Covalent Organic Frameworks as Electrode Materials for Metal Ion Batteries: A Current Review

Construction of High-Performance Anode of Potassium-Ion Batteries by Stripping Covalent Triazine Frameworks with Molten Salt

Covalent triazine frameworks (CTFs) are promising battery electrodes owing to their designable functional groups, tunable pore sizes, and exceptional stability. However, their practical use is limited because of the difficulty in establishing stable ion adsorption/desorption sites. In this study, a melt-salt-stripping process utilizing molten trichloro iron (FeCl3) is used to delaminate the layer-stacked structure of fluorinated covalent triazine framework (FCTF) and generate iron-based ion storage active sites. This process increases the interlayer spacing and uniformly deposits iron-containing materials, enhancing electron and ion transport. The resultant melt-FeCl3-stripped FCTF (Fe@FCTF) shows excellent performance as a potassium ion battery with a high capacity of 447 mAh g-1 at 0.1 A g-1 and 257 mAh g-1 at 1.6 A g-1 and good cycling stability. Notably, molten-salt stripping is also effective in improving the CTF's Na+ and Li+ storage properties. A stepwise reaction mechanism of K/Na/Li chelation with C═N functional groups is proposed and verified by in situ X-ray diffraction testing (XRD), ex-situ X-ray photoelectron spectroscopy (XPS), and theoretical calculations, illustrating that pyrazines and iron coordination groups play the main roles in reacting with K+/Na+/Li+ cations. These results conclude that the Fe@FCTF is a suitable anode material for potassium-ion batteries (PIBs), sodium-ion batteries (SIBs), and lithium-ion batteries (LIBs).

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.

Enhanced As-COF nanochannels as a high-capacity anode for K and Ca-ion batteries

Covalent organic frameworks can be used for next-generation rechargeable metal-ion batteries due to their controllable spatial and chemical architectures and plentiful elemental reserves. In this study, the arsenic-based covalent organic framework (As-COF) is designed by employing the geometrical symmetry of a semiconducting phosphazene-based covalent organic framework that uses p-phenylenediamine as a linker and hexachorocyclotriphosphazene as an As-containing monomer in a C3-like spatial configuration. The As-COF with engineered nanochannels demonstrates exceptional anodic behavior for potassium (K) and calcium (Ca) ion batteries. It exhibits a high storage capacity of about 914(2039) mA h g-1, low diffusion barriers of 0.12(0.26) eV, low open circuit voltage of 0.23(0.18) V, and a minimal volume expansion of 2.41(2.32)% for K (Ca) ions. These attributes collectively suggest that As-COF could significantly advance high-capacity rechargeable batteries.

Chemistry Aspects and Designing Strategies of Flexible Materials for High-Performance Flexible Lithium-Ion Batteries

In recent years, flexible and wearable electronics such as smart cards, smart fabrics, bio-sensors, soft robotics, and internet-linked electronics have impacted our lives. In order to meet the requirements of more flexible and adaptable paradigm shifts, wearable products may need to be seamlessly integrated. A great deal of effort has been made in the last two decades to develop flexible lithium-ion batteries (FLIBs). The selection of suitable flexible materials is important for the development of flexible electrolytes self-supported and supported electrodes. This review is focused on the critical discussion of the factors that evaluate the flexibility of the materials and their potential path toward achieving the FLIBs. Following this analysis, we present how to evaluate the flexibility of the battery materials and FLIBs. We describe the chemistry of carbon-based materials, covalent-organic frameworks (COFs), metal-organic frameworks (MOFs), and MXene-based materials and their flexible cell design that represented excellent electrochemical performances during bending. Furthermore, the application of state-of-the-art solid polymer and solid electrolytes to accelerate the development of FLIBs is introduced. Analyzing the contributions and developments of different countries has also been highlighted in the past decade. In addition, the prospects and potential of flexible materials and their engineering are also discussed, providing the roadmap for further developments in this fast-evolving field of FLIB research.

Utilization of 2D materials in aqueous zinc ion batteries for safe energy storage devices

Aqueous rechargeable battery has been an intense topic of research recently due to the significant safety issues of conventional Li-ion batteries (LIBs). Amongst the various candidates of aqueous batteries, aqueous zinc ion batteries (AZIBs) hold great promise as a next generation safe energy storage device due to its low cost, abundance in nature, low toxicity, environmental friendliness, low redox potential, and high theoretical capacity. Yet, the promise has not been realized due to their limitations, such as lower capacity compared to traditional LIB, dendrite growth, detrimental degradation of electrode materials structure as ions intercalate/de-intercalate, and gas evolution/corrosion at the electrodes, which remains a significant challenge. To address the challenges, various 2D materials with different physiochemical characteristics have been utilized. This review explores fundamental physiochemical characteristics of widely used 2D materials in AZIBs, including graphene, MoS2, MXenes, 2D metal organic framework, 2D covalent organic framework, and 2D transition metal oxides, and how their characteristics have been utilized or modified to address the challenges in AZIBs. The review also provides insights and perspectives on how 2D materials can help to realize the full potential of AZIBs for next-generation safe and reliable energy storage devices.

Recent Progress in Design Principles of Covalent Organic Frameworks for Rechargeable Metal-Ion Batteries

Covalent organic frameworks (COFs) are acknowledged as a new generation of crystalline organic materials and have garnered tremendous attention owing to their unique advantages of structural tunability, frameworks diversity, functional versatility, and diverse applications in drug delivery, adsorption/separation, catalysis, optoelectronics, and sensing, etc. Recently, COFs is proven to be promising candidates for electrochemical energy storage materials. Their chemical compositions and structures can be precisely tuned and functionalized at the molecular level, allowing a comprehensive understanding of COFs that helps to make full use of their features and addresses the inherent drawback based on the components and functions of the devices. In this review, the working mechanisms and the distinguishing advantages of COFs as electrodes for rechargeable Li-ion batteries are discussed in detail. Especially, principles and strategies for the rational design of COFs as advanced electrode materials in Li-ion batteries are systematically summarized. Finally, this review is structured to cover recent explorations and applications of COF electrode materials in other rechargeable metal-ion batteries.

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

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

One-Dimensional Covalent Organic Framework as High-Performance Cathode Materials for Lithium-Ion Batteries

Covalent organic frameworks (COFs) have emerged as a new class of cathode materials for energy storage in recent years. However, they are limited to two-dimensional (2D) or three-dimensional (3D) framework structures. Herein, this work reports designed synthesis of a redox-active one-dimensional (1D) COF and its composites with 1D carbon nanotubes (CNTs) via in situ growth. Used as cathode materials for Li-ion batteries, the 1D COF@CNT composites with unique dendritic core-shell structure can provide abundant and easily accessible redox-active sites, which contribute to improve diffusion rate of lithium ions and the corresponding specific capacity. This synergistic structural design enables excellent electrochemical performance of the cathodes, giving rise to 95% utilization of redox-active sites, high rate capability (81% capacity retention at 10 C), and long cycling stability (86% retention after 600 cycles at 5 C). As the first example to explore the application of 1D COFs in the field of energy storage, this study demonstrates the great potential of this novel type of linear crystalline porous polymers in battery technologies.

Organic Cathode Materials for Rechargeable Aluminum-Ion Batteries

Organic electrode materials (OEMs) have shown enormous potential in ion batteries because of their varied structural components and adaptable construction. As a brand-new energy-storage device, rechargeable aluminum-ion batteries (RAIBs) have also received a lot of attention due to their high safety and low cost. OEMs are expected to stand out among many traditional RAIB cathode materials. However, how to improve the electrochemical performance of OEMs in RAIBs on a laboratory scale is still challenging. This work reviews and discusses the uses of conductive polymers, carbonyl compounds, imine polymers, polycyclic aromatic hydrocarbons, organic frameworks, and other organic materials as the cathodes of RAIBs, as well as energy-storage mechanisms and research progress. It is hoped that this Review can provide the design guidelines for organic cathode materials with high capacity and great stability used in aluminum-organic batteries and develop more efficient organic energy storage cathodes.

Sulfide-Bridged Covalent Quinoxaline Frameworks for Lithium-Organosulfide Batteries

The chelating ability of quinoxaline cores and the redox activity of organosulfide bridges in layered covalent organic frameworks (COFs) offer dual active sites for reversible lithium (Li)-storage. The designed COFs combining these properties feature disulfide and polysulfide-bridged networks showcasing an intriguing Li-storage mechanism, which can be considered as a lithium-organosulfide (Li-OrS) battery. The experimental-computational elucidation of three quinoxaline COFs containing systematically enhanced sulfur atoms in sulfide bridging demonstrates fast kinetics during Li interactions with the quinoxaline core. Meanwhile, bilateral covalent bonding of sulfide bridges to the quinoxaline core enables a redox-mediated reversible cleavage of the sulfursulfur bond and the formation of covalently anchored lithium-sulfide chains or clusters during Li-interactions, accompanied by a marked reduction of Li-polysulfide (Li-PS) dissolution into the electrolyte, a frequent drawback of lithium-sulfur (Li-S) batteries. The electrochemical behavior of model compounds mimicking the sulfide linkages of the COFs and operando Raman studies on the framework structure unravels the reversibility of the profound Li-ion-organosulfide interactions. Thus, integrating redox-active organic-framework materials with covalently anchored sulfides enables a stable Li-OrS battery mechanism which shows benefits over a typical Li-S battery.

Organic Anode Materials for Lithium-Ion Batteries: Recent Progress and Challenges

In the search for novel anode materials for lithium-ion batteries (LIBs), organic electrode materials have recently attracted substantial attention and seem to be the next preferred candidates for use as high-performance anode materials in rechargeable LIBs due to their low cost, high theoretical capacity, structural diversity, environmental friendliness, and facile synthesis. Up to now, the electrochemical properties of numerous organic compounds with different functional groups (carbonyl, azo, sulfur, imine, etc.) have been thoroughly explored as anode materials for LIBs, dividing organic anode materials into four main classes: organic carbonyl compounds, covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and organic compounds with nitrogen-containing groups. In this review, an overview of the recent progress in organic anodes is provided. The electrochemical performances of different organic anode materials are compared, revealing the advantages and disadvantages of each class of organic materials in both research and commercial applications. Afterward, the practical applications of some organic anode materials in full cells of LIBs are provided. Finally, some techniques to address significant issues, such as poor electronic conductivity, low discharge voltage, and undesired dissolution of active organic anode material into typical organic electrolytes, are discussed. This paper will guide the study of more efficient organic compounds that can be employed as high-performance anode materials in LIBs.

Molecular and Morphological Engineering of Organic Electrode Materials for Electrochemical Energy Storage

Organic electrode materials (OEMs) can deliver remarkable battery performance for metal-ion batteries (MIBs) due to their unique molecular versatility, high flexibility, versatile structures, sustainable organic resources, and low environmental costs. Therefore, OEMs are promising, green alternatives to the traditional inorganic electrode materials used in state-of-the-art lithium-ion batteries. Before OEMs can be widely applied, some inherent issues, such as their low intrinsic electronic conductivity, significant solubility in electrolytes, and large volume change, must be addressed. In this review, the potential roles, energy storage mechanisms, existing challenges, and possible solutions to address these challenges by using molecular and morphological engineering are thoroughly summarized and discussed. Molecular engineering, such as grafting electron-withdrawing or electron-donating functional groups, increasing various redox-active sites, extending conductive networks, and increasing the degree of polymerization, can enhance the electrochemical performance, including its specific capacity (such as the voltage output and the charge transfer number), rate capability, and cycling stability. Morphological engineering facilitates the preparation of different dimensional OEMs (including 0D, 1D, 2D, and 3D OEMs) via bottom-up and top-down methods to enhance their electron/ion diffusion kinetics and stabilize their electrode structure. In summary, molecular and morphological engineering can offer practical paths for developing advanced OEMs that can be applied in next-generation rechargeable MIBs.

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Design and Synthesis of a π-Conjugated N-Heteroaromatic Material for Aqueous Zinc-Organic Batteries with Ultrahigh Rate and Extremely Long Life

Electroactive organic materials with tailored functional groups are of great importance for aqueous Zn-organic batteries due to their green and renewable nature. Herein, a completely new N-heteroaromatic material, hexaazatrinaphthalene-phenazine (HATN-PNZ) is designed and synthesized, by an acid-catalyzed condensation reaction, and its use as an ultrahigh performance cathode for Zn-ion batteries demonstrated. Compared with phenazine monomer, it is revealed that the π-conjugated structure of N-heteroaromatics can effectively increase electron delocalization, thereby improving its electrical conductivity. Furthermore, the enlarged aromatic structure noticeably suppresses its dissolution in aqueous electrolytes, thus enabling high structural stability. As expected, the HATN-PNZ cathode delivers a large reversible capacity of 257 mAh g^-1 at 5 A g^-1 , ultrahigh rate capability of 144 mAh g^-1 at 100 A g^-1 , and an extremely long cycle life of 45 000 cycles at 50 A g^-1 . Investigation of the charge-storage mechanism demonstrates the synergistic coordination of both Zn²⁺ and H⁺ cations with the phenanthroline groups, with Zn²⁺ first followed by H⁺ , accompanying the reversible formation of zinc hydroxide sulfate hydrate. This work provides a molecular-engineering strategy for superior organic materials and adds new insights to understand the charge-storage behavior of aqueous Zn-organic batteries.

Challenges and advances of organic electrode materials for sustainable secondary batteries

Organic electrode materials (OEMs) emerge as one of the most promising candidates for the next-generation rechargeable batteries, mainly owing to their advantages of bountiful resources, high theoretical capacity, structural designability, and sustainability. However, OEMs usually suffer from poor electronic conductivity and unsatisfied stability in common organic electrolytes, ultimately leading to their deteriorating output capacity and inferior rate capability. Making clear of the issues from microscale to macroscale level is of great importance for the exploration of novel OEMs. Herein, the challenges and advanced strategies to boost the electrochemical performance of redox-active OEMs for sustainable secondary batteries are systematically summarized. Particularly, the characterization technologies and computational methods to elucidate the complex redox reaction mechanisms and confirm the organic radical intermediates of OEMs have been introduced. Moreover, the structural design of OEMs-based full cells and the outlook for OEMs are further presented. This review will shed light on the in-depth understanding and development of OEMs for sustainable secondary batteries.

Heterostructure Fe2O3 nanorods@imine-based covalent organic framework for long cycling and high-rate lithium storage

Fe₂O₃ as an anode for lithium-ion batteries has attracted intense attention because of its high theoretical capacity, natural abundance, and good safety. However, the inferior cycling stability, low-rate performance, and limited composite varieties hinder the application of Fe₂O₃-based materials. In this work, an Fe₂O₃@COF-LZU1 (FO@LZU1) anode was prepared via an imine-based covalent organic framework (COF-LZU1) covering on the exterior surface of Fe₂O₃ after rational optimization. With its unique heterostructure, the COF-LZU1 layer not only effectively alleviated the volume expansion during cycling but also improved the charge-transfer capability because of the π-conjugated system. Moreover, the organic functional group (CN, benzene ring) for COF-LZU1 provided more redox-active sites for Li⁺ storage. Under the contributions of both Fe₂O₃ nanorods and COF-LZU1, the FO@LZU1_50% exhibited an ultrahigh initial capacity and long-term cycling performance with initial discharge capacities of 2143 and 2171 mA h g^-1 after 300 cycles under 0.1 A g^-1, and rate performance of 1310 and 501 mA h g^-1 at 0.3 and 3 A g^-1, respectively. In addition, a high retention capacity of 1185 mA h g^-1 was achieved at 1 A g^-1 after 500 cycles. Furthermore, a full-cell with the FO@LZU1_50% anode and LiCoO₂ cathode exhibited superior cycling and rate performance, which still maintained a reversible capacity of 260 mA h g^-1 after 200 cycles even at a current density of 1 A g^-1. The proposed strategy offers a new perspective for exploring the high-rate capability and designability of Fe₂O₃-based electrode materials.

A Redox-active Microporous Organosiloxane Containing a Stable Neutral Radical, Trioxotriangulene

A new silyl-substituted trioxotriangulene ( TOT ) neutral radical and corresponding porous organosiloxanes (POSs) were synthesized. The neutral radical exhibited a peculiarly high stability and formed a diamagnetic π-dimer characteristic to TOT neutral radicals stabilized by the strong multiple SOMO-SOMO interaction in both solution and solid states. POSs including TOT units within the organosiloxane-wall were prepared by polycondensation of the silyl groups, and formed microporous structures with ~1 nm-size diameters. Redox ability of TOT units in the POS was demonstrated by the treatment of oxidant/reductant in heterogeneous suspension condition, where the TOT units were reversibly converted between reduced and neutral radical species. Furthermore, the solid-state electrochemical measurements of the POS revealed the reversible multi-stage redox ability of TOT units involving polyanionic species within the organosiloxane-wall.

Recent Developments and Challenges of Vanadium Oxides (Vx Oy ) Cathodes for Aqueous Zinc-Ion Batteries

The rapid depletion of lithium resources and the increasing demand for electrical energy storage have stimulated the pursuit of emerging electrochemical energy storage. Aqueous zinc ion batteries (ZIBs) are highly sought after for their low cost, high safety, and increased environmental compatibility. However, the search for suitable cathode materials is still tricky for a wide range of researchers. Vanadium oxides (V_x O_y ), with their abundant vanadium valence, easily deformable V-O polyhedrons, and tunable chemical compositions, are of significant advantage in developing emerging materials. This work provides a detailed review of different V_x O_y for the application in aqueous ZIBs. The current problems and optimization strategies of V_x O_y cathode materials are systematically discussed. Finally, the current challenges and possible directions for future research of V_x O_y cathode materials in aqueous ZIBs are presented.

Structure-Performance Relationships of Covalent Organic Framework Electrode Materials in Metal-Ion Batteries

Covalent organic frameworks (COFs) have shown great potential as high-performance electrode materials for metal-ion batteries in view of their relatively high capacity, flexibly designable structure, ordered porous structure, and limited solubility in electrolyte. To develop more attractive COF electrode materials, it is necessary to understand their structure-performance relationships. This Perspective focuses on discussing the relationships between structure (molecular structure, micro structure, and electrode structure) and performance (voltage, capacity, cycling stability, and rate performance) of COF electrode materials in metal-ion batteries. Among the reported COF electrode materials, those with all linkages being redox active based on C═O and C═N groups would be the most promising cathode materials because of their high capacity (∼500 mAh g^-1), moderate working voltage (∼2 V vs Li⁺/Li), and good cycling stability. To accelerate practical application of COF electrode materials in metal-ion batteries, future work should pay more attention to function-oriented molecular structure design via theoretical simulations, as well as full-cell fabrication and evaluation. This Perspective will stimulate high-quality research that might lead to future commercialization.

Progress and Perspectives on Covalent-organic Frameworks (COFs) and Composites for Various Energy Applications

Covalent-organic frameworks (COFs), being a new member of the crystalline porous materials family, have emerged as important materials for energy storage/conversion/generation devices. They possess high surface areas, ordered micro/mesopores, designable structures and an ability to precisely control electro-active groups in their pores, which broaden their application window. Thanks to their low weight density, long range crystallinity, reticular nature and tunable synthesis approach towards two and three dimensional (2D and 3D) networks, they have been found suitable for a range of challenging electrochemical applications. Our review focuses on the progress made on the design, synthesis and structure of COFs and their composites for various energy applications, such as metal-ion batteries, supercapacitors, water-splitting and solar cells. Additionally, attempts have been made to correlate the structural and mechanistic characteristics of COFs with their applications.