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 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 Copyright 2023 Elsevier. Download figure Download PowerPoint Moreover, et the stability of COFs through a typical condensation reaction based on A porous COF was synthesized by linker using COF as a The abundant carbonyl groups in this COF backbone the n-type working principle and are used as redox active sites for charge storage (Figure As a the reversible capacity was mAh g−1, which is the as the specific capacity of PICOF-1 (Figure is a that can into COFs to improve their performance. has a larger it can significantly electron thus improving battery In et achieved the of C=O to bonds through a strategy (Figure which further alleviated the solubility of electrode materials and the of COFs based on the more active bonding with Therefore, in of cycle capacity (Figure the = showed the capacity the The group is of the typical active sites of COFs for alkali metal-ion storage. The design of high-density groups can effectively improve the specific of the For the first time, et synthesized a COF the condensation reaction of and from the synergistic effect of the conjugated structure of and the high density of the C=N the electrode exhibited a high reversible capacity and excellent cycle mAh g−1 at 1 A g−1 after (Figure The unit, a typical component of high-density groups, is also a common cathode material with a high capacity. the solubility of et first designed bonds to with units to improve the structural stability of the positive electrode (Figure Subsequently, were to the to obtain a few layers of COFs, which can improve the conductivity of COFs more active The cathode material, showed a reversible capacity to its mAh in AIBs (Figure a current density of 2 A g−1, a capacity of mAh g−1, which was of the at mA g−1 (Figure Figure | (a) and (b) the cycling performance at 1 A g−1 of Reprinted with permission from ref Copyright 2022 Springer Nature. (c) and electrochemical redox mechanism of (d) performance of and in LIBs. (e) of in KIBs. Reprinted with permission from ref Copyright 2022 Elsevier. (f) Schematic and cycling performance at mA g−1 of Reprinted with permission from ref Copyright 2022 Elsevier. Download figure Download PowerPoint the morphology of COF materials to the exposure of active sites is another strategy to the performance of AIBs. Based on the interaction and strategy (Figure et synthesized a electrode for LIB a structure with a of as by transmission electron (Figure this ion/electron transport and more sites for lithium reactions aromatic C=C groups and The electrode achieved a high capacity of mAh g−1 after cycles at mA g−1 (Figure In by the and of the structure and morphology of COFs can be effectively thereby the number and utilization of active sites, which is conducive to excellent performance. and size design In from designing a structure with multiple active sites, the utilization of active sites can increase the specific capacity of the battery. Therefore, researchers the specific surface and ion by the dimension and size of the material to improve the utilization of the active et first constructed 3D = imine-based 3D triazine and (Figure and then introduced units into the framework structure to provide multiple active sites for ion demonstrating excellent lithium storage performance. The specific surface of and by were and (Figure Owing to the exposure of the active sites and the conductivity of the fully conjugated architecture, exhibited an excellent cycle stability of mAh after cycles (Figure In contrast, with relatively low chemical stability and very fast capacity Figure | (a) and of and (d) performance of and Reprinted with permission from ref Copyright 2022 American Chemical Society. (e) Schematic illustration of (f) The comparison of and of Reprinted with permission from ref Copyright 2022 Elsevier. Download figure Download PowerPoint The efficient of COFs to their buried active sites has been the of research on electrode materials. have been such as and chemical these chemical is because it can new functional units and has a of structural In et designed and synthesized a new 2D COF In 2D can be to the units of under