Background
Covalent organic frameworks (COFs) are a class of crystalline porous materials in which organic building blocks are assembled with atomic precision to construct predetermined skeletons and nanopores. The well-ordered channel structures arising from their unique crystalline features have also stimulated extensive research interest in proton/hydroxide conduction and Li⁺ transport. However, from the perspective of applications in fuel cells or lithium-ion batteries, the primary challenge that must be addressed before COF-like materials can be used as ion-conducting components in cells is the fabrication of membranes. COFs possess a highly cross-linked network structure, which gives rise to their porosity, while the regularity of this network imparts crystallinity. Nevertheless, the highly cross-linked nature also means that COFs cannot be dissolved in solvents or melted at elevated temperatures, rendering common membrane fabrication methods such as solution casting or melt processing inapplicable. Faced with the challenge of fabricating membranes from COFs (and metal organic frameworks, MOFs) while still taking advantage of their nanoporous structures, researchers often resort to polymer binders. However, this inevitably disrupts the ion channels within COF/MOF materials, greatly diminishing the advantages offered by their intricately structured nanopores.

Research Progress
Prof. Hong Xu’s team in Tsinghua University reports a substrate-assisted interfacial polymerization approach to easily prepare a uniform, large-area, and microcontinuous 3D-COF membrane, and demonstrate its great application in Li-metal battery to stabilize Li-metal anode through the electron-rich sites integrated on its channels. During the interface polymerization, porous polymer behaves as a COF growth substrate, regionally enriching the COF building blocks and promoting their condensation reactions. Meanwhile, the hydroxyl and imine groups, which provide hydrogen bond locking during COF synthesis, facilitate the formation of a continuous, large-area COF membrane (15 cm × 25 cm). The COF membrane has sufficient strength and considerable thickness (~4 μm) to be peeled from the substrate to obtain a self-standing membrane. Unlike traditional 3D-COFs prepared by the solvothermal method, this 3D-COF membrane features a non-interpenetrating dia topology, which promotes the formation of 3D continuous ionic pathways at the molecular level, enabling rapid ion transport—a crucial characteristic for lithium batteries.

In addition, when used as a separator in lithium metal batteries, the 3D-COF membrane exhibits lower polarization voltage and improved cycling stability, effectively stabilizing the lithium metal anode and suppressing side reactions and dendrite growth. Compared with conventional commercial Celgard separators, the 3D-COF membrane shows superior electrochemical performance.

Density functional theory (DFT) calculations were employed to further investigate the desolvation mechanism. Firstly, the hydroxyl and imine groups on the framework can act as Li⁺-solvation cages, facilitating the conversion of the Li-solvates to more readily reducible species [e.g., Li(EC)₄⁺ to Li(EC)₃⁺]. Secondly, this 3D-COF membrane features dense nanoporous, which can realize the uniform deposition of lithium on the electrode surface. Therefore, incorporating the dense nanoporous membrane and the Li⁺-solvation cages, the 3D-COF membrane paves an effective method to improve the cycling performance of lithium metal batteries.

Future Prospects
A large-area, uniform, and continuous nanoporous 3D-COF membrane was fabricated via a substrate-assisted interfacial in situ polymerization strategy. The interconnected 3D nanochannels within the membrane facilitate the transport of Li⁺ and electrolytes, while the solvation cages formed by hydroxyl and imine groups on the COF framework suppress lithium dendrite growth and side reactions at the lithium-metal electrode. The Li⁺-solvation cage effect and the dense nanoporous characteristics of the 3D-COF membrane demonstrated in this study may provide new insights into addressing long-standing challenges in the battery field. Moreover, this method enables the fabrication of large-sized, uniform, and continuous COF membranes through a simple process, overcoming the bottleneck in porous membrane fabrication and offering promising prospects for the broader application of COF materials.

The complete study is accessible via DOI:10.34133/research.0926