Electrochemical Properties Of Derived Carbon Fiber Materials In Lithium-Sulfur Batteries

The research on lithium-sulfur battery has developed rapidly for ten years,and the key problems that hinder its application are poor conductivity of the cathode material?S?,the structural damage due to volume expansion and the"shuttle effect"of polysulfides and a series of adverse consequences.This paper focuses on the application in lithium-sulfur battery of MOFs.The porous carbon materials of one-dimensional and three-dimensional structure are successfully prepared by using MOFs as pore-forming agent and polyacrylonitrile or glucose as carbon source,which are used as the sulfur-carrying material of lithium-sulfur battery to improve the conductivity of electrode materials,relieve volumetric effects and limit the dissolution of polysulfides.The morphology,phase and specific surface area of the synthesized materials are characterized and their electrochemical properties are analyzed.The main reserch works of this thesis are summarized as follows:1.Porous carbon fibers?FPC-1?are prepared by using ZIF-8 as template and polyacrylonitrile?PAN?as carbon source by electrospinning and high-temperature carbonization method.The electrochemical performance test results show that the initial discharge specific capacities are 990.7,991.8 and 561.6 mAh/g at 0.2 C,0.5 C and 1 C current densities respectively,and the corresponding specific capacities are maintained at 247.8,243.7 and 210.3 mAh/g after 100 cycles.The low specific surface is the main reasons for its rapid capacity decreasing.2.We have modulate the preparation process.The FPC-2 and FPC-3 materials with higher specific surface area and higher porosity are synthesized successfully,which corresponding specific surface areas are 67.8 and 334.4 m²/g,respectively.Electrochemical test results show that the initial discharge specific capacities of S/FPC-2 and S/FPC-3 composites are 1016.0 and 1000.6 mAh/g at 0.5 C current density,respectively,and the corresponding specific capacity are maintained at 442.1 and 434.5mAh/g after 100 cycles.The coulombic efficiencies of the two kind materials are about99%.The S/FPC-3 material has a first discharge specific capacity of 754.0 mAh/g at 2C current density and 394.6 mAh/g after 100 cycles.It is obvious that the electrochemical stability of the S/FPC-2 and S/FPC-3 composite materials are improved compared with that of S/FPC-1 material.3.Porous carbon material?PC?with high specific surface area?2151.9 m²/g?is synthesized by using MOF-5 and glucose as chemical raw materials and hydrothermal method.Adding a small amount of aluminum fluoride?AlF₃·3H₂O?to PC/S composites can effectively improve the cycle stability,and relieve the dissolution of polysulfides in the electrochemical cycle of lithium-sulfur batteries in some extent.The electrochemical results show that the initial discharge specific capacity of AlF₃·3H₂O/PC/S and PC/S composites are 736.1 and 787.5 mAh/g at 0.5 C current density,and 487.2 and 320.9 mAh/g after 500 cycles,respectively.The first discharge specific capacity of AlF₃·3H₂O/PC/S and PC/S materials are 1420.4 and 925.2 mAh/g at 1 C current density,and the capacity of AlF₃·3H₂O/PC/S material still remains at503.4 mAh/g after 800 cycles,while the corresponding PC/S material remains at 366.9mAh/g after 500 cycles.