Research On The Application Of Carbon Fiber And Its Composite Materials In Lithium-sulfur Batterie

Lithium-sulfur batteries have attracted much attention due to their high energy density,but the shuttle effect of polysulfide is the biggest challenge facing the commercialization of lithium-sulfur batteries.The purpose of this paper is to use the electrostatic spinning technology and heat treatment method combined with the preparation of catalytic materials embedded in porous carbon fiber membrane,and then the active material is placed between the two layers of composite carbon fiber membrane to build an electrode with integral sandwich structure of current collector,cathode material and middle layer,which can maintain the stability of the positive electrode structure,and coordinate the volume change of active substance sulfur in electrochemical reaction.Realized with the spread of polysulfide collection and recycling,and embed the polarity of the catalytic materials for polysulfide has strong chemical adsorption and catalysis,strengthen the capture of polysulfide and promote the electrochemical reaction,which inhibited the shuttle of polysulfide and effectively alleviated the shuttle effect of lithium sulfur batteries.The morphology,structure and composition of the composite carbon fibers embedded with electrocatalytic materials Co,CoP/Coand WC,as well as the adsorption and catalytic properties of polysulfide were investigated respectively.The sandwich structure electrodes were prepared,the electrochemical properties were characterized,and the mechanism of their interaction with polysulfide was revealed.Through the above research,an independent positive electrode design method with functions of both current collector and interlayer was constructed,which provides a reference for the development and application of lithium-sulfur batteries with long cycle life and high specific energy.Specific studies are as follows:（1）Co-embedded porous carbon fiber（Co-PCNF）was prepared by combining electrostatic spinning technology and heat treatment,and the influence of the embedding of Conanoparticles on the properties of carbon fiber was explored.The electrochemical properties of Co-PCNF sandwich electrode,carbon fiber（CNF）sandwich electrode and traditional aluminum foil current collector electrode were compared and analyzed.It is found that Co-PCNF sandwich electrode can effectively inhibit the shuttle of polysulfide and improve the utilization rate of active materials,thus improving the electrochemical performance of the battery.The Co-PCNF sandwich electrode with the initial discharge capacity of 1013.3 mAh g^-1at 0.5 C current density,the discharge capacity remains 794.3 mAh g^-1after 500 cycles,and the average capacity decay rate is only 0.04%.In addition,the average capacity decay rate is only 0.08%at 1 C for 500 cycles.After 150 cycles at 0.2 C,the capacity is still up to 1009.8 mAh g^-1when the sulfur loading is increased to 2.2 mg cm^-2.（2）On the basis of（1）,the prepared Co-PCNF was phosphating to obtain porous carbon fiber（CoP/Co-PCNF）Co-embedded with cobalt phosphide.The electrical conductivity,catalytic performance and polysulfide adsorption of the composite carbon fibers before and after phosphating were compared and analyzed,and the effect of Co/CoP co-embedding on the electrochemical performance of sandwich electrode after phosphating was discussed.The results show that the electrochemical performance of CoP/Co-PCNF sandwich electrode is better than that of Co-PCNF sandwich electrode,and the specific discharge capacity,rate performance and cycle stability of CoP/Co-PCNF sandwich electrode are improved.The CoP/Co-PCNF sandwich electrode still has excellent cycle stability even under a large sulfur load.At1 C current density,the initial specific capacity of CoP/Co-PCNF sandwich electrode is 993.5 mAh g^-1,the discharge capacity remains 814.18 mAh g^-1after 300 cycles,and the decay rate per cycle is only 0.06%.When the sulfur load is increased to 4.5 mg cm^-2,the initial discharge capacity at 0.2 C is 700.1 mAh g^-1.After 200 cycles,the capacity retention rate reaches 91%.（3）Different proportions of porous carbon fibers（WC-PCNF）embedded with tungsten carbide were prepared by changing the mass ratio of PAN to ammonium metatungstate by electrostatic spinning and heat treatment.Through the characterization of morphology,structure,composition,adsorption of lithium polysulfide and other properties,as well as the electrochemical performance of the sandwich electrode,it was found that the optimal mass ratio of PAN to ammonium metatungstate was 2:1.At this time,the prepared WC-PCNF₂has a hollow and porous structure with high porosity and large surface area,and a large number of WC nanoparticles are distributed in the fiber,which increases the active sites of polysulfide redox reaction.The electrocatalytic performance of WC can also promote the conversion reaction of polysulfide and reduce the shuttle phenomenon.The initial discharge capacity of WC-PCNF₂sandwich electrode at 1 C is 1030 mAh g^-1,the discharge capacity remains 872.5 mAh g^-1after 200 cycles,and the capacity retention rate is up to 85%.When the sulfur loading is increased to 4 mg cm^-2,the initial discharge specific capacity of 723.4 mAh g^-1is obtained at 0.2 C.After 200 cycles,the discharge specific capacity of 636.62 mAh g^-1is maintained,and the capacity retention rate reaches 88%.