| As an energy storage device with high energy conversion efficiency,long cycle life and high energy density,lithium-ion batteries have been widely used in portable electronic devices.When lithium-ion batteries are used as the power source of electric vehicles,the power density and energy density cannot meet the practical application requirements.Therefore,it has important theoretical significance and practical application value to explore a new chemical power source different from the existing lithium-ion batteries technology.Recently,lithium sulfur batteries have developed rapidly because of their large capacity,high energy density,rich source of sulfur,as well as environmentally friendly.The main obstacles to the commercialization of lithium sulfur batteries are low conductivity of sulfur and lithium sulfide,the shuttling effect of soluble lithium polysulfides species(LiPSs),volume variation caused by conversion of LiPSs,etc.To solve these problems,this thesis attempted to combine reduced graphene oxide(rGO)with the virtues of high conductivity,high mechanical stability,and large specific surface area,and bismuth oxyhalides compounds owing to the strong adsorption and catalysis of LiPSs to prepare a high-loading sulfur cathode material which can effectively inhibit the shuttling effect of LiPSs and promote the conversion kinetics of LiPSs.The main work contents are as follows:(1)A two-dimensional lamellar BiOBr/rGO composite is synthesized by onestep solvothermal method using rGO as substrate with BiOBr depositing.The BiOBr/rGO composite can provide the large specific surface area and high pore volume,which are required for a high sulfur loading.The three-dimensional network structure of rGO constitutes a physical barrier to effectively inhibit the shuttling effect of LiPSs.Moreover,the uniform distribution of BiOBr on rGO is conducive to its full contact with LiPSs to play the roles of adsorption and catalytic effect.The characterization results show that BiOBr/rGO composite can bear high sulfur loading,effectively inhibiting the shuttling effect of LiPSs and promoting its conversion reaction kinetics,leading to enhanced electrochemical properties as a sulfur host material.As a result,the S@BiOBr/rGO electrode shows a special capacity of 882 mA h g-1 after 1000 cycles at 0.5 C.When sulfur loading reaches 4.9 mg cm-1,the reversible specific capacity of 424.6 mA h g-1 can be maintained even after 400 cycles at 0.5C.Moreover,the multiple conversion reactions of sulfur are investigated by insitu X-ray diffraction and in-situ Raman spectroscopy,the results display that the sulfur undergoes a process of α-S8→Li2S8→ Li2S6→Li2S3→Li2S2→Li2S during the discharge process,while the charge process is corresponding reverse reactions with a final product of β-Ss.(2)Due to the higher electronegative of F element than that of Br element,BiOF is expected to exhibit the enhanced adsorption and catalytic effect of metallic Bi.Therefore,two-dimensional BiOF/rGO composite material was designed and synthesized,and its electrochemical performance as a host material of lithium sulfur batteries was investigated.The results show that BiOF/rGO has a high specific surface area and large pore volume,which are beneficial for achieving high sulfur loading.In addition,the rGO can alleviate the volume expansion/extraction during the conversion process of LiPSs.The spectrum of ultraviolet-visible absorption show that BiOF/rGO has excellent adsorption capacity of LiPSs.And the symmetric cells tests and Li2S electrodeposition experiments prove that BiOF/rGO can promote the conversion process of LiPSs.When BiOF/rGO is used as a sulfur host material of lithium sulfur batteries,the electrode can reach the specific capacities of 610.9 and 501.1 mA h g-1 at 3 C and 5 C after 1000 cycles,respectively(the capacity loss rates are 0.044%and 0.048%per cycle),showing a good rate electrochemical performance.In addition,the processes of sulfur conversion are investigated by in situ X-ray diffraction. |
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