Preparation And Adsorption Of Iodine And Lithium Polysulfide By Cyclotriphosphoacinyl Covalent Organic Framework

Two-Dimensional Covalent Organic Frameworks（2D COFs）have excellent material characteristics such as low density,ordered pore,and high thermal and chemical stability,making them show excellent adsorption performance in the adsorption field.The results show that most of the COFs are composed of rigid elements,and the study of COFs with flexible frame structures is not deep enough.Therefore,it is necessary to expand flexible construction units,especially in which N,O,P,and S atoms are both flexible sites and electron-rich active sites of flexible construction units,showing excellent adsorption capacity for iodine and lithium polysulfide.However,there are few reports about the synthesis of COFs with different pore sizes by flexible construction units,especially the synthesis of cyclotriphosphonitrile based COFs with triangular pore topology using（C₆+C₂）as structural configuration.Based on this,the main research content of this paper is as follows:（1）To explore the synthesis of a series of cyclotriphosphonitrile based COFs using a flexible construction unit with a symmetrical structure of C₆ as the parent.Hexadecyl-formyl-phenoxy cyclotriphosphazonitrile（CTP-6）was selected as the node of the C₆-symmetric ligand containing a phosphorus-oxygen bond.It is linked to four amino groups of p-phenylenediamine（PDA）,4,4-diaminodiphenyl ether（ODA）,5,5-diamino 2,2-bipyridine（BPY）,and 4,4-diaminotribiphenyl（DPT）with different molecular lengths.A series of cyclotriphosphonitrile-based COFs（CTP-X-COF,X=PDA,ODA,BPY,DPT）with triangular pore topology were synthesized by the solvothermal method.Through Materials Studio（MS）simulation and Pawley finishing,the P1 space group and AA stacking structure of four cyclotriphosphonitrile based COFs were determined.The successful synthesis of cyclotriphosphonitrile based COFs was confirmed by solid-state nuclear magnetic（NMR）characterization.Finally,the stability of the four prepared cyclotriphosphazonitril-based COFs compounds was studied,showing that they have excellent thermal stability and solvent resistance.（2）Explore the influence of abundant active sites and triangular pore structure of cyclotriphosphonitrile-based COFs on the adsorption performance of iodine.The adsorption properties of four cyclotriphosphonitrile based COFs on iodine vapor were studied by gravimetric method.It was found that the adsorption capacities of CTP-PDA-COF,CTP-ODA-COF,CTP-BPY-COF,and CTP-DPT-COF for iodine vapor were 4.65 g g^-1,4.18 g g^-12.38 g g^-1 and 3.57 g g^-1,respectively.In addition,the adsorption properties of four cyclotriphosphonitrile based COFs in cyclohexane solution were studied by isothermal adsorption experiments.The results showed that the adsorption capacities of CTP-PDA-COF,CTP-ODA-COF,CTP-BPY-COF,and CTP-DPT-COF for iodine in cyclohexane were 281.50 mg g^-1,221.67 mg g^-1,161.14 mg g^-1 and 222.67 mg g^-1,respectively.The results showed that cyclotriphosphonitrile COFs exhibited good adsorption properties in both gas and liquid phases.Through isothermal adsorption,kinetic adsorption,and chemical state analysis of iodine adsorption,the adsorption mechanism of cyclotriphosphazonitril-based COFs on iodine molecules was investigated.It was shown that the adsorption process was a mixed adsorption process with physical adsorption as the main and chemical adsorption as the auxiliary.In addition,the chemical state analysis of the adsorbed iodine also found that the adsorbed iodine in the material exists in the form of molecules and polyiodide anions,which confirmed that the guest molecular iodine forms a strong coulomb force with the electron-rich subject COFs material,which will make the flexible frame distorted and collapsed.Finally,it was confirmed that all four COFs had excellent performance of at least 5 repeated adsorption.（3）To explore the influence of covalent organic frame nanosheets（CONs）and multi-walled carbon nanotubes（MWCNT）on the performance of Li-sulfur batteries.CTP-PDA-COF and CTP-ODA-COF based COFs were separated into nanosheets by solvent ultrasonic stripping method.It is known that electronic-rich groups in CONs can effectively anchor lithium polysulfide in the electrolyte,and its microporous triangular pores can physically adsorb lithium polysulfide.The CONs are then blended with MWCNTS that provide efficient conductive channels.The experimental results show that the composite exhibits better cycle stability and rate performance than the unmixed MWCNT and CONs.The specific discharge capacity of CTP-PDA-COF/MWCNT/S is still maintained at 511 m Ah g^-1,which corresponds to the capacity retention rate of 85.9%under the current of 0.5 C for 200 cycles.The specific discharge capacity of CTP-ODA-COF/MWCNT/S was increased to 636 m Ah g^-1.It is proved that CONs can further optimize the physical and chemical constraints on poly-lithium sulfide by its abundant triangular pore size and electron-rich groups.MWCNT can also improve the electrical conductivity of composite matrix materials.