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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 3: 430 - 447

DOI: https://doi.org/10.7536/PC230720

Review

Covalent Organic Frameworks as Cathode Materials for Metal Ion Batteries

  • Wenbo Zhou ,
  • Xiaoman Li ,
  • Min Luo , *
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  • State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China
* e-mail:

Received date: 2023-07-20

  Revised date: 2023-10-08

  Online published: 2024-02-26

Supported by

National Natural Science Foundation of China(21965027)

Central Guidance on Local Science and Technology Development Fund of Ningxia Province(2023FRD05031)

National First-rate Discipline Construction Project of Ningxia: Chemical Engineering and Technology(NXY-LXK2017A04)

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Abstract

Covalent organic frameworks (COFs) are porous organic materials with periodic two-dimensional or three-dimensional network structures consisting of two or more organic molecules connected by covalent bonds. COFs have attracted considerable interest in energy storage due to their beneficial properties, including low skeletal density, high surface area, high porosity, structural designability and functional modifiability. COFs offer unique advantages as positive electrode materials for metal ion batteries due to their rich redox active sites and open framework structure. However, their application in energy storage is limited by challenges such as poor conductivity, low energy density, limited number of available active sites, and blockage of ion transport channels. This article provides a comprehensive review of recent research on COFs as positive electrode materials for metal ion batteries, discussing their types, design strategies, and synthesis methods. Additionally, it presents an overview of the electrochemical energy storage mechanisms from the perspective of different active groups, and the applications of COFs in various metal ion batteries. Finally, it highlights the prospects and challenges of using COFs in energy storage.

Contents

1 Introduction

2 Types of COFs

2.1 B-C containing

2.2 C-N containing

2.3 C=N containing

2.4 C=C containing

3 Synthesis method of COFs

3.1 Solvothermal synthesis

3.2 Ionic thermal synthesis

3.3 Microwave-assisted synthesis

3.4 Mechanochemical synthesis

3.5 Sonochemical synthesis

4 Microstructure design strategy for COFs

4.1 Introduction of redox active sites

4.2 Crystallinity adjustment

4.3 Interlayer stripping strategy

5 Application of COFs in different metal ion batteries

5.1 Lithium-ion batteries

5.2 Sodium-ion batteries

5.3 Potassium-ion batteries

5.4 Aqueous zinc batteries

6 Conclusion and prospect

Key words： covalent organic frameworks; energy storage; redox active sites; metal ion batteries

Cite this article

Wenbo Zhou , Xiaoman Li , Min Luo . Covalent Organic Frameworks as Cathode Materials for Metal Ion Batteries[J]. Progress in Chemistry, 2024 , 36(3) : 430 -447 . DOI: 10.7536/PC230720

1 Introduction

With the increasing depletion of fossil fuels, the effective use of renewable energy such as wind, solar, tidal and biomass energy has gradually come into public view. Renewable energy is affected by natural conditions and climate, and there is a problem of unstable supply. The intermittency of renewable energy can be better addressed through the rational application of energy storage technologies[1]. As the main sustainable energy storage technology, electrochemical energy storage technology has been widely used in electric vehicles, smart grid and other fields in recent years, which is of great significance to promote the development of clean energy and improve energy efficiency. Metal ion battery has become a research hotspot because of its high energy density, high rate, long life and safety, and is considered to be the most promising energy storage system[2,3]. Metal ion battery is essentially an ion concentration battery. When charged, ions are extracted from the positive electrode and inserted into the negative electrode through the electrolyte. At the same time, electrons are supplied to the negative electrode from the external circuit to ensure charge balance. When discharged, the opposite is true[4]. In the whole battery system, the cathode material is the main component of the electrochemical redox reaction, and its physical and chemical properties have an important impact on the performance of the electrochemical energy storage system[5]. For the cathode materials of metal ion batteries, the current research focuses are still on transition metal oxides, polyanionic compounds and Prussian blue derivatives[6]. Inorganic cathode materials, such as transition metal oxides, generally have excellent characteristics such as low cost, low toxicity and high specific discharge capacity. However, the rapid capacity fading rate, poor structural stability and easy solubility in electrolyte seriously affect the electrochemical performance of metal ion batteries. Therefore, the development of new cathode materials with long cycle life and stable structure is an important research direction at present. In recent years, inorganic/organic hybrid electrode materials and organic electrode materials have attracted much attention[7,8]. Compared with traditional inorganic materials, organic electrode materials, especially covalent organic frameworks (COFs), have attracted wide attention due to their unique advantages, and have great potential applications in the positive and negative electrodes and separators of ion batteries[9⇓~11]. Table 1 summarizes the performance comparison of representative conventional cathode materials with COFs in metal ion batteries. It can be seen that COFs electrode materials generally have higher specific surface area, and also have significant advantages in energy density and cycle stability.
表1 具有代表性的电极材料在金属离子电池中的性能比较

Table 1 Performance comparison of representative electrode materials in metal ion batteries

Cathode material Application
Scenarios		 Specific surface
area (m2·g-1)		 Discharge specific capacity (mAh·g-1) Cycle life (capacity
retention/cycle/rate)(A·g-1)		 ref
MnO2 LIBs 43.601 148/0.05 A·g-1 44%/100/0.1 12
α-MnO2 AZIBs 70.8 240/0.1 A·g-1 58.3%/300/0.1 13
K+/γ-MnO2 SIBs 148.2 300/0.1 A·g-1 60%/200/0.1 14
Mn3O4@rGO LIBs 83 741/0.1 A·g-1 65.9%/300/0.5 15
MgMn2O4 AZIBs — 243/0.1 A·g-1 80%/500/0.5 16
V2O5 AZIBs 43 224/0.1 A·g-1 75%/400/0.1 17
NaV3O8 RMBs 201 184/0.1 A·g-1 88.3%/100/0.5 18
Co/LiNi0.5Mn1.5O4 LIBs — 120/1 A·g-1 81%/2000/5 19
V2O3@rGO AZIBs 133.36 240/0.5 A·g-1 80%/1000/10 20
NaFeⅢ[FeⅢ(CN)6] SIBs 189.19 120.3/0.01 A·g-1 59.1%/50/0.075 21
Na2Fe(C2O4)SO4 SIBs — 80/0.2 A·g-1 85%/500/5 22
CuHCF(Fe2+) CIBs 100.2 54.5/0.02 A·g-1 90.43%/1000/0.02 23
DAAQ-ECOF LIBs 216 148/0.02 A·g-1 74%/1800/0.5 24
TP-PTO-COF AZIBs 601 301.4/0.2 A·g-1 95%/1000/2 25
HATN-AQ-COF LIBs 725 319/0.179 A·g-1 80%/3000/ 3.58 26
TPDA-PMDA-COF LIBs 2669 233/0.5 A·g-1 57.1%/1800/5 27
TQBQ-COF SIBs 94.36 452/0.02 A·g-1 96.4%/1000/1 28
COF-TMT-BT AZIBs 342.5 283.5/0.1 A·g-1 96.2%/2000/0.1 29
Covalent organic framework materials (COFs) are composed of lightweight elements (carbon, hydrogen, oxygen, and nitrogen) and strong covalent bonds, which enable the organic units to form tightly packed conjugated structures. COFs are a new class of crystalline porous polymeric materials whose two-dimensional or three-dimensional topology is pre-engineered by precisely integrating building blocks. COFs as cathode materials for metal ion batteries have the following advantages: (1) abundant redox active sites for energy storage, which can store more charges and provide higher energy density; (2) The special π-π conjugated structure can reduce the energy barrier of intermolecular charge transport, which is more conducive to ion transport and electron conduction, and helps to improve the rate performance of the battery; (3) It has good structural designability, which can be adjusted and optimized by selecting different organic groups and coordination ions. This allows COFs to be flexibly designed according to the needs of specific applications to improve the overall performance of the battery[30,31]. Fig. 1 outlines the development history of COFs as cathode materials for metal ion batteries. Since the first appearance of COF-1 in 2005, various COFs materials have been applied to sodium-ion batteries, lithium-ion batteries, zinc-ion batteries and potassium-ion batteries[32⇓⇓⇓~36].
图1 COFs作为金属离子电池正极材料的发展史(COF-1[32], BPOE-COF[33], DTP-ANDI-COF[34], HqTp-COF[35], DAAQ- COF@CNT[36])

Fig. 1 Timeline of COFs as cathode materials for metal-ion batteries (COF-1 [32], BPOE-COF [33], DTP-ANDI-COF [34], HqTp-COF [35], DAAQ-COF@CNT [36])

Although COFs have many advantages, there are still some problems that can not be ignored: (1) Most COFs contain a large number of benzene rings, and the lowest unoccupied molecular orbital (LUMO) of their π-π conjugated system has a higher energy level, which makes their redox potential lower[37]; (2) There are insufficient redox active sites available in the nanochannels of COFs, which is not conducive to efficient ion transport; (3) Small organic molecules are easily dissolved in the electrolyte, resulting in poor cycling stability[38,39]. The electrochemical behavior of COFs electrodes depends not only on the number of active functional groups, but also on their special two-dimensional or three-dimensional topology in material design. To deepen the understanding of the relationship between materials and properties, it is necessary to expand the perspective from molecular design to structure shaping[40]. The framework of COFs can determine its pore size, shape, and interfacial position by introducing functional groups during monomer design or modifying functional groups after synthesis[41⇓⇓⇓~45].
In this paper, the types, synthesis methods, design strategies of COFs and the research progress of COFs in different metal-ion batteries, such as lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), potassium-ion batteries (KIBs) and aqueous zinc-ion batteries (AZIBs), are introduced. Based on the characteristics of COFs and their performance as cathode materials in metal ion batteries, the prospects and challenges of COFs in electrochemical energy storage in the future are summarized and prospected.

2 Types of COFs

COFs are a class of three-dimensional structural materials formed by organic monomers connected by covalent bonds, which are stacked by specific aromatic organic monomers through reversible polycondensation. Due to the different types of organic monomers and the different types of bonds between the monomers involved in the reaction, the types of COFs formed after polycondensation are also different. Since the first successful synthesis of COFs (COF-1 and COF-5) in 2005, the following COFs have been successfully developed: triazines, boronic acids, phenylhydrazones, imines, polyimides, and keto-enamines[32]. According to the different types of bonds, it can be divided into B-C type, C-N type, C = N type and C = C type. Fig. 2 briefly summarizes the classification of COFs according to different connecting bonds.
图2 根据不同连接键对COFs种类的区分

Fig. 2 Differentiation of COFs types according to different connection keys

2.1 B-C type

Boronic acid COFs are formed by the self-dehydration polycondensation of aromatic compounds with boronic acid groups or the dehydration polycondensation between boronic acid groups and aromatic compounds with hydroxyl groups[46]. The polymerization of boric acid COFs is formed by B — C bond. Because boron is an electron-deficient atom, it can release H+ by adding OH- in water molecules, so it is easy to realize the co-intercalation mechanism of metal cations /H+ dual ions in the charge-discharge process, and H+ with high reactivity may also participate in the reaction and contribute additional specific capacity. In addition, the boronic acid group has a strong ability to accept electrons, and the polymer formed by introducing the polyhydroxy compound has excellent structural stability[47].

2.2 C-N type

The polymerization between keto-enamine and polyimide COFs is usually formed by C — N bond. (1) Keto-enamines: Keto-enamines COFs are intermediate COFs (enol-imine COFs) with the expected structure synthesized by the Schiff base reaction of s-trialdehyde phloroglucinol and some aromatic amines, and the enol-imine COFs undergo irreversible proton tautomerization and are converted into keto-enamine COFs with more stable chemical properties. Such compounds usually have good chemical stability and tunable optoelectronic properties, so they have attracted much attention in the fields of energy storage, catalysis and sensing[48⇓~50]. Dichtel et al. Condensed 2,6-diaminoanthraquinone (DAAQ) with s-trialdehyde phloroglucinol to obtain 2D-COFs linked by β-ketoenamine (DAAQ-TFP-COF) and used them as energy storage materials[51]. The anthraquinone subunit exhibits a reversible electrochemical process and excellent chemical stability in strong acidic electrolytes. (2) Polyimides: Polyimide COFs are formed by the imidization reaction of aromatic amine and aromatic dianhydride through the connection of imide rings (-CO-N-CO-). Compared with other monomers, these two monomers have a wider range of sources and are easier to synthesize. Fang et al. Used solvothermal synthesis to synthesize polyimide COFs (PI-COF) with all-aromatic characteristics. Compared with other polyimide COFs (aliphatic and semi-aromatic), PI-COF has more excellent heat resistance and superior structural stability[52]. Polyimide-based COFs are one of the polymers with the highest thermal stability so far, and are also considered to be one of the most promising organic electrode materials in the future[53].

2.3 C = N type

The polymerization of triazine, imine and phenylhydrazone COFs is formed by C = N bond. (1) Triazines: Triazine COFs are formed by self-dehydration polymerization of primary and above aromatic cyanide compounds and connection of triazine rings (-C3N3-). They can be divided into three groups according to the position of the N group in the aromatic ring, and the aromaticity is judged by the delocalization energy: s-triazine > unsym-triazine > bi-triazine[54]. Triazine COFs are expected to be used in the fields of catalysis, adsorption and separation due to their high crystallinity, ultra-stability, porous nature and controllable synthesis[55]. (2) Imines: Imine COFs are formed by Schiff base polycondensation of aromatic amines with aromatic aldehydes or aromatic ketones through C = N covalent bonding. In general, the structure and properties of imines obtained by condensation of primary amines with aldehydes/ketones are relatively more stable. Due to the rich variety and high selectivity of monomers for the synthesis of imine COFs, the research reports on imine COFs have increased significantly in recent years. (3) Phenylhydrazones: Phenylhydrazone COFs are formed by connecting hydrazide aromatic compounds with aromatic aldehydes through hydrazone bonds. Fernando et al. Generated two new COFs (COF-42 and COF-43) by condensing 2,5-diethoxyterephthaloyl hydrazide with 1,3,5-triformylbenzene or 1,3,5-tris (4-formylphenyl) benzene, whose organic building blocks are connected by hydrazone bonds to form an extended two-dimensional porous framework with high crystallinity and excellent thermal stability[56].

2.4 C = type C

COFs connected by C = C bonds are usually sp2c- conjugated skeleton materials formed by condensation of aromatic aldehydes and aromatic cyanogens or aromatic amines[57]. Xu et al. Synthesized a novel two-dimensional sp2c- conjugated framework material (CCP-HATN) with nitrogen doping and periodic double-pore structure under different solvothermal conditions, and applied it to cathode materials for lithium-ion batteries[58]. Benefiting from the stable framework and abundant redox active sites (C = N groups), CCP-HATN exhibits excellent cycling and rate performance. Zhang et al. Carried out C = C condensation reaction of 2,4,6-tris (4-formylphenyl) -1,3,5-triazine (TFPT) and 1,3,5-tris (4-cyanomethylphenyl) benzene (TCPB) under solvothermal conditions, and successfully synthesized triazine sp2c- conjugated COFs(TA-sp2c-COF) with uniform microporous structure and high crystallinity[59]. This kind of sp2c- conjugated COFs connected by C = C bonds usually have a close-packed π-π conjugated structure with high chemical stability.

3 Synthesis of COFs

Due to the high selectivity of organic monomers of COFs, the synthesis methods of COFs formed by different linkages are also different. As the types of COFs are constantly updated and enriched, the commonly used synthesis methods are: solvothermal synthesis, ionic synthesis, microwave synthesis, mechanochemical synthesis and sonochemical synthesis. Fig. 3 outlines the chemical synthesis reactions of different kinds of COFs.
图3 不同种类COFs的化学合成反应示意图

Fig. 3 Schematic diagram of chemical synthesis of different type

3.1 Solvothermal synthesis

Solvothermal method, as the earliest method for the synthesis of COFs, has become increasingly mature, and is mostly used for the synthesis of boronic acid, imine and keto-enamine COFs. The solvothermal method begins by placing a suitable monomer material, catalyst, and solvent or mixture thereof in a Pyrex tube. After freeze-pump-thaw cycle degassing, sealing, reaction at proper temperature, cooling at room temperature, and washing with solvent or Soxhlet extraction to obtain the target product[60,61]. In solvothermal synthesis, the type of organic solvent, the solubility and ratio of monomers, the reversibility of reaction, reaction time and temperature are the key factors for the successful synthesis of target COFs. In most solvothermal methods, the mixed solution of 1,4-dioxane and mesitylene is used, the reaction temperature is 100 ~ 150 ℃, and the reaction time is 72 ~ 120 H[32,60,61]. Hesham et al. successfully synthesized the target materials Cz-BD-COF and Cz-DHBD-COF by adding glacial acetic acid solution at 120 ℃ for 4 days with 3,3 ', 6,6' -tetraformyl-9,9 '-bicarbazole (Cz-4CHO) and benzidine (BD)/1,4-dihydroxybenzidine (DHBD) as monomer materials and 1,4-dioxane and mesitylene as organic solvents (Fig. 4)[62]. The addition of glacial acetic acid can improve the synthesis efficiency of the reversible reaction of the monomer material. The reaction time of solvothermal synthesis is longer, which is beneficial to the formation of COFs with high crystallinity. However, it should be noted that the selection of solvent ratio has a great influence on the crystallinity and porosity of COFs after synthesis.
图4 Cz-BD COF和Cz-DHBD COF的合成及结构示意图[62]

Fig. 4 The synthesis and structure diagram of Cz-BD COF and Cz-DHBD COF [62]

3.2 Ionothermal synthesis

Ionothermal method is usually used to synthesize triazine COFs. Compared with solvothermal method, ionothermal synthesis requires higher temperature, but takes less time, is more environmentally friendly, and has higher yield of target products. Ionothermal method usually chooses ZnCl2 as the heating medium, which is due to the good solubility of aromatic cyanide compounds in molten ZnCl2. Secondly, ZnCl2 is a good catalyst for trimerization, and the reversible trimerization reaction can be fully carried out by using ZnCl2 as a Lewis acid catalyst[63]. Thomas et al. Successfully synthesized triazine COFs (CTF-2) with 2,6-naphthalenedicarbonitrile as monomer material at 450 ° C using ZnCl2 as catalyst (Fig. 5)[64]. Although the ionothermal synthesis method has the above advantages, it cannot be used for the synthesis of other types of COFs at present. Ionothermal synthesis needs to choose a more suitable ionic medium to meet a variety of reaction conditions in order to expand its scope of application.
图5 离子热合成CTF-2[64]

Fig. 5 Ionic thermal synthesis of CTF-2[64]

3.3 Microwave-assisted synthesis

Because the solvothermal synthesis takes a long time and requires harsh reaction conditions, researchers need to find a simpler, easier and more efficient method to synthesize COFs. In order to solve the problem of time-consuming synthesis of COFs, Campbell et al. Successfully synthesized boric acid COFs (COF-5) by microwave irradiation, which took only 20 minutes[65]. In addition, compared with the solvothermal method, the COFs synthesized by microwave irradiation method have higher specific surface area and porosity, mainly because the by-products and impurities produced in the synthesis process can be effectively removed by microwave extraction method. Ulcuango et al. Synthesized TPPA-1-COF by both conventional solvothermal and microwave irradiation methods (fig. 6)[66]. The yield of the target product was 90% and the specific surface area was 1007 m2·g-1 after heating at 170 ℃ for 3 days in the solvothermal method, while the specific surface area of the target product was relatively stable and the yield was as high as 98% in the microwave irradiation method with the reaction time shortened to 30 min, which indicated that the synthesis of COFs by microwave irradiation method had made a breakthrough. Compared with the conventional solvothermal method, the microwave-assisted method can shorten the reaction time, improve the yield and have better reproducibility.
图6 微波辐射合成TpPa-1[66]

Fig. 6 Microwave radiation synthesis TpPa-1[66]

3.4 Mechanochemical synthesis

Mechanochemical synthesis COFs were prepared by placing the monomers in a mortar and grinding at room temperature. It is a simple and practical preparation technology of COFs because it does not require complex reaction conditions (sealed container, inert environment, complex solvents and high temperature). For the first time, Heine et al. Synthesized three COFs (TpPa-1, TpPa-2 and TpBD) with good chemical stability by mechanochemical grinding at room temperature without solvent (Fig. 7), and the yield was up to 90%[67]. But its crystallinity and specific surface area are low. Karak et al. Successfully synthesized ketoenamine COFs with high crystallinity and ultra-high specific surface area （3000 m2·g-1） within 1 min by liquid-assisted grinding method[68]. It is worth mentioning that COFs contain a large number of active sites, but due to the strong π-π superposition effect between layers, the resistance of ion transport is large, and the active sites deep in the pore wall are difficult to be effectively utilized. Therefore, exfoliation can be performed by mechanical grinding to expose more active sites and improve ion diffusion efficiency.
图7 机械研磨法合成TpPa-1、TpPa-2、TpBD[67]

Fig. 7 Mechanical grinding synthesis TpPa-1, TpPa-2, TpBD [67]

3.5 Sonochemical synthesis

Sonochemical synthesis is an improved alternative to traditional solvothermal synthesis, which usually uses ultrasound to form bubbles in an organic solvent, and then uses the temperature and pressure difference formed by the formation and fracture of bubbles in the solvent to accelerate the chemical reaction. Zhao et al. Successfully synthesized imine COFs in acetic acid aqueous solution within 1 H by sonochemical method, and the crystallinity and porosity were higher than those by solvothermal method (Fig. 8)[69]. In addition, this method has also been successfully applied to COFs and graphene composites[70].
图8 声化学合成亚胺类COF及其二维孔道示意图[69]

Fig. 8 Sonocheical synthesis of imide COF and its two-dimensional pore diagram [69]

The process parameters such as reaction time, reaction temperature and the specific surface area of the products for several typical synthesis methods are summarized in Table 2. Solvothermal method is still the most mature and widely used method for the synthesis of COFs. However, the preparation process is complicated and takes a long time, which is also an urgent problem to be solved in the synthesis and preparation of COFs. Compared with the traditional solvothermal method, ionothermal method, microwave synthesis method, mechanical grinding method and sonochemical method have their own advantages in shortening the reaction time, reducing the reaction temperature, and improving the crystallinity and specific surface area. However, these methods have some limitations, and can not be widely used in the synthesis of COFs at present, so it is necessary to develop more efficient methods for the synthesis of COFs.
表2 不同方法合成COFs的性能指标

Table 2 Performance indexes of COFs synthesized by different methods

Synthesis
method		 Typical
material		 Recation
time		 Recation
Temperature (℃)		 Specific surface
area (m2·g-1)		 ref
Solvothermal COF-1 72 h 120 711 32
TAPB-PZI 72 h 150 598.3 53
TFPM-PDAN 72 h 100 728.4 54
Ionic thermal CTF-1 40 h 400 791 71
TAPB-PTCDA 48 h 300 1250 72
FCTF 40 h 400 — 73
Microwave TTA-DPF 30 min 110 900 74
LZU-1 30 min 120 729 75
AEM-COF-2 40 min 120 1487 76
Mechanochemical TpPa-1 40 min — 61 67
TpBpy-MC 1.5 h — 293 77
NUS-9 45 min — 102 78
Phonochemistry COF-1 1-2 h — 719 79
COF-1 NN 48 h 100 — 70

4 Microstructure design strategy of COFs

COFs are planar networks formed by monomeric materials connected by covalent bonds, and these networks are arranged into multiple layers of compact frameworks by π-π stacking effect, thus forming fixed one-dimensional nanochannels[42]. In order to deeply understand the influence of microstructure of COFs on ion transport, charge migration and energy storage sites during redox reaction, a series of micro-design strategies for COFs are summarized, including the introduction of redox active sites, crystallinity adjustment and interlayer exfoliation strategies.

4.1 Introduced redox active site

The redox reaction of metal ion battery during charge and discharge is actually a process of reversible deintercalation of cations/protons from the positive and negative electrodes and electron transfer from the external circuit. Compared with other electrode materials (such as metal oxides, carbon-based materials and organic polymers), COFs have incomparable advantages[80⇓~82]. Its abundant redox-active sites can interact with metal cations, generating capacity through a cation/proton insertion mechanism during redox reactions. The introduction of active functional groups into COFs is an effective strategy to adjust the ion transport efficiency and improve the ion conductivity and cation transfer number. So far, there are two main methods to introduce active functional groups into COFs: (1) target functional groups (C = N, C = O, etc.) Are introduced into the monomer design process, and then the functional monomers are assembled into target COFs through covalent attachment. This design strategy can precisely regulate the number of active functional groups and precisely design the structure of COFs for specific target tasks. Zheng et al. Introduced the redox active site (C = O group) into the target monomer material 2,7-diaminopyrene-4,5,9,10-tetraone （PTO-NH2）, and then successfully synthesized the target COFs with abundant active sites (BT-PTO-COF) by solvothermal reaction with 1,3,5-benzenetricarboxaldehyde (BT) (Fig. 9)[83].
图9 (a) 2，7-二氨基芘-4，5，9，10-四酮(PTO-NH2)的合成路线；(b) BT-PTO COF合成示意图[83]

Fig. 9 (a) Synthetic route of 2, 7-diaminopyrene-4, 5, 9, 10-tetraone (PTO-NH2); (b) schematic diagram of BT-PTO COF synthesis [83]

Peng et al. Synthesized a structurally robust COFs linked by alkenes (COF-TMT-BT) through the condensation reaction between 2,4,6-trimethyl-1,3,5-triazine (TMT) and 4,4 '- (benzothiadiazole-4,7-diyl) dibenzaldehyde (BT) (Fig. 10)[29]. The authors investigated the redox properties of COF-TMT-BT materials using the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). The HOMO orbital of the COF-TMT-BT unit is mainly distributed on the benzothiadiazole group, while the electron cloud in the LUMO orbital is mainly distributed around the benzothiadiazole group, indicating that it has a better electron affinity and a higher reduction potential. Combined with DFT theoretical calculations, the electrochemical mechanism of COF-TMT-BT in the reversible process of Zn2+ intercalation/deintercalation is proposed as follows: COF-TMT-BT stores charge through an ionic coordination process, and each Zn2+ binds to an S atom in the benzothiadiazole unit to form an S — Zn — S bond in the adjacent COF-TMST-BT[84]. Electrochemical tests showed that COF-TMT-BT exhibited excellent Zn2+storage capability, exhibited a high specific capacity of 283.5 mAh·g-1 at a current density of 0.1 A·g-1, and showed good cycling performance with a capacity retention of 65.9% after 2000 cycles. It should be noted that in the synthesis process of this design strategy, further uncontrollable side reactions may occur between the active sites, and the types of active functional groups that can be embedded in the framework of COFs are limited, and the density of functional groups per unit specific surface area is also challenging[85]. (2) The target COFs can be further synthesized through the interaction between the groups (such as — OH, — CHO, —NH2 and —CN3) in the existing monomer materials and other functional side chains. This strategy of post-synthetic modification can precisely adjust the pore wall environment of COFs and design functional COFs with different specific surface areas and porosities[86].
图10 (a) COF-TMT-BT的合成示意图；(b) COF-TMT-BT的充放电曲线；(c)长期循环性能；(d)重复单元的HOMO和LUMO轨道[29]

Fig.10 (a) Synthesis diagram of COF-TMT-BT; (b) charge/discharge curves at different current densities; (c) long-term cycling performance; (d) HOMO and LUMO orbitals of COF-TMT-BT repetitive unit [29]

Li et al. Successfully synthesized COFs (HATTA) with C = N groups as redox active sites using 3,4-diaminobenzoic acid (3,4-DBA) and hexa-n-cyclohexane as monomer materials, and applied them to ZIBs cathode materials (Fig. 11)[87]. Due to the scalable π-π conjugated structure and abundant C = N active sites, the electrode has a high specific capacity of 225.8 mAh·g-1 at a current density of 0.05 A·g-1 and can still deliver a specific capacity of 136.1 mAh·g-1 at a current density of 25 A·g-1 with a capacity retention of 84.07% after 10 000 cycles, demonstrating excellent rate and cycling performance. The infrared spectrum shows that the active unit (C = N group) changes with the insertion of Zn2+ during the discharge process, and then the C = N group returns to the initial state during the charge process. This indicates that the active part (C = N group) is restored when the Zn2+ is removed from HATTA, and that the C = N group is the active unit of HATTA.
图11 (a) HATTA的结构示意图；(b) HATTA的倍率性能；(c)不同电流密度下HATTA的充/放电曲线；(d) 在25 A·g-1下的长期循环性能[87]

Fig. 11 (a) Diagram of the structure of HATTA-COF; (b) the rate performance of the HATTA; (c) charge/discharge curves at different current densities; (d) long-term cycling performance at 25 A·g-1 [87]

Lei et al. prepared imine-based COFs composites using benzenetricarboxaldehyde and p-diaminobenzene as monomer materials, and applied them to lithium-ion batteries (Fig. 12)[88]. Electrochemical analysis showed that the Li+storage mechanism of the COFs was 14 Li+ per repeating unit, and one Li+ could be inserted into each C = N group and six Li+ could be inserted into each benzene ring during the redox process.
图12 (a) COFs的合成示意图；(b) 碳纳米管外表面覆盖的孔道结构示意图；(c) 长期循环性能；(d)锂化过程中的结构演变[88]

Fig.12 (a) Diagram of the composition of COFs; (b) the surface of carbon nanotubes outside channel structure diagram; (c) long-term cycling performance; (d) structural evolution during lithification [88]

The electrode can deliver an ultrahigh reversible capacity of 153.6 mAh·g-1 at a current density of 0.1 A·g-1, and the capacity remains stable after 500 cycles with excellent cycling performance. In addition, the target functional group can also be introduced into a monomer material, which can be further synthesized and modified into target COFs with groups in another monomer material (such as — OH, — CHO, —NH2 and —CN3) to form dual-active redox sites.
Lin et al. Prepared an aromatic microporous structure-resistant COFs by condensation reaction using tetraamino-p-benzoquinone (TAQ) and p-benzoquinone (BQ) as monomer materials, and used them as cathode materials for ZIBs (Fig. 13)[89]. The active functional group changes on TAQ-BQ during charge-discharge were investigated by ex situ FT-IR. Upon discharge, the intensity of the peak at 1103 cm−1 (orange) becomes higher, and the C — O and C — N bonds vibrate in this region. At the same time, the intensity of the C = N peak at 1643 cm-1（ green) and the C = O peak at 1680 cm-1（ pink) decreased. This indicates that the discharge process of TAQ-BQ is related to the reduction of C = O and C = N to C — O and C — N, confirming that the C = N group and the C = O group are the redox active sites of TAQ-BQ. The abundant active sites promote the transport efficiency and ionic conductivity of Zn2+, so that it can deliver a specific discharge capacity of 208 mAh·g−1 at a current density of 0.1 A·g−1, with a capacity retention of 87% after 1000 cycles at a current density of 1 A·g−1.
图13 (a) TAQ-BQ的合成及结构示意图；(b) 非原位FT-IR；(c) 不同电流密度下的充电/放电曲线；(d) 1 A·g−1下的长期循环性能[89]

Fig. 13 (a) Synthesis and structure diagram of TAQ-BQ; (b) out-of-situ FT-IR; (c) charge/discharge curves at different current densities; (d) long-term cycling performance at 1 A ·g-1[89]

4.2 Regulation of crystallinity

The crystallinity of COFs indicates the degree of periodic structure of long-range ordered arrangement of atoms in materials, which is one of the important characteristics to judge the characteristics of COFs and the performance of different devices. The high crystallinity of COFs is not only beneficial to their structural determination, including many structural parameters such as pore size, atomic position, bond length and angle, but also significantly improves their performance in electron conduction, energy storage and so on. COFs with high crystallinity will provide a fast charge carrier (hole or electron) transport pathway through the tight π-π stacking interaction between layers, which is very important for the realization of high-performance metal-ion batteries[90,91]. At present, crystallinity adjustment is widely used in metal-organic frameworks (MOFs), and the crystallinity adjustment strategy is also suitable for the fast ion conduction of COFs. However, compared with MOFs, the crystallinity of COFs is more difficult to adjust, mainly because the covalent bond in COFs is stronger and the reversibility of bonding is lower than that in MOFs[67,92,93]. So far, the strategies to improve the crystallinity of COFs can be divided into two aspects: the optimization of polycondensation reaction conditions and the improvement of interlayer packing density. (1) Polycondensation reaction conditions are one of the main factors affecting the crystallinity of COFs.The control of reaction time and temperature, the selection of different solvent ratios and catalysts, and the addition rate of monomer materials during the reaction are all important factors affecting the crystallinity of COFs. Liu et al. Synthesized covalent triazine framework COFs (CTF-HUST-HC1 and CTF-HUST-HC2), and controlled the nucleation and crystal growth rate by adjusting the feeding rate of monomers, thus improving the crystallinity of CTFs[94]. It is found that high crystallinity COFs with abundant bare facets have better charge mobility. (2) The side chains of two-dimensional COFs are generally divided into rigid chains and flexible chains, and the rigid planar COFs form a close π-π stacking interaction between layers, which makes their crystallinity higher than that of general organic materials. In the synthesis of COFs, the monomer materials containing flexible chain groups have low crystallinity due to the large degree of distortion of their side chains and low interlayer packing density, and may even form amorphous organic polymeric materials[95]. At present, the interlayer packing density can be enhanced by introducing nitrogen atoms to replace the C — H bond in the benzene ring, so that it has a close π-π stacking effect, thus improving the crystallinity of COFs[96]. However, it should be noted that not all COFs with high crystallinity have satisfactory electrochemical performance. Most of the COFs with high crystallinity have a strong π-π stacking structure, and their interlayer arrangement is too close, which leads to the failure to make full use of the active sites in COFs and has a great impact on the electrochemical performance of batteries. Therefore, the crystallinity of COFs can be effectively adjusted by using the interlayer exfoliation strategy.

4.3 Interlayer stripping strategy

Generally, COFs with high crystallinity have stronger π-π interaction and closer interlayer arrangement. However, this also leads to a long ion transport distance and a large transport resistance, which makes it difficult for ions to penetrate into the active sites inside the framework through the channels of COFs, which greatly affects the electrochemical performance of COFs electrodes. Interlayer exfoliation can reduce the packing density between layers, prepare thin layers of COFs with smaller thickness, and shorten the ion diffusion distance, which is an effective strategy to regulate ion diffusion. Current exfoliation strategies are mainly divided into physical and chemical exfoliation methods. Lu et al. Prepared COFs with different stack thicknesses (DAAQ-COF) by physical exfoliation method (Fig. 14)[36]. Nanosheets with a thickness of 100-250 nm and 100-180 nm were obtained by grinding or ball milling the sample in a mortar; Nanosheets with a thickness of 50 ~ 85 nm were obtained by dissolving the sample in methanesulfonic acid and reprecipitating with water; While the nanosheets with a thickness of 4 – 12 nm were obtained by dissolving the sample in methanesulfonic acid and reprecipitating with methanol. In addition to the physical exfoliation strategy, chemical exfoliation has also become an effective strategy for the preparation of few-layer COF nanosheets.
图14 物理剥离法制备不同厚度DAAQ-COF纳米片的示意图[36]

Fig. 14 Schematic diagram of DAAQ-COF nanosheets stacked layer by layer with different thickness by physical stripping method [36]

Chen et al. Developed a chemical exfoliation strategy to obtain thin-layer COF nanosheets (E-TFPB-COF) and applied them to lithium-ion batteries (Fig. 15)[97]. Potassium permanganate is added into a mixed solution of TFPB-COF and perchloric acid for water-bath stirring, then E-TFPB-COF/MnO2 is obtained after ultrasonic treatment, and finally the E-TFPB-COF/MnO2 is etched in a hydrochloric acid aqueous solution to obtain the E-TFPB-COF. Compared with TFPB-COF, the stripped E-TFPB-COF exhibited faster ion/electron dynamics, exhibiting lithium-ion active storage sites associated with conjugated aromatic π electrons. After 300 cycles, the reversible capacities of the E-TFPB-COF/MnO2 and E-TFPB-COF electrodes were 1359 and 968 mAh·g-1, respectively, with good rate capability.
图15 (a) 剥离后的E-TFPB-COF结构示意图；(b) E-TFPB- COF的长期循环性能；(c) E-TFPB-COF的倍率性能[97]

Fig. 15 (a) The structure diagram of E-TFPB-COF after stripping; (b) Long-term cyclic performance of E-TFPB-COF; (c) The magnification performance of E-TFPB-COF [97]

5 Application of COFs in different metal ion batteries

In recent years, there are more and more reports on the application of COFs in different metal ion battery cathode materials, and COFs show great development prospects in the field of energy storage. Table 3 illustrate that performance of COFs as cathode material in different metal ion batteries.
表3 COFs在不同金属离子电池正极材料中的应用

Table 3 COFs application in different metal ion battery cathode material

Name of the COFs Batteries Voltage
Window (V)		 Discharge specific
capacity (mAh·g-1)		 Cycle life (capacity retention/cycle/rate) ref
TPPDA-CuPor-COF
USTB-6-COF@G
BFPPQ-COF@CNT
IISERP-COF22
COF-N
TAQ-BQ-COF
HAQ-COF
S@TAPT-COF
GOPH-COF
BT-PTO-COF
TP-TA-COF
SCNMC-COF
TFPPy-ICTO-COF
HATN-HHTP@CNT
HATN-HHTP@CNT
BAV-COF-Br-
HATN-AQ-COF
TPF-1S-COF
DAPO-TpOMe-COF
TPDA-PMDA-COF
HTAQ-COF
PT-COF50
E-TP-COF
TPPDA-PI-COF
NTCDI-COF
PICOF-1
F-COF
TP-COF/CNTs
QPP-FAC-Pc-COF
COF-CRO
Tp-DANT-COF
PI-ECOFs/rGO
PD-NDI-Lp
PPTODB-COF
PIBN-G
TpBpy-COF		 LIBs
LIBs
LIBs
AZIBs
MIBs
AZIBs
AZIBs
SIBs
AZIBs
AZIBs
LIBs
LIBs
LIBs
LIBs
KIBs
SIBs
LIBs
LIBs
LIBs
LIBs
AZIBs
LIBs
LIBs
LIBs
LIBs
SIBs
KIBs
KIBs
KIBs
LIBs
LIBs
LIBs
LIBs
LIBs
LIBs
AIBs		 1.5~4.2
1.2~3.9
1.7~3.3
0.2~1.6
0.3~2.5
0.4~1.6
0.26~1.5
1.5~3.2
0.2~1.6
0.4~1.5
1.2~4.3
3.6~4.2
0.05~3.0
1.2~3.8
1.2~3.8
1.4~3.9
1.2~3.9
0.01~3
1.5~4.2
1.2~4.3
0.1~1.45
1.5~3.5
1.5~3.5
2.6~4.1
1.5~3.5
0.01~3
0.01~3
0.01~3
0.01~3
0.5~4.5
1.5~4.0
1.5~3.5
1.5~3.5
1.5~3.5
1.5~3.5
0.01~2.3		 142/0.06 A·g-1
285/0.2 C
87.5/0.2 C
690/1.5 A·g-1
120/0.05 A·g-1
208/0.1 A·g-1
339/0.1 A·g-1
109/0.1 A·g-1
70.2/0.015 A·g-1
225/0.1 A·g-1
207/0.2 A·g-1
160.5/1 C
338/0.1 A·g-1
231/0.05 A·g-1
218/0.1 A·g-1
152/0.05 A·g-1
319/0.5 C
1563/0.08 C
81.9/0.1 A·g-1
233/0.5 A·g-1
305/0.04 A·g-1
280/0.2 A·g-1
110/0.2 A·g-1
47/0.2 A·g-1
212/0.1 A·g-1
237/0.1 C
248/0.05 A·g-1
290/0.1 A·g-1
424/0.05 A·g-1
268/0.1 C
144.4/0.34 C
124/0.1 C
77/0.5 C
198/0.02 A·g-1
271/0.1 C
307/0.1 A·g-1		 85%/3000/1 A·g-1
70%/6000/5 C
86%/600/5 C
83%/6000/5 A·g-1
99%/300/0.2 A·g-1
87%/1000/1 A·g-1
99%/10000/5 A·g-1
76%/2000/2 A·g-1
82%/500/0.015 A·g-1
98%/10000/5 A·g-1
93%/1500/5A·g-1
87.5%/200/1 C
100%/1000/1 A·g-1
100%/6900/0.5 A·g-1
86.5%/2400/0.5 A·g-1
76.5%/500/0.25 A·g-1
80%/3000/10 C
43.5%/1000/2 C
94%/200/0.1 A·g-1
57.1%/1800/5 A·g-1
87%/1000/2 A·g-1
82%/3000/2 A·g-1
87.3%/500/0.2 A·g-1
65%/3000/1 A·g-1
86%/1500/2 A·g-1
84%/175/0.3 C
99.7%/5000 /1 A·g-1
80%/500/0.2 A·g-1
99.9%/10000/2 A·g-1
99%/100/0.1 C
95%/600/7.5 C
72.6%/300/1 C
80%/400/2.5 C
68.3%/150/0.02 A·g-1
86%/300/5 C
100%/13000/2 A·g-1		 98
99
100
101
102
88
103
104
105
82
106
107
108
109
109
110
26
111
112
27
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128

5.1 Lithium ion battery

The energy density of lithium-ion batteries (LIBs) is higher than that of traditional nickel-metal hydride and lead-acid batteries, but their poor cycling stability is a key challenge for their application in future energy storage. The structural stability and ion transport efficiency of cathode materials in the electrochemical process are the key factors affecting the cycle life of LIBs[129,130]. COFs are promising organic cathode materials for LIBs due to their inherent advantages of high thermal stability, long-range ordered structure and high porosity. To design cathode materials with high reversible capacity and good cycling stability, it is an effective strategy to introduce maximized redox-active sites into COFs[131]. Yang et al. Synthesized polyimide-based COFs with multiple redox active sites (HATN-AQ-COF) using 2,6-diaminoanthraquinone and 2,6,8,9,14,15-hexa-carboxyhexaazatrinaphthalene trianhydride as monomer materials, and used them as LIBs cathode materials (Fig. 16)[26]. HATN-AQ-COF has a large pore size of 3.8 nm, which is combined with abundant redox active sites for stable and fast ion transport. With a high reversible capacity of 319 mAh·g-1 at a current density of 0.5 C(1 C=358 mA·g−1). With a high active site utilization of 89% and good cycling performance, the HADN-AQ-COF electrode is one of the best-performing COF electrodes in known reports. Due to the inherent low conductivity of COFs and the hindrance of ion transport channels, the performance of COFs in LIBs can be significantly improved by adding highly conductive carbon matrices (such as graphene oxide and carbon nanotubes) into composites.
图16 (a) HATN-AQ-COF的合成示意图；(b) HATN-AQ-COF的电化学氧化还原机理；(c) 不同电流密度下的充放电曲线；(d) HATN-AQ-COF的长期循环性能[26]

Fig. 16 (a) Schematic diagram of HATN-AQ-COF synthesis; (b) the electrochemical redox mechanism of HATN-AQ-COF; (c) charge/discharge profiles for varied current densities; (d) long-term cycling performance of HATN-AQ-COF[26]

Liu et al. Prepared COFs cathode materials with C = O and C = N active groups (TH-COF) using 2,3,5,6-tetraamino-p-benzoquinone and hexa-n-cyclohexane as monomers (Fig. 17)[132]. Each TH-COF repeating unit contains 6 C = O groups and 12 C = N groups, which can provide 18 Li+ for deintercalation and can theoretically achieve a high reversible capacity of 773 mAh·g-1. However, insufficient contact between the active electrode material and the electrolyte hinders the capacity improvement. In addition, the conductivity of TH-COF is relatively low, which is not conducive to the rapid charge transfer during the redox reaction. Graphene oxide (rGO) was therefore complexed with TH-COF to reduce the resistance and strengthen the interaction between the active electrode material and the electrolyte. The specific discharge capacity of the TH-COF/rGO material after 600 cycles at a current density of 1 C（1 C=0.76 A·g-1） is 135 mAh·g-1. In addition, after 100 cycles at high current densities of 5 C and 20 C, the capacity is still maintained at 118 mAh·g-1 and 73 mAh·g-1, showing excellent cycle stability.
图17 (a) TH-COF的合成示意图；(b) TH-COF的电化学氧化还原机理；(c) TH-COF的长期循环性能；(d) TH-COF在5 C和20 C电流密度下的循环性能[132]

Fig. 17 (a) Schematic diagram of HATN-AQ-COF synthesis; (b) the electrochemical redox mechanism of HATN-AQ-COF; (c) long-term cycling performance of TH-COF; (d) long-term cycling performance of TH-COF at 5 C and 20 C [132]

5.2 Sodium ion battery

Sodium-ion batteries (SIBs) are considered to be a potential alternative to LIBs due to the limited lithium resources and the problems caused by their cost and safety. It has attracted wide research interest because of its low cost, wide distribution of resources, and similar reaction potential to lithium (Na/Na+ is 2. 71 V, Li/Li+ is 3. 04 V)[133,134]. However, it is not feasible to simply use inorganic transition metal oxides as electrode materials in SIBs, mainly because the Na+ radius (0. 102 nm) is larger than the Li+ radius (0. 076 nm), and the limited ion channel causes the volume expansion of inorganic materials and the slow kinetics of Na+ transport, which leads to poor cycle and rate performance. In contrast, the pore size of COFs is large enough to accommodate large metal ions such as Na+ without causing significant volume expansion, which makes Na+ easy to transport, so COFs are also considered as promising electrode materials for high-performance SIBs. Shehab et al. Synthesized a microporous COFs with phenazine modified channels (Aza-COF) (Fig. 18) by condensation reaction using hexa-n-cyclohexane and 1,2,4,5-tetraaminobenzene as monomer materials, and applied it to the cathode material of SIBs[135]. The one-dimensional channel and π-π conjugated structure of Aza-COF enhance the diffusion efficiency of Na+ and promote the reaction of Na+ with the aza-redox active site. The cell showed outstanding rate performance at different current densities of 0.1~40 C(1 C=0.6 A·g-1), and exhibited excellent cycling performance with 87% capacity retention after 500 cycles at a current density of 5 C.
图18 (a) Aza-COF的合成示意图；(b) 通过模型预测Aza-COF孔径；(c) Aza-COF的倍率性能；(d) Aza-COF的长期循环性能[135]

Fig. 18 (a) Schematic diagram of Aza-COF synthesis; (b) Aza-COF aperture was predicted by the model; (c) rate performance of Aza-COF; (d) long-term cycling performance of Aza-COF[135]

Shi et al. Developed a honeycomb TQBQ-COF (Fig. 19)[28]. TQBQ-COF has a high reversible capacity of 515 mAh·g-1 because it contains a large number of C = O and C = N active groups and a very small amount of inactive groups. The Na+storage mechanism of TQBQ-COF was investigated by in situ infrared spectroscopy and ex situ XPS, which confirmed that 12 Na+ could be accessed in each TQBQ-COF unit. Abundant N atoms in the framework can reduce the energy gap between LUMO and HOMO, thus improving the ionic and electronic conductivity. Therefore, the TQBQ-COF exhibited a high rate capability of 134.3 mAh·g-1 at a current density of 10 A·g-1 and a capacity retention of 96% after 1000 cycles at a current density of 1 A·g-1, demonstrating excellent cycling stability.
图19 (a) TQBQ-COF的化学结构和可能的电化学氧化还原机理；(b) TQBQ-COF在0.02 A·g-1下的充放电曲线；(c) TQBQ-COF的原位FTIR光谱；(d) TQBQ-COF在不同充/放电状态下的C 1s XPS谱；(e) TQBQ-COF的倍率性能；(f) TQBQ-COF的长期循环性能[28]

Fig. 19 (a) The chemical structure and possible electrochemical redox mechanism of TQBQ-COF; (b) Discharge/charge profiles of TQBQ-COF electrode at 0.02 A·g-1; (c) In-situ FTIR spectra of TQBQ-COF; (d) the C1s XPS spectra of TQBQ-COF electrodes at different charge/discharge states; (e) rate performance of TQBQ-COF; (f) long-term cycling performance of TQBQ-COF [28]

5.3 Potassium ion battery

Potassium-ion batteries (KIBs), like sodium-ion batteries, have attracted increasing attention in the field of energy storage because of their abundant metal resources and lower cost, and their standard potential （K/K+ of 2.92 V) is closer to that of lithium. Similar to SIBs, the application of general transition metal oxide materials such as MnO2 and V2O5 in KIBs is limited due to the following reasons: (1) The large ionic radius ：K+ radius (0.133 nm) is larger than that of Li+, so it occupies a larger space in the lattice. However, the lattice structure of most transition metal oxide materials has poor ionic compatibility with larger ionic radius, which will cause lattice expansion and even crystal structure collapse in the electrochemical process, thus reducing the cycle and rate performance of batteries[136,137]. The reactivity of （2）K+ ：K+ is high in the battery, which is easy to react with other substances in the electrolyte, such as water decomposition in the electrolyte. Some transition metal oxide materials may not be able to withstand this high activity environment and lead to their structural instability[138]. (3) Working potential: Some transition metal oxide materials are suitable for the use of LIBs, but show lower working potential in KIBs. Due to the different physicochemical properties of K+ and Li+, different materials have different potential requirements for their deintercalation, and higher or lower potentials are required to achieve the reaction, resulting in their limited application in KIBs[139]. Therefore, it is necessary to develop new materials with high ionic compatibility and structural stability to meet the needs of potassium-ion batteries in terms of high cycle life and high safety. Compared with transition metal materials, such COFs with highly controllable pore structure are beneficial to the transport and storage of K+ in batteries. The organic framework of COFs has a large number of C — C and C — H bonds with low bond energy and low ion transport resistance, which is beneficial to the rapid diffusion of K+. Duan et al. prepared DAAQ-COF @ CNTs by in situ growth of DAAQ-COF on CNTs (Fig. 20)[140]. The conductivity of DAAQ-COF @ CNT is nearly 500 times higher than that of DAAQ-COF. The number of DAAQ-COF layers on the surface of carbon nanotubes is small, which is beneficial to the effective utilization of active sites.
图20 (a) DAAQ-COF和DAAQ-COF@CNT合成示意图；(b) DAAQ-COF和DAAQ-COF@CNT的倍率性能；(c) DAAQ- COF@CNT的长期循环性能；(d) DAAQ- COF@CNT的电化学氧化还原机理[140]

Fig. 20 (a) Schematic illustration of synthesis of the DAAQ-COF and DAAQ-COF@CNT; (b) rate performances of the DAAQ-COF and DAAQ-COF@CNT; (c) long-term cycling performance of DAAQ-COF@CNT; (d) electrochemical redox mechanism of DAAQ-COF@CNT in charge and discharge process [140]

Therefore, the DAAQ-COF @ CNT exhibits a good rate capability of 111.2 mAh·g-1 at a current density of 1 A·g-1, with a capacity retention of 77.6% after 500 cycles at 0.5 A·g-1, exhibiting excellent cycling stability, which is significantly better than that of DAAQ-COF. The study shows that the storage mechanism of DAAQ-COF @ CNT is based on the reversible reaction of redox-active C = O groups with potassium ions in two consecutive steps.

5.4 Aqueous zinc-ion battery

In order to meet the requirements of green, safe and sustainable development in the field of energy storage in the process of energy transformation, water-based batteries led by water-based zinc-ion batteries (AZIBs) have sprung up in recent years.With the advantages of abundant storage resources, low redox potential (-0.76 V), high safety, low cost and environmental friendliness, it is called the most promising green environmental battery system in the future, and is expected to achieve large-scale applications in the field of electrochemical energy storage[141,142]. However, the main problems that hinder its development are also obvious. At present, transition metal oxides are the most widely used cathode materials for AZIBs[143,144]. Because the radius of Zn2+ (0. 074 nm) is comparable to that of Li+, the reversible deintercalation of divalent cations will lead to strong electrostatic interaction with the cathode material, resulting in a high activation barrier, which hinders the transport of Zn2+ and even leads to the collapse of the crystal structure[145]. Therefore, the current research direction of AZIBs is to find cathode materials with excellent structural stability. COFs have been gradually applied in AZIBs in recent years due to their highly ordered ion transport channels, good structural designability and abundant specific surface area. Ma et al. Successfully synthesized keto-enamine COFs (Tp-PTO-COF) by polycondensation using trialdehyde phloroglucinol (TP) and 2,7-diaminopyran-4,5,9,10-tetraone (DAPTO) as monomer materials, and introduced them into AZIBs for the first time (Fig. 21)[25]. Tp-PTO-COFs have multiple carbonyl active sites due to the conversion of hydroxyl (— OH) to carbonyl (C = O) during the irreversible proton tautomerization of keto-enamine COFs, and one DAPTO molecule contains four C = O groups. It exhibits a high reversible capacity of 301.4 mAh·g-1 at a current density of 0.2 A·g-1, while maintaining a capacity retention of 95% after 1000 cycles at a current density of 2 A·g-1, demonstrating excellent cycling stability.
图21 (a) Tp-PTO-COF的合成及结构的示意图；(b) 不同电流密度下的充电/放电曲线；(c) 在2 A·g-1下的长期循环性能[25]

Fig. 21 (a) Schematic diagram of synthesis and structure of Tp-PTO-COF; (b) Charge/discharge curves at different current densities; (c) Long-term cycling performance at 2 A·g-1[25]

6 Conclusion and outlook

COFs are a kind of porous organic framework materials with periodic two-dimensional or three-dimensional networks, whose structure is formed by two or more organic molecules connected by covalent bonds. Compared with transition metal oxides and common polymers, COFs exhibit unique advantages as cathode materials for metal ion batteries.
(1) High energy density: COFs have a highly controllable porous structure, which provides more active sites for the reversible deintercalation of metal ions in the cathode material during the charge-discharge process, increasing the energy density of the battery; The high specific surface area can increase the contact area between the battery cathode material and the electrolyte, thereby improving the electrochemical reaction rate. In addition, COFs are composed of light elements (C, H, N, O and B, etc.) and have low skeleton density, which is very important to improve the energy density of batteries[30].
(2) High electrochemical stability: The tightly packed conjugated aromatic ring structure in COFs makes them highly chemically inert. The conjugated interactions and hydrogen bonds make COFs show high stability in high temperature and acid-base environment. The unique periodic two-dimensional or three-dimensional structure of COFs makes them insoluble in electrolyte solution and can maintain structural stability during charging and discharging, thus improving the cycle life of batteries and reducing the occurrence of side reactions.
(3) Green sustainability: The synthetic raw materials of COFs are mainly some natural organic compounds, such as aromatic compounds, aliphatic compounds, etc. These compounds are widely available and most of them are renewable. The synthesis of conventional COFs mainly relies on chemical synthesis methods such as solvothermal. However, with the continuous exploration of synthetic means, more and more synthetic methods do not require the use of organic solvents or other harmful substances. The production of organic waste can be effectively reduced by using efficient and environmentally friendly synthesis methods.
So far, COFs have played an increasingly important role in the field of energy storage. However, it is undeniable that its application in metal ion batteries still faces many challenges.
(1) Low charge-discharge efficiency: Although COFs have abundant redox active sites, the tight π-π stacking effect between their layers makes most of the active groups not effectively utilized, resulting in low ion transport efficiency, which is also the main reason for the poor battery capacity. Exfoliation of COFs into few-layer nanosheets can expose more active sites to metal ions. In addition to the exfoliation strategy, COFs and MOF materials composite is also an effective means to improve the electrochemical performance of batteries[146,147].
(2) Poor ionic conductivity: Although the ordered pores in COFs can promote the diffusion of ions in the material, the diffusion rate in COFs is slow due to the long transmission path of ions. Secondly, the structure of COF monomer is usually composed of aromatic rings or imine groups, which determines the long chemical bonds of COFs, which also limits the efficiency of ion transport. COFs are usually an insulator or semiconductor because of their large band gap, through which only a small number of carriers pass at room temperature. In order to improve their ionic conductivity, it is an effective strategy to add highly conductive materials such as carbon nanotubes, graphene oxide or metal oxides to COFs[148,149].
(3) Complicated synthesis process: At present, COFs are mostly synthesized by solvothermal synthesis method, which requires strict reaction conditions and environment, and takes a long time to synthesize, so the steps and difficulties of synthesis are greatly increased. In addition, there are many by-products after synthesis, and the post-treatment is more complex, resulting in low yield. It is worth mentioning that with the in-depth research on the synthesis of COFs in recent years, new synthesis methods have been developed, such as mechanochemical method, microwave radiation method, sonochemical synthesis method and so on. However, these synthesis methods are not universal and can not be applied to the synthesis of COFs on a large scale. Therefore, the development of green, simple and universal synthesis methods is a key work in the study of COFs[150,151].

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