Application Of Core-Shell Structure COF-Coated MOF-Derived Materials As Cathode Materials For Lithium-Sulfur Batteries

The theoretical energy density of the lithium-sulfur（Li-S）batteries can reach2600Wh/kg,and it is recognized as the most promising next-generation lithium secondary battery.However,the development of lithium-sulfur batteries is hindered by the shuttle effect of lithium polysulfide（Li PS）and the slow reaction kinetics of sulfur anodes.In response to the above problems,this thesis starts with the structural design of cathode materials and combines the latest knowledge of the roles of porous carbonaceous materials,metal oxides,transition metals and covalent organic frameworks in lithium-sulfur batteries to develop two multipurpose sulfur hosts.Two kinds of core-shells,Co/Ti-NC@Tp Pa and Co/Zr-NC@Tp Pa,were prepared by a multi-step reaction to coat metal-organic framework（MOF）porous carbon/transition metal-derived materials with covalent organic frameworks（COFs）.Structure nanoporous high-performance cathode materials,study their microscopic morphology,thermal stability,specific surface area,etc.and conduct electrochemical performance tests on them.The research work of this paper is as follows:（1）In this paper,MIL-125-NH₂was prepared by hydrothermal reaction,followed by Co doping and carbonization treatment,followed by organic Schiff base reaction to prepare Co/Ti-NC@Tp Pa composite.In the micro morphology analysis,the average particle size of Co/Ti-NC@Tp Pa is about 500 nm.The nitrogen doped carbon matrix not only improves the conductivity of the material,but also acts as a physical barrier or even a slight chemical trap to block polysulfides.The Ti O₂and Co nanoparticles in the carbon matrix enhance the interaction with polysulfides,reduce the shuttle effect,and accelerate the conversion kinetics of polysulfides.In addition,the microporous nature of Tp Pa-1 allows Li⁺to migrate but inhibits the penetration of polysulfides.The COF shell with ultra-small pores and polar functional groups further reduces the shuttle of Li PS through screening.The Co/Ti-NC@Tp Pa composite material combines the advantages of Tp Pa-1,catalytic cobalt,and metal oxide conductive carbon,and shows good electrochemical performance.For example,the S/Co/Ti-NC@Tp Pa positive battery with excellent cycle stability provides a high initial specific capacity of 1135 m Ah g^-1at 0.2C and a high specific capacity of 1020m Ah g^-1after 100 cycles.In addition,the high capacity retention rate in 500 cycles at 1C is75%.More importantly,the S/Co/Ti-NC@Tp Pa cathode shows a high area capacity(3.6m Ah cm^-2)under a high sulfur load of 4.46 mg cm^-2.（2）UIO-66-NH₂is doped with Co and carbonized to form a MOF-derived material Co/Zr-NC.In the analysis and test of the microscopic morphology,it can be clearly seen that the inner core Co/Zr-NC is a dense particle size of about Co/Zr-NC@Tp Pa composites were prepared by controlling the time and temperature of the Schiff base reaction to control the micropores of COF materials.Its core perfectly inherits the structure of the MOF material.As the main conductive chain,it has structural stability,which is beneficial to the rate and stability of the battery.The doping of Co on the carbon matrix endows the composite with excellent catalytically active sites,which significantly promotes the rapid redox conversion of sulfur species,thereby delaying the lithium polysulfide shuttle.The improved Tp Pa-1 shell not only retains its ability to selectively pass through,but also greatly increases the overall number of micropores in the material,thereby increasing the overall specific surface area and adsorption of the material.The S/Co/Zr-NC@Tp Pa positive battery can provide a high initial specific capacity of 792 m Ah g^-1at 1 C and a high capacity retention rate of 76.4%after 500 cycles.More importantly,the S/Co/Zr-NC@Tp Pa anode shows a high area capacity(4.15 m A h cm^-2)under a high sulfur load of 5.1 mg cm^-2.In addition,the S/Co/Zr-NC@Tp Pa positive electrode assembled soft-pack battery exhibits excellent flexibility in a flat or curved state,and can provide a high capacity of 751.5 m Ah g^-1after 50 cycles at 0.1 C.