2.1 Topological structure of COFs

The architectural diversity of COFs is dictated by the geometric symmetry of building blocks, primarily manifesting as two-dimensional (2D) or three-dimensional (3D) frameworks (Figure 1). For 2D COFs, common topological designs are primarily based on combinations such as C₂ + C₂, C₂ + C₃, C₃ + C₃, and C₂ + C₄ symmetries. Extending the polymeric framework into a 3D network to construct 3D COFs generally requires the use of building blocks with orthogonal geometries, such as T₄ or C₂ units. Thus, by strategically selecting molecular components with specific geometric shapes and connectivity, it is feasible to design porous materials that possess both structural and functional diversity. 2D COF structures typically display hexagonal, rhombic, square, and triangular [37]. In contrast, the development of 3D COFs remains comparatively limited, primarily due to challenges in synthesis and the scarcity of suitable molecular building blocks capable of forming 3D networks.

Figure 1 2D and 3D topologies of COFs

2.2 Linkage of COFs

The structural integrity and functional properties of COFs are fundamentally governed by their linkage chemistry [38]. The early connection methods mainly include reversible borate esters and imines, but these connection methods have poor chemical stability. This spurred the development of irreversible linkages, including covalent triazine frameworks (CTFs), keto-enamines (via imine tautomerization), and carbon-carbon bonds. Particularly, reactions like Knoevenagel condensation or Aldol condensation form sp² carbon-carbon bonds, resulting in COFs with exceptional stability against harsh chemical environments. More recently, the highly polar and stable linkage bonds, such as amides, have attracted much attention for their potential applications (such as ion transport). Consequently, the strategic choice of monomers and links is essential to the rational design of COFs since it allows for accurate modification of the framework's symmetry, topology, and porosity to satisfy particular application requirements.

2.3 Synthesis methods of COFs

The selection of a synthetic pathway is dictated by the targeted linkage, the desired morphology (e.g., powders and thin films), and the specific application requirements, driving continuous innovation in synthetic methodologies. Currently, the primary synthetic methodologies for COFs can be categorized into solvothermal, ionothermal, interfacial polymerization, mechanochemical, and microwave-assisted approaches [39–41] (Table 1).

Solvothermal synthesis stands as a pioneering methodology in the field of COFs [33]. This technique is typically conducted in a sealed vessel at temperatures above the normal boiling point of the solvent, generating autogenous pressure. Its key advantage lies in the thermal energy provided, which facilitates the reversible reactions characteristic of dynamic covalent chemistry. Seminal examples, such as the boroxine-linked COF-1 and the imine-linked COF-300, were successfully synthesized via this route. However, the method is also associated with notable limitations, including prolonged reaction times (typically 3–7 days), high energy consumption, and potential safety hazards arising from the high-pressure conditions.

Ionothermal synthesis is primarily employed for the fabrication of CTFs [42]. This method utilizes molten inorganic salts, such as zinc chloride, which function simultaneously as both solvent and catalyst. The Lewis acidity of ZnCl₂ efficiently catalyzes the cyclotrimerization of nitrile-based monomers, leading to the formation of stable triazine linkages. A notable advantage of the ionothermal approach is its capability to produce CTF materials with exceptionally high thermal and chemical stability. However, this method also presents significant drawbacks, including extremely high reaction temperatures (typically exceeding 400°C), a strongly acidic reaction medium that may degrade acid-labile functional groups, and considerable difficulty in the complete removal of metal salt residues post-synthesis.

Interfacial polymerization has emerged as a pivotal strategy to address the challenge of processing COF into continuous thin films [43]. This approach confines the polymerization reaction at the interface between two immiscible phases. Typically, one contains an amine-based monomer and the other an aldehyde-based monomer. By restricting the reaction to a narrow liquid–liquid or liquid–gas interfacial region, it enables the bottom-up growth of continuous, large-area, and thickness-tunable COF films. Nevertheless, challenges remain in precisely controlling interfacial stability and reaction kinetics, as well as in ensuring film uniformity during scale-up.

Mechanochemical synthesis significantly reduces reaction time and eliminates solvent waste generation [44]. The microwave-assisted synthesis leverages efficient, volumetric microwave heating to dramatically accelerate COF nucleation and crystal growth. This method serves as a highly effective approach for rapid parameter optimization and powder production, although its scalability remains a challenge.

3.1 COF-based separators

To design a separator capable of sustaining stable cycling in challenging conditions for LMBs, it is crucial to ensure superior ion conduction for efficient Li deposition/stripping and the ability to foster a stable electrode-electrolyte interface that can protect the electrodes under extreme operational circumstances. As a key component in direct contact with electrodes, the separator's properties are decisive for interfacial stability in LMBs. Consequently, functional modification of the separator has evolved from merely fulfilling the basic requirement of physical isolation into an effective strategy for actively regulating interfacial chemistry and guiding uniform lithium deposition. By designing functional separators to induce a stable electrode-electrolyte interface, it is possible to simultaneously optimize Li-ion transport kinetics and deposition homogeneity, thereby enhancing overall battery performance. Nevertheless, the precise design and selection of modification materials capable of achieving a highly stable interface remains a core challenge in this field.

The COFs possess significant structural tunability, enabling the rational design of frameworks with tailored physical and chemical properties to satisfy specific application demands by strategies such as monomer selection and post-synthetic ionic exchange. On this basis, Hu et al. fabricated a SnF₂ and NO₃⁻-rich composite separator (PCS) by coating a polypropylene (PP) substrate with a functional EB-COF:NO₃@SnF₂ material, aiming to establish a stable SEI for effective dendrite suppression [12]. First, the EB-COF:Br@SnF₂ precursor was prepared using ethidium bromide (EB), 1,3,5-triformylphloroglucinol (TFP), and nano SnF₂. Then the obtained precursor was subjected to ion exchange by dispersing it in 15 mL of a saturated lithium nitrate solution in methanol, yielding the target product EB-COF:NO₃@SnF₂. The synthesized COF and polyvinylidene difluoride (PVDF) were dispersed in N-Methylpyrrolidone for forming a slurry to prepare the PCS (Figure 2A). The interaction between the EB-COF:NO₃ framework and Li⁺ not only facilitated Li⁺ desolvation, thereby promoting its migration, but also enabled the release of NO₃⁻ anions to participate in the formation of a stable SEI on the Li anode (Figure 2B). Furthermore, the positively charged frameworks immobilize anions and polar solvent molecules, which can effectively decrease their decomposition at the electrode surface and enhance overall electrolyte stability. The SnF₂ provided F-containing species to form a robust SEI, while the in-situ formed Li–Sn alloy concurrently reduced the energy barrier for Li⁺ transport. Thus, the application of the PCS facilitated the in-situ formation of a reinforced SEI, containing beneficial species such as Li₃N, LiN_xO_y, LiF, and Li₅Sn₂ alloy, which collectively contributed to the significantly enhanced electrochemical performance of the LMBs. The Li||NCM811 cell, configured with a high loading of 3.9 mAh cm⁻² and a lean electrolyte (6 μL mAh⁻¹), demonstrated the enhanced capacity retention of 95.9% after 150 cycles (Figure 2C).

Figure 2 (A) The synthesis route to PCS. (B) Schematic illustration of PCS for improving the electrochemical performance of LMBs. (C) Cycling performance of Li||NCM811 full cells using PP and PCS under a voltage range of 3.0–4.4 V at 30°C. Reproduced with permission from Ref. [12]. Copyright 2023, RSC. (D) Design principles for dendrite-free LMBs with redox MCOFs. (E) Li nucleation overpotentials of Li||Cu half cells at a current density of 0.1 mA cm⁻² with a capacity of 0.1 mAh cm⁻². (F) Cycle performance of Li||Li symmetric cells at a current density of 0.5 mA cm⁻² for 0.5 mAh cm⁻². (G) Cycling performance of the Li||LFP cells with TAPA-MCOF at 0.5C. Reproduced with permission from Ref. [13]. Copyright 2025, Wiley

The introduction of dual electron-rich groups into COFs provides an effective strategy for optimizing their electronic structure. This molecular engineering approach homogenizes the local charge distribution at active sites and refines the coordination microenvironment for lithium ions, thereby enhancing electrochemical performance. Based on the aforementioned strategy, Zhai and colleagues designed and synthesized two types of redox-active metal-covalent organic frameworks (TAPA-MCOF and NTBCA-MCOF) [13]. These materials structurally integrated an electron-rich trinuclear copper cluster and redox-active diarylamine units with differentiated electron-donating capabilities (Figure 2D). When coated onto separator surfaces, these MCOFs functioned as interfacial layers to cooperatively regulate lithium nucleation behavior and lithium-ion transport kinetics. Experimental evidence confirms that the active sites in the Cu₃Py₃ and the diarylamine units act synergistically to optimize the local lithium-ion coordination microenvironment, thereby guiding uniform lithium nucleation. The optimized electrode interface delivered multifaceted performance enhancements. The Li||Cu cell exhibited a low Li nucleation overpotential of only 16 mV (Figure 2E), and the Li||Li symmetric cell sustained stable cycling for over 1,600 hours at 0.5 mA cm⁻² (Figure 2F). The LiFePO₄-based battery achieved a remarkable capacity retention of approximately 98% (Figure 2G), underscoring its practical potential.

3.2 COF-based artificial SEIs

Constructing an artificial SEI on lithium metal anodes represents a promising approach to enhance interfacial stability and curtail detrimental side reactions with electrolytes, thereby mitigating dendrite formation. An ideal artificial SEI should demonstrate exceptional electrochemical, chemical, and mechanical robustness. Furthermore, its architecture should be both uniform and compact to enable homogeneous Li⁺ flux and provide high lithium-ion conductivity (σLi⁺). A fundamental limitation of conventional liquid electrolytes is their low Li⁺ transference number, wherein the dominant contribution of anions to the total ionic conductivity (σ) leads directly to Li⁺ depletion during cycling [46, 47]. Thus, engineering an electrolyte-derived interphase capable of facilitating selective and high-rate Li⁺ transport to address the critical issue of Li⁺ depletion, effectively inhibiting dendrites without compromising overall cell performance.

COFs functionalized with ion-conducting groups are excellent electrolyte-based interphasial materials because of their strong Li⁺ conductivities and stabilities. Kim et al. designed a COF with redox-active and ionic groups integrated into its backbone, enabling simultaneous Li⁺ and electron transport and its successful deployment as an SEI in LMBs [18]. The redox-active molecule 2,3,6,7-tetrahydroxy-9,10-anthraquinone (THAQ) served as the building block for AQ-Si-COF, which featured a three-fold symmetric structure formed by hypervalent silicate linkages (Figure 3A). The AQ-Si-COF-based SEI functioned to selectively adsorb and store Li⁺ ions from the electrolyte during cycling (Figure 3B). This unique capability contributed to an enhanced specific capacity and improved cycling stability. When paired with a LiCoO₂ cathode and operated at high voltage, the cells delivered a maximum reversible capacity of 188 mAh g⁻¹ at 0.25C (Figure 3C). The cell also exhibited excellent cycling stability, with a capacity fade of less than 3% over 100 cycles, along with suppressed Li dendrite growth (Figure 3D).

Figure 3 (A) Synthesis of AQ-Si-COFs and schematic illustrations of Li⁺ transport in the AQ-Si-COFs (blue, yellow, gray, and red represent Li, Si, C, and O, respectively). (B) Schematic illustration showing the complexation of Li⁺ and e^– around the AQ-Si-COFs during the charging and discharging processes. (C) Comparison of cycle performance of LiCoO₂||AQ-Si-COFs/Li with COFs SEI layers of various thicknesses (15, 35, and 100 μm). Reproduced with permission from Ref. [18]. Copyright 2023, American Chemical Society. (D) In situ optical microscopy images of the Li electrodeposition process on the bare Li anode and Li anode with a 15-μm AQ-Si-COFs coating as an SEI. (E) Synthetic scheme of the urea-linked COF-531. (F) Voltage profile of cycling performances of symmetric cells with bare Li and COF@Li at 10 mA cm⁻², 10 mAh cm⁻². Reprinted with permission from Ref. [16]. Copyright 2025, Wiley

The artificial SEI with high Li⁺ conductivity (> 10⁻⁴ S cm⁻¹) can be constructed by incorporating abundant Li─N bond species [48, 49]. These bonds have been demonstrated to effectively lower the energy barrier for Li⁺ migration and boost Li⁺ flux [50]. Accordingly, increasing the amount of nitrogen within artificial SEI is a direct and effective strategy to enhance LMBs' performance. A central challenge in designing Li─N-rich artificial SEIs is overcoming their inherent structural disorder to achieve uniform Li⁺ transport. COFs, with their intrinsic ordered pores, offer a promising platform, yet their practical application is constrained by a trade-off between high nitrogen content and crystallinity [51]. The low reactivity of N-rich building blocks like melamine often results in poorly crystalline materials, making the integration of chaotic nitrogen-rich units into periodic crystalline networks a pivotal and demanding goal in the field [52–55]. A strategy reported by Ke et al. utilized the high dynamicity of N,N'-Carbonyldiimidazole (CDI)-derived urea linkages to overcome the low reactivity of nitrogen-rich melamine, achieving the synthesis of a novel urea-linked COF (COF-531) to construct high-performance artificial SEI [16] (Figure 3E). The COF-531, featuring regularly ordered pore structures and an exceptionally high nitrogen content of 50.9 wt%, provided abundant lithiophilic sites. When deployed as an interfacial layer on lithium metal anodes, it served as a Li⁺-guided channel that enhanced desolvation kinetics, facilitated rapid transport, and promoted uniform Li deposition, thereby effectively suppressing dendrite growth. The efficacy of this approach was underscored by the outstanding performance of COF@Li symmetric cells, which achieved ultra-stable cycling for over 11,500 hours at 10 mA cm⁻² in a carbonate-based electrolyte (Figure 3F).

3.3 COF-based solid-state electrolytes

The solid-state LMBs based on solid polymer electrolytes (SPEs) represent a promising technological direction, owing to their intrinsic safety and favorable interfacial properties. Within this context, poly(vinylidene fluoride co-hexafluoropropylene) (PVDF-HFP) has garnered significant attention as a matrix for advanced SPEs, primarily due to its remarkable electrochemical stability, interfacial compatibility, and film-forming processability [56, 57].

However, the high rigidity of PVDF-HFP induces poor interfacial contact, severely limiting practical battery performance by fostering unstable Li deposition. To mitigate this, a strategy focused on optimizing the electron density of the PVDF-HFP-based SPE has been developed. This strategy concurrently enhances Li⁺ transport kinetics and promotes the formation of a LiF-rich SEI from TFSI^- decomposition, leading to uniform lithium deposition and dendrite suppression. It's worth noting that the topologically ordered columnar π-arrays within COFs facilitate rapid charge carrier transport [58], making them exceptional materials for precisely modulating the electron density of PVDF-HFP-based SPEs. Thus, Liu and co-workers designed and synthesized donor-acceptor (D-A) COFs to modulate the electron density of PVDF-HFP-based SPEs, thereby achieving high-performance solid-state LMBs [23]. The D-A COF (Br-COF) was synthesized from tetra(p-aminophenyl)porphyrin (TAPP) and 2,5-dibromo-terephthalaldehyde (TA-Br). The D-A structure enabled rapid electron transfer via a push-pull effect, creating an electron-rich environment that promoted TFSI⁻ decomposition to yield a LiF-rich SEI (Figure 4A). Concurrently, the lithophilic sites weaken TFSI⁻–Li⁺ interaction for accelerating Li⁺ migration. Therefore, the Li||Li symmetric cells with the Br-COF@PVDF-HFP electrolyte demonstrated an extended cycling stability of 3,000 hours (Figure 4B). Furthermore, Li|Br-COF@PVDF-HFP|LFP cell exhibited remarkable long-term cyclability, maintaining 71.4% capacity retention over 2,000 cycles (Figure 4C).

Figure 4 (A) Schematic illustrations of Br-COF catalyzing LiF formation and comparison of Li deposition behavior between PVDF-HFP and Br-COF@PVDF-HFP. (B) Galvanostatic cycling curves of the Li||Li symmetric cells with PVDF-HFP, H-COF@PVDF-HFP, and Br-COF@PVDF-HFP at 0.1 mA cm⁻². (C) Long-term cycling performances of LFP at 1C. Reproduced with permission from Ref. [23]. Copyright 2025, Wiley. (D) Mechanisms of ion transport in solid-state electrolyte. (E) Arrhenius plots of ionic conductivities. (F) Cycling performance of LCO|CF₃−COF@PVDF−HFP|Li pouch cell at 0.5C. (G) The abuse experiment of NCM523|CF₃−COF@PVDF−HFP|Li pouch cell. Reproduced with permission from Ref. [21]. Copyright 2025, Wiley

Furthermore, there is an urgent need for designing efficient ion transport pathways within PVDF-HFP based SPEs to concurrently boost ionic conductivity, structural robustness, and interfacial stability for the practical application of LMBs. Chen and his colleagues added a fluorinated COF (CF₃-COF) as a functional filler into the PVDF-HFP matrix to build efficient ion transport channel [21] (Figure 4D). The uniform incorporation of PVDF-HFP into the CF₃-COF pores generated a nanoconfined environment that disrupts polymer crystallinity and selectively enhances Li⁺ mobility [54]. This configuration established a continuous conductive pathway along the COF-polymer interface, capitalizing on the strengths of both materials. A synergistic effect between the −CF₃ groups of both components further created stable transport sites, reducing the polymer's affinity for Li⁺ and thereby facilitating ion-pair dissociation to yield a stable network for efficient ion conduction. Consequently, the ultra-thin SPE showed a remarkably high ionic conductivity of 1.21 × 10⁻³ S cm⁻¹ (Figure 4E), endowing the symmetric cells with outstanding cycling stability. Moreover, a corresponding LCO/Li pouch cell delivered a high specific capacity of 760 mAh and retained 96% of its capacity after cycling (Figure 4F, G).