Study On Modification Of Fe-based MOFs And Its Derived Meral Oxide Composite As Cathode For Lithium Sulfur Battery

With the continuous development of human society,the issue of fossil energy consumption has attracted much attention.The widespread use of electronic equipment has made people's demand for energy storage batteries increasing rapidly,and their performance requirements are becoming more and more stringent.On the one hand,researchers are looking for electrode materials with higher specific capacity to increase energy density,and on the other hand,they are developing new battery systems to solve the high cost caused by over-reliance on lithium resources.Nowadays,lithium-ion batteries have been widely used in modern portable equipment,but due to the limited storage of lithium resources and high cost,it is difficult to use them in energy storage devices on a large scale.However,due to the high specific capacity(1675 mAh·g-1)and high energy density(2600 Wh·kg-1),lithium-sulfur batteries are lithium-ion batteries(?360Wh·kg-1 about 7 times)and other advantages.In addition,elemental sulfur(positive electrode active material)is non-toxic,low cost,and rich in natural resources.It has broad prospects in other energy storage fields such as electric vehicles and drones.However,it has problems such as poor electrode conductivity,dissolution of polysulfides in the electrolyte,and large volume changes during charging and discharging.Therefore,Fe-based MOFs and their derived metal oxides have been prepared for the positive electrode of lithium-sulfur batteries,and their weak conductivity has been modified to study their structural morphology and electrochemical performance.The specific work content is as follows:(1)Using terephthalic acid(C8H6O4)and ferric trichloride hexahydrate(FeC13·6H2O)as precursors,the Fe-based MOFs material of MOF-235 was prepared by solvothermal method,and the metal oxide(235-Fe3O4/C)was obtained by heat treatment of MOF-235,then 235-Fe3O4/C/S composite material was prepared by loading sulfur with high temperature melting method and the possible energy storage mechanism of the composite material were studied.According to SEM and TEM characterization.The 235-Fe3O4/C obtained after heat treatment basically retains the morphology and pore structure of the precursor MOF-235,and its crystal grain edges are clear and transparent.From the analysis of TGA and BET,it can be known that the negative temperature sulfur by the high-temperature melting method makes more sulfur occupy the pore structure of 235-Fe3O4/C,and exerts its adsorption of sulfur to a certain extent.Compared with MOF-235/S,the first discharge specific capacity of the 235-e3O4/C/S composite at room temperature 0.1 C is 949.7 mAh·g-1,and after 400 cycles the discharge specific capacity is 156.7 mAh·g-1.The battery has good cycle performance.the 235-Fe3O4/C/S composite material has high coulombic efficiency,indicating that the energy storage of the electrode has good reversibility,which is mainly attributed to the large specific surface and pore structure of the Fe3O4/C material provides good ion deintercalation storage sites and transmission channels.In the process of charge and discharge,the iron ions in the metal center can form Fe-S bonds with sulfur,which further strengthens the binding effect on sulfur.(2)The structure of the material determines the performance.In order to explore the effect of iron-based MOF with different microstructures on the performance of the battery.Using fumaric acid(C4H4O4)and ferric chloride hexahydrate(FeCl3·6H2O)as precursors,Fe-based MOFs material MIL-88A was successfully prepared by solvothermal method,MIL-88A is heat-treated to obtain metal oxide(88A-Fe3O4/C),and then loaded with sulfur by high temperature melting method to obtain 88A-Fe3O4/C/S composite material.Compared with MIL-88A/S,the first discharge specific capacity of 88A-Fe3O4/C/S composite at 0.1 C is 940.3 mAh·g-1,the reversible capacity after 100 cycles is 307.9 mAh·g-1,and the coulombic efficiency above 98%.the current density at 0.1,0.2,0.5 and 1C,the reversible capacity reaches 940.3,433.0,107.4 and 0.4 mAh·g-1,respectively.When the current density returns to 0.1 C,the specific capacity can still reach 282.9 mAh·g-1.It shows that the composite material has good cycle performance and stable rate performance.From the electrochemical test results,it is speculated that the porous structure of the composite material can provide a rich transmission channel for Li+and effectively relieve the volume change of the electrode.(3)By adding graphene oxide(GO)-derived metal oxide-Fe3O4/C/rGO during the preparation of Fe-based MOFs,the effect of adding GO on the structure and morphology of Fe-based MOFs-derived oxides and the battery influence of electrochemical performance.The electrochemical performance test of the 235-Fe3O4/C/rGO/S composite derived from MOF-235 showed that its specific discharge capacity was 1519.2 mAh·g-1 for the first time at 0.1 C,and the specific discharge capacity after 50 cycles was 632.0 mAh·g-1.Adding GO during the synthesis of MIL-88A to obtain the precursors,the rest of the steps are the same as above,its specific discharge capacity is 1047.9 mAh·g-1 for the first time at 0.1 C,and the specific discharge capacity is 461.8 mAh·g-1 after 50 cycles,the battery has good cycle performance.It can be seen from the experimental data that the addition of GO significantly improves the specific capacity of the electrode material,This is because the GO and MOFs crystals form a network structure,The rGO sheet conductive structure formed by GO after heat treatment can better achieve ion transmission and electronic conductivity improvement,So that more active materials can participate in the electrode reaction and exert their capacity.