Recent Advances in Covalent Organic Framework Electrode Materials for Alkali Metal-Ion Batteries

Open AccessCCS ChemistryMINI REVIEW30 May 2023Recent Advances in Covalent Organic Framework Electrode Materials for Alkali Metal-Ion Batteries Jianlu Sun, Yifan Xu, Yanqi Lv, Qichun Zhang and Xiaosi Zhou Jianlu Sun School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023 , Yifan Xu School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023 , Yanqi Lv School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023 , Qichun Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR 999077 Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Kowloon, Hong Kong SAR 999077 and Xiaosi Zhou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023 https://doi.org/10.31635/ccschem.023.202302808 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Owing to the shortcomings of traditional electrode materials in alkali metal-ion batteries (AIBs), such as limited reversible specific capacity, low power density, and poor cycling performance, it is particularly important to develop new electrode materials. Covalent organic frameworks (COFs) are crystalline porous polymers that incorporate organic building blocks into their periodic structures through dynamic covalent bonds. COFs are superior to organic materials because of their high designability, regular channels, and stable topology. Since the first report of DTP-ANDI-COF as a cathode material for lithium-ion batteries in 2015, research on COF electrode materials has made continuous progress and breakthroughs. This review briefly introduces the characteristics and current challenges associated with COF electrode materials. Furthermore, we summarize the basic reaction types and active sites according to the categories of covalent bonds, including B–O, C=N, C–N, and C=C. Finally, we emphasize the perspectives on basic structure and morphology design, dimension and size design, and conductivity improvement of COFs based on the latest progress in AIBs. We believe that this review provides important guidelines for the development of high-efficiency COF electrode materials and devices for AIBs. Download figure Download PowerPoint Introduction With the increasing demand for electronics and electric vehicles, electrochemical energy storage technology is expected to play a pivotal role in our daily lives.1–5 Since the first commercialization of lithium-ion batteries (LIBs) in 1990, alkali metal-ion batteries (AIBs), including LIBs, sodium-ion batteries (NIBs), and potassium-ion batteries (KIBs), have been extensively studied because of their advantages of high energy density, rechargeability, and environmental benignity.6–8 Nevertheless, the challenges of capacity fading and fast charging and discharging under high current densities remain.9–11 Electrode materials with high capacity, high rate capability, long cyclic stability, and low cost still must be explored.12–14 In comparison with conventional inorganic materials, such as transition metal oxides and alloy-based materials, organic materials possess a designable molecular architecture and are composed of lightweight elements that can be used as both positive and negative electrodes.15–17 Although organic electrode materials are very attractive, several issues, such as easy solubility in organic electrolytes based on the principle of compatibility, poor electronic conductivity, and low discharge voltage, hinder their commercial application.18,19 As a new class of crystalline porous polymers, covalent organic frameworks (COFs) were first synthesized in 2005.20,21 Their regular network structures with strong covalent bonds effectively alleviated the issue of rapid dissolution of electrode materials and provided a new opportunity for the development of organic electrode materials.22 Subsequently, as illustrated in Figure 1a, the COF electrode material was first utilized as a cathode for LIBs in 2015, showing that they can undergo reversible redox reactions. Since then, various COF electrode materials have been investigated and reported for use in AIBs. Research on COF-based electrode materials has shown an upward trend from January 2019 to February 2023 (Figure 1b). COF electrode materials have been most studied in LIBs, based on the extensive research on LIBs (Figure 1c). Both COF cathode and anode materials have been reported (Figure 1d); however, due to their low discharge voltage, novel electrode materials with high performance are still highly desirable. Figure 1 | (a) Timeline of COF electrode material in AIBs. Reprinted with permission from refs 23–28. Copyright 2015–2022 Springer Nature, Royal Society of Chemistry, Wiley-VCH, American Chemical Society. (b) The growing trend of COF electrode material in AIBs. (c) The research article number of the application of COF electrode material in AIBs. (d) The proportion of research papers published on LIBs, NIBs, and KIBs. Download figure Download PowerPoint In this mini review, we first summarize the main characteristics of COF electrode materials. We then list the reaction types and electrochemically active sites of COF electrode materials in AIBs developed in recent years. Subsequently, the latest developments in multifunctional COF electrode materials for AIBs are discussed in detail. Finally, the opportunities and challenges of basic and applied research on COF electrode materials are briefly analyzed. We believe that this review will bridge the gap between experimental research and future practical applications and provide valuable guidance for exploring efficient COF electrode materials in the field of energy storage. Merits and Issues of COFs as Electrode Materials Currently, COF synthesis is mainly achieved using solvothermal methods. By selecting appropriate building units, COFs can be successfully constructed into two-dimensional (2D) or three-dimensional (3D) structures.29–31 For AIBs, 2D COFs have been widely investigated. The main properties of 2D COFs as electrode materials are summarized in Figure 2. Figure 2 | The inherent characteristics of COFs in rechargeable AIBs and major considerations as electrode materials. Download figure Download PowerPoint Periodically open channels COFs are usually prepared through the condensation of monomers to form stable ring structures with periodic open channels.32,33 These open channels facilitate ion transport and lower the energy barrier for ion diffusion, as well as endowing COFs with a high porosity and specific surface area,34,35 where the latter is beneficial to the exposure of the active sites of electrode materials.36 In addition, the covalently bonded structure can be finely tuned by virtue of monomer size selectivity, which is conducive to the transmission of alkali metal-ions with different sizes and the construction of multiple active sites in the framework, thereby achieving a large reversible specific capacity of the battery. However, a majority of the currently reported COF electrode materials have a 2D architecture similar to the layered structure of graphite. Excessive π–π interactions between COF layers cause the formation of bulky solids with long channels and poor conductivity, resulting in slow transport of electrolyte ions and electrons, making it difficult for ions and electrons to reach deeply buried active sites.22 Due to the particularity of organic reactions, COFs often possess some defects that also hinder electron conduction, leading to low utilization of active sites.37 Additionally, the excessive construction of active sites can easily lead to structural damage of electrode materials during charging and discharging and reduce the reversibility of the redox reaction, resulting in a rapid decline in battery capacity.38 Therefore, designing more active sites in COF electrode materials with stable structures is necessary. Considering long-term cycling stability, the existence of inactive linking groups is necessary. Designable covalent backbone Designable covalent skeletons are another advantage of COFs. During preparation, diverse frameworks can be designed at the molecular level by utilizing different monomers or different types of organic reactions.39,40 This is favorable for us anchoring different functional groups at preselected positions on the framework and provides COF electrode materials with preferentially designed redox sites.32,41,42 However, in the synthesis of COF materials, there are challenges that cannot be ignored, such as difficult monomer preparation, low crystallinity, long synthesis time, and complicated purification. As with most of the 2D COFs reported so far, the design and synthesis of some building units with 3D configurations can lead to the emergence of 3D COF materials with larger specific surface areas.43,44 Nonetheless, owing to the instability of the 3D architecture, there are few reports on these materials as electrodes in batteries.45,46 Constructing larger conjugated structures or weakening 2D overlapping layers can allow the adjustment of the overall electron distribution and thus the plateau voltage and conductivity of the battery.47 Furthermore, the structural design at the molecular level enables the material to have a special chemical structure, which is beneficial for analyzing the structural changes of the materials during the charging/discharging process of the battery, thereby assisting with in-depth analysis of the reaction mechanism. Clearly, the exploration and elucidation of the reaction mechanism can provide the possibility of designing more ideal electrode materials. Lightweight elements and insolubility Unlike ordinary electrode materials, COF electrode materials usually do not contain metal elements but are composed of lightweight elements such as C, N, H, O, and B.32,48 Lightweight elements reduce the density of the material, which is beneficial for increasing the energy density of the battery. Moreover, such elements are abundant in nature and are not influenced by metal resources, which would theoretically decrease the comparative cost of COF electrode materials.49 However, due to the requirement for long-range ordered covalent structures, to ensure excellent thermal and chemical stability, complex preparation and tedious purification processes are currently adopted, making the cost relatively high.50 Unlike other small-molecule organic electrodes, COFs are insoluble, reducing their dissolution in organic electrolytes.49,51 However, their insolubility also leads to adverse effects on the commonly used binders for AIBs, which affects the electronic transportation between electrode materials and current collectors. When the mass loading of a COF electrode is increased, it becomes easy to peel off, resulting in the poor cycle stability of the battery. Moreover, the nature of insoluble powder crystals makes it difficult to determine the actual structure of COFs and the true content of the active groups. Consequently, it is difficult to analyze the utilization rate and mechanism of the active groups. Low conductivity COF electrodes usually suffer from low electronic conductivity, preventing batteries from achieving advanced rate performance.52 Since the π–π stacking interaction can significantly improve the conductivity of 2D COFs, constructing larger conjugated structures has become another common method besides the recombination with conductive substrates.53,54 Nonetheless, excessive stacking is not conducive to the exposure of active sites.55 In contrast, some nonplanar architectures, such as the random conformation of amine structures and the nonplanar orientation of sp3 C, can affect the extension of the conjugated structure and hinder charge transfer. Therefore, they are usually limited to a few delocalized aromatic structures.49 Overview of COFs as Electrode Materials in AIBs As mentioned above, COFs have attracted the attention of researchers because of their inherent channelability, designability, insolubility, and use of lightweight elements. In particular, their diversity at the molecular level has been extensively studied, and various COF electrodes have been reported successively.39,56 To further design electrode materials suitable for AIBs with high specific capacity, advanced rate performance, and long-term cyclic stability, in-depth studies on their electrochemically active sites have become the focus.50,57,58 In this section, we systematically summarize the recently reported active organic groups for COF electrodes and their design principles, as well as synthesis strategies for AIBs. Electrochemically active sites in COFs COF electrodes enable charge storage through the combination of positive/negative charges and active groups for redox reactions during the charging/discharging process.59 According to the redox-active groups, the currently reported COFs can generally be classified into borates, borazines, triazines, imines and enamines, imides, ketoenamines, and hexazines.37,60–62 Figure 3 shows the COF electrodes in the research of AIBs in recent years. Figure 3 | The structures of COF electrodes employed in the research on AIBs during past years. Download figure Download PowerPoint The bonding structures of COFs have been widely studied and can be roughly divided into boron-based63–65 (boroxine and boronate), amine-based66–68 (Schiff bases, imides, and ketoenamides), triazine-based,69 and sp2 (C=C) linkages.70,71 The formation of B–O involves the dehydration and condensation of phenylboronic acid; however, this is easily hydrolyzed, leading to structural instability.72 Therefore, it has been the subject of relatively few recent studies in the field of energy storage. The formation of C=N is most commonly achieved through condensation of aldehydes and amines. It follows a dynamic reversible reaction for continuous repair to increase crystallinity.73–75 Due to their excellent stability, imine-based COFs have been the most investigated. C–N often exists in the imide structure obtained through the condensation of aromatic amine and phthalic anhydride.76 Such COFs are stable but difficult to prepare.77 In addition to C–N units, covalent triazine frameworks (CTFs) obtained by cyanide polymerization contain C=N units.78 The formation of C=C bonds is generally through the Knoevenagel reaction, which helps to further expand the conjugated structure, thereby improving the conductivity.79 C=O groups mainly exist in organic compounds such as quinones, acid anhydrides, and ketones, which can be introduced into COFs as building blocks. Through research on the mechanism of charging and discharging in batteries, it was found that C=N and C=O are typical active sites (Table 1), formed through the interaction of O and N with alkali metal-ions (Li+, Na+, and K+). Although the benzene skeleton can be used as the active site in a specific structure, in most cases, it is considered as an inactive component to connect the active group to ensure the stability of the architecture.50 It is worth noting that hexaazatrinaphthalene (HATN) structures have been widely studied because their conjugated structures and high-density C=N groups are beneficial for improving the specific capacity of the battery. Table 1 | Electrochemically Active Sites Involved in AIBs Type Sample Active Site Battery References Boron-based linkage PPTODB COFCOF-10 C=OBenzene LIBs 80 KIBs 28 Amine-based linkage COF TPDA-PMDAUSTB-6PGF COFHAB-COFHATN-HHTPPICOF-1MPc-2D-cCOFAza COFP-COFTAEB-COF C=O & benzeneC=N & C=OC=NC=NC=NC=OC=NC=NC=O & benzeneC=N & alkynyl LIBsLIBsLIBsLIBsLIBsNIBsNIBsNIBsKIBsKIBs 81 82838485868788 89 90 Triazine-based linkage DCB-COF-450CTF-2 triazinetriazine & benzene LIBsLIBs 9192 Sp2 (C=C) linkage TFPPy-ICTO-COF C=O LIBs 93 Current design principles and synthesis strategies To improve the specific capacity, cycle stability, and rate capability of batteries, diverse strategies for synthesizing and modifying COFs have been proposed, such as chemical structure and morphology design, dimensionality and size design, and conductivity enhancement.94 The following is a systematic overview of the recent exploratory work done. Chemical structure and morphology design To develop multiple active sites or improve the utilization of active sites, innovation in monomer design and synthesis methods becomes the primary goal. The excellent stability of the rigid triazine linker unit and its benzene ring can provide rich active sites for high-capacity lithium storage, making CTFs an attractive electrode material for LIBs.95 In 2022, in order to solve the problems of poor structural stability and low capacity, Jiang et al.92 synthesized biphenyl-based CTFs (CTF-2) for the first time as a LIB anode (Figure 4a,b). In the study, CTF-2 showed a superlithiation of 13.2 Li+ insertions per repeat unit, which is different from the Li+ insertions into benzene rings previously shown by Fourier transform infrared (FTIR) spectroscopy, thus demonstrating high lithium storage capacity. To obtain an anode for LIBs with excellent rate performance, Wu et al.97 designed a piperazine-terephthalaldehyde (PA-TA) COF with superlayer spacing. Owing to the tetrahedral piperazine unit, the PA-TA COF exhibited fast ion/electron transport and delivered a specific capacity of 207 mAh g−1 even at a high current density of 5.0 A g−1 (Figure 4c,d). Figure 4 | (a) Schematic illustration of CTF-2 and (b) the color-mapped profiles of in situ FTIR spectra. Reprinted with permission from ref 92. Copyright 2022 Elsevier. (c) Schematic and (d) rate performance of PA-TA COF. (e) Simulated packing structure of MPc-2D-cCOFs. (f) Representation of the interaction between Na-ions and CuPc-2D-cCOF. (g, h) Electrochemical performance of as-synthetized MPc-2D-cCOFs and reference electrode. Reprinted with permission from ref 87. Copyright 2022 Springer Nature. (i) Schematic illustration of the construction of FAC-Pc-COFs. (j) Structural evolution during the potassiation process and (k) long cycling stability of QPP-FAC-Pc-COF//PTCDA@450 potassium-ion full battery at 2 A g−1. Reprinted with permission from ref 96. Copyright 2022 Wiley-VCH. Download figure Download PowerPoint N-rich phthalocyanine (Pc) units partially fused with benzene can be used to synthesize complete π-conjugated COF electrode materials with good rate capabilities and excellent cycle stabilities. For example, Yang et al.23,96 successfully designed a series of COF electrode materials by preenrichment of N and conjugated pyrrole, such as MPc-2D-cCOF (M = Cu, Fe, Co, and Ni; Figure 4e) as the anode for NIBs, and BB-FAC-Pc-COF and QPP-FAC-Pc-COF (Figure 4i) as the anode for KIBs. The results show that Na+ can be stored through the interaction of N···Na and Na–π, and all MPc-2D-cCOF samples showed excellent sodium storage performance (Figure 4f–h). Similarly, in KIBs, K+ gradually interacts with C=N and benzene rings to produce K···N and K–π (Figure 4j). In addition, a QPP-FAC-Pc COF//PTCDA@450 organic potassium-ion full battery was constructed to further explore K+ storage performance. Even at a high current density of 2 A g−1, the assembled full cell maintained a reversible capacity of 125 mAh g−1 after 1000 cycles (Figure 4k). In recent years, some polyimide-based COFs have been repeatedly synthesized; however, it is often difficult to obtain COFs with high crystallinity due to the rapid reaction between monomers and the poor reversibility of the reaction. To improve the crystallinity, the reaction-solvent ratio and temperature are often modulated for optimization.98 Based on the discovery that the addition of water promotes the reversibility of the polyimidization reaction, Yao et al.81 designed a water-assisted fabrication strategy to regulate the reaction rate of polyimidization. Their work reported the successful synthesis of COFTPDA-PMDA through N,N,N',N'-tetra (4-aminophenyl)-1,4-phenylenediamine (TPDA) and pyromellitic dianhydride (PMDA) ligands. The COF has an ordered layered porosity, including triangular micropores (1.2 nm) and hexagonal mesopores (2.7 nm) (Figure 5a). This unique architecture helps Li+ and TFSI– to fully interact with the active sites on the COF backbone (Figure 5b). COFTPDA-PMDA is a carbonyl compound that balances the negative charge of the COF electrode by the movement of Li+. A typical n-type working principle is shown based on the charged state in the redox process. Furthermore, the synergistic effect of the C=N and C=O redox groups expands the operating voltage of COFTPDA-PMDA and increases the specific capacity of the cathode materials for LIBs. Figure 5 | (a) Synthesis and (b) mechanism during the reversible electrochemical process of COFTPDA-PMDA. Reprinted with permission from ref 81. Copyright 2022 American Chemical Society. (c) Redox mechanism of PICOF-1 with sodium and (d) charge-discharge profiles of potential versus capacity at 23 mA g−1. Reprinted with permission from ref 86. Copyright 2023 Wiley-VCH. (e) Schematic illustration of the preparation and charge-discharge mechanism of S@TAPA-COFs and (f) cycling performance comparison. Reprinted with permission from ref 99. Copyright 2023 Elsevier. Download figure Download PowerPoint Moreover, Shehab et al.86 improved the crystal stability of polyimide COFs through a typical condensation reaction based on joint exchange. A porous polyimide COF (PICOF-1) was synthesized by linker exchange using imine-linked COF as a template. The abundant carbonyl groups in this COF backbone demonstrate the n-type working principle and are used as redox active sites for charge storage (Figure 5c). As a result, the reversible capacity was approximately 230 mAh g−1, which is the same as the theoretical specific capacity of PICOF-1 (Figure 5d). Postsynthetic modification is a technique that can introduce desired functionalities into COFs to improve their performance. Because sulfur has a larger atomic radius than oxygen, it can significantly alter electron mobility, thus improving battery performance.100 In 2023, Shi et al.99 achieved the conversion of C=O to C=S bonds through a synthetic modification strategy (Figure 5e), which further alleviated the solubility problem of electrode materials and enhanced the activity of COFs based on the more active C=S bonding with sodium. Therefore, in terms of cycle capacity (Figure 5f), the postvulcanized TAPT-COF (TAPT = 2,4,6-tris(4-aminophenyl)-1,3,5-triazine) hybrid showed the highest capacity among the NIBs.99 The imine group (C=N) is one of the typical active sites of COFs for alkali metal-ion storage. The design of high-density imine groups can effectively improve the theoretical specific capacities of the batteries. For the first time, Zhao et al.84 synthesized a COF (HAB-COF) via the condensation reaction of hexaaminobenzene trihydrochloride (HAB·3HCl) and terephthalaldehyde (Td). Notably, benefiting from the synergistic effect of the conjugated structure of HAB-COF and the high density of the C=N group, the HAB-COF electrode exhibited a high reversible capacity and excellent cycle life (1255 mAh g−1 at 1 A g−1 after 1100 cycles) (Figure 6a,b). The HATN unit, a typical component of high-density imine groups, is also a common cathode material with a high theoretical capacity. Given the solubility problem of HATN, Li et al.85 first designed ether bonds to combine with HATN units to improve the structural stability of the positive electrode HATN-hexahydroxytriphenylene (HHTP) (Figure 6c). Subsequently, carbon nanotubes (CNTs) were doped to guide the growth to obtain a few layers of COFs, which can improve the conductivity of COFs while exposing more active sites. The cathode material, HATN-HHTP@CNT, showed a reversible capacity close to its theoretical value (231 mAh g−1) in AIBs (Figure 6d,e). At a current density of 2 A g−1, HATN-HHTP@CNT retained a capacity of 130 mAh g−1, which was 60% of the value at 50 mA g−1 (Figure 6d). Figure 6 | (a) Structure and (b) the cycling performance at 1 A g−1 of HAB-COF. Reprinted with permission from ref 84. Copyright 2022 Springer Nature. (c) Architecture and electrochemical redox mechanism of HATN-HHTP. (d) Rate performance of HATN-HHTP and HATN-HHTP@CNT in LIBs. (e) Galvanostatic discharge-charge curves of HATN-HHTP@CNT in KIBs. Reprinted with permission from ref 85. Copyright 2022 Elsevier. (f) Schematic illumination, (g) TEM image, and (h) cycling performance at 100 mA g−1 of Sn@COF-hollow composite. Reprinted with permission from ref 101. Copyright 2022 Elsevier. Download figure Download PowerPoint Controlling the morphology of COF materials to achieve the exposure of internal active sites is another effective strategy to optimize the performance of AIBs. Based on the tin–nitrogen (Sn–N) coordination interaction and amino-modified SiO2 template strategy (Figure 6f), Tang et al.101 synthesized a Sn@COF hollow electrode for LIB anodes. Sn@COF displayed a hollow microsphere structure with a shell thickness of ∼20 nm, as confirmed by transmission electron microscopy (TEM) (Figure 6g); this promoted ion/electron transport and offered more sites for lithium reactions involving activated aromatic C=C groups and Sn ions. The Sn@COF electrode achieved a high capacity of 1080 mAh g−1 after 100 cycles at 100 mA g−1 (Figure 6h). In summary, by controlling the synthetic conditions and selection of monomers, the structure and morphology of COFs can be effectively manipulated, thereby regulating the number and utilization of active sites, which is conducive to obtaining excellent AIB performance. Dimension and size design In general, apart from designing a structure with multiple active sites, boosting the utilization of active sites can considerably increase the specific capacity of the battery. Therefore, researchers enhanced the specific surface area and accelerated ion diffusion by changing the dimension and size of the material to help improve the utilization of the active sites.43,50 Chen et al.102 first constructed all-π-conjugated 3D PYTRI-COFs (PYTRI-COF = imine-based "[3 + 4]" 3D COF) containing triazine and pyrazine (Figure 7a) and then introduced vinyl units into the framework structure to provide multiple active sites for ion adsorption, demonstrating excellent lithium storage performance. The specific surface areas of PYTRI-COF-1 and PYTRI-COF-2, determined by N2 adsorption–desorption analysis, were 894 and 317 m2 g–1, respectively (Figure 7b,c). Owing to the exposure of the active sites and the improved conductivity of the fully conjugated architecture, PYTRI-COF-2 exhibited an excellent cycle stability of 227.3 mAh g–1 after 450 cycles (Figure 7d). In striking contrast, PYTRI-COF-1, with relatively low chemical stability and π-coupling, displayed very fast capacity decay. Figure 7 | (a) Structures and (b, c) N2 adsorption–desorption isotherms of PYTRI-COF-1 and PYTRI-COF-2. (d) Cycling performance of PYTRI-COF-1 and PYTRI-COF-2. Reprinted with permission from ref 102. Copyright 2022 American Chemical Society. (e) Schematic illustration of CON-UV-1. (f) The comparison of COF-935, CON-UV-0.5, CON-UV-1, CON-UV-2, and CON-UV-33. (g) Mechanistic study of CON-UV-1. Reprinted with permission from ref 103. Copyright 2022 Elsevier. Download figure Download PowerPoint The efficient exfoliation of COFs to expose their buried active sites has been the focus of much research on electrode materials. Various exfoliation techniques have been reported, such as self-exfoliation, liquid-assisted exfoliation, mechanical exfoliation, and chemical exfoliation.22,104 Among these approaches, chemical exfoliation is prominent because it can introduce new functional units and has a higher degree of structural retention than mechanical exfoliation. In 2023, Wang et al.103 designed and synthesized a new 2D COF (COF-935). In oligo(phenylene vinylidene) (OPV)-based 2D COF, thiols can be added to the vinylene units of OPV under ul