有机电极材料以其绿色环保、低成本等特点引起了许多研究者关注，被认为是下一代最有潜力的电极材料之一。有机电极材料可以通过引入不同功能化的官能团来提高能量密度和功率密度。另外，有机电极的储能机理被人们逐渐知晓，从而可以针对性地开发出大量的新型有机电极材料。然而，有机电极也存在一些比较明显的缺陷，比如导电性差、易溶解等，这严重影响了其进一步广泛应用。 共价有机框架(covalent organic frameworks，COFs)材料是通过强大的共价键连接成的二维或三维多孔聚合物，主要含有C，N，B，H等元素。COFs已经被广泛的应用在多相催化、气体分离、半导体和光电子器件等领域。然而，COFs作为电极材料的研究还较少，主要是由于其导电性差和层层堆积的结构阻碍了离子／电子的传输。因此提高COFs的导电性、改善其离子／电子传输能力，是提高COFs电极材料性能的主要研究手段。通常来讲，通过在COFs的合成过程中引入导电材料(碳纳米管，石墨烯，石墨烯量子点等)，可以显著提高其导电性能。此方法在其它有机电极材料体系的研究中已经被广泛应用。另外，可以对二维共价有机框架进行有效地剥离。剥离后的少层有机纳米片可以提供更多活性位点和有效缩短其活化时间，这是改善COFs储能性能的有效方法。 基于上述背景，本论文以探索和开发具有优异电化学性能的COFs负极材料为研究目标，通过引入碳纳米管，石墨烯量子点等导电材料来改善COFs材料的导电性。同时，采用机械剥离、化学剥离和可控原位生长等方法制备基于少层二维共价有机框架的复合材料，以期改善其储能动力学速率。最终，对这一系列的复合材料进行了相应的分析表征，并通过原位／非原位拉曼、非原位红外／X射线光电子谱(XPS)和密度泛函理论(DFT)计算方法等手段探究了相应的储能机理。 本论文的具体研究内容如下： (1)通过溶剂热反应得到两种三嗪基共价有机框架材料(CIN-1和SNW-1)。在合成的过程中引入碳纳米管，得到堆积结构的复合材料(CIN-1／CNT和SNW-1 ／CNT)，并采用机械球磨的方法对复合材料进行改性得到E-CIN-1／CNT和E-SNW-1／CNT。这两种共价有机纳米片是通过共价键连接的多孔聚合物，也可以被认为是无序多孔结构的无定形COF。E-CIN-1／CNT和E-SNW-1／CNT展现出了优异的锂离子储存性能。这是由于薄层结构提供了更多的活性位点，缩短了离子／电子的传输距离。另外，碳纳米管的引入提高了材料的导电率并且稳固了材料的结构。电化学和材料表征揭示了E-CIN-1／CNT和E-SNW-1／CNT的储锂机理，分别是涉及到11个和16个电子的氧化还原反应。锂离子不仅与常见的有机基团(C=N和-NH-基团)还与不常见的三嗪环，哌嗪环和苯环等发生氧化还原反应。在100 mA g⁻¹的电流密度下循环250圈后，E-CIN-1和E-SNW-1在E-CIN-1／CNT和E-SNW-1／CNT复合材料中的容量贡献分别达到1005和920 mA h g⁻¹。机械剥离的策略为开发共价有机框架材料作为高性能锂离子电池电极材料，提供了一种有效的策略。 (2)在室温条件下制备了一种亚胺基二维共价有机框架材料。同时，在合成过程中加入导电剂(碳纳米管)得到一种层状堆积结构的复合材料(COF)。为改善复合材料堆积结构，我们采用机械球磨的方法，可以得到薄层结构的共价有机框架材料(E-COF)。由于薄层结构的E-COF材料结构稳定性弱，因此我们采用水热法复合羧基化的石墨烯量子点(GQDs)来稳固其结构，得到E-COF／GQDs复合物。E-COF／GQDs作为锂有机电池的负极材料，展现出了优异的电化学性能。在100 mAg⁻¹的电流密度下循环300圈之后，容量高达1386 mAh g⁻¹。基于COF贡献(根据COF在复合材料中的质量比)实现了高达1687 mAh g⁻¹的可逆充电容量。同时，E-COF／GQDs具有良好的倍率性能。通过各种表征手段(非原位FTIR，原位拉曼，非原位XPS)和DFT计算，我们研究了每个COF重复单元在少层超共轭E-COF／GQDs材料中的锂存储机理。我们发现每个COF重复单元具有超锂化机理(LiC)，可以储存33个锂，主要是发生在三个C=N基团和五个苯环上。我们发现LiC的超锂化机理也适用于其他具有类似超共轭结构的COF (E-SCOF-ICI)材料，证明了其高容量有机电极的分子设计策略具有一定的普遍性。 (3)基于Schiff反应机理设计了一种新的亚胺基二维共价有机框架材料(TFPB-COF)。由于TFPB-COF的多层堆叠结构的层间是通过弱的范德瓦尔斯力连接而不是共价键连接。首次通过KMnO₄和HClO₄剥离，得到少层结构的E-TFPB-COF和E-TFPB-COF／MnO₂复合材料，将其作为锂离子电池的负极材料。 E-TFPB-COF／MnO₂复合材料表现出了优异的电化学性能。在100 mA g⁻¹电流密度下，前10圈循环中发生容量衰减，随后从第10圈循环到第52圈循环容量出现明显的增加，这是通过逐渐活化过程改善了Li⁺扩散动力学和电解质渗透所致。循环108圈后放电／充电容量达到1361／1349 mA h g⁻¹，稳定性可达300圈。对于E-TFPB-COF，循环300圈后同样实现了高达968 mAhg⁻¹的可逆容量。由于其促进了离子／电子扩散，周期性开放通道和大的比表面积，E-TFPB-COF表现出强的π-Li阳离子效应和优异的锂离子存储性能。同时，MnO₂纳米粒子可以作为“间隔物”，有效防止少层E-TFPB-COF纳米片在反复循环过程中的团聚，进一步提高复合材料的循环稳定性。 (4)以2，3，6，7，10，11-六羟基三苯(HHTP)和4，4'-联苯二硼酸(BPDA)为前驱体，在均三甲苯和二恶烷(v／v=1／1)的混合溶剂中合成了含硼COFs(COF-10)。同时我们合成了基于CNT表面原位生长的介孔共价有机框架(COF-10@CNT)，将其作为高性能钾离子电池的负极材料。COF-10@CNT表面的COF-10可以提供更多活性位点，缩短扩散距离并增强钾离子的嵌入／脱出动力学速率。同时， COF-10材料的多孔结构促进钾离子的传输并提供足够的空间以有效地缓冲可逆充放电过程中的体积变化。此外，CNT交织的三维网络结构可以大大提高电子传导性和结构稳定性。因此，COF-10@CNT显示出超高的钾储存性能(在100 mA g⁻¹的电流密度下500圈循环后，可逆容量为288 mAh g⁻¹)。更值得注意的是，在1000 mA g⁻¹的高电流密度下，循环4000圈后COF-10@CNT负极可以保持161 mA hg⁻¹的可逆容量。 关键词：共价有机框架，剥离少层纳米片，锂离子电池，钾离子电池，储能机理

 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 applications. Covalent 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-C0F/MnO₂ composites were obtained by exfoliating with KMnO₄ and HClO₄, which were subsequently used as anode materials for lithium ion batteries. The E-TFPB-C0F/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-C0F/MnO₂ electrode from the 10〓 to 52〓 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 π-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. Keywords: covalent organic framework, exfoliated nanosheets, lithium ion batteries, potassium ion batteries, energy storage mechanism