Design And Synthesis Of Phosphorus-containing Covalent Organic Frameworks And Their Application In Lithium Sulfur Battreries

Covalent organic frameworks（COFs）stand out among polymer materials due to their well-arranged porous structures.The unique topological structures design has the advantage of adjustable structural aperture,and the introduction of building blocks with specific functions makes the functional design of COFs possible.At present,the materials show extremely high value in the fields of gas adsorption,optoelectronic materials,and energy storage.COFs are a new class of crystalline porous organic materials linked by dynamic covalent bonds,and their ordered channels are conducive to ion transport.In recent years,COFs,as the main sulfur loading material for lithium sulfur batteries（Li-S）,have attracted extensive attention.Lithium-sulfur batteries use elemental sulfur with abundant reserves as the cathode material,which have the characteristics of high theoretical specific capacity,low cost as well as environmental friendliness.Several problems such as the flammability of elemental sulfur and organic ether electrolytes,the non-conductivity of active materials and discharge intermediates,the shuttle effect of lithium polysulfides and the volume effect of sulfur cathodes have impeded their development speed.To address the above problems,in this paper,four imine-bonded COFs were designed and synthesized by introducing phosphorus-containing building blocks with flame retardant properties,and employed as host materials for lithium-sulfur batteries.The chemical structures,morphological features,pore structures,thermal stability,flame retardancy and electrochemical properties of the four COFs were investigated in detail.The main research contents are as follows:（1）Using tris（4-formylphenyl）phosphonate（TFP）with flame retardant function as the building block,we designed and synthesized four novel phosphorus-containing covalent organic frameworks linked by imine bonds.Fourier transform infrared（FT-IR）spectra,solid-state ¹³C cross-polarization magic-angle spinning（CP MAS NMR）spectra,X-ray photoelectron spectroscopy（XPS）and other tests demonstrated the successful preparation of four COFs.All of these four COFs possess high crystallinity,regular pore structure,good thermal stability and morphology.The good flame retardant property of the four COFs materials were proved by the combustion test and the flame retardant mechanism was analyzed.（2）Using COFs material with flame retardant property as elemental sulfur carrier,four kinds of composite materials with flame retardant property were prepared as cathode materials for Li-S batteries.The prepared four composites have higher specific ignition time（SIT）and lower specific self-extinguishing time（SET）compared with elemental sulfur,among which COF-P4/S composite exhibits the fastest self-extinguishing behavior and has the best flame retardant performance.Due to the good crystallinity,porous structure and heteroatoms（N,O）of COFs materials,all four composites show good physicochemical synergistic adsorption effect of polysulfide,which inhibite the"shuttle effect"and improve the utilization of sulfur.In addition,the regular pores of the crystalline COFs can limit the volume change of sulfur and facilitate the rapid transport of lithium ions.Compared with pure sulfur cathode,all four composite cathodes exhibit higher initial specific capacity,good cycling stability and rate capability.Among them,the COF-P4/S composite cathode has an initial discharge specific capacity of 868 m Ah g^-1at a high current density of 1 C,and still retains a discharge specific capacity of 805 m Ah g^-1after500 cycles,with a low average attenuation rate of 0.035%per cycle.