3D Crown Ether Covalent Organic Framework as Interphase Layer toward High-Performance Lithium Metal Batteries

Electronegative Strategic Positions in Covalent Organic Frameworks: Unlocking High-Efficiency Gold Recovery

Gold (Au) concentrations accumulated from electronic waste (e-waste) and industrial leachates far surpass those found in natural ores, a highly valuable resource if efficient recovery methods can be developed. Despite advancements in covalent organic frameworks (COFs), achieving adsorbents with high selectivity, large capacity, and rapid adsorption kinetics remain challenging because of limitations in partial pore wall sites. Here, we present hexaazatriphenylene-based COFs (HATP-COFs) with an electronegative skeleton, specifically designed for selective Au recovery. The hexaazatriphenylene centers, imine linkages, and pyridine linkers within the COFs introduce electron-rich sites that extend across strategic positions-vertex, linkages, and linkers-thereby enhancing the overall structural integrity. These features facilitate efficient Au capture through electrostatic interactions, achieving an exceptional adsorption capacity exceeding 2366 mg g-1 with rapid kinetics, making HATP-COFs one of the most efficient pure COFs reported to date. Moreover, these HATP-COFs demonstrate remarkable selectivity, stability, and scalability. Theoretical calculations reveal that the electronegative skeleton introduces critical binding sites, promoting strong electrostatic interactions with Au3+ ions and improving adsorption kinetics. This work highlights the potential of charge-interface engineering in COFs as a transformative strategy for developing next-generation materials.

Rational Design of Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production

Photocatalytic production of hydrogen peroxide (H2O2) represents a significant approach to achieving sustainable energy generation through solar energy, addressing both energy shortages and environmental pollution. Among various photocatalytic materials, covalent organic frameworks (COFs) have gained widespread attention and in-depth research due to their unique advantages, including high porosity, predesignability, and atomic-level tunability. In recent years, significant progress has been made in the development, performance enhancement, and mechanistic understanding of COF-based photocatalysts. This review focuses on the latest advancements in photocatalytic H2O2 production using COFs, particularly emphasizing the rational design of COF structures to regulate catalytic performance and exploring the fundamental processes involved in photocatalysis. Based on current research achievements in this field, this paper also discusses existing challenges and future opportunities, aiming to provide a reference for the application of COFs in photocatalytic H2O2 production.

Multi-Shelled Hollow Covalent Organic Framework Nanospheres for Stable Potassium Storage

Multi-shelled hollow covalent organic framework nanospheres (MH-COFs) with at least two shells integrate the merits of the porous crystalline covalent organic framework (COF) matrix with the complex hollow architecture, which can motivate new functions for exceptional performance. However, the fabrication of MH-COFs is still uncultivated and remains a formidable challenge. Herein, we reported a facile template-free protocol for the general synthesis of different MH-COFs by controlling the simultaneous processes of surface crystallization and core etching of the crystalline-inhomogeneity nanospheres of COFs precursor. The crystalline-inhomogeneity solid covalent organic polymer nanospheres (COPs) with robust crystalline surface but vulnerable amorphous core were designed. Subsequently, an acetic acid aqueous solution treatment of crystalline-inhomogeneity COPs not only induced selective etching of the vulnerable cores but also promote the further crystallization of the surface layers, thereby producing hollow COFs. A further step-by-step expansion of seeded growth for inhomogeneous COP layers and then similar acid solution treatment can output the intriguing MH-COFs. An extraordinarily stable K-ion battery anode with high capacity was demonstrated with a MH-COFs/S nanocomposite fabricated by covalently bonding chain sulfur into the MH-COFs matrix. This work opened a simple but powerful avenue in designing complex hollow COFs architectures to boost their potential for applications.

Soluble Covalent Organic Frameworks as Efficient Lithiophilic Modulator for High-Performance Lithium Metal Batteries

Lithium metal batteries (LMBs) are regarded as the potential alternative of lithium-ion batteries due to their ultrahigh theoretical specific capacity (3860 mAh g-1). However, severe instability and safety problems caused by the dendrite growth and inevitable side reactions have hindered the commercialization of LMBs. To solve them, in this contribution, a design strategy of soluble lithiophilic covalent organic frameworks (COFs) is proposed. By introducing polyethylene glycol as the side chains, two COFs (CityU-28 and CityU-29) not only become soluble for the facile coating technique, but also can facilitate the lithium-ion migration in batteries. Furthermore, when coated on the lithium anode of LMB, both COFs can act as artificial solid electrolyte interphase to prevent dendrite growth thus enabling the long-term stability of the cells. Notably, the symmetric CityU-29@Li cell can work for more than 5000 h at a current density of 2 mA cm-2 and an areal capacity of 1 mAh cm-2. A remarkable capacity retention of 78.9 % after 1500 cycles and a Coulombic efficiency of about 99.9 % at 1.0 C can also be realized in CityU-29@Li||LiFePO4 full cell. This work could provide a universal design strategy for soluble COFs and enlighten their application in diverse scenarios, especially energy-related fields.

Redox-Active Metal-Covalent Organic Frameworks for Dendrite-Free Lithium Metal Batteries

Lithium (Li) metal has gained attention as an anode material for lithium-metal batteries (LMBs) owing to its low electrochemical potential, high specific capacity, and low density. However, the accumulation of Li dendrites and unstable solid electrolyte interphases, caused by sluggish Li+ migration and uneven Li deposition, limit practical LMB applications. This study presents the first report on redox-active metal-covalent organic frameworks (MCOFs) with dual-active centers as functional separators for LMBs. These MCOFs facilitate homogeneous Li nucleation and accelerate Li+ ion transport. The synergistic effects of redox-active diarylamine units and trinuclear copper clusters modulate local electron-cloud density, regulating microenvironment of Li+ ions and ensuring homogeneous Li nucleation. The MCOF-based separator's well-defined 1D channels in MCOF-based separator enable uniform Li+ flux, and promote homogeneous Li deposition, resulting in high Li+ transference number of 0.93 and an ionic conductivity of 2.01 mS cm-1 at room temperature. The Li|Cu cell demonstrates a low Li nucleation barrier of 16 mV, while the Li symmetric cell exhibits stable Li plating/stripping for over 1600 h at 0.5 mA cm-2. When coupled with LiFePO4 cathodes, the assembled LMB exhibits stable capacity retention of ≈98%. This work paves the way for dendrite-free Li metal anodes in high-performance LMBs.

Tuning steric hindrance of cyclic ether electrolytes enables high-voltage lithium metal batteries

Ether-based electrolytes are known for their high stability with lithium metal anodes (LMAs), but they often exhibit poor high-voltage stability. Structural optimization of ether-based solvent molecules has been proven to effectively broaden the electrochemical window of these electrolytes, yet the optimization rules within cyclic ethers remain unclear. Herein, we investigate the impact of methyl substitution positions on the molecular properties of 1,3-dioxolane (DOL), a commonly used cyclic ether. The results show that the introduction of methyl groups can effectively inhibit the ring-opening polymerization of DOL. Besides, 4-methyl-1,3-dioxolane (4-Me DOL), with larger steric hindrance compared to 2-methyl-1,3-dioxolane (2-Me DOL), exhibits weaker solvation ability, favoring the formation of anion-rich inner solvation sheath layers and anion-derived interfaces. Even at conventional concentrations, 1 M LiFSI in 4-Me DOL (LiFSI/4-Me DOL) electrolyte demonstrates good LMA stability and an expanded electrochemical window up to 4.6 V. The Li-LiNi0.5Co0.2Mn0.3O2 (NCM523) cell using LiFSI/4-Me DOL could stably cycle over 300 times. This work reveals a new design principle for solvent molecules.

Functional crystalline porous framework materials based on supramolecular macrocycles

Crystalline porous framework materials like metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) possess periodic extended structures, high porosity, tunability and designability, making them good candidates for sensing, catalysis, gas adsorption, separation, etc. Despite their many advantages, there are still problems affecting their applicability. For example, most of them lack specific recognition sites for guest uptake. Supramolecular macrocycles are typical hosts for guest uptake in solution. Macrocycle-based crystalline porous framework materials, in which macrocycles are incorporated into framework materials, are growing into an emerging area as they combine reticular chemistry and supramolecular chemistry. Organic building blocks which incorporate macrocycles endow the framework materials with guest recognition sites in the solid state through supramolecular interactions. Distinct from solution-state molecular recognition, the complexation in the solid state is ordered and structurally achievable. This allows for determination of the mechanism of molecular recognition through noncovalent interactions while that of the traditional recognition in solution is ambiguous. Furthermore, crystalline porous framework materials in the solid state are well-defined and recyclable, and can realize what is impossible in solution. In this review, we summarize the progress of the incorporation of macrocycles into functional crystalline porous frameworks (i.e., MOFs and COFs) for their solid state applications such as molecular recognition, chiral separation and catalysis. We focus on the design and synthesis of organic building blocks with macrocycles, and then illustrate the applications of framework materials with macrocycles. Finally, we propose the future directions of macrocycle-based framework materials as reliable carriers for specific molecular recognition, as well as guiding the crystalline porous frameworks with their chemistry, applications and commercialization.

Stabilizing Li-Metal Electrode via Anion-Induced Desolvation in a Covalent Organic Framework Separator

Although Li-metal batteries have been widely used as high-capacity batteries, they are highly susceptible to electrolytes that lead to dendritic or dead Li growth, which significantly reduces the stability of Li-metal electrodes. Herein, we report an anionic covalent organic framework (sulfonate COF: Bd-COF) as a Li+-solvate dissociator that strips solvent molecules from encapsulated Li+ to stabilize Li-metal electrodes. The homogeneous and dense ionic COF separator was prepared using a template-assisted interface in-suit polymerization engineering. Notably, the well-developed anionic groups within the COF channels could as counter-charge ligands to Li+, that adsorb Li+-solvates and induce their partial desolvation. Meanwhile, the ordered anionic groups on the surface of COF pores provide continuous ion channels for Li+ migration, facilitating the removal of solvent molecules from Li+-solvated species. Combined with the dense nanoporous feature, the COF membrane was found to be effective in suppressing Li-dendrites and parasitic reactions. The Bd-COF/Celgard membrane realizes uniform Li deposition on Li-metal electrodes, exhibiting excellent cycling performance in Li-symmetric batteries and high-voltage Li-metal batteries with LiNi0.6Mn0.2Co0.2O2 cathodes, showcasing the application prospects of ion-conductive covalent organic frameworks in lithium battery separators.

Covalent Organic Frameworks for Green Energy: Synthesis, Properties, and Applications

Covalent organic frameworks (COFs) are a new type of porous organic crystalline material, which have become an emerging platform for promoting the development of green energy technology due to their high surface area, adjustable pores, low skeleton density, and easy functionalization. In recent years, with the continuous advancement of synthesis technology, the synthesis efficiency and sustainability of COFs have been significantly improved, from traditional solvothermal methods to the emergence of various green synthesis strategies such as ion thermal, mechanochemical, and ultrasound assisted methods. This article reviews the main synthesis methods of COFs and explores their applications in the field of green energy, such as photocatalysis, gas adsorption and separation, electrocatalysis, battery, supercapacitor and Proton exchange membrane fuel cell. By analyzing the performance and mechanism of COFs in these applications in detail, this article further looks forward to the challenges and future development trends faced by COFs in green energy technology, aiming to provide valuable reference and inspiration for researchers in related fields.

Three-Dimensional Covalent Organic Framework with Dense Lithiophilic Sites as Protective Layer to Enable High-Performance Lithium Metal Battery

Lithium (Li) metal batteries with remarkable energy densities are restrained by short lifetime and low Coulombic efficiency (CE), resulting from the accumulative Li dendrites and dead Li during cycling. Here, we prepared a new three-dimensional (3D) covalent organic framework (COF) with dense lithiophilic sites (heteoatom weight contents of 32.32 wt %) as an anodic protective layer of Li metal batteries. The 3D COF was synthesized using a [6+4] synthesis strategy by inducing flexible 6-connected cyclotriphosphazene derivative aldehyde and 4-connected porphyrin-based tetraphenylamines. Both phosphazene and porphyrin rings in the COF served as electron-rich and lithiophilic sites, enhancing a homogeneous Li+ flux via 3D direction towards highly smooth and compact Li deposition. The Li/Por-PN-COF-Cu cells achieved a record average CE of 99.1 % for 320 cycles with smooth Li deposition. Meanwhile, the abundant lithiophilic sites can promote fast Li+ transport with Li+ transference number of 0.87, enabling LiFePO4 full cell with stable stripping/plating processes even at a harsh rate of 5 C. Theoretical calculations revealed that the strong interaction force between Li+ and the COF facilitated the dissolution of Li+ from the electrolyte, and the low migration barrier of 1.08 eV indicated a favorable interaction between the Li+ ions and the π-electron system.

Designing Current Collectors to Stabilize Li Metal Anodes

Rechargeable batteries employing Li metal anodes have gained increasing attention due to their high energy density. Nevertheless, low stability and reversibility of Li metal anodes severely impeded their practical applications. Designing current collectors (CCs) with reasonable structure and composition is an efficient approach to stabilizing the Li metal anodes. However, an in-depth comprehensive understanding about the design principles and modification strategies of CCs for realizing stable Li metal anodes is still lacking. Herein, a critical review focusing on the rational design of CCs for Li metal anodes is summarized. First, the requirements for CCs in Li metal anodes are elucidated to clarify the design objectives of CCs. Then, the modification strategies of CCs including lithiophilic site modification, 3D architecture construction, protective layer modification, and crystalline plane engineering, as well as the corresponding principles are highlighted. On this basis, the recent progress in the development of CCs for Li metal anodes is discussed. Finally, future directions are suggested to focus on developing operando monitoring technology, and designing the CCs and cells under practical conditions close to the requirements of commercial applications. This review will spur more insightful researches toward advanced CCs, and promote their commercialization.

Nano-Metal-Organic Frameworks and Nano-Covalent-Organic Frameworks: Controllable Synthesis and Applications

Nanoscale framework materials have attracted extensive attention due to their diverse morphology and good properties, and synthesis methods of different size structures have been reported. Therefore, the relationship between different sizes and performance has become a research hotspot. This paper reviews the controllable synthesis strategies of nano-metal-organic frameworks (nano-MOFs) and nano-covalent-organic frameworks (nano-COFs). Firstly, the synthetic evolution of nano-frame materials is summarized. Due to their special surface area, regular pores and adjustable structural functions, nano-frame materials have attracted much attention. Then the preparation methods of nanostructures with different dimensions are introduced. These synthetic strategies provide the basis for the design of novel energy storage and catalytic materials. In addition, the latest advances in the field of energy storage and catalysis are reviewed, with emphasis on the application of nano-MOFs/COFs in zinc-, lithium-, and sodium-based batteries, as well as supercapacitors.

An In Situ Generated Organic/Inorganic Hybrid SEI Layer Enables Li Metal Anodes with Dendrite Suppression Ability, High-Rate Capability, and Long-Life Stability

High-quality solid electrolyte interphase (SEI) layers can effectively suppress the growth of Li dendrites and improve the cycling stability of lithium metal batteries. Herein, 1-(6-bromohexanoyl)-3-butylurea is used to construct an organic/inorganic hybrid (designated as LiBr-HBU) SEI layer that features a uniform and compact structure. The LiBr-HBU SEI layer exhibits superior electrolyte wettability and air stability as well as strong attachment to Li foils. The LiBr-HBU SEI layer achieves a Li+ conductivity of 2.75 × 10-4 S cm-1, which is ≈50-fold higher than the value measured for a native SEI layer. A Li//Li symmetric cell containing the LiBr-HBU SEI layer exhibits markedly improved cyclability when compared with the cell containing a native SEI layer, especially at a high current density (e.g., cycling life up to 1333 h at 15 mA cm-2). The LiBr-HBU SEI layer also improves the performance of lithium-sulfur cells, particularly the rate capability (548 mAh g-1 at 10 C) and cycling stability (513 mAh g-1 at 0.5 C after 500 cycles). The methodology described can be extended to the commercial processing of Li metal anodes as the artificial SEI layer also protects Li metal against corrosion.

H3PO4-Impregnated Covalent Organic Framework Membrane on Separators to Prevent Lithium Metal Anode Dendrite Growth

Inhibiting the growth of lithium dendrites is crucial for battery safety. For separators, their favorable electrolyte wettability, uniform current density, and high ionic conductivity are beneficial for avoiding Li dendrite growth. In this work, we propose a separator (PA@COF/PP) by modifying a polypropylene separator with H3PO4-functionalized covalent organic frameworks. The uniform channels of the covalent organic frameworks and H3PO4 can homogenize the current and act as ionic conductors for efficient Li+ migration. The synthesized separator effectively suppresses the growth of lithium dendrites and improves the stability of the batteries. A symmetric cell with the PA@COF/PP separator exhibits a stable life span over 4000 hours at a high current density of 5 mA cm-2, compared to the commercial PP separator, which lasts only 159 hours. This work provides an efficient method and novel inspiration for the construction of dendrite-free lithium metal batteries.

Covalent Triazine Based Frameworks with Donor-Donor-π-Acceptor Structures for Dendrite-Free Lithium Metal Batteries

The appearance of disordered lithium dendrites and fragile solid electrolyte interfaces (SEI) significantly hinder the serviceability of lithium metal batteries. Herein, guided by theoretical predictions, a multi-component covalent triazine framework with partially electronegative channels (4C-TA0.5TF0.5-CTF) is incorporated as a protective layer to modulate the interface stability of the lithium metal batteries. Notably, the 4C-TA0.5TF0.5-CTF with optimized electronic structure at the molecular level by fine-tuning the local acceptor-donor functionalities not only enhances the intermolecular interaction thereby providing larger dipole moment and improved crystallinity and mechanical stress, but also facilitates the beneficial effect of lithiophilic sites (C-F bonds, triazine cores, C=N linkages and aromatic rings) to further regulate the migration of Li+ and achieve a uniform lithium deposition behavior as determined by various in-depth in/ex situ characterizations. Due to the synergistic effect of multi-component organic functionalities, the 4C-TA0.5TF0.5-CTF modified full cells perform significantly better than the common two/three-component 2C-TA-CTF and 3C-TF-CTF electrodes, delivering an excellent capacity of 116.3 mAh g-1 (capacity retention ratio: 86.8 %) after 1000 cycles at 5 C and improved rate capability. This work lays a platform for the prospective molecular design of improved organic framework relative artificial SEI for highly stable lithium metal batteries.

Fabrication of Azacrown Ether-Embedded Covalent Organic Frameworks for Enhanced Cathode Performance in Aqueous Ni-Zn Batteries

Crown ethers (CEs), known for their exceptional host-guest complexation, offer potential as linkers in covalent organic frameworks (COFs) for enhanced performance in catalysis and host-guest binding. However, their highly flexible conformation and low symmetry limit the diversity of CE-derived COFs. Here, we introduce a novel C3-symmetrical azacrown ether (ACE) building block, tris(pyrido)[18]crown-6 (TPy18C6), for COF fabrication (ACE-COF-1 and ACE-COF-2) via reticular synthesis. This approach enables precise integration of CEs into COFs, enhancing Ni2+ ion immobilization while maintaining crystallinity. The resulting Ni2+-doped COFs (Ni@ACE-COF-1 and Ni@ACE-COF-2) exhibit high discharge capacity (up to 1.27 mAh ⋅ cm-2 at 8 mA ⋅ cm-2) and exceptional cycling stability (>1000 cycles) as cathode materials in aqueous alkaline nickel-zinc batteries. This study serves as an exemplar of the seamless integration of macrocyclic chemistry and reticular chemistry, laying the groundwork for extending the macrocyclic-synthon driven strategy to a diverse array of COF building blocks, ultimately yielding advanced materials tailored for specific applications.

Covalent Organic Framework-Based Materials for Advanced Lithium Metal Batteries

Lithium metal batteries (LMBs), with high energy densities, are strong contenders for the next generation of energy storage systems. Nevertheless, the unregulated growth of lithium dendrites and the unstable solid electrolyte interphase (SEI) significantly hamper their cycling efficiency and raise serious safety concerns, rendering LMBs unfeasible for real-world implementation. Covalent organic frameworks (COFs) and their derivatives have emerged as multifunctional materials with significant potential for addressing the inherent problems of the anode electrode of the lithium metal. This potential stems from their abundant metal-affine functional groups, internal channels, and widely tunable architecture. The original COFs, their derivatives, and COF-based composites can effectively guide the uniform deposition of lithium ions by enhancing conductivity, transport efficiency, and mechanical strength, thereby mitigating the issue of lithium dendrite growth. This review provides a comprehensive analysis of COF-based and derived materials employed for mitigating the challenges posed by lithium dendrites in LMB. Additionally, we present prospects and recommendations for the design and engineering of materials and architectures that can render LMBs feasible for practical applications.

Recent Progress in Using Covalent Organic Frameworks to Stabilize Metal Anodes for Highly-Efficient Rechargeable Batteries

Alkali metals (e.g. Li, Na, and K) and multivalent metals (e.g. Zn, Mg, Ca, and Al) have become star anodes for developing high-energy-density rechargeable batteries due to their high theoretical capacity and excellent conductivity. However, the inevitable dendrites and unstable interfaces of metal anodes pose challenges to the safety and stability of batteries. To address these issues, covalent organic frameworks (COFs), as emerging materials, have been widely investigated due to their regular porous structure, flexible molecular design, and high specific surface area. In this minireview, we summarize the research progress of COFs in stabilizing metal anodes. First, we present the research origins of metal anodes and delve into their advantages and challenges as anodes based on the physical/chemical properties of alkali and multivalent metals. Then, special attention has been paid to the application of COFs in the host design of metal anodes, artificial solid electrolyte interfaces, electrolyte additives, solid-state electrolytes, and separator modifications. Finally, a new perspective is provided for the research of metal anodes from the molecular design, pore modulation, and synthesis of COFs.

Covalent organic frameworks and their composites for rechargeable batteries

Covalent organic frameworks (COFs), characterized by well-ordered pores, large specific surface area, good stability, high precision, and flexible design, are a promising material for batteries and have received extensive attention from researchers in recent years. Compared with inorganic materials, COFs can construct elastic frameworks with better structural stability, and their chemical compositions and structures can be precisely adjusted and functionalized at the molecular level, providing an open pathway for the convenient transfer of ions. In this review, the energy storage mechanism and unique superiority of COFs and COF composites as electrodes, separators and electrolytes for rechargeable batteries are discussed in detail. Special emphasis is placed on the relationship between the establishment of COF structures and their electrochemical performance in different batteries. Finally, this review summarizes the challenges and prospects of COFs and COF composites in battery equipment.

Engineering Covalent Organic Frameworks Toward Advanced Zinc-Based Batteries

Zinc-based batteries (ZBBs) have demonstrated considerable potential among secondary batteries, attributing to their advantages including good safety, environmental friendliness, and high energy density. However, ZBBs still suffer from issues such as the formation of zinc dendrites, occurrence of side reactions, retardation of reaction kinetics, and shuttle effects, posing a great challenge for practical applications. As promising porous materials, covalent organic frameworks (COFs) and their derivatives have rigid skeletons, ordered structures, and permanent porosity, which endow them with great potential for application in ZBBs. This review, therefore, provides a systematic overview detailing on COFs structure pertaining to electrochemical performance of ZBBs, following an in depth discussion of the challenges faced by ZBBs, which includes dendrites and side reactions at the anode, as well as dissolution, structural change, slow kinetics, and shuttle effect at the cathode. Then, the structural advantages of COF-correlated materials and their roles in various ZBBs are highlighted. Finally, the challenges of COF-correlated materials in ZBBs are outlined and an outlook on the future development of COF-correlated materials for ZBBs is provided. The review would serve as a valuable reference for further research into the utilization of COF-correlated materials in ZBBs.