Preparation Of BMZIF Derrived Carbon Materials And Study On Application And Electrochemical Properties In The Cathode Of Lithium-Sulfur Batteries

As a kind of efficient energy storage system,the theoretical specific energy of lithium-sulfur battery is 2600Wh/kg,which is much higher than the current commercial energy storage secondary battery.With abundant sulfur reserves,low price,safety and environmental protection,lithium-sulfur battery has a great application prospect in the future commercialization.However,there are still many problems in lithium-sulfur batteries.The elementary sulfur as the positive electrode and the final discharge product lithium sulfide are insulated,and the bulk expansion phenomenon in the process of charge and discharge and the"shuttle effect"of the intermediate product lithium sulfide limit its application.Metal-organic framework MOFs are polymer materials with network structure independently assembled by organic ligands containing nitrogen and oxygen and metal ions.Metal-organic framework has a large specific surface area and abundant pore pore channels.Moreover,the pore size and structure of the metal-organic framework can be adjusted through experiments.In addition,the metal-organic framework can obtain different physical and chemical properties by introducing different functional groups.However,the conductivity of metal organic skeleton is very poor,but the metal organic skeleton as a carbon before body,through direct carbonation method is transformed into the heteroatom doping carbon material,in this way,organic metal skeleton material not only has good electrical conductivity,also inherited the porous structure of the precursor,has a larger specific surface area,Good electrochemical performance can be obtained when used in lithium sulfur batteries.In order to improve the performance of sulfur cathode materials,carbon matrix composites with porous carbon doped transition metal nanoparticles were designed to improve the electrochemical performance of lithium-sulfur battery cathode materials by taking advantage of the good conductivity of porous carbon materials and the strong absorption and catalysis of transition metal particles to charge and discharge intermediates.Specific research contents are as follows:（1）Bimetallic organic skeleton（BMZIF）was prepared by solvothermal reaction of Zn（NO₃）₂ and Co（NO₃）₂ with 2-methylimidazole at room temperature.The bimetallic organic skeleton was used as the precursor for sintering at different carbonization temperatures.The effects of different carbonization temperatures on the morphology and properties of BMZIF materials were studied.The material（Co（Zn）-C）prepared at 900°carbonization temperature has a high degree of carbonization,and presents uniform hollow polyhedron morphology by SEM test.The cathode material loaded with sulfur has excellent electrochemical performance,and the initial specific capacity of cyclic charge-discharge is 1100.2 mAh/g when the current density is 0.1C.（2）in preparation of zinc with Co doped double metal organic skeleton（BMZIF）as substrate,the hard template method is adopted to SiO₂ as templates,in forming a bilayer BMZIF surface coated SiO₂ structure,the organic compound is encased secondary octadecyl silane(C₁₈TMS)three oxygen radicals,finally through high temperature carbonization and Na OH etching SiO₂ was prepared on the surface of the drop BMZIF a high conductivity layers structure of porous carbon materials Co（Zn）-C-C.Compared with the single structure of BMZIF,Co,Zn-C-C materials,they have more complex carbonization structure,which can increase the sulfur carrying rate of carbon-based materials and alleviate the volume expansion of cathode materials in the process of charge and discharge.In addition,Co（Zn）-C-C materials also have the advantages of BMZIF itself.Co,Zn nanoparticles can interact with polysulfide intermediates in charge and discharge,which reduces the diffusion of polysulfide in electrolyte and thus inhibits the"shuttle effect".Through the characterization of the morphology of Co（Zn）-C-C materials and the electrochemical test of Co,Zn-C-C/S materials,it is found that the coated Co（Zn）-C-C materials have better rate performance and cycling stability than BMZIF materials.（3）Metal-organic skeleton derived carbon materials are prepared by direct carbonization.However,there are many ways to prepare carbon matrix composites with porous carbon doped transition metal nanoparticles.In order to study the advantages of preparing carbon materials,a comparative test was set-the preparation method of carbonization followed by doping.Taking yeast as the preliminary carbonization precursor,carbonization at high temperature generated carbonization material（CY）material.Then Fe SO₄and CY material were evenly stirred,and Fe ions were doped into the carbon material through hydrothermal reaction.Finally,iron-doped yeast carbonization material（Fe-CY）was obtained by high temperature sintering.By direct carbonization of biological materials with complex structure,the original pores are retained,and the sulfur carrying rate of the materials is improved.At the same time,doping a certain amount of Fe ions is conducive to the adsorption of sulfur.Iron has a strong absorption and catalytic effect on lithium sulfide,which can effectively inhibit the shuttle effect and improve the rate performance and cycling performance of the electrode.At the first cycle of 0.1C,the average capacity of ten cycles of S@Fe-CY material is 1100 mAh/g.After 50 cycles,the current drops again to 0.1C,with an average capacity of 1085 mAh/g,which is almost the same as the capacity after the first cycle.