Ionic Covalent Organic Frameworks for Solid-State Polymer Electrolytes: Synthesis, Ion-Transport Regulation, and Interfacial Stabilization

Abstract

Solid-state polymer electrolytes are considered promising alternatives to conventional liquid electrolytes for improving the safety and energy density of lithium batteries. However, their practical application is still limited by insufficient room-temperature ionic conductivity, low lithium-ion transference number, poor mechanical robustness, and unstable electrode/electrolyte interfaces. Ionic covalent organic frameworks (iCOFs), as charged and crystalline porous polymers, provide a unique platform for regulating ion transport and interfacial behavior through ordered channels, tunable ionic sites, and designable framework structures. In this review, recent progress in iCOF-based solid-state polymer electrolytes is summarized from a charge–structure–function perspective. First, representative synthetic strategies for iCOFs are discussed, including direct synthesis and postmodification, with emphasis on how ionic-site distribution, crystallinity, pore accessibility, and membrane processability influence electrolyte performance. Subsequently, the applications of cationic, anionic, and zwitterionic COFs in solid-state electrolytes are reviewed. Cationic COFs mainly promote lithium-salt dissociation and anion immobilization. Anionic COFs enable quasi-single-ion or single-ion conduction. Zwitterionic COFs provide dual-charge regulation for Li⁺ transport and interfacial stabilization. The structure–performance relationships of representative iCOF-based electrolytes are critically compared in terms of ionic conductivity, lithium-ion transference number, electrolyte architecture, and battery performance. Finally, the remaining challenges and future opportunities are discussed, including the construction of continuous ion-transport pathways, improvement of polymer–COF compatibility, scalable membrane fabrication, standardized electrochemical evaluation, and clarification of intrinsic ion-conduction mechanisms. This review highlights the potential of iCOFs as functional ion-transport frameworks for safer and higher-performance solid-state lithium batteries.

1. Introduction

In recent years, with the rapid advancement of battery technology, conventional liquid electrolytes have been widely employed. However, the organic solvent-based electrolytes commonly used in commercial lithium batteries are typically volatile and flammable, giving rise to safety concerns such as leakage and even combustion. , In addition, liquid electrolytes show poor compatibility with high-energy-density lithium metal anodes, where severe interfacial side reactions can be readily triggered, leading to the formation of an unstable solid electrolyte interphase (SEI). , Meanwhile, the nonuniform lithium-ion flux in liquid electrolytes tends to induce the uncontrolled growth of lithium dendrites, which may further penetrate the separator and cause internal short circuits. , In contrast, solid-state electrolytes offer the advantages of nonflammability, leakage-free operation, and higher mechanical strength. , Therefore, replacing liquid electrolytes with solid-state electrolytes is widely regarded as an effective strategy for improving both the safety and energy density of batteries.

Despite these advantages, currently available solid-state electrolytes still suffer from fundamental limitations in practical applications. , In the case of inorganic solid-state electrolytes, although they generally exhibit high ionic conductivity and mechanical strength, the high solid–solid interfacial resistance arising from the rigid contact between the electrolyte and electrode severely restricts charge-transfer efficiency, while their intrinsic brittleness also leads to poor processability and difficulties in fabrication. − Moreover, some inorganic systems, particularly sulfide-based electrolytes, are highly sensitive to moisture and air, thereby imposing stringent requirements on manufacturing and operating environments. , By comparison, polymer solid-state electrolytes possess desirable flexibility, better solid–solid interfacial contact, and ease of processing. − Nevertheless, their practical application is still limited by low ionic conductivity at room temperature due to restricted polymer segmental motion, as well as by low lithium-ion transference numbers and limited capability to suppress interfacial polarization. ,, Therefore, simultaneously achieving efficient ion transport, good interfacial compatibility, and practical processability has become a central challenge that urgently needs to be addressed in the field of solid-state electrolytes.

Covalent organic frameworks (COFs) are a class of long-range-ordered crystalline porous polymers constructed from organic building blocks linked by covalent bonds. , Owing to their periodic and well-defined frameworks, as well as their highly tunable structures, COFs provide a unique platform for ion transport and electrolyte design. When incorporated as fillers into polymer matrices, neutral COFs primarily function through their porous structures, polar functional groups, and rigid frameworks. On the one hand, polar sites within the COF framework can regulate the local coordination environment of lithium salts through Lewis acid–base interactions, dipole–ion interactions, or hydrogen bonding, while their ordered channels provide pathways for rapid lithium-ion transport. , On the other hand, their rigid skeletons can effectively enhance the mechanical strength of soft polymer matrices and may contribute to homogenizing Li⁺ flux, mitigating interfacial polarization, and improving electrode/electrolyte interfacial stability. , As a functional subclass of COFs, ionic covalent organic frameworks (iCOFs) not only retain the structural advantages of COFs but also contain immobilized charged sites within their frameworks, thereby creating a tunable local electrostatic environment within the pores. Through electrostatic interactions, iCOFs can therefore more directly regulate the spatial distribution of Li⁺ and anions as well as their transport behavior. , Consequently, iCOFs offer an expanded structural and chemical design space for developing composite solid polymer electrolytes with high ionic conductivity, high Li⁺ transference numbers, and stable interfaces with lithium–metal electrodes (Figure ). ,

1.

Functions of ionic COFs in solid-state polymer electrolytes.

Compared with other porous framework-based electrolyte platforms, iCOFs offer a distinctive combination of immobilized ionic sites, long-range structural order, and molecular-level chemical tunability. MOFs similarly possess well-defined pore architectures and coordinatively active metal centers that can promote lithium-salt dissociation and facilitate ion transport; however, their chemical and electrochemical stability is often governed by the nature of the metal–ligand coordination. Conventional porous organic polymers (POPs), by contrast, generally feature stable covalent networks and synthetic versatility, but often lack the long-range periodicity characteristic of COFs. The convergence of these structural and chemical attributes enables iCOFs to exert more precise control over Li⁺ coordination and transport. Nevertheless, challenges related to crystallinity control, processability, and solid–solid interfacial compatibility remain to be addressed.

Although several recent reviews have summarized COF-based solid-state electrolytes or single-ion-conducting COF electrolytes, , the specific role of ionic sites in regulating ion transport, polymer–filler interactions, and electrode/electrolyte interfacial stability has not yet been systematically discussed from the perspective of ionic COF structural design. In particular, the charge polarity, charge density, counterion exchangeability, and spatial distribution of ionic sites within COF frameworks can strongly influence lithium-salt dissociation, Li⁺ migration pathways, anion immobilization, and Li⁺ flux homogenization. These features distinguish iCOFs from neutral COFs and provide unique opportunities for designing high-performance solid-state polymer electrolytes. Therefore, a focused review that correlates the synthesis strategies, charged-framework structures, and electrolyte functions of iCOFs is timely and necessary. Different from previous reviews that mainly emphasize COFs as porous fillers or single-ion conductors, this review highlights the structure–function relationship of cationic, anionic, and zwitterionic COFs in solid-state electrolytes, with particular attention to their roles in enhancing ionic conductivity, lithium-ion transference number, mechanical robustness, and lithium–metal interfacial stability.

On the basis of this charge–structure–function perspective, this review first summarizes the common strategies for the synthesis of ionic covalent organic frameworks, which can be broadly categorized into direct synthesis and postsynthetic modification. We then discuss the recent applications of cationic, anionic, and zwitterionic COFs in solid-state electrolytes, emphasizing how different ionic sites regulate Li⁺ transport, anion migration, polymer compatibility, and electrode–electrolyte interfacial behavior. Finally, the remaining challenges and future opportunities for iCOF-based composite polymer solid-state electrolytes are discussed.

2. Synthesis of Ionic COFs

During COF synthesis, organic building blocks are organized into long-range-ordered crystalline porous networks through reversible covalent reactions. The thermodynamic reversibility of the synthetic reactions enables self-correction of the crystal structure. , Moreover, choosing different monomers allows for tunability of the topological structure and pore size of COF channels. For ionic COF synthesis, two primary methods are employed: direct synthesis and postmodification, which differ in the introduction of ionic sites. Figure illustrates the synthetic processes of these two methods along with representative monomers and reactions.

2.

Synthesis of ionic COFs. (a) Direct synthesis. (b) Post-modification strategy. (c) Representative monomers. (d) Representative reactions.

From the perspective of solid-state electrolyte design, the synthesis strategy for iCOFs is not only a matter of framework construction but also directly determines their ion-transport behavior and membrane-forming capability. The position, density, and accessibility of ionic sites influence lithium salt dissociation, Li⁺ coordination, anion immobilization, and the formation of continuous ion-conduction pathways. Meanwhile, the crystallinity, pore size, channel orientation, and counterion-exchange capability of iCOFs strongly affect whether their ordered channels can be effectively utilized in polymer electrolytes. In addition, synthetic methods that allow the preparation of nanosheets, oriented films, or freestanding membranes are particularly attractive because they may reduce random filler aggregation and improve the interfacial contact with polymer matrices. Therefore, the following discussion emphasizes not only how ionic COFs are synthesized but also how different synthetic strategies may influence their subsequent use in solid-state polymer electrolytes.

2.1. Direct Synthesis

In the direct synthesis method, monomers with ionic groups react with neutral monomers to fabricate iCOFs; the ionic group may be located either on the skeleton or on the side chain. , Common synthetic strategies for COFs currently include solvothermal synthesis, interfacial synthesis, microwave-assisted synthesis, and in situ growth. Among them, solvothermal synthesis is the most mature and widely used method. In this method, monomers react in a sealed vessel containing solvents at elevated temperature. For example, in 2021, Zhuang et al. reported the solvothermal synthesis of a guanidinium-based β-ketoenamine-linked covalent organic framework (Tp-DG_Cl) from 1,3-diaminoguanidine hydrochloride and 1,3,5-triformylphloroglucinol (Figure a). In 2023, Du et al. employed a similar reaction of triaminoguanidinium chloride with another monomer, 5,5′-(1,4-phenylene)­dipicolinaldehyde, to obtain an imine-linked cationic COF. In the same year, Cui et al. reported the synthesis of an olefin-linked cationic covalent organic framework (BDBI-TMT) using 5,6-bis­(4-formylbenzyl)-1,3-dimethyl-benzimidazolium bromide (BDBI) as the ionic building block (Figure b). In 2025, Zarei et al. synthesized a novel viologen-based covalent organic framework (VZ-COF) through the Zincke reaction between 4,4′,4″,4‴-(quinoxalino­[2′,3′:9,10]­phenanthro­[4,5-abc]­phenazine-6,7,15,16-etrayl)­tetraaniline (QPPTA) and 1,1′-bis­(2,4-dinitrophenyl)-4,4′-bipyridinium dichloride (Figure c).

3.

Synthesis of ionic COFs via the solvothermal method. (a) Tp-DG_Cl. Adapted with permission. Copyright 2021, Elsevier. (b) BDBI-TMT. Adapted with permission. Copyright 2023, Elsevier. (c) VZ-COF. Adapted with permission. Copyright 2025, Royal Society of Chemistry. (d) Synthesis of SJTU-COF-Br using microwave-assisted methods. Adapted with permission. Copyright 2020, Elsevier. (e) Synthesis of COFs using the interfacial synthesis method. Adapted with permission. Copyright 2022, Springer Nature.

Solvothermal synthesis often affords COFs with high crystallinity, but it is generally time-consuming; by contrast, microwave-assisted synthesis can yield crystalline products in a much shorter time. For example, Ding et al. synthesized SJTU-COF-Br by subjecting ethidium bromide and 1,3,5-triformylphloroglucinol to microwave heating at 100 °C for 3 h (Figure d). In addition, COF membranes can be prepared by methods such as interfacial synthesis and in situ growth, rather than as powder materials. Moreover, oriented COF membranes can convert random and discontinuous Li⁺ transport pathways into continuous and directionally aligned ion-conduction channels. In 2022, Wang et al. reported the fabrication of an ionic COF membrane via a dual-activation interfacial polymerization method, in which the ionic amine monomer was activated by a Brønsted base in the water phase and the aldehyde monomer was activated by a Brønsted acid in the organic phase (Figure e). In 2025, Liu et al. reported guanidinium-based covalent organic framework membranes prepared via an in situ growth strategy, in which the monomers slowly nucleated and grew on an indium tin oxide (ITO) substrate. Directly prepared freestanding COF films show great potential for applications in ion separation, gas separation, ion transport, and chemical sensing. −

In addition to the direct construction of ionic COFs from monomers that already contain ionic moieties, there is another distinctive scenario in which neither monomer bears an ionic group, yet ionic sites are generated at the linkage during framework formation. For example, in 2022, Ding et al. introduced a third neutral aldehyde monomer, TPA, into the reaction between 4,4′,4″,4‴-(pyrene-1,3,6,8-tetrayl)­tetraaniline (TAPPy) and squaric acid (SA). The incorporation of charge-neutral imine-linked segments could mitigate the influence of electrostatic interactions on the eclipsed stacking of the COF layers. Through modulation of the monomer ratio, a highly crystalline COF with exclusively zwitterionic linkages was successfully obtained. In 2025, Liu et al. reported that the reaction between squaric acid and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)­trianiline afforded a zwitterionic covalent organic framework STT COF (Figure ). Owing to the strongly electron-withdrawing and highly delocalized nature of the squaric four-membered ring, condensation accompanied by proton transfer led to the formation of zwitterionic linkages containing CNH⁺ and C–O^– moieties.

4.

Synthesis of STT COF from TAPT and SQ, and PTT COF from TAPT and PDA. Adapted with permission. Copyright 2025, Springer Nature.

For solid-state electrolyte applications, direct synthesis offers the advantage of introducing ionic sites into the framework during COF formation, which can ensure a relatively uniform distribution of charged centers within ordered channels. Such structural uniformity is beneficial for regulating Li⁺ migration and suppressing local concentration polarization. Moreover, direct synthesis methods that generate continuous COF membranes or oriented films are especially valuable because they may provide more efficient ion conduction pathways than randomly dispersed COF powders. However, the use of ionic monomers may also weaken crystallization because strong electrostatic interactions and bulky charged groups can interfere with reversible bond formation and ordered layer stacking. Therefore, when direct synthesis is used for electrolyte-oriented iCOF design, a balance should be achieved among the ionic-site density, crystallinity, pore accessibility, and membrane processability.

2.2. PostModification

Postmodification is a strategy in which a precursor COF is first constructed, followed by the introduction of an ionic site on the skeleton or on the side chains through subsequent reactions, including sulfonation, phosphorylation, quaternization, and ion exchange. , This approach can effectively maintain the crystallinity of COFs. For example, Zhai et al. prepared a neutral TP-PDA-COF from 1,3,5-triformylphloroglucinol (TP) and pyridine-2,5-diamine (PDA) and subsequently converted it into an ionic COF (C-TP-PDA-COF) using bromoethane as a postmodification reagent. In 2022, Yang et al. obtained N⁺-COF by ionizing the in situ ammonium groups in an interfacially polymerized TAPA-TFP COF membrane through the Menshutkin reaction, thereby introducing ionic sites into the COF skeleton (Figure a). In 2023, Liu et al. obtained a doubly modified amine-linked ionic COF through a two-step postsynthetic modification of a presynthesized parent COF. In this work, the postsynthetic modification strategy addressed the difficulty of directly synthesizing the target COF arising from electrostatic repulsion, thus enabling precise control over the COF structure.

5.

Postmodification synthesis of (a) N⁺-COF. Adapted with permission. Copyright 2022, American Chemical Society. (b) MEICOFs. Adapted with permission. Copyright 2025, John Wiley and Sons. (c) TAPT-COF-AB. Adapted with permission. Copyright 2024, Elsevier.

Similarly, ionic sites can in some cases be introduced by postsynthetic modification at the linkage positions of COFs. In 2023, Grunenberg et al. first reduced a preformed imine-linked COF to the secondary amine-linked framework rPI-3-COF using formic acid and then oxidized the secondary amine linkages in rPI-3-COF to nitrone linkages with m-chloroperoxybenzoic acid, ultimately affording the ionic COF NO-PI-3-COF. In 2025, Mu et al. reported that prepolymerization of 2,5-diaminobenzene-1,4-dithiol dihydrochloride with M-tetraanhydrides-2,3,9,10,16,17,23,24-octacarboxy-phthalocyanine (M = Cu, Co, or Fe) gave a thiazolium-linked COF precursor, which subsequently underwent self-ionization at 200 °C. During this process, the sulfhydryl and amide carbonyl groups underwent dehydration condensation to form cationic thiazolium rings, yielding the cationic covalent organic framework MEICOF (Figure b).

These studies focus on the introduction of ionic sites directly into the COF skeleton. By contrast, installation of ionic sites through side chains is also a common postsynthetic strategy. , Although the introduction of side chains may partially reduce the effective pore aperture or accessible pore volume, it is generally more favorable for preserving the original COF framework structure. , In 2022, Yu et al. synthesized a neutral covalent organic framework (COF-DT) from 2,5-dimethoxyterephthalaldehyde and 1,3,5-tris­(4-aminophenyl)­benzene. Subsequent postsynthetic reaction with 1,3-propanesultone immobilized alkyl chains bearing terminal sulfonic acid groups onto the COF skeleton. Finally, ion exchange with sodium hydroxide afforded the anionic COF, COF-DT-SO₃Na. In 2024, Qi et al. first prepared TAPT-COF-OH by a solvothermal method, followed by reaction of the hydroxyl groups on the framework with (2-bromomethyl)­trimethylammonium bromide to afford a cationic COF, TAPT-COF-AB (Figure c).

Compared with direct synthesis, postsynthetic modification provides greater flexibility for electrolyte-oriented structural regulation. A highly crystalline neutral COF can first be constructed, followed by the introduction of ionic groups through reactions, such as quaternization, sulfonation, phosphorylation, or ion exchange. This strategy is beneficial for preserving the framework order while tuning the charge polarity and counterion chemistry of the COF. In particular, postmodification allows the same parent COF framework to be converted into cationic, anionic, or zwitterionic derivatives, thereby providing a useful platform for comparing different ion-transport mechanisms. Nevertheless, excessive postmodification may partially block the pores, reduce accessible channel volume, or introduce structural defects, which can negatively affect Li⁺ transport. Therefore, the key challenge is to achieve sufficient ionic functionality while maintaining open, continuous, and polymer-compatible channels. These synthesis-dependent structural features directly determine the performance of iCOFs in solid-state electrolytes, as discussed below.

3. Applications of Ionic COFs in Solid-State Electrolytes

Owing to their structural stability, tunable topology, and easily functionalized pore walls, COFs have demonstrated great potential in practical applications. , This review mainly focuses on the recent advances in the application of COFs in polymer solid electrolytes. The introduction of COFs can not only disrupt the regular packing of polymer chains and suppress their crystallization but also provide fast transport pathways for lithium ions through their well-defined pore channels. , Moreover, COFs can enhance the mechanical strength of polymer electrolytes, while their ordered channels facilitate the uniform deposition of lithium ions at the anode. The incorporation of ionic sites into COFs can further promote lithium-salt dissociation and improve ionic conductivity. , In practical applications, different ionic COFs exhibit distinct mechanistic features.

It should be noted that the electrochemical performance of iCOF-based solid-state electrolytes cannot be evaluated solely by comparing the reported ionic conductivities or lithium-ion transference numbers. These values are strongly affected by electrolyte type, polymer matrix, lithium salt, testing temperature, membrane thickness, filler loading, and cell configuration. For example, room-temperature conductivity values obtained from dry composite polymer electrolytes are not directly comparable to those measured at elevated temperatures or in quasi-solid-state systems containing ionic liquids. Similarly, a high Li⁺ transference number does not necessarily guarantee stable battery operation, unless it is accompanied by sufficient total ionic conductivity, good mechanical integrity, and stable electrode/electrolyte interfaces. Therefore, in the following discussion, the performance of cationic, anionic, and zwitterionic COFs is evaluated not only in terms of electrochemical parameters but also from the perspectives of ion-transport mechanisms, structural continuity of COF channels, polymer compatibility, and practical membrane processability.

As summarized in Table , different iCOF systems should not be evaluated solely by their reported conductivity or Li⁺ transference number, because these values are closely related to the electrolyte architecture, testing temperature, polymer matrix, and cell configuration. Therefore, a meaningful comparison requires simultaneous consideration of the ion-transport mechanism, membrane structure, interfacial compatibility, and practical operating conditions.

1. Representative Ionic COF-Based Solid-State Electrolytes and Their Structure–Performance Relationships.

iCOF Type	Representative system	Main ion-regulation mechanism	σLi+ (S/cm)	t Li+	Battery/cell performance	Refs
Cationic COFs	Im-COF-TFSI/LiTFSI	Cationic framework sites immobilize TFSI– and promote Li-salt dissociation	4.64 × 10–4 S cm–1 (80 °C)	0.62	The Li/Im-COF-TFSI@Li/LiFePO4 all-solid-state Li-ion battery maintains 91.6% of its capacity after 100 cycles at 353 K.
	COF@PDV/LiTFSI	The cationic COFs inhibit the mobility of anions and promote the mobility of lithium ions.	1.3 × 10–4 S cm–1 (25 °C)	0.62	The LiCoO2|C-SPE|Li cell displays initial capacity of 160.3 mAh g–1 (0.1C) and the capacity retention reaches 83.9% at 1C after 200 cycles.
	LITFSI/EO-BIm-iCOF	Rigid COF channels provide structural order, while EO chains assist Li+ coordination and migration	1.08 × 10–4 S cm–1 at room temperature	0.69	The SSLMBs operates stably under both 60 °C and –10 °C conditions.
	EB-COF-TFSI/PEO/LiTFSI	Cationic COF as a fast Li+ pathway in PEO matrix accelerates Li+ conduction and helps the Li salt dissociation	2.19 × 10–4 S cm–1 (50 °C)	0.316	The LFP/1%EB-COF/C cell showed little capacity loss at an increased current density
	HC–COF/PEO/LiTFSI	Guanidinium sites anchor TFSI– through hydrogen bonding and weaken Li+-TFSI– ion pairing	6.15 × 10–4 S cm–1 (30 °C)	0.786	Li symmetric cell cycled for 450 h with an overpotential of 90 mV
	Dha-COFim-IL	Ordered channels and imidazolium sites regulate ion transport and promote LiF/Li3 N-rich SEI formation	1.74 × 10–3 S cm–1 (25 °C)	0.72	Improved Li metal battery performance
Anionic COFs	ETPTA-COF-S/LiFSI	SO3 – groups coordinate Li+, suppress bulky anion migration, and homogenize Li+ flux	1.29 mS cm–1 (20 °C)	0.83	Li||ETPTA-COF-S||LFP remains stable for 150 cycles (∼95.2% capacity retention) with the initial capacity of 1.96 mAh cm–2.
	TpPa-SO3Li/PEO/LiTFSI	Sulfonated COF acts as a Li+-conducting pump and promotes Li+ migration	6.28 × 10–4 S cm–1 (60 °C)	0.78	Solid-state Li–S battery stably cycled for 600 cycles at 60 °C with a high Coulombic efficiency of approximately 98%
	Li-COF@P	Fixed anionic COF framework immobilizes anions, confined PEGDA weakens Li+–anion association	8.9 × 10–5 S cm–1 at room temperature	0.95	88.3% capacity retention after 2000 cycles in all-solid-state Li organic batteries
	TpBD-(SO3Li)2	Near absence of mobile anions enables single-Li+ conduction through ordered COF channels	4.0 × 10–5 S cm–1 (30 °C)	0.92	/
Zwitterionic COFs	Zwitt-COF/PVDF/LiTFSI	Promote the effective dissociation of strong lithium ions in electrolytes and accelerate the transport of dissociated Li+	1.65 × 10–4 S cm–1 at room temperature	0.31	Maintains 99% discharge capacity and 98% Coulombic efficiency in all-solid-state lithium metal batteries
	PEO-10%HUT4/LiTFSI	Zwitterionic sites regulate local electric field and facilitate Li+ transport	5.3 × 10–4 S cm–1 (90 °C)	0.62	Li–S battery retained 498 mAh g–1 after 500 cycles at 1C
	LLZTO@HUT4/PEO/LiTFSI	HUT4 coating improves LLZTO/polymer compatibility and promotes Li-salt dissociation	7.33 × 10–4 S cm–1 (60 °C)	0.74	Li–S battery retained about 92.1% capacity after 100 cycles
	QZwiCOF/PVDF-HFP/LiTFSI	Dual-polarization effect and amphiphilic nanochannels promote aligned Li+ transport	3.44 × 10–4 S cm–1 (30 °C)	0.56	After 200 charge–discharge cycles at 3.0C, the battery maintained a specific capacity of 108 mAh g–1 with a capacity retention rate as high as 87.8%.

3.1. Cationic COFs

Cationic COFs can immobilize the anions from lithium salts by coordinating the cationic sites on the framework with salt anions, such as TFSI^– in LiTFSI. ,, This helps promote salt dissociation and increase the concentration of free Li⁺, thereby improving both the lithium-ion transference number and the ionic conductivity. For example, in 2020, Li et al. constructed a cationic covalent organic framework (Im-COF-Br) via a Schiff-base reaction between an imidazolium-based monomer (TIBBr) and benzidine (BZ), and then obtained Im-COF-TFSI by replacing Br^– with TFSI^– through ion exchange. Compared with the corresponding neutral COF, this cationic COF significantly enhanced the lithium-ion conductivity (2.92 × 10^–5 S cm^–1 at 303 K). In 2024, Song et al. first constructed a rigid BIm-COF through the condensation of 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)­tetraaniline (ETTA) and 4,4′-(1-methyl-1H-benzo­[d]­imidazole-4,7-diyl)­dibenzaldehyde (BImDB). Subsequently, a one-step postsynthetic modification was employed to simultaneously introduce flexible oligo­(ethylene oxide) (EO) chains and convert the originally neutral benzimidazole sites into benzimidazolium moieties (Figure a). The cationic sites within the framework were able to trap TFSI^– anions, while the flexible side chains facilitated the coordination and transport of lithium ions, thereby further promoting Li⁺ migration. Owing to this rigid–flexible synergistic effect, the resulting solid electrolyte delivered a high lithium-ion conductivity of 1.08 × 10^–4 S cm^–1 at room temperature (Figure b), together with an improved t _Li+ of 0.69.

6.

Structure and electrochemical properties of cationic COF solid-state electrolytes. (a–b) Structures and impedance plots of BIm-COF and its postmodified derivative EO-BIm-iCOF. Adapted with permission. Copyright 2024, Royal Society of Chemistry. (c–e) Schematic illustration of HC–COF synthesis, binding energies of pure PEO and HC–COF@PEO for anchoring TFSI^– and adsorbing Li⁺, time-voltage profile of the HC–COF@PEO symmetric cell at 0.1 mA cm^–2. Adapted with permission. Copyright 2025, Elsevier. (f–h) Schematic illustration of the bottom-up strategy for synthesizing Dha-COF and Dha-COF_im, Arrhenius plots of Dha-COF-IL and Dha-COF_im-IL, current–time curve of the Li∥Dha-COF_im-IL∥Li cell at 25 °C. Adapted with permission. Copyright 2025, Elsevier.

Meanwhile, incorporating cationic COFs into polymer matrices can also homogenize the interfacial space-charge distribution and mitigate local polarization, thereby promoting uniform Li deposition, improving interfacial stability, and suppressing dendrite growth. In 2023, Chen et al. demonstrated that the addition of only 1 wt % EB-COF-TFSI to a PEO electrolyte increased the Li⁺ transference number from 0.115 to 0.316. In addition, the cycling lifetime of the Li-symmetric cell was extended to approximately three times that of the pristine PEO electrolyte, while lithium dendrite formation on the Li metal surface was markedly reduced. In 2025, Liu et al. interfacially synthesized HC–COF from 2,6-diformylpyridine (DFP) and 1,2,3-triaminoguanidinium hydrochloride (TGH·Cl), and subsequently fabricated an electrolyte membrane by solution casting a composite of the COF powder with PEO and LiTFSI (Figure c). The guanidinium units in the COF immobilized anions through hydrogen-bonding interactions. A comparison of the binding energies for TFSI^– anchoring and Li⁺ adsorption between pristine PEO and HC–COF@PEO further demonstrated that HC–COF@PEO could weaken Li⁺–TFSI^– ion pairing and increase the Li⁺ transference number (Figure d). In symmetric cells, this composite electrolyte enabled uniform Li deposition/stripping for up to 450 h at a low overpotential of 90 mV (Figure e). In full-cell tests, it also achieved stable long-term cycling, exhibiting excellent interfacial properties.

In recent years, COFs have also been combined with ionic liquids to construct quasi-solid-state electrolytes with high ionic conductivity, further demonstrating the role of their ordered channels and functional sites in regulating ion transport and interfacial stability. In 2025, Tan et al. first synthesized a neutral Dha-COF, followed by the introduction of functionalized imidazolium groups and subsequent ion exchange to obtain Dha-COFim (Figure f). These two COFs were then separately combined with ionic liquids. Among these, Dha-COFim-IL delivered an excellent ionic conductivity of 1.74 × 10^–3 S cm^–1 and a high Li⁺ transference number of 0.72 (Figure g–h). Moreover, a stable SEI layer enriched with LiF and Li₃N was formed at the lithium metal/electrolyte interface, which facilitated Li⁺ transport, promoted uniform lithium deposition, and improved the interfacial performance.

Overall, cationic COFs are particularly effective in immobilizing salt anions and promoting lithium-salt dissociation, which explains the frequently observed increase in both the ionic conductivity and Li⁺ transference number. However, their performance advantages depend strongly on whether the cationic sites are sufficiently accessible within the COF channels and whether continuous ion-transport pathways can be maintained after incorporation into the polymer matrix. Excessive cationic site density may also strengthen electrostatic interactions with anions and reduce framework crystallinity or pore regularity, thereby limiting long-range ion transport. In addition, many reported cationic COF electrolytes still rely on PEO-based matrices or ionic-liquid-containing quasi-solid systems, meaning that their apparent high conductivity should be interpreted together with the contribution of polymer segmental motion or liquid-like ion mobility. Therefore, future evaluation of cationic COFs should place greater emphasis on separating the intrinsic contribution of the charged COF framework from that of the polymer or ionic liquid phase.

3.2. Anionic COFs

In polymer solid electrolytes containing anionic COFs, if no additional lithium salt is introduced, the system contains no mobile negative charge carriers, and ion transport relies primarily on Li⁺, resulting in a state close to that of a single-ion conductor. , If extra lithium salt is added, the anionic COF pores can attract Li⁺ while repelling anions such as TFSI^–, which likewise facilitates lithium-salt dissociation and enhances the lithium-ion transference number.

In 2024, Zhuang et al. constructed a COF framework through the reaction of 2,5-diamino-1,4-benzenedisulfonic acid with 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde, followed by postsynthetic modification to introduce branched sulfonate chains, yielding COF-S. COF-S was then mixed with ethoxylated trimethylolpropane triacrylate (ETPTA) and LiFSI, and the ETPTA-COF-S composite gel electrolyte membrane was obtained via in situ UV-induced curing (Figure a). In this system, the abundant SO₃ ^– groups serve as Li⁺-coordinating transport sites, while the branched chains suppress the migration of bulky anions and homogenize the Li⁺ flux. As a result, although this electrolyte is not a single-ion conductor, it still delivered a high Li⁺ transference number of 0.83 and a high ionic conductivity of 1.29 mS cm^–1, together with excellent cycling stability (Figure b).

7.

Structure and electrochemical properties of anionic COF solid-state electrolytes. (a–b) Schematic illustration of anionic COF with tailored ion channels, Arrhenius plot of ETPTA-COF-S. Adapted with permission. Copyright 2024, Springer Nature. (c–e) Chemical structure of Li-COF@P_X% and conceptual design of their pore functionalization, Li⁺ transport mechanism through the PEGDA-embedded 1D channels in Li-COF@P_X%, electrochemical impedance spectroscopy (EIS) profiles of Li-COF-2@P_75%. Adapted with permission. Copyright 2024, Springer Nature. (f–h) Structure of TpBD-(SO₃Li)₂, Nyquist plots of EIS measurements using blocking electrodes, time-dependent response of potentiostatic DC polarization for the Li|TpBD-(SO₃Li)₂|Li symmetric cell. Adapted with permission. Copyright 2025, American Chemical Society.

In 2025, Zhang et al. synthesized TpPa-SO₃H-COF via a solvothermal method and subsequently dispersed it in an aqueous Li₂CO₃ solution for ion exchange, obtaining a lithium-sulfonated COF, TpPa-SO₃Li. This material was employed as a filler in PEO-based solid-state electrolytes for lithium–sulfur batteries. The PEO composite electrolyte containing 5 wt % TpPa-SO₃Li exhibited an ionic conductivity of 6.28 × 10^–4 S cm^–1 at 60 °C and an excellent Li⁺ transference number of 0.78. The resulting solid-state Li–S battery operated stably for 600 cycles at 60 °C, with a high Coulombic efficiency of approximately 98%.

Notably, beyond the incorporation of anionic COFs as functional fillers into polymer matrices, recent studies have also explored alternative architectures in which the ordered channels of ionic COFs serve as the primary ion-transport framework, while polymeric components are confined within the COF pores to simultaneously regulate the local ion-coordination environment and improve membrane processability. For instance, in 2024, Li et al. reported a solvent-free COF-based single-ion conductor, denoted as Li-COF@P. In this work, PEGDA monomers were introduced into the channels of the anionic Li-COF through vacuum-assisted infiltration, followed by UV-induced cross-linking and thermal treatment to obtain a self-standing Li-COF@P membrane containing pore-confined PEGDA (Figure c). The immobilized anionic sites in the COF framework ensured single-ion conduction, whereas the oxygen atoms in the confined PEGDA interacted with Li⁺ through weak ion–dipole interactions (Figure d). This interaction weakened the strong Li⁺–anion association typically present in conventional anionic COF single-ion conductors, thereby promoting Li⁺ dissociation and migration along the ordered COF channels. As a result, Li-COF-2@P_75% exhibited a Li⁺ conductivity of 8.9 × 10^–5 S cm^–1 and a high Li⁺ transference number of 0.95 at room temperature (Figure e). Moreover, Li-COF@P achieved 88.3% capacity retention after 2000 cycles in all-solid-state Li-organic batteries.

The above studies demonstrate that integrating anionic COFs with polymer components represents an effective strategy for enhancing the performance of solid-state electrolytes. Beyond this composite approach, recent efforts have also focused on the intrinsic ion-transport capability of anionic COFs, in which pure COF membranes are constructed by interfacial synthesis or in situ polymerization and directly used as electrolyte membranes. This strategy allows the advantages of COFs, such as ordered continuous channels and an anion-rich framework, to be more fully exploited. For example, in 2025, Kato et al. first prepared a TpBD-(SO₃H)₂ membrane via in situ polymerization using a previously reported self-catalyzed solution-casting method. Subsequently, TpBD-(SO₃H)₂ was reacted with a LiOAc solution, and the resulting membrane was immersed in 1 M LiOH solution for less than 5 min, followed by washing and drying to afford the TpBD-(SO₃Li)₂ membrane (Figure f). This membrane delivered a high Li⁺ conductivity of 4.0 × 10^–5 S cm^–1 at 30 °C (Figure g). Moreover, owing to the near absence of mobile anions in the system, the TpBD-(SO₃Li)₂ membrane exhibited an ultrahigh Li⁺ transference number of 0.92 (Figure h).

Compared with cationic COFs, anionic COFs are more closely related to single-ion-conducting electrolyte design because the negative sites are fixed to the framework, while Li⁺ serves as the dominant mobile carrier. This feature is beneficial for suppressing concentration polarization and homogenizing the Li⁺ flux. Nevertheless, anionic COFs also face a fundamental trade-off between a high Li⁺ transference number and sufficient ionic conductivity. Strong coordination between fixed anionic sites and Li⁺ can effectively restrict anion migration, but it may also slow Li⁺ hopping if the binding strength is too high or if the ion transport channels are discontinuous. Therefore, the design of anionic COFs should balance the charge density, Li⁺ binding strength, pore size, and channel connectivity. In addition, while pure anionic COF membranes provide a clearer platform for studying intrinsic single-ion conduction, their mechanical flexibility, large-area fabrication, and interfacial contact with electrodes still require further improvement before practical battery applications.

3.3. Zwitterionic COFs

In recent years, a few studies have begun to explore the application of zwitterionic COFs in solid-state electrolytes. Unlike anionic or cationic COFs, which regulate ion transport mainly through a single type of charged site, zwitterionic COFs contain both positively and negatively charged centers within the same framework. Such a structural feature enables the construction of a more balanced local electric field and allows for more comprehensive and synergistic regulation of ion-transport behavior. As a result, zwitterionic COFs show promising potential for promoting Li⁺ transport, restricting anion migration, and stabilizing the electrolyte/electrode interface.

In 2022, Wang et al. synthesized a zwitterionic COF (HUT4) from melamine and squaric acid, and subsequently dispersed it in PEO to obtain a PEO-HUT4 electrolyte. Among these electrolytes, PEO-10%HUT4 exhibited a Li⁺ transference number of up to 0.62 at 60 °C. Moreover, the Li–S battery assembled with this electrolyte retained a discharge capacity of 498 mAh g^–1 after 500 charge–discharge cycles at 1C. In 2023, Wang et al. further employed the zwitterionic COF HUT4 in a different design, in which HUT4 was synthesized in situ on the surface of LLZTO nanoparticles, followed by incorporation of the COF-modified LLZTO nanoparticles into a PEO electrolyte (Figure a). By converting the lithiophobic LiOH/Li₂CO₃ layer on the LLZTO surface into an HUT4 coating, lithium salt dissociation and Li⁺ transport were effectively promoted. Meanwhile, the COF layer coated on the LLZTO surface improved the interfacial compatibility between the inorganic filler and the organic polymer matrix. Consequently, the modified composite electrolyte, LLZTO@HUT4–15%/PEO, delivered a high ionic conductivity of 0.73 mS cm^–1 and a high Li⁺ transference number of 0.74 (Figure b). In addition, the corresponding Li–S battery maintained a capacity retention of approximately 92.1% after 100 cycles, indicating effective suppression of the polysulfide shuttle effect (Figure c).

8.

Structure and electrochemical properties of zwitterionic COF solid-state electrolytes. (a–c) The synthetic procedure and mechanism of LLZTO@HUT4 in a polymer composite electrolyte, Arrhenius plots of the ionic conductivity of PEO, LLZTO-15%/PEO, and LLZTO@HUT4/PEO PCEs, cycling performance of S@CNT|PCEs|Li at 0.2C. Adapted with permission. Copyright 2023, Royal Society of Chemistry. (d) Cycling performance of the Li∥NCM811 cells at 6.0C, 80 °C. Adapted with permission. Copyright 2025, Royal Society of Chemistry. (e–f) Synthetic reaction scheme and structure of Z-COF, galvanostatic cycling of Li|Li symmetric cells at 2 mA cm^–2/2 mAh cm^–2. Adapted with permission. Copyright 2026, Springer Nature.

In 2025, Xu et al. synthesized an extended π-conjugated zwitterionic COF (QZwiCOF) via a one-pot strategy using 2,4,6-tris­(4-aminophenyl)-1,3,5-triazine (TATT), 1,3,5-tris­(4-formylphenyl)-1,3,5-triazine (TATB), and a vinylimidazole-based zwitterionic ionic liquid (VZwILs). Compared with the anionic and cationic COFs with the same framework synthesized in the same series, the zwitterionic framework of QZwiCOF generated a dual-polarization effect, which enhanced the dipole moment and structural regularity of PVDF-HFP. In addition, its amphiphilic nanochannels enabled selective Li⁺ transport. Owing to this synergistic effect, the QZwiCOF-modified composite electrolyte achieved a high ionic conductivity of 3.44 × 10^–4 S cm^–1, and the corresponding cell still delivered a capacity of 96 mAh g^–1 after 200 cycles at 80 °C and 6.0C (Figure d).

Existing studies have mainly focused on the role of zwitterionic COFs in regulating ion transport within solid-state electrolytes. However, their unique sites bearing opposite charges, designable pore structures, and tunable interfacial chemistry also endow them with considerable potential for interfacial engineering. In particular, on the lithium metal anode side, nonuniform Li⁺ flux, interfacial side reactions, and dendrite growth remain critical factors that severely limit cycling stability. Therefore, recent efforts have extended the application of zwitterionic COFs to the construction of artificial protective layers on lithium metal surfaces. In 2026, Rong et al. developed a zwitterionic COF (Z-COF) incorporating dual functional groups of sulfonate and ethidium, and employed it as an artificial protective layer for lithium metal anodes (Figure e). In this design, the cationic sites served to capture anions and promote lithium salt dissociation, while the anionic sites acted as Li⁺ binding and transport sites, facilitating rapid Li⁺ migration along the COF channels. Electrochemical results showed that the Z-COF protective layer enabled Li|Li symmetric cells to achieve ultralong stable cycling for over 6300 h at 2 mA cm^–2/2 mAh cm^–2, demonstrating that Z-COF effectively promoted uniform lithium deposition and improved interfacial stability (Figure f).

Zwitterionic COFs provide a more integrated ion-regulation platform than purely cationic or anionic COFs because their positively and negatively charged sites can simultaneously participate in anion immobilization, Li⁺ coordination, and interfacial electric-field regulation. This dual-charge feature is particularly attractive for constructing directional Li⁺ transport pathways and stabilizing lithium–metal interfaces. However, research on zwitterionic COF electrolytes remains at an early stage, and several key issues still need clarification. First, the relative contribution of cationic and anionic sites to Li⁺ transport is often difficult to distinguish experimentally. Second, the spatial arrangement of the two charged centers may strongly influence local ion-pair dissociation, but this structure–function relationship has not been systematically established. Third, many reported zwitterionic systems are evaluated under different temperatures, cathode chemistries, and current densities, making direct performance comparison difficult. Therefore, more standardized electrochemical testing, combined with theoretical simulations and in situ characterization, is needed to reveal the intrinsic advantages of zwitterionic COFs in solid-state electrolytes.

4. Conclusions

Overall, ionic COF-based composite polymer solid electrolytes have attracted an increasing amount of attention in recent years. The incorporation of ionic COFs not only enhances the mechanical properties of polymer solid electrolytes but, more importantly, the presence of ionic sites enables the COF channels to achieve selective and rapid Li⁺ transport, homogenize Li⁺ flux, and mitigate concentration polarization, thereby substantially improving the ionic conductivity, lithium-ion transference number, and interfacial properties of the electrolyte membranes. However, constructing continuous ion-transport pathways in iCOFs and fabricating homogeneous electrolyte membranes remain major challenges for the processing and practical application of iCOF/polymer composite solid electrolytes. These challenges are mainly manifested in the following aspects: (1) compared with neutral COFs, ionic COFs often exhibit lower crystallinity. On the one hand, the electron-withdrawing effect and electrostatic repulsion introduced by ionic sites negatively affect the reversible error-correction crystallization process of COFs. On the other hand, the presence of charged centers interferes with the interlayer arrangement, making it difficult to achieve well-ordered layer stacking. As a result, ionic COFs often suffer from insufficient long-range order, which compromises the continuity of their ion-transport channels. (2) Although the orientation of COF channels in electrolytes has received increasing attention in recent years, it has still not been adequately resolved. Ideally, ions should migrate rapidly through the continuous COF channels. However, when used as polymer fillers, COFs are typically present in the form of polycrystalline powders that are randomly dispersed within the polymer matrix, making it difficult to form continuous channels with uniform direction. Consequently, ion transport often falls short of expected performance. (3) COF powders are also prone to local aggregation within the polymer matrix, which disrupts the continuity and homogeneity of the electrolyte membrane and limits its performance and practical applicability. Because COFs generally possess poor dispersibility, they are often difficult to disperse uniformly and stably in common solvents. In addition, the interfacial compatibility between COFs and polymers is often insufficient, so conventional physical blending methods tend to result in local agglomeration and nonuniform dispersion during membrane fabrication. Such spatial heterogeneity not only deteriorates the mechanical properties of the membrane at the macroscopic level but also prevents the formation of a continuous ion-transport network at the microscopic level, thereby reducing the overall ionic transport capability of the electrolyte membrane and weakening interfacial contact.

To address these limitations, future studies should place greater emphasis on the rational coupling of charged-framework design with electrolyte membrane engineering. First, the ionic-site density, charge polarity, counterion chemistry, and Li⁺ binding strength of iCOFs should be systematically optimized to balance salt dissociation, anion immobilization, and Li⁺ mobility. Excessively strong ion–framework interactions may increase the lithium-ion transference number but reduce the overall ionic conductivity, whereas insufficient interaction may fail to suppress concentration polarization. Second, strategies for constructing continuous and oriented ion-transport pathways should be further developed. Compared with randomly dispersed COF powders, oriented COF nanosheets, vertically aligned channels, in situ-grown COF networks, and freestanding COF membranes may provide more efficient pathways for directional Li⁺ migration. Third, improving the compatibility between iCOFs and polymer matrices is essential for obtaining homogeneous, mechanically robust, and scalable electrolyte membranes. Surface engineering, covalent coupling, polymer-grafted COFs, and in situ polymerization within COF channels may be effective approaches to reduce filler aggregation and enhance interfacial adhesion.

In addition to material design, more standardized and practical electrochemical evaluations are required. Because iCOF-based electrolytes differ greatly in polymer matrix, lithium salt, filler content, membrane thickness, testing temperature, and cell configuration, a direct comparison of reported ionic conductivity or Li⁺ transference number can be misleading. Future reports should provide complete testing conditions and evaluate electrolytes under more device-relevant conditions, including room-temperature operation, high-voltage cathode compatibility, long-term lithium plating/stripping stability, mechanical strength, thermal stability, and cycling performance at practical areal capacities and current densities. In addition to conventional characterization techniques, such as powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, and electrochemical testing, advanced characterization techniques, such as in situ spectroscopy, solid-state NMR, cryo-TEM, neutron scattering, and computational simulations, are required to elucidate Li⁺ coordination environments, ion-hopping pathways, interfacial evolution, and the actual contribution of ionic COF channels to overall ion transport. These techniques are particularly important for gaining deeper insights into the ion-transport mechanisms of ionic COF-based composite polymer electrolytes. In situ spectroscopy and solid-state NMR can reveal Li⁺ coordination behavior and dynamic exchange among the COF, polymer matrix, and their interfaces, whereas cryo-TEM and neutron scattering can probe interfacial structures, confined polymer dynamics, and nanoscale transport behavior. When combined with computational simulations, these approaches can further clarify Li⁺ hopping mechanisms and, importantly, distinguish the respective contributions of the COF channels, the polymer matrix, and interfacial regions to the overall ionic conductivity.

Despite these challenges, iCOF/polymer composite solid-state electrolytes remain a highly promising platform for next-generation solid-state lithium batteries. Their ordered nanoscale channels, tunable charged frameworks, and designable polymer–filler interfaces provide unique opportunities to simultaneously regulate ion transport, mechanical reinforcement, and electrode/electrolyte interfacial stability. Looking forward, the development of high-performance iCOF-based electrolytes will rely on the integration of precise ionic-site design, oriented channel construction, scalable membrane fabrication, and standardized electrochemical assessment. With continued progress in these directions, iCOFs are expected to evolve from functional electrolyte fillers into structurally integrated ion-transport frameworks, providing important support for safer, higher-energy-density, and more durable solid-state batteries.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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S.C. and Y.L. contributed equally to this work. S.C.: Writingreview and editing, Supervision, Resources. Y.L.: Writingoriginal draft, Validation, Investigation. Y.L.: Supervision, Resources, Project administration, Conceptualization.

The authors declare no competing financial interest.

Published as part of ACS Omega special issue “Microporous Organic Frameworks”.