Study On Functional Regulation And Performance Of Covalent Organic Frameworks

Covalent organic frameworks(COFs)are a class of crystalline polymers,which are constructed by light elements and connected by covalent bonds.COFs have attracted tremdous attentions and developed rapidly in the last decade.COFs which possess good thermal/chemical stabilities,controllable structures,adjustable pore structures and high surface area values.With these advantages,they have been utilized in many application fields,such as gas storage and separation,catalysis,optoelectronics and biomedicine.Based on different design strategies,the structure and functionalization of COFs can be precisely controlled.According to the applications of the materials in different fields,COFs with specific structures can be precisely controlled and designed,which makes COFs based materials show great application potentials in terms of functionalization.Although numerous of COFs have been reported,there are still have major challenges in certain fields.Such as,fewer types of ionic COFs,preparation of COFs containing active anchor points via one-step synthesis,employing COFs as solid electrolyte material still relatively weak in the applications of all-solid lithium-ion batteries and fewer building blocks for construction of new three-dimensional COFs.To solve the above mentioned challenges,we employed different methods and strategies to regulate the structure and functional of COFs to satisfy the needs in different fields.Based on bottom-up strategy,a new two-dimensional imidazolium-based cationic COFs were constructed through imine linkages under condensation via precisely controlled the reaction conditions(Im-COF-Br),which possesses charge characteristic,good stability and highly ordered porous structure.Furthermore,by adopting an ion-exchange strategy,the counter-ion Br^–was replaced by bis(trifluoromethylsulfonyl)imide(TFSI^–)ions,which endows the COF based material with good suitability for improving the lithium ion conductivity as well as good crystallinity and highly ordered porous structure.For optimizing the process of exploring reaction conditions for COFs,several 2D COFs(dCOFs)with different defect degree were synthesized via three-component condensation strategy,which possess good crystallinity,porosity,and active amine as anchoring points for further post-functionalization.By employing the post-modification strategy,imidazolium functional groups were introduce into pore walls of COFs via Schiff-base reaction,and by using ion-exchage strategy,final product for all-solid-state electrolytes were obtained(dCOF-ImTFSI-60).Considering the higher symmetry of building blocks to construct of COFs,buliding block linker exchange strategy was employed to transform the structure of COFs for realising the structure modulation.Relying on the principle of dynamic covalent chemistry,the structural transformation between three-dimensional COFs(COF-300 andCOF-320),and dimensional transformation from 3D(COF-301)to 2D(TPB-DHTP-COF)were performed.The successful realization of these structural transformations were confirmed by a series of measurements.The conversion of COFs structure was precisely controlled by adding different equivalents of linker exchange building blocks under solvothermal conditions,and the influences of different reaction time on the crystallization behaviours were also conducted,and the COF-to-COF transformation may happen at the solid-liquid interface.At last,we employ Im-COF-TFSI and dCOF-ImTFSI-60 as COFs-based all-solid-state electrolyte materials to explore the application potential in the field of lithium-ion batteries.COFs-based electrolyte materials exhibit excellent lithium ion conductivity,wide operating temperature range(303 to 423 K),wide electrochemical window,good thermal stability,and interfacial stability.The assembled Li/COFs-based electrolyte material/Li Fe PO₄ all-solid-state lithium-ion battery exhibited stable and excellent battery performance.