Study On Modification Of Cathode And Properties Of Lithium Sulfur Battery Based On Metal-organic Frameworks

Both the lithium metal anode(3860 mA·h/g)and sulfur cathode(1675 mA·h/g)in lithium-sulfur battery have high theoretical specific capacity,which is considered to be one of the most promising candidates for next-generation energy storage devices,but its disvantsges,such as poor conductivity of the electrodes,high polysulfide dissolution,and large changes in electrode volume,lead to a decrease in the electrochemical performance of the battery.Metal-organic frameworks(MOFs)are inorganic-organic hybrids,which can physically and chemically adsorb polysulfides through their high specific surface area,porous structure and chemical bonding.Moreover,a variety of complexes can be synthesized based on MOFs,which can improve the conductivity of the electrode and adsorb the polysulfide.In this study,MOFs an d their derivatives were prepared as the cathode hosts of lithium-sulfur battery and their electrochemical properties were studied.The specific work content is as follows:Firstly,MIL-101(Cr)with high specific surface area and micro-mesopor structure was prepared by hydrothermal method,then S/MIL-101(Cr)was obtained by chemical precipitation and heat treatment,and its electrochemical performance was tested.From the weight loss and content of sulfur,the specific surface area and the micropore volume of the composites can be obtained.The heat treatment can make more sulfur occupy the pore structure of MIL-101(Cr),and greatly exert its physical and chemical adsorption on sulfur.In contrast,S/MIL-101(Cr)-3 has good electrochemical performance,the initial discharge specific capacity as 879 mA·h/g at 0.1 C,and after 50 cyclesathe specific discharge capacity is 517 mA·h/g.The battery retains a discharge specific capacity for 302 mA·h/g after 200 long cycles at 0.5 C.Secondly,ZIF-67 prepared by rapid ultrasonic method is a precursor of high temperature calcination to synthesize nitrogen-doped porous carbon-oxide(NC-Co3O4).The presence of porous carbon in NC-Co3O4 can enhance the conductivity of electrode and provide a transport channel for Li+.The doping of heteroatoms and oxides in NC-Co3O4 facilitates chemisorption of polysulfide and catalytic electrode charge transferuthereby improving the specific capacity,cycle stability and rate performance of the lithium-sulfur battery,SEM,TEM images and BET characterization show that NC-Co3O4 retains the morphology and pore structure of precursor ZIF-67,and its particle edge is slightly thin and light.It is concluded that NC-Co3O4 is a structure of oxide encapsulated by porous carbon thin layer.The first discharge specific capacity of S/NC-Co3O4 composite is 1018 mA·h/g at 0.1 C,and after 50 cycles,the specific discharge capacity is 643 mA·h/g.The battery has good cycle and rate performance.Finally,ZIF-67 was used as template and cobalt source,and thioacetamide was used as vulcanizing agent to synthesize Co3S4 and Co9S8 with hollow porous polyhedron structure,Co3S4 and Co9S8 not only improve the conductivity of the electrode,to reduce the electrical resistance of the electron transfer,but also have a high binding energy with the polysulfide and a polar chemisorption of the polysulfide.The hollow porous polyhedral structure of Co3S4 and Co9S8 can provide abundant Li+ transmission channels and effectively alleviate the volume change of the electrodes.The hollow structure and polyhedral shape of Co3S4 and Co9S8 can be observed by SEM and TEM images.S/Co9S8 composite has excellent cycle stability and rate performance,with its initial discharge specific capacity of 884 mA·h/g at 0.1 C,and the discharge specific capacity of 554 mA·h/g retained after 50 cycles.