Covalent organic frameworks as electrode materials for rechargeable metal‐ion batteries

Covalent organic frameworks (COFs), as a class of crystalline porous polymers, featuring designable structures, tunable frameworks, well‐defined channels, and tailorable functionalities, have emerged as promising organic electrode materials for rechargeable metal‐ion batteries in recent years. Tremendous efforts have been devoted to improving the electrochemical performance of COFs. However, although significant achievements have been made, the electrochemical behaviors of developed COFs are far away from the desirable performance for practical batteries owing to intrinsic problems, such as poor electronic conductivity, the trade‐off relationship between capacity and redox potential, and unfavorable micromorphology. In this review, the recent progress in the development of COFs for rechargeable metal‐ion batteries is presented, including Li, Na, K, and Zn ion batteries. Various research strategies for improving the electrochemical performance of COFs are summarized in terms of the molecular‐level design and the material‐level modification. Finally, the major challenges and perspectives of COFs are also discussed in the aspect of large‐scale production and electrochemical performance improvements.


| INTRODUCTION
Ever-increasing energy demand and global warming require the pursuit of next-generation sustainable energy storage systems. Rechargeable metal-ion batteries have played a critical role in human society due to their wide applications for portable electronics and high-power electric vehicles. [1,2] In recent decades, extensive and innovative research efforts have been devoted to developing lithium-ion batteries (LIBs), and tremendous advancements have been made in battery performance and technologies. [3][4][5] At present, the commercial LIBs are mainly assembled by graphite anodes and transition-metal oxide/phosphate cathodes, such as LiCoO 2 , LiMn 2 O 4 , LiFePO 4 , or LiNi x Mn y Coz O 2 . [6][7][8][9][10] However, the development of LIBs based on conventional electrode materials has been approaching their limits, making it difficult to achieve greater breakthroughs in energy density, thus failing to satisfy ever-growing energy demands. Moreover, the excessive utilization of transition metals for energy storage has inevitably highlighted the issues of resource shortage and environmental pollution. Besides, high cost and safety problems also have to be concerned. Consequently, exploring renewable and ecofriendly electrode materials has become the most significant issue and attracted worldwide attention towards pursuing higher energy density and sustainable rechargeable batteries.
Covalent organic frameworks (COFs), as a class of crystalline porous polymers constructed from organic building monomers via covalent bonds, were first reported by Yaghi and co-workers in 2005. [11] Since then, a variety of COFs has been synthesized and reported. [12][13][14] The emergence of COFs successfully expands the scope of porous organic polymers and provides a new platform for the development of materials science, especially in the interdisciplinary materials field. Compared with conventional organic polymers, COFs possess many distinguishing advantages of diverse structures, adjustable pore sizes, designable skeletons, and tailorable functionalities. [15][16][17] Owing to their characteristics, COFs have been studied in many research fields ranging widely from gas adsorption, catalysis, and photoelectric devices to energy conversion and storage. [16][17][18][19][20][21][22][23][24][25][26][27][28] As an emerging materials platform, COFs possess many distinct merits when applied as electrode materials for rechargeable metal-ion batteries: (1) the diversity of organic building monomers and linkages, together with the availability of different chemical reactions and synthesis methods, offer many feasible strategies for developing desired COFs with specific active sites and functions; (2) the large framework of COFs and the strong covalent bond linkages constructed by organic building monomers can ensure their high chemical and thermal stability during the redox process; (3) the porous architecture and large surface areas are beneficial to the ion diffusion and penetration of electrolytes; (4) the long-range ordered structures of COFs make it possible to predict their properties and in-depth study their reaction mechanism by theoretical simulations; (5) some functional groups in the skeleton of COFs can be modified by post functionalization, which endows them with the capability to modify the molecule structures and properties; (6) the diverse redox-active groups and tailored molecular structures of COFs endow the charge carrier with great flexibility, suggesting that other cations such as Na + , K + , and Zn 2+ are also applicable to COFbased electrode materials for the batteries beyond LIBs. Benefiting from the above features, COFs have drawn growing interest and have been extensively studied in the field of rechargeable metal-ion batteries. [29][30][31] Nowadays, many reviews have involved the development of COFs. However, there are few systematic reviews summarizing the redox-active groups used in COFs, hierarchical research strategies from the molecular level to the material level, and the development of COFs in LIBs as well as other rechargeable metal-ion batteries. In this review, we systematically introduce the representative redox-active moieties for constructing COF-based electrode materials, together with the corresponding redox reaction mechanisms. Then, we review the principles in molecular-level design and modifications at the material-level engineering to improve the electrochemical performance of COFs as electrode materials, emphasizing the great significance of establishing structural-performance relationships and the necessity of the modifications at the material level. Subsequently, the latest developments in COFs for other rechargeable metal-ion batteries are also summarized and discussed. Towards the end, the main challenges and prospects of COFs as electrode materials for rechargeable metal-ion batteries are discussed and analyzed from the perspective of electrochemical performance limitation and large-scale production.

| REDOX-ACTIVE GROUPS AND REACTION TYPES
On the basis of the redox mechanisms, organic electrode materials can be roughly classified into n-, p-, and bipolar types. [32] COFs as electrode materials participate in the electrochemical process mainly through the reversible redox reaction of active organic groups. Representative redox-active moieties for each reaction type and their redox mechanisms for LIBs are displayed in Scheme 1. N-type materials typically first undergo reduction, during which they serve as the electron acceptor and combine with metal cations such as Li + , Na + , or K + , resulting in a negatively charged state from its original neutral state. [33] N-type materials can act as cathodes or anodes, which are dominated by their redox potentials. Most reported n-type materials as cathode materials for LIBs can offer remarkably high capacity, well above those of commercial inorganic cathodes. [34][35][36][37][38][39] Nevertheless, the redox potentials of n-type materials are generally below 3.0 V versus Li/Li + , significantly lower than those of conventional cathodes, which limits the overall energy density of LIBs. [40] Carbonyl (C=O) compounds are representative of n-type materials for rechargeable metal-ion batteries, which usually exhibit high specific capacity and good redox reaction reversibility, and have been one of the most widely investigated organic electrode materials. [41][42][43] Quinones and imides are the most commonly used structural units to provide the redox-active C=O groups in COFs. [44][45][46][47][48] As shown in Scheme 1, when used as electrode materials for LIBs, the two C=O groups of quinones can gain/release two electrons and Li + , and the structures of the C=O groups are reversibly transformed into C-OLi through enolization reaction. Imides perform a similar redox reaction mechanism, but only half of the C=O groups can be reversibly reduced, decreasing the practical capacity. [49][50][51][52][53] The main reason is that the reduction of the other two C=O groups at a much lower potential will damage the structural stability due to charge repulsion interactions. Organic compounds with conjugated C=N groups are another typical class of n-type materials. As summarized in Scheme 1, phenazine can act as a redox-active moiety in COFs, where the C=N groups undergo a reversible two-electron transfer during Li storage. [34,54] In addition, triquinoxalinylene has attracted increasing attention due to its multiple C=N redox sites, which can react with six Li + and offer a high capacity. [55][56][57][58][59] Moreover, the N-rich and expanded conjugated structure is conducive to enhance rate performance. Besides, the conjugated C=N linkages, formed by Schiff base reactions between -NH 2 and -CHO, can also be used as redox sites in some cases. [60][61][62] Azo (N=N) groups are also the type of redox sites involved in n-type reactions, favoring fast redox reaction kinetics owing to the π-conjugated structure of N=N groups and aromatic rings. [63][64][65] As shown in Scheme 1, each N=N group can reversibly transfer two electrons and transform into an N-N group during the redox process, providing a high capacity. In addition to the conjugated C=O, C=N, and N=N groups, benzene rings can also be applied as redox moieties to store Li + . [66,67] Unlike the graphite redox process based on LiC 6 , many studies have indicated that the intercalation behaviors of Li + in benzene rings for organic anodes are interpreted as overlithiation. [32] As shown in Scheme 1, each benzene ring inserts six Li + to form a Li 6 /C 6 complex reversibly, affording an ultrahigh capacity. Consider that most COFs consist mainly of aromatic rings and possess a large specific surface area. In this way, COF-based anode materials are capable of achieving high capacity that can be assigned to abundant redox sites and surface adsorption. In contrast to n-type materials, p-type materials first experience an oxidation process with the extraction of electrons from their neutral state and combine with anions (PF 6 − , ClO 4 − , or TFSI − ) from the electrolyte to remain electroneutral. [33,68,69] The redox reactions of p-type materials occur between their neutral state and positively charged state. As shown in Scheme 1, N,N′-substituted phenazine and triphenylamine derivatives are common p-type redox moieties in COFs, where the N atoms can be oxidized into N + and combined with anions (A − ) from the electrolyte. Note that the redox potentials of p-type materials are usually higher than those of n-type materials, such that p-type materials are more suitable for battery cathodes. In addition, p-type materials generally exhibit excellent kinetic properties. However, they tend to deliver relatively low specific capacity due to excessive redoxinactive components, failing to obtain high energy density, which makes them less competitive in practical applications. Besides, despite numerous studies on p-type organic small molecules and polymer materials, there are very few reports on COFs that undergo p-type reactions. Bipolar-type organic materials possess the properties of both n-type and p-type materials, that is, they can utilize their negatively or positively charged states to combine with cations or anions, which extends their applications to not only rechargeable metal-ion batteries but also dual-ion batteries. [34,35,[70][71][72] Nitroxyl radical compounds, with bipolar-type features, have been widely investigated for energy storage. [73,74] As displayed in Scheme 1, they can be reversibly reduced to N-O − anions or oxidized to O=N + cations, along with the incorporation of cations or anions. Besides, porphyrin and phthalocyanine (Pc) derivatives have also been reported as bipolar-type redox-active moieties for energy storage. [70,[75][76][77][78][79] As can be seen from Scheme 1, the 18 π-electrons of Pc-Cu can be oxidized or reduced into 16 π-electrons or 20 π-electrons, respectively. The bipolar feature renders them with a wide working potential window, even suitable for all-organic symmetric batteries.
Nevertheless, note that most redox processes for bipolar-type electrode materials involve not only cationic charger carriers but also anions in the electrolyte. Thus, there is no doubt that the types and properties of the electrolytes have a certain degree of influence on the electrochemical behaviors of bipolar-type materials. However, there is a lack of systematic and in-depth studies on this aspect. Future studies on bipolar electrode materials should also take into account the influence of electrolytes. Besides, the involvement of the anions also complicates the reaction mechanism of bipolartype electrode materials, which requires more attention and effort from researchers. In a word, the research on bipolar-type COF-based electrode materials is still at the early stage, and much more efforts need to be devoted to boosting the development.
In this section, we presented three reaction types of organic electrode materials according to their reaction mechanisms. Taking LIBs as an example, we introduced representative redox-active moieties and their redox mechanisms in each reaction type for constructing COFs as electrode materials in detail. The diverse redox-active groups and advanced synthesis techniques have greatly boosted the development of COFs as electrode materials for rechargeable metal-ion batteries.

| RESEARCH STRATEGIES TO IMPROVE ELECTROCHEMICAL PERFORMANCE
With the immense progress in organic chemistry and materials technology, many effective strategies have been proposed to address the inherent defects of organic materials related to poor electrical conductivity, relatively low redox potential, and so on. In this section, the main research strategies for improving the electrochemical performance of COFs are discussed and analyzed in the following two dimensions: principles in molecular-level design and modifications in material-level engineering.

| Principles in molecular-level design
The diversity of COFs endows them with endless possibilities in structural design. Furthermore, the structural designability of COFs offers a great opportunity to improve their electrochemical performance by rational and controllable design at the molecular level. As shown in Figure 1, the main components of COFs (such as redox sites, building units, covalent linkages, and functional groups) and their molecular skeleton as well as conjugated structure can be tailored and have a decisive influence on the electrochemical performance. In terms of the critical performance parameters of rechargeable metal-ion batteries, such as theoretical specific capacity, redox potential and rate performance, several principles in molecular-level design have been proposed to obtain the desired COFs as electrode materials, establishing the structure-performance relationships.

| Improving specific capacity
The theoretical specific capacity (C spec [mAh g −1 ]) of electrode materials can be calculated by the following equation [80] : where n is the number of transferred electrons per molecule or repeating unit, F is Faraday's constant (C mol −1 ), and M is the molar mass of per molecule or repeating unit (g mol −1 ). The theoretical and initial capacities of the COFs developed for cathode materials of LIBs are summarized in Table 1. Many COFs have higher theoretical and initial capacities compared with those of conventional inorganic cathode materials, demonstrating the distinct advantages of COFs. According to the above equation, the most effective approach to improve the theoretical specific capacity is to increase redox-active groups in the structure while minimizing inactive groups to maximize the number of transferred electrons per repeating unit. [38] As shown in Figure 2A, we designed and synthesized a BQ1-COF constructed by maximum active groups and minimal inactive groups. One repeating unit of the as-designed BQ1-COF consisted of 6 C=O and 12 C=N groups, which could reversibly gain/release 18 electrons in theory, thus yielding an ultrahigh theoretical capacity of 773 mAh g −1 . As shown in Figure 2B, BQ1-COF delivered a high capacity of 502.4 mAh g −1 at 0.05 C, corresponding to a maximum of 12 transferred electrons Abbreviations: 2D, two-dimensional; CMP, conjugated microporous polymer; CNT, carbon nanotube; COF, covalent organic framework; CTF, covalent triazine framework; DAAQ, 2,6-diaminoanthraquinone; DAPH, 2,7-diaminophenazine; DAPO, 2,7-diamine-10-methyl-phenoxazine; DAPQ, 2,7-diamino-9, per repeating unit. In addition, owing to the well-defined channel and extended π-conjugated structure, BQ1-COF exhibited a superior rate capacity of 170.7 mAh g −1 even at an ultrahigh current density of 10 C ( Figure 2C). Recently, Yang and co-workers proposed a new two-dimensional (2D) polyimide-linked COF (denoted as HATN-AQ-COF), featuring a high loading density of redox-active sites for Li + storage. [81] As indicated in Figure 2D, six pyrazine C=N, three imide C=O, and three quinone C=O redox-active sites per asymmetric unit of HATN-AQ-COF were synergistically involved in the reversible Li + storage process, which rendered it a high capacity of 319 mAh g −1 at 0.5 C ( Figure 2E). Such a high capacity corresponds to a highly active site utilization of 89%, which could be attributed to its porous channels with large pore sizes. In addition, the large channel size and conjugated 2D network of HATN-AQ-COF were conducive to fast ion diffusion, thus obtaining superior rate capability as displayed in Figure 2F.

| Tuning redox potential
As the energy density of batteries is determined by the discharge specific capacity and the average output voltage together, in addition to improving the specific capacity, elevating the redox potential of cathodes has always been a research hot spot in pursuit of higher energy density. The redox potentials are intrinsically related to their functional groups and molecular skeletons, which opens up the great possibility to tune their redox potential by structural design. Years earlier, our groups proposed that Clar's theory can account for the relationship between the conjugated skeleton and the redox potential of carbonyl-containing polycyclic aromatic hydrocarbons. [93] On the basis of this proposal, we designed several organic compounds for high-voltage cathode materials. As evaluated by density functional density (DFT) calculation and experiments, the average voltages of the as-designed compounds 1 and 3 can reach up to 3.11 and 3.08 V, respectively, indicating the feasibility of our strategy. In addition, many reports have demonstrated that the redox potential of organic cathode materials can be raised by decreasing the lowest unoccupied molecular orbital (LUMO) energy level by introducing electron-withdrawing groups (such as −F, −Cl, and −CN). [94][95][96][97][98][99] For example, Wang and coworkers demonstrated that introducing quinone groups in the structure of HAQ-COF could significantly elevate its capacity and redox potential. [99] When applied as cathode materials in aqueous zinc-ion batteries (ZIBs), The initial three-cycle capacity-voltage profile of BQ1-COF at a current of 0.05 C.
the average discharge voltage of the HAQ-COF was 0.84 V (green line in Figure 3A), while the voltage of 1,4,5,8,9,12-hexaazatriphenylene-based COF (HA-COF) was only 0.53 V (yellow line in Figure 3B). The increased discharge voltage was associated with the grafted quinone groups in HAQ-COF, endowing it with a lower LUMO energy level. DFT computations were performed to further study the electronic structures and relative energy levels of HAQ-COF and HA-COF. As shown in Figures 3C,D, the introduced quinone groups indeed affect the electronic structure of HAQ-COF and result in broader electron delocalization within its backbone, leading to higher redox activity. The LUMO energy level of HAQ-COF (−4.17 eV) was lower than that of HA-COF (−3.62 eV) without quinone groups, indicating stronger electron affinity and higher reduction potential in theory, which accounts for the experimental results. Actually, the above strategy is generally applicable in ntype materials to increase their redox potentials. In the case of p-type materials, they are inherently capable of offering high redox potentials. [68,[100][101][102][103][104][105] Therefore, introducing p-type active moieties into COFs structures, such as N,N′-substituted phenazine, triphenylamine derivatives, and 2,2,6,6-tetramethylpiperdinyloxyl (TEMPO), has also emerged as an effective strategy to obtain COF-based electrode materials with high redox potentials. [68][69][70]106] As shown in Figure 4A, Meng and co-workers designed and synthesized two p-type phenoxazine-based COFs (DAPO-COFs) as cathode materials for LIBs, which can afford high redox potentials (~3.6 V vs. Li/Li + ). [69] The pristine DAPO-TpOMe-COF provided the first discharge capacity of 81.9 mAh g −1 at 100 mA g −1 . Recently, Gu and coworkers have developed a unique star-shaped polyimide COF (TPPDA-polyimide covalent organic framework [PICOF] in Figure 4B) by introducing N-containing N,N,N,N-tetraphenylphenylenediamine (TPPDA) units into the COF structure, which also displayed a high discharge voltage up to 3.6 V. [68] As shown in Figure 4D during discharge/charge process. In the range of 2.6-4.1 V, the COF cathode provided an initial discharge capacity of 40 mAh g −1 at 200 mA g −1 with an obvious discharge voltage platform between 3.1 and 4.0 V ( Figure 4C). After 200 cycles, it yielded a higher capacity of 47 mAh g −1 , and output a higher average discharge potential of 3.6 V.
As can be seen from the above results, although p-type materials show a high redox potential, their energy density was seriously inhibited by the relatively low capacity. The trade-off relationship between the specific capacity and the redox potential in organic electrodes urges the exploration of other feasible solutions. Fortunately, bipolar-type materials can operate in a wide range of potential and experience n-/p-type redox processes, which makes them promising for achieving both high capacity and high potential. [76,[107][108][109] In consequence, constructing COFs with a bipolar feature by molecular design has become an efficient strategy to pursue satisfying capacity and redox potential simultaneously. [62,[70][71][72]77,110] For instance, Yang and co-workers synthesized a series of nanoporous fluorinated covalent quinazoline networks (F-CQNs in Figure 4E) with the bipolar feature as cathode materials for LIBs. [71] As shown in Figure 4F, F-CQNs cathode materials were capable of operating in a large working potential window of 1.5-4.5 V versus Li/Li + , with 1.2 M LiPF 6 in ethyl methyl carbonate and ethylene carbonate as the electrolyte, the p-doping process of F-CQNs occurred in the high potential range of 3.0−4.5 V versus Li/Li + , accompanied by the combination between TCQ cores and PF 6 − anions, whereas the n-doping process was performed in the low potential region of 1.5-3.0 V Li/Li + , involving in the adsorption of Li + into benzene rings. As can be seen from Figure 4G,H, F-CQN-1-600 provided a high capacity of 250 mAh g −1 at 0.1 A g −1 and good rate capability of 105 mAh g −1 at 5.0 A g −1 and enhanced cycle stability, which were associated with its bipolar feature and high surface area and high nitrogen content as well as extended π-conjugated structure.

| Enhancing rate performance
In most cases, the rate performance of rechargeable metal-ion batteries is dominated by the ion and electron conductivity of electrode materials. From the point of view of molecular structure design, narrowing the band gap and expanding the conjugated degree have been common approaches to improve the intrinsic conductivity of organic electrode materials. Generally, introducing functional groups or heteroatoms with strong electron-withdrawing properties in the structure of COFs can effectively reduce their band gap. [64,72,[111][112][113] Zhang and co-workers prepared a 2D fewlayered COF (exfoliated few-layered fluorinated covalent triazine framework [E-FCTF]) by introducing fluorine atoms into the covalent triazine framework (CTF) to decrease the band gap. [113] As expected, the band gap of E-FCTF (1.45 eV) was indeed lower than that of CTF (2.35 eV), further heightening the electrical conductivity ( Figure 5A). As a result, the E-FCTF anode material for LIBs showed excellent rate performance ( Figure 5B). Besides, designing electronconductive linkages like thiazole or thiophene also has a positive effect on the electronic conductivity of COFs. Singh et al. constructed a π-conjugation COF with high chemical stability and good out-of-plane electrical conductivity by incorporating the thiazole-linkage as displayed in Figure 5C, and demonstrated the fast two-electron transfer of the azo redox groups. [64] The COF electrode exhibited over 5000 cycles at 10 C and a high-power density of ≈2800 W kg −1 at 40 C. Recently, Ma and co-workers have designed and synthesized two conjugated microporous polymers (CMPs) as cathode materials for energy storage. [72] As shown in Figure 5D, TzPz with a donor-acceptor (D-A) molecular structure was constructed with an electron-donating dihydrophenazine (Pz) unit and electron-withdrawing 2,4,6-triphenyl-1,3,5-triazine (Tz) unit, while BzPz was obtained from Pz and 1,3,5-triphenylbenzene (Bz) units. Compared with BzPz, the D-A type structure of TzPz efficiently enhanced its conjugated degree and decreased its band gap. It indicated a higher electron conductivity of TzPz, which promoted electron migration along its skeleton during the redox reaction process. As a result, TzPz showed an outstanding rate capacity of 108 mAh g −1 even at 30 A g −1 and good cycle stability, which was superior to that of BzPz ( Figure 5E). As discussed above, porphyrin and Pc compounds can serve as bipolar-type redox-active materials for energy storage, releasing or accepting electrons in a broad potential window. [70,[75][76][77][78][79] In addition, they are typically planar aromatic macrocyclic molecules with large delocalized π-conjugated structures. Their unique structures endow them with high electronic conductivity, and they have been widely selected for the construction of conductive COFs. [114][115][116][117][118][119][120] Accordingly, integrating porphyrin or Pc moieties into COFs to increase the electron conductivity has been a feasible approach to elevate the capacity and rate performance. As shown in Figure 5F, a Pc-based redox-active CMP (named CuPcNA-CMP) with bipolar and double redox-active centers was constructed by integrating CuTAPc and 1,4,5,8-naphthalenetetracarboxylic dianhydride into CMP. [77] DFT computations in Figure 5G fully indicate that CuPcNA-CMP possessed the narrowest energy gap of 1.61 eV, confirming its enhanced electronic conduction. Furthermore, CuPcNA-CMP and Li + /PF 6 − showed large binding energy, revealing the strong electronic interaction between them. Moreover, both CuPcNA-CMP-Li and CuPcNA-CMP-PF 6 exhibited narrowed band gaps, leading to enhanced conductivity. All these advantages highly promoted the electrochemical performance of CuPcNA-CMP. When employed as the cathode material in lithium-based dual-ion batteries (LDIBs), CuPcNA-CMP showed a high reversible capacity of 202.4 mAh g −1 at 0.2 A g −1 and excellent rate performance of 86.1 mAh g −1 at 5 A g −1 ( Figure 5H). Moreover, the bipolar property of CuPcNA-CMP allowed it to be used as a multifunctional electrode material in different storage systems with outstanding performance. The symmetric all-organic batteries with CuPcNA-CMP as both cathode and anode materials even realized a high charge/discharge capacity of 269.4/198.5 mAh g −1 at 0.05 A g −1 together with a high cell voltage of 2.5 V.
As well as improving the electronic conductivity of COFs, accelerating the ion transport is of great significance for boosting the rate performance of COF-based electrode materials. In most cases, when COFs form the long-range ordered channels, the 2D layers usually tend to be arranged densely in a parallel-stacking manner due to strong interlayer π-π interactions. [121] The narrow space between the tightly stacked layers of COFs seriously inhibits ion transport during redox reactions and thus hinders rate performance. From this point of view, it is feasible to improve the rate performance by expanding the interlayer distance of COFs by rational molecular design. By integrating nonplanar building units and linkages into the skeleton of COFs, Wu et al. designed and synthesized a redox-active piperazineterephthalaldehyde (PA-TA) COF with an ultralarge interlayer distance of 6.2 Å. [122] Unlike other COFs with narrow interlayer distances, the ultralarge interlayer spacing of PA-TA COF facilitated the ion diffusion and improved the utilization of redox-active sites, enabling it high capacity and rate performance. When used as the anode material for LIBs, it showed a high capacity of 207 mAh g −1 even at 5.0 A g −1 .
In addition, the pore size of the COFs also has a certain effect on ion diffusion during electrochemical processes. [17,123] Therefore, the porous skeleton of COFs should also be considered when designing the framework structure of COFs. Generally, relatively small pores are not conducive to ion transportation, especially for those charge-carrying ions with large ionic radii. Conversely, a large surface area and pore volume can expose a large number of accessible redox-active sites and reserve sufficient space to accommodate ions during the redox process. [81,123] Nevertheless, excessive surface areas also reduce the volume energy density of COFs. All these factors should be taken into account and well-balanced when designing COF-based electrode materials. Recently, the construction of COFs with dual-pore skeletons or hierarchical pores by combining micro-and meso-and even macropores has been proposed as an efficient strategy to improve their rate performance. [68,124] As an example, we designed and synthesized a TP-TA COF with a micro-meso-porosity as the cathode material for LIBs. [62] The hierarchical pore characteristics of TP-TA COF can greatly boost ion diffusion and facilitate the penetration of electrolytes, yielding an elevated rate performance.

| Modifications at the material level
Although great efforts have been devoted to enhancing the performance of COF-based electrode material by molecular-level design, their performance is far from the requirements for practical applications. Developing feasible strategies from the view of the material level has become another common option to further improve the performance of COFs, which can be divided into two main categories: incorporation with conductive materials and micromorphology optimization.

| Incorporation with conductive materials
As it is difficult to achieve electronically conductive COFs, compositing with conductive additives is the most extensively used strategy to enhance the conductivity of electrodes. Carbon nanotubes (CNTs), [61,86,88,89,125] graphene, [82,83,90] and conductive polymers [54] are the most commonly used conductive additives. Wang and co-workers synthesized a polyarylimide hybrid (2D-PAI@CNT) with abundant π-conjugated redox-active groups by integrating CNTs with highly stable and crystalline 2D polyarylimide (2D-PAI). [88] The capacity of 2D-PAI@CNT was 104.4 mAh g −1 at 0.1 A g −1 , while the pristine 2D-PAI delivered a lower capacity of 28.5 mAh g −1 . Correspondingly, the active site utilization of 2D-PAI@CNT (82.9%) was far higher than that of 2D-PAI (22.6%). Besides, the 2D-PAI@CNT cathode showed excellent rate performance and remarkable cycle stability (100% capacity retention after 8000 cycles). Similarly, as shown in Figure 6A, a series of β-ketoenamine-linked COF composites with tube-type core-shell structures were obtained by the in situ growth on the CNT surface. [86] By varying the content of CNTs, DAPQ-COF50 containing 50 wt% of CNTs showed the highest utilization of the redox-active sites (95%) and thus obtained the highest capacity of 162 mAh g −1 at 500 mA g −1 . Remarkably, DAPQ-COF50 exhibited an ultrahigh rate capacity ( Figure 6B,C), retaining 58% of its capacity even at 50 A g −1 , which was far higher than those of previously reported carbonyl-contained organic electrodes. The excellent performance was attributed to the synergistic effects of DAPQ-COF and CNTs as well as the core-shell structures constructed by in situ growth.
Graphene is another conductive carbon additive for energy storage owing to its outstanding conductivity. Luo and co-workers reported a microporous COF (poly(imidebenzoquinone)-graphene [PIBN-G]) used as cathode materials for LIBs via in situ polymerization on graphene. [90] Such a structure efficiently facilitated the charge transfer in PIBN-G and the complete access of electrons and ions to the rich redox-active carbonyl groups. Thus, the PIBN-G afforded high capacities of 271.0 and 193.1 mAh g −1 at 0.1 and 10 C, respectively, and high retention of more than 86% after 300 cycles. Such in situ integration strategy during the condensation process is universal for obtaining highperformance COF-based electrodes. As shown in Figure 6D, Liu and co-workers also successfully synthesized a surfacesupported dual-porous COF (University of Science and Technology Beijing-6@graphene [USTB-6@G]) by the in situ polymerization of two high-density redox-active building monomers in the presence of graphene. [82] The strong π-π interactions between USTB-6 nanosheets and graphene efficiently improved the electronic conductivity of USTB-6@G, leading to outstanding rate performance. As can be seen in Figure 6E, compared with USTB-6 and USTB-6/G, USTB-6@G realized the highest capacity retention of 188 mAh g −1 at 10 C.
In addition to carbon materials, conductive polymers can greatly enhance the electronic transport of COFs. As shown in Figure 6F, a new phenazine-based 2D COF (DAPH-TFP COF) and its composite with a conductive polymer of PEDOT were prepared and evaluated as cathode materials for LIBs. [54] The DAAQ-TFP COF and its composite were also prepared for comparison. As a result, the rate and cycling performance of PEDOT@DAAQ-TFP were higher than those of DAAQ-TFP without PEDOT ( Figure 6G,H). Interestingly, although the conductivity of DAPH-TFP COF improved upon the addition of PEDOT and showed the fastest electron transport among the four samples, there was no apparent improvement in the performance of PEDOT@DAPH-TFP. Furthermore, the diffusion coefficients of DAPH-TFP COF with and without PEDOT were only slightly changed, so the unimproved performance could be ascribed to the fact that DAPH-TFP COF was suppressed by its ionic transport rather than its electronic transport. Nevertheless, although conductive materials could greatly improve the conductivity of COFbased electrodes, the excessive introduction of these additions can reduce the overall energy density of batteries, limiting their commercial applications.

| Micromorphology optimization
In most cases, the micromorphology of COFs also plays an important role in their electrochemical performance.
Typically, in most bulk COFs, the interior redox-active sites are deeply buried due to the tightly stacking layers, making them inaccessible to be fully utilized for energy storage and prolonging the distance of ion migration, thus resulting in a limited capacity and rate performance. Compared with the bulk COFs, COFs with nanostructure can expose more accessible active sites and minimize the transport length for ions/electrons, which not only increases the capacity but also enhances the rate performance. [121,126] At present, the top-down strategy is a common method to obtain nanostructured COFs by directly breaking their interlayers interactions, such as mechanical exfoliation, [60,84,92] chemical exfoliation, [127][128][129] and self-exfoliation. [121,130] As shown in Figure 7A, Zhao and co-workers developed an atomiclayer COF (E-TP-COF) with a dual-active-center of C=O and C=N groups through the mechanical exfoliation. [84] As displayed in the atomic force microscopy (AFM) image ( Figure 7B), after exfoliation, the micromorphology of E-TP-COF was completely transferred into nanosheets with an ultrathin thickness of~2.6 nm ( Figure 7C), corresponding to~14 atomic layers. When employed as the anode material for LIBs, E-TP-COF afforded a high capacity of 110 mAh g −1 , whereas TP-COF anodes showed a relatively low capacity of 25 mAh g −1 . The large performance gap suggests that the atomic-layer structure of E-TP-COF could render it with high utilization of redox-active sites. In addition, the rate performance of E-TP-COF also greatly improved as shown in Figure 7D,E. These results fully demonstrated the advantages of the atomic-layer thick structure for enhancing the electrochemical performance of COFs.
The above results prove that introducing metal/metal oxide nanoparticles into COFs is a feasible strategy to prevent the agglomeration of organic sheets/particles and improve the electrochemical performance of COFs. Recently, a 2AQ-MnO 2 composite has been prepared by introducing a petal-like nanosized MnO 2 layer into the conjugated polymer of 2AQ (two ligands with active quinone groups), and then a Co@2AQ-MnO 2 composite was prepared by coordinating with Co centers ( Figure 8A). [131] Figure 8B-G displays the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-obtained material. Clearly, after 2AQ was integrated with the nanosized petal-like MnO 2 layer, both 2AQ-MnO 2 and Co@2AQ-MnO 2 displayed a unique yolk-shell morphology, whereas 2AQ displayed smooth nanospheres. As shown in Figure 8H, compared with 2AQ and 2AQ-MnO 2 , the Co@2AQ-MnO 2 anode showed a higher capacity of 1534.4 mAh g −1 at 100 mA g −1 together with outstanding rate performance ( Figure 8H) and remained a higher capacity of 596.0 mAh g −1 at 1000 mA g −1 after 1000 cycles ( Figure 8I), suggesting its superior cycle and rate performance. The improvement in electrochemical performances is attributed to the incorporation of nanosized MnO 2 and Co ions, which improved the utilization of the redox-active sites of 2AQ and facilitated the charge transfer. Furthermore, they efficiently reduced the polymer agglomeration, and the resulting interior void space in unique yolk-shell structures could accommodate the volume change caused by Li + insertion/deinsertion, enhancing the long-time cycling performance.
In addition to the top-down strategy, the bottom-up approach is another important strategy to prepare nanostructured COFs, which can more directly modulate the morphology of COFs by controlling the reaction conditions (such as monomers, solvents, and substrates). [121] Solvothermal synthesis is the most commonly used bottom-up method to synthesize COFs. [28,132,133] For example, Hu and co-workers for the first time reported a 3D hierarchical porous COF (COF-MF) with microflower superstructures by a bottom-up solvothermal route, which consisted of porphyrin-rich and conjugated ultrathin COF nanosheets. [124] The formation and assembly of these ultrathin nanosheets are closely related to the strictly controlled reaction conditions, such as the reaction solvent and the dosage and concentration of the catalyst. When applied in energy storage, the microflower morphology of COF-MF efficiently restrained the nanosheet to be restacked and fully exposed to the existing active sites as well as highly shortened the ion transportation length, which greatly enhanced the energy storage performance of COF-MF. We also successfully constructed a 3D flowerlike TP-TA COF with flexible building units via a facile bottom-up strategy. [62] The resulting TP-TA COF was directly assembled by hexagonal 2D ultrathin nanosheets without any postprocessing. The unique morphology of TP-TA COF played an important role in improving its electrochemical performance. Unfortunately, the bottomup approaches cannot be widely applied to a variety of COFs with desired morphologies because both the reaction conditions and the chemical properties of monomers have an important influence on the eventual morphology of the COFs.
To expand the applicable scope of bottom-up approaches in morphology optimization, some general strategies have been proposed, such as introducing growth templates or substrates via in situ reactions. [82,86,134] CNTs or graphene are commonly used as substrates or templates for COF growth, as the aforementioned COFs in Section 3.2.1. The introduction of 2D graphene sheets or 1D CNTs provides an available template for the direct growth of COFs, improving the electrical conductivity of COFs while also obtaining a more favorable morphology. Nevertheless, although various strategies for the micromorphology optimization of COFs have been developed, it remains a great challenge to find a simple and versatile method to directly prepare ideal COFs with optimized micromorphology as electrode materials for rechargeable metal-ion batteries.
In this section, we summarized and highlighted the main research strategies for improving the electrochemical performance of COFs as electrode materials from the perspective of molecular-level design and material-level engineering. These principles in molecular-level design focused on the relationships between the chemical structure of COFs and their electrochemical performance. The intrinsic properties of COFs can be greatly improved by rational molecular design, such as electrical conductivity and theoretical capacity, and thus the electrochemical performance of COFs can be enhanced. These design principles help establish the full structure-performance relationships of COFs and then predict and evaluate the electrochemical behavior, which in turn can guide the design of high-performance COF-based electrode materials. In terms of material-level engineering, we mainly discussed two types of modification strategies for optimizing the electrochemical performance of COFs: incorporation with conductive materials and micromorphology optimization. Compared with the molecular-level design, these strategies place more emphasis on the preparation methods and postprocessing of COFs, such as in situ growth on the surface of conductive materials, exfoliation, and so on. In a sense, the modifications at the material level are of great significance for the large-scale production and application of COFs as electrode materials for rechargeable metal-ion batteries.

| DEVELOPMENT FOR OTHER RECHARGEABLE METAL-ION BATTERIES
Considering the growing energy demand and the limited lithium source, other rechargeable metal-ion batteries have been widely investigated as alternative energy storage devices to supplement the current energy systems, such as sodium-ion batteries (SIBs), potassiumion batteries (PIBs), ZIBs, and so on. [135][136][137][138][139][140] The redoxactive groups and pore structures of COFs can be flexibly tailored towards the desired requirements and applications, allowing them to be versatile for a wide range of batteries. In addition, the excellent chemical stability of COFs in a variety of aqueous and organic electrolytes also makes them a platform for other rechargeable metalion batteries. Recent advances in the development of COFs for other metal-ion batteries are discussed in this section.

| Sodium-ion batteries
SIBs, as one of the most promising alternatives to current LIBs, have attracted considerable attention owing to the abundance and low cost of sodium resources. [141][142][143] Typically, the working principle of SIBs is similar to that of LIBs, based on the "rocking chair" mechanism where Na + reversibly migrates from the anode to the cathode during the discharge process and from the cathode to the anode when charging. Nevertheless, the larger ionic radius of Na + (1.02 Å) than Li + (0.76 Å), leads to more sluggish ions transportation and bigger volume changes during the reversible cycle process, remaining the challenge of high rate and cycle performance. [144] In this regard, benefiting from the tunable pore sizes and chemical structures, many COFs have been reported as electrode materials for SIBs to address huge volume expansion and sluggish ions transport. [65,110,[145][146][147][148][149] As shown in Figure 9A, Shi and co-workers successfully synthesized an N-rich COF (TQBQ-COF), showing honeycomb-like and containing multiple carbonyls, as cathode materials for high-performance SIBs. [146] As displayed in SEM and AFM images ( Figure 9B,C), the as-obtained TQBQ-COF exhibited the porous-honeycomb morphology, and its layer height was 5 nm. In situ Fourier transform infrared spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) together with DFT computations (Figure 9D-G) were performed to analyze the redox mechanism. The results well demonstrated that both pyrazines and carbonyl groups of TQBQ-COF were involved in the Na + storage. As indicated in Figure 9G, the redox reaction of up to 12 Na + ions per repeating unit of TQBQ-COF proceeded through the successive two-step sodiation/desodiation process. The first insertion of six Na + ions could be depicted as each Na + was stored between two adjacent oxygen and nitrogen atoms inside the ring plane of TQBQ-COF, and the next six Na + could be combined outside the ring plane of TWBQ-COF. The multiple redox-active sites endow it with a high capacity of 452.0 mAh g −1 at 0.02 A g −1 . Besides, the electronic conductivity of TQBQ-COF was enhanced due to its reduced band gap, arising from the introduction of N atoms. High conductivity together with the poroushoneycomb-like morphology greatly promoted the rapid ion/electron transport, resulting in the remarkable rate performance of 134.3 mAh g −1 at 10.0 A g −1 .
Inspired by COF-based anode materials for LIBs, COFs have also been utilized as anode materials for SIBs. [150][151][152][153][154][155] As shown in Figure 10A-C, Gu and coworkers reported a β-ketoenamine-based DAAQ-COF and prepared four samples with different layer thicknesses as anode materials for SIBs. [152] Figure 10D displayed the electrochemical performance of these samples, where the thinnest 4-12-nm sample exhibited the highest capacity and superior rate performance. Besides, a high capacity of 198 mAh g −1 at 5 A g −1 with a capacity retention of 99% after 10 000 cycles can be achieved ( Figure 10E), suggesting its excellent cycle performance. These results imply that weak stacking interaction of COFs could render them high electrochemical performance by suppressing the interlayer electron self-exchange and offering more available redox-active sites. In addition, ex situ experiments and DFT computations were performed to analyze the redox mechanism of DAAQ-COF. As indicated in Figure 10F, the structure evolution of DAAQ-COF was associated with the initiation formation and transformation of the C-O· and α-C radical intermediates during the Na + insertion/extraction process. Furthermore, the authors clarified that reducing the thickness of DAAQ-COF was beneficial to inhibit the self-discharge behaviors and enhancing the stability of radical intermediates in COFs, which sufficiently accounted for the optimal electrochemical performance deriving from the thinnest DAAQ-COF sample.

| Potassium-ion batteries
PIBs are another type of alkali metal-ion battery that has been studied in recent years. The electrochemical potential K/K + (−2.94 V vs. Standard Hydrogen Electrode [SHE]) is very close to that of Li/Li + (−3.04 V vs. SHE), endowing KIBs with high operation voltage and desirable energy density. [20,156] Moreover, K + presents weaker Lewis acidity and lower desolvation energy in comparison with Li + , thus leading to higher ionic conductivity and faster diffusion kinetics in the liquid electrolyte. Unfortunately, the ionic radius of K + (1.38 Å) is even larger than that of Na + , thus there are few electrode materials that can reversibly accommodate K + during the redox process. In particular, such large K + tends to cause electrode degradation, bringing large volume expansion and suffering from capacity decay rapidly, which put higher demand for the structural stability of electrode materials for high-performance PIBs. In this regard, COFs demonstrate significant advantages due to their robust and porous conjugated crystalline structures. [57,113,[157][158][159][160][161] As indicated in Figure 11A, Chen and co-workers reported a boronic ester-based COF composite (COF-10@CNT) with a few-layered structure. [157] The π-cation interaction between K + and a π-electron cloud of  [146] Copyright 2020, Nature Publishing Group. AFM, atomic force microscopy; COF, covalent organic framework; FTIR, Fourier transform infrared spectroscopy; SEM, scanning electron microscopy; TQBQ, triquinoxalinylene and benzoquinone; XPS, X-ray photoelectron spectroscopy. benzene rings enables it to be used as anode material for K + storage, delivering a large capacity of 288 mAh g −1 at 0.1 A g −1 and excellent cycle stability of 161 mAh g −1 after 4000 cycles at 1 A g −1 . Similarly, Duan and co-workers constructed a nanofiber-structured DAAQ-COF@CNT as the cathode material for PIBs by integrating carboxylated CNTs with a few-layered DAAQ-COF. [160] As shown in Figure 11B, the 12 quinone groups in the DAAQ-COF units could be combined with 12 K + via a two-step multipleelectron transfer reaction. The DAAQ-COF@CNT as the anode for PIBs provided a high specific capacity of 157.7 mAh g −1 at 0.1 A g −1 , superior rate capability of 111.2 mAh g −1 at 1 A g −1 , and outstanding cycle performance with a capacity retention of 77.6% after 500 cycles at 0.5 A g −1 . Figure 11C shows an alkynyl-based COF (TAEB-COF), which was also designed as the anode material for PIBs in a localized high-concentration electrolyte. [158] The alkynyl ligands were devoted to facilitating the cation-π interaction with K + . In addition, the flexible conformation of the 1,3,5-tris(arylethynyl)benzene (TAEB) linkers allows TAEB-COF to insert K + compatibly, thus yielding a higher capacity of 254.0 mAh g −1 after 300 cycles.
Recently, Luo and co-workers have prepared a polyimide COF composite (P-COF@SWCNT) by in situ growth P-COF on the surface of SWCNTs ( Figure 11D). [161] Subsequently, the exfoliation of P-COF@SWCNT was conducted to offer access to active sites and shorten ion routes, achieving fast K + transport. When acted as anode materials for PIBs, the electrochemical performance of P-COF@SWCNT was much higher than that of P-COF, which could deliver a high capacity of 438 mAh g −1 at 0.05 A g −1 , and excellent rate performance of 158 mAh g −1 at 1.0 A g −1 , as well as good cycle stability of 114 mAh g −1 (E) Cycle performance of the 4-12-nm thick sample at a current density of 5000 mA g −1 . (F) Sodiation procedure of DAAQ-COF of the 4-12-nm thick sample. [152] Copyright 2019, American Chemical Society. 2D, two-dimensional; COF, covalent organic framework; DAAQ, 2, 6-diaminoanthraquinone.
after 1400 cycles. The rate performance of P-COF@SWCNT was ascribed to its exfoliated fewlayered structure and the forming of cross-linked conductive networks by the incorporation of SWCNTs. Furthermore, the K + storage kinetic behavior of P-COF@SWCNT was investigated by the b values obtained from the CV curves at various scan rates ( Figure 11E). As shown in Figure 11F, all the b values of P-COF@SWCNT are close to 1, indicating that the K + storage was mainly determined by a capacitive-controlled process. As a result, P-COF@SWCNT anodes afforded faster K + storage kinetics and higher capacity. Furthermore, the schematic illustration of its redox mechanism is presented in Figure 11G based on DFT computations, where the C=O groups in P-COF can experience reversible redox reaction with K + to form C-O⋯K, along with the electron transfer during the redox process.
In contrast to rechargeable alkali metal-ion batteries, the electrochemical behaviors of ZIBs occur in aqueous electrolytes in most cases. Note that their n-type redox reactions usually involve not only Zn 2+ but also H + . For example, Zheng and co-workers synthesized a novel orthoquinone-based COF (BT-PTO COF) as the cathode material for aqueous ZIBs using pyrene-4,5,9,10-tetraone (PTO) active monomer ( Figure 12E). [134] The carbonylrich groups deriving from PTO served as redox-active sites to combine with Zn 2+ and H + , and exhibited high rate capacity owing to its pseudocapacitance process. The Zn 2+ and H + co(de)insertion mechanism of BT-PTO COF was confirmed by in/ex situ characterizations. Figure 12F displayed the in situ Raman spectra of the COF in 3 M ZnSO 4 /D 2 O electrolyte, where the reversible change of the C=O peak at 1665 cm −1 showed its redox activity. Besides, the peaks of zinc sulfate hydroxide hydrate located at 166 and 435 cm −1 also appeared and disappeared regularly, indicating the H + insertion/deinsertion during the discharge/charge process, as evidenced by ex situ X-ray powder diffraction and XPS results. Furthermore, the authors revealed that the COF underwent first Zn 2+ insertion and then two H + coinsertion, with the dominant process changing from H + and Zn 2+ coinsertion to more H + insertion at high current densities.
All the above-designed COFs for ZIBs undergo n-type redox reactions, together with the insertion of cations during the discharge process. Recently, Zhang and co-workers have reported a COF-like microporous polytriphenylamine (m-PTPA in Figure 12G) containing N-rich tertiary and secondary amines, which can be used as a cathode material for zinc dual-ion batteries by combining with anions. As indicated in Figure 12H, the redox-active N atoms in the skeleton of m-PTPA can react with Cl − ions in a pseudocapative manner. As shown in Figure 12I, such a p-type redox mechanism gave rise to a higher voltage of 1.15 V at 0.5 A g −1 , leading to high energy density and superior rate performance.
In this section, the recent advances in the development of COFs for other metal-ion batteries, including SIBs, KIBs and ZIBs, were summarized and discussed. Nevertheless, though COFs have been considered alternative electrode materials for batteries beyond LIBs, there are several key issues that need to be addressed. First, the larger ionic radius of Na + and K + put forward higher demands on the electrode materials. To accommodate the large ions, COFs should have robust structures and excellent stability together with adequate space to ensure that the structures are not damaged by the cycling ions embedding. Second, from the above studies, it can be seen that in situ growth of COFs on the surface of conducting carbon materials is a common strategy to facilitate the transport of Na + /K + and electrons. This is because these large cations also place a higher standard on the ions and electrons conductivity of the electrode materials. Considering these facts, more considerations should be given when designing COFs as electrodes for SIBs or KIBs to address the problems caused by the larger ionic radius. Thirdly, there is no doubt that many advanced in situ characterization and theoretical calculations have been employed to study the redox reactions of COFs involved in the electrochemical process. However, compared with LIBs, the redox mechanism of COFs for SIBs and KIBs is still not clear exactly, especially for ZIBs involving divalent Zn 2+ and H + in the aqueous electrolyte. Building a complete relationship between the molecular structure of COFs and their electrochemical performance may be helpful for performing more in-depth studies to reveal the redox mechanisms of COFs as electrode materials for rechargeable metal-ion batteries.

| CHALLENGES AND PERSPECTIVES
There is no doubt that various COFs have been developed to obtain high-performance organic electrode materials for rechargeable metal-ion batteries and tremendous achievements have been made in the past few years. However, the development of COFs as electrode materials for energy storage is still at an infant stage, and many challenges remain to be addressed for practical applications. The main challenges and perspectives are discussed in the following subsections, which are divided into two main aspects: electrochemical performance limitation and large-scale production.

| Electrochemical performance limitation
In view of electrochemical performance, there are two main challenges that lie ahead for COF-based electrode materials, namely, unsatisfied energy density and limited rate performance for practical applications. The former is mainly derived from the trade-off relationship between the specific capacity and the redox potential, while the latter is influenced by the inherent poor electronic conductivity of COFs and the diffusion behaviors of ions during the redox process.
As mentioned above, the incorporation of COFs and conductive additives can greatly improve the conductivity of COF electrodes, resulting in enhanced rate performance. However, this strategy does not improve the intrinsic electronic conductivity of COFs, and the additional conductive materials usually increase the cost while decreasing the overall energy density of the batteries. Comparatively, narrowing the band gap or expanding the conjugated system by rational structure design at the molecular level has been proposed to enhance the intrinsic electronic conductivity of COFs. At present, the introduction of π-conjugated or electron-withdrawing moieties into COF structures has been regarded as an effective way to tune their electronic structure and promote electronic transportation. Note that some introduced functional groups or atoms are redox inactive, which decreases the proportion of redox-active groups in the COFs, thus exhibiting a reduced specific capacity. To well balance the rate performance and capacity of COFs, it is suggested to introduce redox-active π-conjugated or electron-withdrawing moieties and minimize the inactive content in COFs, such as aromatic redox-active units and N-rich hexaazatrinaphthalene.
With respect to ion diffusion, micromorphology optimization of COFs by constructing nanostructures is beneficial to facilitate ion transport and expose more available redox-active sites, leading to improved specific capacity and rate performance. Generally, top-down exfoliation to strip bulk COFs into few-layered nanostructures can be used to efficiently improve the lithium transfer kinetics. Nevertheless, of particular note is that there are some other concerns raised from the exfoliation, which have to be considered before the practical applications of COFs. On the one hand, it is difficult to precisely control the layers and sizes of the exfoliated products, thus reducing the homogeneity of the products and the reproducibility of the experiments. Besides, the relatively low yield of few-layered or even single-layered nanosheets further hinders their further large-scale applications. On the other hand, exfoliation may affect the long-range order of the crystalline COFs, resulting in poor crystallinity and infecting their electrochemical performance. Compared with the top-down strategy, the bottom-up strategy is another important approach to obtaining COF nanosheets and thin films, such as solvothermal synthesis, interface synthesis, and onsurface synthesis. However, their reaction conditions involving substrates, monomers, solvents, temperature, and so forth, have strong impacts on the micromorphology of COF nanosheets and films, which limits the universality of the bottom-up strategy. Therefore, it is highly desired to explore alternative methods to construct nanostructured COFs. As the micromorphology of COFs is greatly influenced by their structure in some way, more attention should be paid to building up the structure-performance relationships so as to successfully optimize the morphology of COFs by tailoring their structure at the molecular level.
When it comes to the energy density of COFs, further efforts are required to bridge the gap with inorganic battery systems, where the central issue is how to well balance the specific capacity and redox potential. For ntype materials, improving the redox potential without sacrificing the capacity is of great significance for realizing high energy density. Conversely, the development of p-type materials should focus on increasing redox-active sites or decreasing redox-inactive sites so as to improve their capacity and then afford high energy density. Meanwhile, considering the advantages of p-type materials in redox potentials, it is necessary to explore innovative p-type materials for rechargeable metal-ion batteries. Developing bipolar-type materials is one of the alternative pathways to mitigate the incompatible relationship between the high capacity and high redox potential of COFs. Some dual-ion batteries have been reported to realize high capacity and high energy density by using bipolar-type materials as cathode materials. [62,71,72] Besides, a few all-organic symmetric batteries have been proposed by utilizing bipolar-type materials as both the cathode and anode. [35,108] Nevertheless, there are relatively few studies in this respect at present, and further improvements in the electrochemical performance of bipolar materials are urgently needed. In the future, exploring more innovative COFs, and establishing and perfecting the structure-performance relationships of COFs are required to obtain satisfied organic electrode materials that are comparable to commercial inorganic ones. On the other hand, as one of the most important components of rechargeable batteries, electrolytes also have a profound impact on the electrochemical performance of the as-fabricated batteries. Liquid electrolytes have been widely applied in battery systems due to their high ionic conductivity and interfacial wettability. However, their intrinsic defects, such as flammability, leak, and volatility, pose a serious security concern. Besides, the cycle performance of organic electrodes is generally limited by their dissolution in liquid electrolytes. [33,177] Conversely, with the advantage of high safety, and good mechanical and chemical properties, quasi-solid-state electrolytes and all-solid-state electrolytes have been developed as promising candidates for improving safety and cycle performance. [178][179][180] Although the interface issue and other practical problems still need to be addressed for their further applications, solid-state electrolytes pave an alternative way for improving the performance of COF-based rechargeable metal-ion batteries. In conclusion, only by tackling the issues concerning various aspects in a comprehensive way can we gain a great breakthrough in obtaining the desired COF-based electrode materials with remarkable overall electrochemical performance.

| Large-scale production
Taking into account the practical large-scale applications, we have to consider and address some issues related to active material synthesis and electrode preparation. Most COFs are synthesized in sealed glass tubes by typical solvothermal methods, undergoing a long reaction time. The harsh reaction conditions result in most redox-active COFs have yet to be synthesized on a large scale. [28] Besides, the properties of the obtained COFs, such as the porosity and crystallinity, are highly dependent on the reaction environment, which affects the reproducibility of synthesis experiments. It is necessary to explore more feasible methods for largescale synthesis of COFs under mild reaction conditions. When referring to the electrode preparation, COF-based electrodes generally have low active material mass loading of~1 mg cm 2 , which is far below the standard for practical applications. In addition, the prepared electrodes typically contain large amounts of conductive additives, which not only decreases the overall energy density but also increases the production cost. Moreover, many designed COFs as electrodes for LIBs are Li-free, thus they require a prelithiation process if the counter electrodes are also Li-free, leading to complicated operation and inferior electrochemical performance. Additionally, excessive electrolyte usage in organic batteries should be taken into account, which also increases the cost and limits the large-scale application of COFs. In a word, the development of COFs in rechargeable metal-ion batteries is still a long way from the practical large-scale applications. To gain breakthrough, future efforts should not only focus on the properties of active materials but also pay more attention to the electrode and battery assembly as well as full-cell performance.