And Study Of Their Lithium Ion Battery Performance

Recently,transition metal sulfides have been considered as the promising electrode materials for lithium ion batteries because of their high theoretical capacities and power density.However,metal sulfides suffer from poor conductivity and unavoidable large change of volume during charging/discharging processes,which results in the poor rate capability and cyclic stability.The common method to address the problem is to design and synthesize electrode material with nanostructured or combine with carbon materials（such as graphene）to improve their electrochemical properties.Therefore,this work aims to fabricate metal sulfides with high electrical conductivity,high specific surface area,suitable pore structure and size,as well as the improved electrochemical performance as the anode materials for lithium ion batteries.In this thesis,metal organic frameworks（MOFs）are used as templates to synthesize metal sulfides with uniform morphology.However,the general calcining method is always costly,complicated,and easy to destroy the morphology of the precursor.Therefore,Ni_0.2Co_0.8S and FeS₂ with specific morphology were synthesized by simple reflux and solvothermal method,and then combined them with graphene to form composites with good electrical conductivity and high specific surface area via self-assembly method.Specifically,Ni-Co-ZIF precursor was prepared by the reaction of cobalt nitrate hexahydrate and nickel nitrate hexahydrate with dimethyl imidazole.And then Ni_0.2Co_0.8S was generated using sodium sulfide as sulfur source to transfer Ni-Co-ZIF by solvothermal method.Finally,the hollow cobalt-based bimetallic sulfide attached on reduced graphene oxide(Ni_0.2Co_0.8S@rGO)was synthesized by a simple self-assembly process and freeze-drying treatment.The reflux and self-assembly processes were carried out at a relatively low temperature,which was benefit to inheriting the unique morphology of Ni-Co-ZIF precursors.Scanning electron microscopy and transmission electron microscopy showed that the product Ni_0.2Co_0.8S-2.5 nanocage exhibited a uniform morphology and hollow nanostructure,which were well maintained after the introduction of rGO.The composite structure of Ni_0.2Co_0.8S@rGO could effectively inhibit the volume change of electrode materials during the cycling process.And the conductivity of the composites was improved by the introducing bimetals and reduced graphene oxide.The results of infrared and impedance spectroscopy analysis reveal that the increasing capacity during the cycle can be partially attributed to the highly reversible decomposition/formationofsolidelectrolyteinterfacelayerduring de-lithiation/lithiation.In the electrochemical evaluation,the capacity of Ni_0.2Co_0.8S@rGO composite increased to 1585 mA h g^-1 after 250 cycles at current density of 1 A g^-1.It is shown that the increase of reversible capacity was mainly due to the electrochemical activation of porous materials,pseudocapacitance behavior and the highly reversible formation/decomposition of SEI film during cycling.Furthermore,Fe-MOF was prepared by the reaction of FeCl₃·6H₂O with terephthalic acid.FeS₂ was synthesized by solvothermal method using the Fe-MOF as template.Then,FeS₂ was combined with graphene at low temperature by self-assembly method in order to maintain the morphology of precursor.Finally the flower-like FeS₂@rGO composite was successfully synthesized.SEM and TEM images showed that the metal sulfide was uniformly coated by graphene.The composite structure of metal sulfide/graphene can not only effectively alleviate the volume change of sulfide during repeated charge-discharge processes,but also accelerate the transport of lithium ions and electrons within the 3D graphene network.The obtained FeS₂@rGO exhibited good electrochemical performance with high specific capacities of 1406 mA h g^-11 after 100 cycles at a current density of 100mA g^-1.