Preparation Of MOFs Derivative Composite And Its Application In Lithium Sulfur Batteries

Lithium-sulfur（Li-S）batteries with high energy density have broad development prospects.MOFs materials show advantages in solving the problems such as slow reaction kinetics,shuttle effect of polysulfide lithium battery and poor cycling.In this paper,the MOFs derived composite materials with different functional components are combined with the characteristics of different MOFs materials and the different suitable functions,and the research is carried out from the positive electrode modification of lithium sulfur battery and multi-functional interlayer.Firstly,core-shell type Co-NC@TpBD-Me₂cathode material was prepared by wrapping a layer of microporous TpBD-Me₂COF material with hollow structure ZIF-67 derived Co embedded n-doped mesoporous carbon material（CO-NC）.Then,UiO-66-NH₂/CNT interlayer material was constructed by combining CNT membrane with UiO-66-NH₂membrane,which was easy to form MOFs membrane and had hydrophilic properties and amino functional groups.The morphology,structure and specific surface area of the two materials were studied respectively,and the electrochemical performance of different components in Li-S battery and their influence on redox kinetics were analyzed.The research work of this paper is as follows:（1）For the multi-functional sulfur main design strategy with hollow ZIF-67 derived materials and covalent organic framework（COF）constraints,the strategy adopted an internal porous catalyst and an external microporous COF layer.The built-in hollow catalyst,consists of carbon supported cobalt nanodots（Co-NC）derived from ZIF-67metal-organic framework,servers as sulfur accommodation and catalytic center for Li PSs trapping and conversion.The Co-NC is wrapped by a thin microporous TpBD-Me₂COF layer with polar functional groups,providing additional physical and chemical confinement for polysulfides shuttling.With these merits,the cell with S/Co-NC@TpBD-Me₂cathode exhibits a high initial capacity of 1178.15 mAh g^-1at 0.2 C.The cell also delivers a high rate performance(543.13 mAh g^-1)up to 5 C.Even at raised sulfur loading(5.71 mg cm^-2)and lean electrolyte condition(E/S ratio 6.10μL mg^-1),a 4.53 mAh cm^-2high area capacity is achieved after 50 cycles.The Li-S pouch cell also achieves a specific capacity of 860.35m Ah g^-1at 0.1 C after 50 cycles.This COF-wrapped multifunctional composite design provides an alternative strategy for the fabrication of high-performance Li-S batteries.（2）We used carbon nanotube thin film as the substrate and supported continuous UIO-66-NH₂membrane as ion osmotic selective interlayer.The UiO-66-NH₂membrane with appropriate pose size can act as an ionic sieve to constitute a physical barrier to polysulfides while allowing the transport of lithium ions during electrochemical cycles,therefore validly hindering the shuttle of polysulfides.Moreover,a fast catalytic conversion of polysulfides is also achieved by employing the as-developed interlayer.Therefore,lithium-sulfur batteries with this MOFs-based interlayer exhibit an admirable initial capacity of 999.21 mAh g^-1at 1 C,and a durable cyclic stability with a limited decay rate of0.04%per cycle for 300 cycles.Besides,even under increased sulfur loading(4.79 mg cm^-2),a high area capacity of 4.29 mAh cm^-2is also gained.（3）By exploring the characteristics of different MOFs materials,different MOFs-derived composite materials are constructed for Li-S battery cathode modification and multi-functional interlayer respectively.Both materials show excellent electrochemical performance.It is of great significance for the development of lithium sulfur battery to construct composite materials for different metal sites,structures and functional groups of MOFs materials for different parts of lithium sulfur battery.