Covalent Organic Frameworks as Model Materials for Fundamental and Mechanistic Understanding of Organic Battery Design Principles

Abstract

Redox-active covalent organic frameworks (COFs) have recently emerged as advanced electrodes in polymer batteries. COFs provide ideal molecular precision for understanding redox mechanisms and increasing the theoretical charge-storage capacities. Furthermore, the functional groups on the pore surface of COFs provide highly ordered and easily accessible interaction sites, which can be modeled to establish a synergy between ex situ/in situ mechanism studies and computational methods, permitting the creation of predesigned structure–property relationships. This perspective integrates and categorizes the redox functionalities of COFs, providing a deeper understanding of the mechanistic investigation of guest ion interactions in batteries. Additionally, it highlights the tunable electronic and structural properties that influence the activation of redox reactions in this promising organic electrode material.

1. Introduction

Covalent organic frameworks (COFs) are an emerging class of ordered polymers and are among the most designable members of the family of porous organic materials, constructed by using modular chemistry, wherein the molecular building blocks can be decorated with a variety of redox-active groups connected via covalent bonds.^1,2 Depending on the geometry and connectivity of the monomeric modules, the network of the COFs can propagate in two or three dimensions with adjustable pore sizes and tunable topologies, which enables easy percolation of guest ion pathways during electrochemical investigations to understand the applicability in charge storage devices, particularly in batteries.^3,4 In many 2D COFs, being regarded as a new type of layered materials and resembling the stacked structure of graphite, the strong interlayer π–π interaction generates one-dimensional nanoporous channels (Figure 1A,B)^5,6 These nanochannels can be decorated with functional groups containing heteroatoms, allowing facile interaction with guest ions from the electrolyte under applied potential, and are suitable candidates for rechargeable battery systems, constructed by a counter metal electrode and the COF as a working electrode (Figure 1A, Figure 2A).⁷ Additionally, weakening of the π–π stacking interactions enables the exfoliation of the multilayer COFs to a few atom-thick layers, called covalent organic nanosheets (CONs),⁸⁻⁹⁵⁰ where the constituting redox-functionalities are more exposed than in typical COF-derived electrodes, for improved interactions with the guest ions (Figure 1B).^10,11 The COFs with substantial structural rigidity and planarity, propelled by a highly conjugative vinylene linkage^12,13 and/or fused aromatic ring,¹⁴ show facile electron transfer to its redox functionalities. Hence, designing electroactive COFs with reasonable electronic conductivity and ample redox centers delivers promising electrode performance. Interestingly, the nanoporous channels of COF improve the diffusion kinetics of the charges that flow in and out of it.¹⁵ Furthermore, due to the modular construction principle and the vast variety of organic reactions to construct COFs, the molecular building blocks are precisely assembled into a crystalline framework, align donor–acceptor moieties, control the planarity, and retain the conjugation of the framework, which all tune the polymeric band structure and electronically activate the redox functionalities of the framework.¹⁶⁻¹⁸ Owing to these fascinating properties and additionally the controllable pore structure, large surface area, simple surface/structural modification, and high thermal and chemical stability, COFs have been investigated as intriguing electrode materials in reversible energy storage, especially in secondary ion batteries (Li⁺,¹⁹ Na⁺,²⁰ K⁺,²¹ Mg²⁺,²² Ca²⁺,²³ Zn²⁺,^24,25, Al³⁺,²⁵⁵⁰ etc.) (Figure 1A).²⁶

Figure 1.

(A) (i) and (ii) The n-type and p-type redox mechanisms of the COF derived electrode in a metal ion battery. (B) The tunable structural, physical, and chemical properties of the COF for potential advantages as an electrode in the battery. Reprinted in part with permission from refs (27, 28), and¹²⁹. Copyright 2020 Royal Society of Chemistry and Copyright 2019, 2018 John Wiley & Sons, Inc.

Figure 2.

(A) Illustration of the construction of 2D and 3D COF with redox-active linkers/linkages. (B) The general electron-transfer mechanism of the redox-active π-electronic system is based on Hünig’s principle.

Since rechargeable batteries are ranked highest among all energy storage systems, considering their capability of delivering stored energy for a long time, the universal target is to obtain the maximum energy output with high durability.²⁹ Of the various approaches, improving battery performance by manipulating the chemistry around the electrode materials is an effective strategy. For example, integrating the electrode materials with ample electroactive functionalities can enhance the theoretical capacity for charge accumulation, both electrostatically and electrochemically, particularly in metal-ion batteries.^30,31 Within this materials platform, the modular design approach of COFs allows the introduction of electronically active segments into the backbone to make it suitable for high-energy-density electrodes. Utilizing the scope of synthetic development of monomer synthesis can enrich COFs with multiple redox centers per repeating unit (Figure 1B). This opens up new avenues to fabricate high-performance electrode materials in the field of batteries, something which is cumbersome in heteroatom-doped carbonaceous materials or amorphous polymers, as the intrinsic heterogeneity leads to a wide distribution of redox-active sites without a clear spectroscopic signature.

Based on recent research, the three key factors that impact the performance of COF-derived electrodes include (i) the nature, density, proximity, and orientation of the redox-active functional groups present on the walls of the pores, (ii) the electronic and pore-size driven diffusion of the guest ions toward the bulk of the framework structure, and (iii) the electronic activation of redox-functionalities under applied potential (Figure 1B).³²⁻³⁴ In addition, the ions from the electrolytes follow the insertion mechanism through the nanochannels of the COFs and electrosorb or intercalate in between the π-stacked aromatic building units, whereas the functional groups undergo the redox-conversion under potential, rendering the COF suitable for a wide variety of battery mechanisms.³⁵ Hence, the high-level designability, atom-precise manipulation, tunable structural complexity, and computational modeling of the periodic structure propose COFs as the ideal model system for understanding advanced electrode materials and their fabrication.³⁶⁻³⁹ The crystalline nature and periodicity of the building units simultaneously aid in the good anticipation of charge storage mechanisms. The operando and ex situ experimental studies during stepwise electrochemical change of the COFs can provide information to establish the mechanism of the charge carriers’ interactions with the framework (Figure 1B).^36,40,41 Moreover, one can unravel a multitude of insights that feed the possibility of developing attractive lightweight energy storage devices through detailed experimental and theoretical investigations of the charge-storage ability of COF materials with benchmark performance. Hence, a wonderful coherence of the experimental in situ analysis with the computational modeling, along with thorough MD simulation,⁴² may eventually draw the roadmap for the future development of the redox-active COFs as per requirement.

2. COF-Electrodes: A Platform for Designed Molecular Construction of Polymeric Structures

2.1. Redox Behavior of COFs in Batteries, Periodically Tethered with Different Elements

COFs with 2D and 3D periodic structures made from π systems can incorporate various heterocyclic moieties for redox activity in electrode materials (Figure 2A). The π systems containing p block elements (X/Y: N, O, S, Se, and P) can interact with guest ions through their accessible lone pair electrons, enabling redox-functionality and charge transfer steps that follow Hünig’s classification (Figure 2B).⁴³ X/Y containing π-systems with carboxylate, anhydride, or amide functional groups can be integrated into the cyclic structure, allowing for precise manipulation of the COF backbone or side functionalities for the desired redox activity. COFs containing heterocycles, such as pyridine, pyrrole, thiophene, and thianthrene, have higher theoretical capacities than those with fused benzene rings and are known to be high-performing electrode materials. The stability of the heterocycles even in their radical or cationic form during interactions with guests results in prolonged stability of COFs featuring such heterocycles during electrochemical oxidation–reduction cycles.²⁶ Although transition metals (Ni, Co, Fe, Sn, and Cu), lanthanides (La, Ce), and alkaline earth metals (Mg, Ca, and Be) undergo intrinsic redox conversion, making them suitable for developing profound redox-electrodes, integrating them into the organic skeleton of of the COF is challenging and has only been reported in a few examples.⁴⁴⁻⁴⁶ However, COF structures containing chelating cores such as porphyrins,⁴⁷ phthalocyanines,⁴⁸ quinaxolines,⁴⁹ phenanthrolines,⁵⁰ bipyridines,⁵¹ and keto–enols⁵² can contain these highly redox-active elements. Other chelators like salen^53,55 or crown ethers^54,56 have also been integrated into COFs, but experimental battery studies are still lacking.

2.2. COFs with Capacitive Behavior and Redox Functionalities

The high surface area of COFs allows for easy polarization of π electrons by electrolyte ions, generating an EDLC under applied potential.^57,58 Heteroatoms facilitate on-surface charge transfer and attract metal ions, leading to pseudocapacitive behavior. However, this does not result in a prominent voltage plateau during charge–discharge due to the absence of electrochemical conversion.⁵⁹

There are also examples of a combined effect of pseudocapacitive behavior and battery characteristics in COFs. DeBlase et al.⁶⁰ and Vitaku et al.⁶¹ integrated cyclic carbonyls, such as the anthraquinone moiety, into DAAQ-TFP COF for successive implementation in both of those systems.

The strategic design of electroactive COFs with integrated redox-functional groups results in prominent redox peaks in cyclic voltammetry and a stable voltage plateau in charge–discharge cycles,⁶² crucial for determining battery working potential and understanding COFs’ ongoing redox mechanisms via ex situ and in situ characterization techniques.

2.3. P-Type and N-Type Redox-Functionalities in COF-Based Electrodes for Batteries

The behavior of COFs in electrochemical processes depends on the type of organic redox centers involved in charge transfer. These electroactive functional segments can be classified into three categories (p-type, n-type, and ambipolar) based on their charge during redox reactions (Figure 3).⁶³ N-type redox groups receive electrons in their neutral state, become negatively charged, and then undergo subsequent oxidation back to their neutral state, driving the redox process. COFs containing electron-rich functionalities such as quinones,⁶⁴ phenazine,⁶⁵ imides,^66,57 triazine,⁵⁸ and tetrazine³⁷ generally behave as n-type electrodes and are likely to interact with metal ions from electrolytes under applied potential (Figure 3, red-shaded area).⁶⁷ P-type COFs, on the other hand, participate in redox processes through facile transformation between their neutral state and positively charged state and induce interactions with anionic species. COFs containing tertiary amine,³⁵ viologene,⁶⁸ phenaxozine,⁶⁹ or thianthrene salts can store charge via a p-type mechanism (Figure 3, yellow shaded area). Unlike n-type organics, the anions (such as ClO₄^–, PF₆^–, BF₄^–, and TFSI^–) balance the positively charged functional groups by a pseudofaradaic process in the p-type mechanism. Conducting polymers such as polythiophene,^70,71 polypyrrole,⁷² and polyacetylene,⁷³ which follow typical p-type organics, can also be integrated into COF structures to promote hole transport mechanisms.⁷⁴ Bipolar organic functionalities⁷⁰ (Figure 3, blue shaded area), which achieve either a reduced state to a negative charge or can be oxidized to a positively charged state, are rare but offer advantages in terms of energy and power density. This mechanism is achievable via n- and p-doping in the same COF by applying different potential ranges. A recent report by Gong et al. showed the construction of a bipolar redox-active TPPDA-CuPor-COF containing p-dopable sp³ nitrogens and n-type Cu-porphyrin moieties, which demonstrated stable multiple electron transfer through the use of DFT calculation.⁷⁵

Figure 3.

(A–C) Representative types of redox-active moieties, either through COF-linkage or substitution on COF-linkers, as well as their respective theoretical specific capacities, are considered for constructing COF-derived electrodes in metal-ion batteries. The red and yellow highlighted moieties represent n-type and p-type mechanisms, respectively, while the blue highlighted moieties represent ambipolar mechanisms.

2.4. Electroactive COF-Electrodes Constructed from Redox-Active Linkers

2.4.1. COFs with Nitrogen-Rich Functional Groups

COFs with pyridinic nitrogen linkers reduce the adsorption energy of positively charged ions due to nitrogen’s low-lying lone-pair electrons, similar to ammonia or amino groups (Figure 3A). This creates strong interaction sites for chemical coordination with metal ions, making pyridine-rich COFs advantageous for improving ion-interaction kinetics in various batteries such as Li⁺, Na⁺, K⁺, and aqueous Zn–I₂ batteries.^18,76−78 The aromatic phenazine rings with two pyridinic-N sites are well-known for reversible redox activity at high potential (E_reduction: 1.9–2.2 V vs Li/Li⁺), and this strategy was successfully implemented in DAPH-TFP-COF by Vitaku et al. to enhance the energy density of the fabricated lithium ion battery (LIB) cathode (Table 1, Scheme 1).⁶¹ In this class of COFs, the phenazine-containing linker offers energetically favorable quinoxaline sites where the nitrogens are oriented in a fashion amenable for stepwise redox reaction (E_reduction: 1.2–2.45 V vs Li/Li⁺) via facile metal ion chelation.⁷⁹ Wu et al. used DFT calculations and ex situ XPS studies to show that electroactive pyridinic N in a π-conjugated C=N system of the BQ1-COF strongly coordinates with lithium, resulting in high-capacity lithium storage through multiple electron redox reactions by accepting three lithium ions that bond with three pyridinic nitrogen atoms.¹⁸ Recently, Gu et al. constructed tertiary amine-containing TPPDA-PI-COF for high-voltage (E_reduction: 3.53–3.83 V vs Li/Li⁺) organic batteries and verified the formation of a p-type radical cationic intermediate by operando-EPR spectroscopy.⁴¹ Furthermore, the highly nitrogen-rich aromatic rings such as tetrazine were tethered in IISERP-COF18 by Haldar et al. to trigger the n-type reduction followed by metalation.²⁷

Table 1. Redox Potentials of the Functional Groups in Particular COF Systems^a.

^aN-rich functional groups are highlighted in sky blue, O-rich functional groups are highlighted in pink, S-rich functionalities are highlighted in yellow, and other functionalities are highlighted in green. Entries marked with an asterisk denote functional groups that are p-type in nature.

Scheme 1. Reduction Potential Window of the Redox-Functionalities of the COFs with Respect to the Different M/M⁺ Reference Electrodes.

N-rich functional groups are highlighted in sky blue, O-rich functional groups are highlighted in pink, S-rich functionalities are highlighted in yellow, and other functionalities are highlighted in green. Entries marked with an asterisk denote functional groups that are p-type in nature.

2.4.2. COFs with Oxygen-Rich Functional Groups

Quinone- and imide-based COFs are popular as organic carbonyl electrodes in metal-ion batteries because of their n-type properties (Figure 3B). The carbonyl groups undergo a reversible redox mechanism during discharge/charge by opening and reconstructing C=O bonds, participating in a nucleophilic addition reaction. Enolization is a key step for interaction with metal ions.^74,80 To enhance metal-ion affinity and homogeneous ion flux during the electrochemical process, COFs are designed to have the maximum amount of carbonyl groups per repeating unit and a highly aligned arrangement.^81,82 Among the quinone family, the ortho-diketone (E_reduction: 2.65–2.9 V vs Li/Li⁺, Table 1, Scheme 1) functionalities in COFs create cooperative metal ion binding sites and facilitate the chelation mechanism.⁸³ Several other related moieties such as diketopiperazines (E_reduction: 2.48–2.6 V vs Li/Li⁺) with appropriate substituents well-known for stabilization of the resulting radical anion or dianion could also be explored in COF chemistry for high-energy electrode fabrication.⁸⁴ Construction of COFs with β-ketoenamine (E_reduction: 0.2–0.6 V vs Li/Li⁺) or quinone functionalities with fused aromatic systems results in effective radical carbanion stabilization during a stepwise radical mechanism and effortlessly drives metalation with enolized oxygen.⁸⁵ Meanwhile, cyclic polyimide carbonyls when fused with the aromatic π-current of COFs stabilize radical intermediates during the electrochemical reduction and afterward accelerate the interaction with Li⁺, Na⁺, and K⁺.⁸⁶⁻⁸⁹ DFT calculation and XPS studies by Luo et al. on P-COF confirmed that the metal ions are engaged with the carbonyls of imides and naphthalene during the storage mechanism through the enolization and the π-cation stabilization effect of the aromatic rings.⁸⁶ Hence, the theoretical capacities and working potential window (E_reduction: 1.6–2.6 V vs Li/Li⁺) of such polyimide-containing COFs also vary with the number of aromatic rings (benzene < naphthalene < pyrene)⁶⁷ connected to these moieties. Though the loss of resonance in the core of the aromatic π-ring of the conjugated carboxylic acid decreases the redox reactivity to the anodic potential (E_reduction: 0.5–0.8 V vs Li/Li⁺), the reversible two-electron redox reaction, whereby the resulting dianion is stabilized by a conjugated π-system of the COF, offers an opportunity for deriving promising anode materials (Scheme 1).⁹⁰

2.4.3. COFs with Sulfur-Containing Functional Groups

Sulfur-containing heterocycles are very promising for constructing hole-conducting polymers (polythiophene, poly(3,4-ethylenedioxythiophene, PEDOT)) owing to their facile conversion to positive bipolaron states which tunes the valence band and conduction band leading to partial overlap across its Fermi level (Figure 3C).⁹¹ If this type of conducting backbone can be explored for the construction of COFs, it will certainly play an important role in the improvement of electronic conjugation to decrease the overpotential of the battery, especially since it is known to be stable in a radical cationic form and undergoes a p-type mechanism. Considering the low electronegativity of the sulfur in the heterocycle, the lone pair electrons from the C–S bond can interact with positively charged metal ions, and the sulfur’s vacant d-orbitals can accommodate guest ions for the charge storage mechanism. Based on this, Chen et al. described the improved anodic (E_reduction: 0.9–1.25 V vs Li/Li⁺, Table 1, Scheme 1) kinetics of 3,4-ethylenedioxythiophene (EDOT)-bridged PTT-O@C–COF networks in LIBs by comparing them with a homologous COF constructed from thiophene units.⁹² Another very interesting and promising nonaromatic sulfur heterocycle, thianthrene, typically shows anionic interaction at high potential (E_reduction: 1.6–1.9 V vs Li/Li⁺) via interconversion between aromatic and nonaromatic states.⁹³ The first example of a thianthrene-connected COF named DUT-177 delivered cathodic capacity against a lithium counter electrode and eventually degrades with cycling due to the irreversible breaking of the C–S bond.^94,95 Synthetic modification by substitutions in the proximity of the thianthrene-ring and its optimized compatibility with the new electrolyte system improved the battery performance. Among the other sulfur linkages in COFs, the organosulfides are well-known for the electrochemically reversible breaking and reconnection of the S–S bond during metalation (E_reduction: 2.25–2.51 V vs Li/Li⁺). The mechanism was investigated by Haldar et al. by combining a computational approach with in situ Raman studies on redox-active COF-electrodes named DUT-185-S₂ and DUT-185-S_n.¹⁰⁵ Meanwhile, a new class of covalently anchored sulfurized COFs has recently been explored to suppress the commonly observed polysulfide shuttle problem during charge–discharge in lithium–sulfur (Li–S) batteries. Strategic substitution of halo atoms in the halobenzenes of the COF-building unit or inverse-vulcanization at vinylene backbones of COFs at high temperatures enables this polysulfide linking to the framework’s backbone.^96,97 This induces the on-surface redox reaction and improves the durability of the engineered electrodes in a Li–S battery (E_reduction: 2–2.4 V vs Li/Li⁺) setup.

2.5. COFs with Radical-Stabilizing Functional Groups

The electron transfer between COF-electrode and electrolytes to conduct any redox reaction is a heterogeneous interfacial process that may be expected to be kinetically slower than a homogeneous electrochemical reaction.⁹⁸ The highly redox-active organic radicals which are well-known as redox mediators are capable of undergoing a single-electron-transfer (SET) mechanism and can be integrated into the COF skeleton to facilitate this transfer of electrons. Designing the COFs by coupling with such strong redox-active groups (nitroxyl, viologen, etc.) is growing the interest of synthetic organic chemists to develop next-generation high-energy batteries.^98,99 An interesting work by Wang et al. showcases a strong single-electron reduction of DAAQ-ECOF functionalized by an oxoammonium cation. There the nitroxide radical in 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) converts to an aminoxy anion by a p-doping mechanism for anionic charge balance, producing a stable and reversible voltage plateau (E_reduction: 3.4–3.6 V vs Li/Li⁺) with very low overpotential during charging and discharging (Figure 3D).^10,100 Perticarari et al. investigated the phase change of the TEMPO-based materials during operando XRD studies and confirmed the appearance of the new band during the operando UV–vis measurements.¹⁰⁰ The wide variety of COFs constructed from pyridinic nitrogen after successful conversion to pyridinic N-oxide or pyridinium ions is also expected to show similar behavior. In this context, the viologen-based BAV-COF by Tong et al. showed the potential of single-electron reduction with tailored anionic redox chemistry and a selective ion-transport mechanism in the fabricated high-performing metal ion cathodes.⁶⁸ The coherence of the electrochemical analysis with ab initio molecular dynamics (AIMD) establishes the impact of the counteranion coordination to the pyridinium ions for the transport kinetics of the electrolytes in the confined nanochannels of the COF. In another way, the strong redox activity can be introduced in COF-electrodes by installing multivalence metal centers which can also stabilize the SET phenomena. Porphyrinic COFs with chelated metal centers or COFs constructed from metal–dithiolene and metal–cyanide-containing chelating sites (Co,⁴⁴ Sn,⁴⁵ Ni⁴⁶) participate in multistep electron-oxidation reduction processes depending on the possible conversion of the metal-coordination sites or the delocalization probability of the accommodated charge through the metal-chelated ring. The delocalization of the π-electron cloud in the aromatic ring of the COF allows for multiple redox couples to form during the electrochemical SET process, which can lead to enhanced stability during reversible charge–discharge. Additionally, the cation−π interaction, which occurs between positively charged metal ion and the π-electron cloud of the aromatic ring, can also contribute to the stabilization of the coordinated metal centers in the COF. Zhao et al.⁴⁴ established this electron-delocalization effect in aromatic rings of COF-Co during guest metal ions interactions with a Co-chelated ring by detecting a broadening of the ex-situ C 1s XPS signals and observing the weakening of characteristics of the benzene ring’s peaks in FT-IR spectra.

2.6. Redox-Active Linkages in COFs to Trigger Electrochemical Behavior

Substituting functional side groups on COFs may not fully activate redox-active centers due to insufficient electron flow, leading to a higher overpotential and slower kinetics. To address this, constructing COFs with redox-active linkages such as imine, azide, phenazine, or polyimides allows for easy through-bond electron conjugation and electronic transformations under applied potential (Figure 3A, Table 1, Scheme 1). Followed by a reversible two-electron reduction (E_reduction: 1–1.75 V vs Li/Li⁺) by Bai et al., the imine-linked N2-COF and N3-COF have been exploited for anodic activity.¹⁰¹ However, the localized electron conjugation pathway inferred by the polarity of the imine bond often induces slow kinetics and hence poor rate performance of the battery. The azo bond (E_reduction: 0.3–1.9 V vs Li/Li⁺) containing COFs Azo-CTF and Tp-Azo-COF, which follow a similar mechanism, could be a potential alternative, as considered by Wu et al.³⁵ and Zhao et al.¹⁰² Their studies combining in situ FTIR as well as operando Raman spectroscopy verified the redox process occurring in the azo linkages during metal ion interactions. The reversible appearance and disappearance of the sp² nitrogen signals of azo bonds during the charge–discharge process indicated a pure bianionic interaction with two metal ions. Computational molecular orbital optimization of the COFs indicates the possibility of electronic structure tuning by synthetic strategies. The aforementioned cyclic phenazine⁵⁵ linkage in COFs is also very promising for fostering highly reversible two-electron reduction–oxidation (E_reduction: 1.95–2.4 V vs Li/Li⁺) procedures driven by the facile transformation between aromatic to nonaromatic states. The detailed study by Shehab et al.⁶⁵ showed that the ex situ C 1s XPS spectrum of the phenazine-linked Aza-COF produced a strong signal of C=N which was seen to disappear during metalation. In contrast, a new peak for the reduced C–N appeared for dearomatization of the phenazine ring. The electrically insulating n-type polyimide linkages, well-known for the stepwise four-electron reduction process, are commonly used for the fabrication of COF-based cathodes by mixing with conducting carbon nanotubes (CNTs).⁸⁷ The cooperative metal binding degrades the polyimide rings during cycling unless they are connected to stabilizing polycyclic aromatic π systems such as naphthalene, pyrene, or quinoxaline.²⁰ The weakening of breathing vibration of the imide-carbonyl of PIBN-COF during in situ FTIR studies by Luo et al.⁸⁷ and the complete disappearance of its signal in solid-state ¹³C NMR at the expense of a new signal indicated the profound lithiation process of the imide centers.

3. Tunable Battery Redox Potential by Systematic Design of COF Functionalities

3.1. Estimation of Redox Potentials COF: A Guiding Factor for Cathodic or Anodic Activity

The computationally calculated binding energy E_B(x₁,x₂) of any metal ion (M⁺) to any organic functional group is correlated to the redox potential for the subsequent electron transfer, as derived by the equation of Lee et al.¹⁰⁷ Followed by this, Haldar et al. also analyzed the mechanism of M⁺ interaction to COF in a stepwise manner by evaluating the energy profile diagram for the system. In consideration of the typical metal-ion battery reaction mechanism, the binding energy was calculated using the following equation:^105,108,109

1

Here, E_total(x₂) and E_total(x₁) are the total energies of the system at two adjacent low-energy concentrations x₂ and x₁, e is the electronic charge, and E_M is the energy per metal atom in its bulk structure. Since reduction potential reflects the electrochemical potential required for electron transfer in a chemical reaction, the metalation and demetalation reactions in COFs were shown by Singh et al. using the following equation:¹¹⁰

2

When the reaction is in equilibrium, the chemical potential of the electron can be calculated from the Gibbs free energies of other chemical species:

3

Using the Nernst equation, the potential required for the metalation reaction can be written as the following equation:

4

Here, e is the elementary charge and F is the Faraday constant. As we measure the reduction potential with M/M⁺ reference electrode, the potential values can be calculated using the following equation:

5

Hence, COFs provide an open platform for fabricating functionalized electrodes that can be introduced systematically as a cathode or anode depending on the redox potential of the electroactive segments integrated into it.¹¹¹ The experimentally observed electron-transfer potential of organic functions of COFs against NHE or SHE matches well with the theoretically calculated value, even when covalently attached to a polymeric or periodic material.^112,113 These potentials are roughly interconvertible to the redox potential of the metallic counter electrode following the electrochemical series (Scheme 1), guiding the selection of COFs as either a highly energetic cathode with higher redox potentials or as an easy-to-metalate anode at a lower applied potential (Table 1).¹¹⁴

3.2. Potential Window of Batteries Guided by Frontier Molecular Orbitals of Functional Units of COFs

The functional groups on organic building blocks or covalent linkages enable precise modulation of COF energy states in systematic synthesis.^115,116 The HOMO and LUMO energy positions of COF building units correlate with electrochemical potential¹¹⁷ and redox stability in batteries (Figure 4). Stabilization of frontier molecular orbitals is crucial for interactions with electrolyte ions. Molecular building units and band structure can stabilize oxidized HOMO or reduced LUMO states and influence metal ion binding (Figure 4A).¹¹⁸ Strategies for fine-tuning of COF-derived electrodes involve adjusting electrochemical potential by changing organic substituents with +R/–R (resonance effect) or +I/–I (inductive effect) on π-aromatic building units (Figure 4A).

Figure 4.

(A) The redox potentials of COF-derived electrodes can be tuned using the frontier orbital energy approach by functionalizing them with electron-donating or electron-withdrawing groups (EDGs or EWGs). HOMO energy increases from blue to red, while LUMO energy increases from pink to green. (B) The oxidation and reduction potential trend of COFs in metal-ion batteries follows the increase in HOMO and LUMO energy levels of the building units, respectively. (C) COFs with cathodic or anodic activity in metal-ion batteries exhibit an oxidation and reduction potential that follows the trend of HOMO and LUMO energy levels, as indicated by the consistent color code used throughout.

Building upon this, the organic substituents (OH, SH, OMe, NMe₂) with +R or +I effects on the π-aromatic cloud of COF enrich electron density and trigger interactions with positively charged ions in lower potential. Conversely, −R or −I effects of substituents (−C=O, NO₂, CN, N₂⁺Cl^–) withdraw electron density from the COF backbone (Figure 4A). This probably makes the COFs useful for interacting in relatively high applied potential windows considering the electrolyte–ion interaction with COFs is not energetically favorable in those cases.¹¹⁹ In general, the ease of metalation of COFs constituting anthraquinones,⁸⁵ β-ketoenamines,^85,89 azo^35,102 groups, etc. with a stabilized HOMO structure shows anodic behavior in the presence of metallic counter electrodes (Scheme 1). Meanwhile, the COFs decorated with imides, phenazine, organosulfides, etc. possessing higher energetics with less stable LUMO require a high applied potential for the interactions with metal ions as a cathode (Scheme 1). Also, the COFs undergoing a p-doping mechanism in the electrode are generally positively charged and ionic in character (such as viologen,⁶⁸ phenoxazine,⁶⁹ and nitroxyl radicals^10,11) with high energy in nature. This induces prominent cathodic activity at high redox potential (E_reduction: 2–4 V vs Li/Li⁺) (Table 1 and Scheme 1). By looking at the energy levels of certain building blocks of the COFs that can undergo oxidation and reduction, one can predict how they will behave during cyclic voltammetry for both the negative and positive electrodes. (Figure 4B,C). Higher HOMO energy levels of certain building blocks can enhance the positive electrode character, and higher LUMO energy facilitates electro-activity as a negative electrode. However, the estimated redox potential of the individual electroactive segments of the COFs sometimes differs from the overall oxidation–reduction potential of the polymeric band structure considering the delocalized energy levels of the VB or CB. Charge-transfer or electron push–pull mechanisms between the covalently connected electroactive segments and interactions between the layers and inter/intralayer hydrogen bonding in the COFs cause such deviation of the redox activity. Hence, the electroactivity of the COF under applied potential and the corresponding redox potential can be determined precisely only by band structure analysis and with the help of MD simulation.

4. Improved Redox-Kinetics in Batteries with COF-Derived Electrodes

4.1. COF Pore Size Control for Facile Ion Percolation

Since COFs consist of well-defined and easily tunable porous nanochannels, mass transfer of target electrolyte-ions with different polarities and dimensions easily proceeds and endows these systems with great potential for different metal-ion batteries. The pore apertures of COFs dictate the size of various guest ions that can permeate into the pores, which also provide active sites and confined space to perform the necessary redox reactions (Figure 5A). The size of the metal ions used in metal ion batteries is very small, allowing for easy percolation through porous structures and n-type redox interactions. However, the diameter of the solvated metal ions and the counteranions that balance the overall charge can sometimes hinder the entire ion transportation process, including the p-type redox reactions.¹²⁰ Considering M⁺B^– as the solvated electrolyte in a typical metal ion battery, the solvation diameter of the M⁺ varies, and the size of B^– changes with counteranions, as shown in Figure 5A.¹²¹ Therefore, it is generally understood that choosing COFs with at least pore diameters in the microporous (up to 20 Å) to mesoporous (20–50 Å) range is important.³⁶ Li et al. compared battery performance using molecular mechanistic simulations that accounted for the pore size effect by tuning the reticular structures of CTF-0 and CTF-1.¹²² The study showed that optimal pore sizes facilitate energetically favorable exothermic processes, while larger pore sizes inhibit reversibility. Two-dimensional COFs with small pores have more rigid and stable structures with precise layer stacking but can conduct charge for the electronic activation of functional groups. Larger pore dimensions reduce the electroactive mass of the COFs, resulting in lower cell voltages. However, some COFs with small pore sizes still have restricted mass transfer of electrolyte ions, hindering performance.¹²³ Therefore, choosing optimal pore apertures with redox-active functionalities is crucial for improving the COF-derived battery performance. The 2D flow path of electrolytes in a porous medium of a 2D COF has fewer geometric degrees of freedom than the 3D flow path. This difference leads to a smaller accessible volume for electrolyte transportation in the 2D COF compared to the 3D version. Hence, three-dimensional COFs with functionalized pores may improve the mass transport of guest ions^124,125 (Figure 5B). However, 3D COFs lack electrochemical activity and have poor electronic conductivity, so the proper structural design is necessary for improvement.

Figure 5.

(A) The ionic radius of the metal ions (M⁺) and counteranion (B^–) and solvated radius or Stokes radius of the metal ions (M⁺). (B) The construction of tunable-pore 2D and 3D-COFs facilitates facile ion diffusion to redox sites in COF-derived electrodes, while exfoliation assists the facile redox interaction of functional groups with guest ions in 2D-COFs. Reprinted in part with permission from refs (120) and¹²¹. Copyright 2019 Elsevier B.V. and Copyright 2018 John Wiley & Sons Inc.

4.2. Tuning Layer Thickness of COF-Electrodes: Toward Diffusion Control of the Metal Ions

2D COFs exhibit unidirectional diffusivity of electrolyte ions through their one-dimensional nanochannels due to uniform π-stacking of building blocks. This preserves framework rigidity, allowing for electronic flow through the XY plane and Z-direction for essential electrochemical activation of functional groups during targeted redox reactions (Figure 5). In certain cases, the binding mode of the electrolyte ions to the redox functionalities present on the pore walls can have a greater influence on the pore size. Therefore, experimental evidence for facile ion percolation and diffusion kinetics is always more reliable than a theoretical prediction. Different literature sources have taken two different approaches for estimating the diffusion coefficients of the ions within the porous nanostructure of COFs.

(1) The ion diffusion coefficient (D) was obtained by electrochemical impedance spectroscopy (EIS) and derived from following equation:^10,28

6

where R is the gas constant (8.314 J mol^–1 K^–1), T is the temperature (298.5 K), A is the area of the electrode surface, F is the Faraday constant (9.65 × 10⁴ C mol^–1), C is the molar concentration of the ions, and σ is the Warburg coefficient. The Warburg coefficient σ can be obtained from

7

where σ is the slope for the plot of Z_re vs the reciprocal square root of the lower angular frequencies (ω^–0.5).

(2) The diffusion coefficient within the COF-electrode was verified by the galvanostatic intermittent titration technique (GITT) measurement.²⁰ The D can be calculated by the GITT method based on the following equation

8

where τ is the pulse time, n_m is the mole number, V_m is the molar volume, S is the electrode–electrolyte interface area, ΔE_S is the voltage difference between the steady state and the initial state of every step, and ΔE_t is the change of the total voltage during a pulse step excluding the IR drop.

While higher order stacking of functionalized 2D-layers of COFs can enhance the diffusion path length, it negatively affects battery kinetics, especially with high scan rates and current densities, which reduce the interaction time. The redox-active functional groups are often buried between stacked layers and do not get enough exposure to interact with guest ions, limiting the overall battery performance, especially at faster reaction times. To improve performance, controlling the thickness of the COF stack appears to be an effective strategy for regulating redox activity and stabilizing charged species. An interesting finding by Gu et al.⁸⁵ indicates that the stabilization of the radical intermediates of DAAQ-COF results in enhanced reversibility by modulating the thickness of COF nanosheets. This observation inspired many researchers working in the field to exfoliate the 2D-COFs into the corresponding few-layer nanosheets^104,127 (covalent–organic nanosheets) to derive the electrode materials for addressing the aforementioned issues (Figure 5) as it provides the opportunity for better control over the kinetics of the battery and shows significant improvement in the electrode performance.^10,28 Multiple exfoliation strategies, including mechanical exfoliation,¹⁰ chemical exfoliation,^28,127 and self-exfoliation,^126,128,129 have been utilized in the synthesis of covalent organic nanosheets for electrode fabrication. However, the statistical distribution plot of the nanosheets for a bulk synthesis shows a random aspect ratio distribution (thickness/length). So scaling up the synthesis toward large amounts of COF nanosheets using a straightforward approach or synthesizing it in bulk quantity with controlled thickness is still challenging. Sometimes the postsynthetic modification of COFs for exfoliation can destroy the order of COF layers or even decrease the connectivity along the layers.¹³⁰ Hence, it is essential to design one-pot synthetic strategies that can produce the nanosheets with controlled thickness without sacrificing the crystallinity of the COFs.^131−134

5. COF-Derived Electrodes for Systematic Investigation of Structure–Property Relations

5.1. Approximation of Charge Storage Capacity

Understanding the structure-dependent electroactivity of periodically installed redox-active functional groups in COFs facilitates the systematic estimation of the energy storage capacity in fabricated electrodes by modeling crystalline COFs and theoretically determining unit cells (Figure 6A). This can be aided by techniques such as powder or single-crystal diffraction (i.e., through cryo-transmission electron microscopy),^135,136 as well as pore width distribution analysis and simulating its energetically stable crystallographic repeating unit using first principle-DFTB, CASTEP or Dmol, etc. with appreciable accuracy.¹³⁷

Figure 6.

(A) Charge–discharge mechanism of a highly redox-active phenazine-COF and its periodic unit-cell showing the possibilities of mechanism studies with computational modeling. (B) The adsorption energy per metal ion at different sites of the unit cell. (C) Differential charge density distribution of the COF after adsorbing metal ions in which the green area is for the accumulation of electrons and cyan is the dissipation area of electrons. (D) The stepwise structural evolution during the metal ion incorporation in COF. (E) The obtained lithiation pathway from theoretical simulations. Reprinted in part with permission from ref (111). Copyright 2022 John Wiley & Sons Inc.

Thereafter, it is possible to calculate the theoretically achievable storage capacity (C_t) of the COFs by determining the number of redox-active groups present per unit cell using the following equation:^{129,138−140}

9

Here M_wq is the molar mass of active species of the COF, n is the number of charge carriers, and F is the Faraday constant.

Hence, the determination of repeating unit cells in COFs through structural analysis provides a foundation for computational modeling to understand the charge-storage mechanism (Figure 6A–D). Such prediction is challenging in amorphous or glassy polymers due to their lack of crystallinity.

5.2. Density of Redox-Centers and Framework Topology

In addition to redox-active sites, non-redox-active sites of COFs also contribute significantly to electrode mass loading and can ultimately suppress gravimetric charge-storage ability. Therefore, a strategy for enhancing the electrode performance is to decorate the repeating unit of the COF with multiple redox functionalities. Precise topological control can accommodate multiple redox centers in a crystalline COF by choosing building linkers with different symmetries. COFs offer a wonderful platform for regulating the density of redox-active constituents per unit cell, which ultimately impacts the gravimetric charge storage capacity as described in eq 9. Theoretically, a higher accumulation of redox-active centers per unit-cell of a crystalline COF with lesser pore volume should enhance the charge-storing capacity. Optimal nanopores of COFs with various topologies for facile binding of ions from electrolytes can be created by choosing the appropriate symmetry of redox-active linkers, which construct smaller pore dimensions and connect via a redox-active linkage. For example, it has been noticed that redox-active 2D-COFs with C₄ by C₂, C₃ by C₃, and C₆ by C₂ symmetry¹⁴¹ generate COFs with smaller pore widths of kagome, hexagonal, and trigonal lattice, respectively, and increase the density of redox units per unit cell. In addition to the topology guided by the choice of symmetry of linkers, the length of the linker also regulates pore size and density of redox centers in the COF.² Practically, the optimal pore diameter for facile percolation of ions and accessibility of redox centers for electrolyte interactions mainly control the overall performance of the COF-derived electrodes. In the future, a combined theoretical and experimental approach with precise topological control for adjusting the optimal pore diameter may show the strategic enhancement of the density of redox centers of the COFs for improved battery performance.

5.3. Advantages for Mechanism Studies

Computational approximations combined with a series of ex situ and in situ characterization methods on COF-derived electrodes including Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron (XPS) spectroscopy, Raman spectroscopy, and solid-state nuclear magnetic resonance (NMR) have been employed to provide detailed information on the interactions of the electrolyte-ions with the COFs and the corresponding structural changes during redox interactions or for simple ion insertion (Figures 7 and 8). An operando Raman study carried out on an Azo-CTF derived lithium-ion battery by Wu et al. indicated a stronger Li interaction with azo nitrogens compared to the triazine nitrogen centers and investigated the charge–discharge mechanism of the COF (Figure 7A).³⁵ Using a different strategy, Zhao et al. explained the lithiation mechanism of a carbonyl-rich Tp-Azo-COF by combining the theoretical approach with operando XPS analysis. This certainly confirmed the lowering of the azo link peak intensity and showed the generation of a new C–O---Li⁺ binding energy during the strong lithiation of the COF (Figure 7B). Since many functional groups undergo an electrochemical oxidation–reduction process via radical transformation, a properly engineered setup for in situ electron paramagnetic resonance (EPR) displays the stepwise electronic behavior of the COFs functional groups.⁴¹ To track such a mechanism by operando EPR Gu et al.⁸⁵ predesigned TPPDA-PICOF with a p-type N,N,N′,N′-tetraphenyl-1,4-phenylenediamine linker which undergoes a single-electron-transfer (SET) mechanism via a radical intermediate during the lithiation (Figure 8A). The redox-active functional groups of interest integrated into the COF skeleton can also be monitored intermittently during the electrochemical transformation by in situ NMR through proper isotope labeling of the atoms in the functional groups.^35,102 Very often, the NMR active nuclei of the electrolytes also assist in tracking the mechanistic pathway of the new ionic bond formation between the functional groups and the electrolyte ions (Figure 8B).

Figure 7.

(A) (i) Schematic diagram of the metalation/demetalation process of an azo-linked covalent triazine framework obtained via the MESP (kcal mol^–1) method. (ii) The electrochemical mechanism of the redox constituents of the same COF in the presence of the lithium counter-electrode. (iii) Instrumentation and technical representation of the operando Raman studies on the COF-derived coin-cell batteries. (iv) In situ Raman spectroscopy on the COF-electrode during the charge–discharge. (v) Contour and response surface analysis corresponding to it. (B) (i) After complete metalation (lithiation) of azo-linked phloroglucinol COF, structural evolution occurs, with the binding sites between Li and the C=O and N=N groups indicated by blue and red spheres, respectively. (ii) The instrumentation and technical representation of the operando XPS studies on the COF-derived coin-cell batteries. (iii) Comparison of XPS spectra of N 1s and O 1s for the COF, discharged to 0.5 and 0.01 V and charged to 0.5 and 3.0 V. Reprinted in part with permission from refs (35), (40), and (102). Copyright 2021 Elsevier B.V. Copyright 2021 Royal Society of Chemistry. Copyright 2020 American Chemical Society.

Figure 8.

(A) (i) The modeled structure of the polyimide COF was constructed from a p-type N,N,N′,N′-tetraphenyl-1,4-phenylenediamine linker. (ii) The p-type redox activity of the tetraphenylphenylenediamine linker shows the formation of a radical intermediate. (iii) The instrumentation and technical representation of the operando EPR studies on the COF-derived coin-cell batteries. (iv) The voltage plateau in the charge–discharge profile of the COF-derived electrode indicates the p-type redox activity of the constituting linker. (v) The operando EPR spectra of the COF-electrode at different voltages determine the formation of the paramagnetic intermediate. (B) (i) A representative structural unit of heteroatom-rich COF showing the possible interaction with NMR active metal ions (M⁺, here Na⁺) and voltage plateau in the charge–discharge profile. (ii) The instrumentation and technical representation of operando NMR studies on COF-derived coin-cell batteries show that the configuration of a COF-derived pouch cell allows for static NMR measurements inside the NMR coil. Reprinted in part with permission from refs (41) and (27). Copyright 2022, 2020 Royal Society of Chemistry.

The density functional theory (DFT) is another crucial tool for understanding electro-redox processes in COF-electrode materials by investigating electronic band structures (Figure 6) and can provide insight into charged species-intercalation mechanisms in layered COFs and ion dispersion in pores (Figure 6B–D).³⁶ DFT computation complements experimentation by unveiling spectroscopic signatures of the COF redox functionalities and plausible conversion reactions of functional groups, revealing the structural evolution of COF-derived electrodes during operation. Computational monitoring of COFs under electrochemical conditions can establish the conformational change of the COF, which is critical to ion mobility and electronic conductivity (Figure 9A). The orientation of the functional groups can affect the mobility of guest ions. Molecular Dynamics (MD) simulations can predict the charge transfer mechanism in the COF structures. Simulated PXRD patterns can be analyzed in conjunction with in situ-experimental techniques to correlate stacking distances and layer arrangements in 2D-COF systems during the movements of charge carriers.⁸⁸ Selecting the appropriate theory for computational calculation is crucial, as DFT calculations using the PBE functional can result in an overestimation of COF interlayer spacing during ion intercalation.¹⁴² In contrast, PBE with the DFT-D3 method of Grimme¹⁴³ or optPBE-vdW¹⁴⁴ provides greater accuracy in assessing interlayer distance and configuration of COFs. The first-principles approximation investigating COFs needs to consider the long-range dispersion interactions of ions with COFs for more accurate electronic and configurational predictions.

Figure 9.

(A) (i) Some examples of π-stacking linkers and conjugative linkages which allow the electron flow through the 2D-COF skeleton in the XY plane as well as in the Z direction. (ii) A schematic representation of the in-layer and out-of-layer electron flow in a highly π-stacked 2D-COF. (iii) Some examples of the in-plane and out-of-plane charge transfer in redox active 2D-COFs aid the ion hopping mechanism. (B) (i) Unit cell geometry of a thiazole-azo linked COF. (ii) High symmetry k-points of the COFs. (iii) Calculated Frontier orbitals of the COF obtained from the three-layer model configuration showing out-of-plane p-orbital overlap. (iv) Band structures analysis of COF calculated using the HSE06 function shows a narrowing of the bandgap owing to out-of-plane orbital overlap. Reprinted in part with permission from ref (110). Copyright 2021 John Wiley & Sons Inc.

In another approach, molecular model compounds are easy to synthesize and useful for correlating chemical properties with redox phenomena in COFs. Unlike polymeric COFs, they follow equilibrium electrochemistry principles, allowing for the quantitative estimation of electrochemical activity. The well-defined structure aids in studying structure–property relations, and three electrode measurements help estimate redox-active centers. Solubility in electrolytes assists in better characterization and systematic COF design for mechanistic elucidation.

6. COF to Satisfy the Electronic Demand for Electrodes

6.1. Tunable Band Structure and Conductivity of COFs for Improved Electrode Kinetics

The activation of redox functionalities and COF-electrode kinetics depends on the delocalized band structure or band energy, which reflects the integrated energy of the functional moieties present in the COF structure (Figure 9A). Dispersion of the bands, as opposed to flat bands in the energy axis, indicates delocalization and facilitates charge carrier transport. This determines the ease of electron delocalization and activation of redox functionalities. Singh et al.¹¹⁰ systematically engineered the band structure of 2D-COFs by adjusting the overlap of p-orbitals, both in and out of the plane, to enhance conductivity and reduce charge/discharge overpotential (Figure 9B). Using the DFTB approximation, Zhu and Meunier found that the abundance and rigidity of the π-aromatic constituents of COFs can reduce the activation energy for electron transfer in the band structure.¹⁴⁵ They also observed that the tunability of the phenylene ring nodes affects the valence band maximum (VBM) but has minimal influence on the conduction band minimum (CBM), whose electronic density is not delocalized enough. 2D-COFs consisting solely of fused aromatic rings, such as thiazole, benzoxazole, and phenazine, exhibit remarkable planarity due to strong π-stacking, resulting in the necessary semiconductivity to facilitate electronic flow toward redox functionalities. This is in contrast to frameworks with free rotation along single bonds or localized polarity in the imine-bonded skeleton, which introduce an enormous degree of freedom in framework structures and disrupt electronic conjugation in the layers. Jin et al.¹⁴⁶ found that the olefin-connected linkage of the sp²c-COF improves electronic conductivity compared to commonly used imine linkages, enabling redox functionalities to be electronically activated for redox participation even with shorter interaction times at high current densities and high scan rates (Figure 9A(i), (ii)).

6.2. Charge-Transfer-Driven Redox Activity in COF-Electrodes

COFs are often designed with a programmed donor–acceptor module orientation to maintain planarity and facilitate through-space and/or through-bond transfer of charge carriers for electronic activity (Figure 9A(ii), (iii)). This charge-transfer capability mitigates the limitations of electronic conductivity, resulting in hopping mechanisms dominating the ionic interactions. Continuous electron flow from electron-rich to electron-deficient centers further assists charge carrier hopping, lowering charge-transfer resistance and enhancing ionic conductance. Tetrazine-rich IISERP-COF18 reported by Haldar et al. has low-lying LUMO orbitals that attract positively charged metal ions, improving redox kinetics.²⁷ This enhances charge transfer from adjacent pyridine centers to electron-deficient tetrazine moieties in COFs, resulting in a reduction potential for tetrazine of 0.45–0.65 V vs Li/Li⁺. Finally, intercalation of iron¹⁴⁷ and first-row transition metals¹⁴⁸ can modulate the electronic conductivity of COFs, allowing for substantial charge transfer.

7. Limitations and Challenges

Computational prediction of the redox mechanism of COF-derived electrodes sometimes lacks experimental verification. COF electrodes with high porosity suffer from low electrode density and limited electronic conductivity. Conductive additives improve electron propagation, but intrinsic or extrinsic amalgamation is required. Also, theoretical prediction of the COF capacity must account for non-redox-active mass loading, which is not well documented in the current literature. Incorporating electroactive organic functional groups into COFs enables on-surface redox activity with metal ions, although complete conversion may not always occur due to slower kinetics. Solid-state NMR, Raman, and EPR require a good signal-to-noise ratio for accurate in situ and/or operando mechanism studies of COF-derived electrodes, which may suffer from the poor resolution of signals due to a low abundance of functional motifs and interference from conducting additives. Deviation from predicted electronic properties can result from agglomeration in different forms and shapes observed in morphologies of bulk powdered COFs. 3D-COFs lack electron transfer due to missing planarity between π-stacking aromatic rings, hampering battery application. However, future designs of highly conductive 2D- and 3D-COFs with proper electrochemical stability can overcome these limitations.

8. Conclusion

COFs are highly controllable platforms in framework chemistry, providing insights into redox processes, charge carriers, and mass transport in organic batteries. Recent studies have improved our mechanistic understanding of COFs as battery electrodes, but the lack of standardized analysis techniques limits systematic tracking of their development and cross-comparison between studies. A more standardized approach that references experimental and theoretical parameters for evaluating the use of COFs as battery electrode materials would be helpful. Developing a wider database for proposing the effective potential window of certain COFs in battery applications and using machine learning and high-throughput approaches to explore COF-derived electrodes can expedite the fine-tuned design strategies, leading toward practical applicability.

The authors declare no competing financial interest.