Design And Energy-Storage Mechanism Of Covalent Organic Framework-based Electrodes

Organic electrode materials with features of environmental-friendliness and low-cost have attracted the attention of many researchers,and are also considered to be one of the most promising next-generation electrode materials.Organic electrode materials of high energy density and high power density can be synthesized at the molecular level by introducing different functionalized organic functional groups.Besides,the energy storage mechanism of the organic electrode generally becomes clear,therefore the novel organic electrode materials can be developed in a targeted manner.However,organic electrodes still exist some obvious defects,such as poor electronic conductivity and easy dissolution in electrolytes,which seriously affect their further applicationsCovalent organic frameworks(COFs)are two-dimensional or three-dimensional porous polymers joined by strong covalent bonds,mainly containing C,N,B,and H elements.COFs have been widely used in heterogeneous catalysis,gas separation,semiconductors,and optoelectronic devices.However,there are few studies on COFs as electrode materials,mainly due to their poor conductivity and layered structure hindering the transport of ions/electrons.Therefore,it is an imperative research direction to enhance the conductivity of COFs and improve their ion/electron transport capabilities.Generally,electrochemical performance of COFs can be improved by introducing conductive materials(carbon nanotubes,graphene,quantum dots,etc.)during the synthesis process.This method has been widely used in the research fields of other organic electrode material systems.In addition,the two-dimensional COFs can be effectively exfoliated with the similar method as what is used to exfoliate graphite.The exfoliated few-layered organic nanosheets can provide more active sites and shorten the activation time,which is an effective method to improve the energy storage properties.Based on the research background mentioned above,the aim of this thesis is to explore and develop COFs as anode materials with excellent electrochemical properties.The electronic conductivity of COFs materials is improved by introducing carbon nanotubes,graphene quantum dots,etc.Meanwhile,several few-layered two-dimensional covalent organic framework composites were prepared by mechanical or chemical exfoliation and in-situ growth methods to improve their kinetics.Finally,a series of composite materials were analyzed and characterized,and the corresponding energy storage mechanism was explored by in-situ/ex-situ Raman,ex-situ FTIR/XPS and density functional theory(DFT)calculation.The specific research contents of this thesis are as follows:(1)Two triazine-based COF materials(CIN-1 and SNW-1)were obtained by a facile solvothermal reaction.Carbon nanotubes were simultaneously added during the synthesis process to obtain composites with stacked structures(CIN-1/CNT and SNW-1/CNT).The composites were modified by mechanical ball milling to obtain exfoliated products,denoted as E-CIN-1/CNT and E-SNW-1/CNT,respectively.These two covalent organic nanosheets are porous polymers connected by covalent bonds,which can also be considered as amorphous COFs with disordered porous structure.E-CIN-1/CNT and E-SNW-1/CNT exhibited excellent lithium ion storage performance,because the thin layered structure can provide more active sites and shorten the ion/electron diffusion distance.Besides,the introduction of carbon nanotubes increased the electronic conductivity and stabilized the structure of composites.Electrochemical measurements and material characterizations revealed the lithium-storage mechanism of E-CIN-1/CNT and E-SNW-1/CNT,which involved redox reactions of 11 or 16 electrons.Lithium ions storage are associated with not only common organic groups(C=N and-NH-groups)but also uncommon organic groups such as triazine rings,piperazine rings and benzene rings.The capacity contribution of the E-CIN-1 and E-SNW-1 in the E-CIN-1/CNT and E-SNW-1/CNT composites were 1005 and 920 mA h g-1 after 250 cycles at 100 mA g-1,respectively.The strategy of mechanical stripping provides an effective strategy for developing COF materials as high-performance electrode materials for lithium ion batteries.(2)The imine-based two-dimensional covalent organic framework material was prepared at room temperature.Meanwhile,a conductive agent(carbon nanotube)was added during the synthesis process to obtain a layered composite(COF).In order to improve the electrochemical kinetics of the composites,covalent organic framework materials(E-COF)with few-layered structure can be obtained by mechanical ball milling.Because the few-layered structure of E-COF material is not stable,carboxylated graphene quantum dots(GQDs)were used to stabilize its structure and E-COF/GQDs composite was obtained.E-COF/GQDs as anode materials for lithium organic batteries exhibited excellent electrochemical performances.The capacity can be maintained up to as high as 1386 mA h g-1 after 300 cycles at 100 mA g-1.Based on COF contribution(according to the mass ratio of COF in composites),a reversible charge capacity up to 1687 mA h g-1 has been achieved.Meanwhile,E-COF/GQDs have good rate performance.Then,we investigated the lithium storage mechanism of each COF monomer in E-COF/GQDs by various characterization methods(ex-situ FTIR,in-situ Raman,ex-situ XPS)and DFT calculations.It is found that each COF monomer has a superlithiation mechanism(LiC)and can store 33 lithium ions,mainly on three C=N groups and five benzene rings.The superlithiation mechanism of LiC is also applicable to other COF(E-SCOF-ICI)with similar hyper-conjugated structure,which proves the universality of molecular design of organic electrodes with high reversible capacity.(3)A new imine two-dimensional COF material(TFPB-COF)was designed based on the Schiff reaction mechanism.The multilayer stack structures of TFPB-COF were connected by weak van der Waals forces rather than covalent bonds.For the first time,few-layered E-TFPB-COF and E-TFPB-COF/MnO2 composites were obtained by exfoliating with KMnO4 and HClO4,which were subsequently used as anode materials for lithium ion batteries.The E-TFPB-COF/MnO2 composites exhibited excellent electrochemical performance.It showed a small capacitydecreased in the first ten cycles at the current density of 100 mA g-1,then an obvious capacity increased for the E-TFPB-COF/MnO2 electrode from the 10th to 52th cycles due to the better electrolyte infusion and improved Li+ diffusion kinetics during repetitive cycling.The discharge/charge capacities of 1361/1349 mA h g-1 were observed after 108 cycles,which was stable for 300 cycles.For E-TFPB-COF,a reversible capacity of up to 968 mA h g-1 was also achieved after 300 cycles.The E-TFPB-COF exhibited strong ?-Li cation effect and excellent lithium ion storage performance due to the periodic open channels and large specific surface area with shortened pathway and facilitated ion/electron diffusion.Meanwhile,MnO2 nanoparticles can be used as"spacer" materials to effectively prevent the agglomeration of E-TFPB-COF during repeated cycles and further improve the cycle stability of the composite(4)2,3,6,7,10,11-hexahydroxytriphenyl(HHTP)and 4,4'-biphenyldiboronic acid(BPDA)were used as precursors.Boron-containing COFs(COF-10)were synthesized in a mixed solvent of mesitylene and dioxane(v/v=1/1).Meanwhile,we synthesized a mesoporous COF based on in-situ growth on CNT(COF-10@CNT)surface as an anode material for high performance potassium ion batteries.The few-layered structure of COF-10 shell on the surface of COF-10@CNT can provide more exposed active sites,shorten the diffusion distance and enhance the insertion/extraction kinetics of potassium ions.The mesoporous structure of the COF-10 material facilitates the transport of potassium ions and provides sufficient void space to effectively buffer the volume change during reversible discharge-charge process.Besides,the three-dimensional network structure of CNT can greatly improve electronic conductivity as well as structural stability.Consequently,the COF-10@CNT anode showed ultra-high potassium storage performance(high reversible capacity of 288 mA h g-1 after 500 cycles at 100 mA g-1).More impressively,a reversible capacity of 161 mA h g-1 could be maintained for the COF-10@CNT anode after 4000 cycles at a high current density of 1000 mA g-1.