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

High-Performance Aqueous Zinc-Organic Battery with a Photo-Responsive Covalent Organic Framework Cathode

Covalent organic framework (COF) materials, known for their robust porous character, sustainability, and abundance, have great potential as cathodes for aqueous Zn-ion batteries (ZIBs). However, their application is hindered by low reversible capacity and discharge voltage. Herein, a donor-acceptor configuration COF (NT-COF) is utilized as the cathode for ZIBs. The cell exhibits a high discharge voltage plateau of ≈1.4 V and a discharge capacity of 214 mAh g-1 at 0.2 A g-1 when utilizing the Mn2+ electrolyte additive in the ZnSO4 electrolyte. A synergistic combination mechanism is proposed, involving the deposition/dissolution reactions of Zn4SO4(OH)6·4H2O and the co-(de)insertion reactions of H+ and SO4 2- in NT-COF. Meanwhile, the NT-COF with a donor-acceptor configuration facilitates efficient generation and separation of electron-hole pairs upon light exposure, thereby enhancing electrochemical reactions within the battery. This leads to a reduction in charging voltage and internal overvoltage, ultimately minimizing electricity consumption. Under ambient weather conditions, the cell exhibits an average discharge capacity of 430 mAh g-1 on sunny days and maintains consistent cycling stability for a duration of 200 cycles (≈19 days) at 0.2 A g-1. This research inspires the advancement of Zn-organic batteries for high-energy-density aqueous electrochemical energy storage systems or photo-electrochemical batteries.

Electrostatically Designing Materials and Interfaces

Collective electrostatic effects arise from the superposition of electrostatic potentials of periodically arranged (di)polar entities and are known to crucially impact the electronic structures of hybrid interfaces. Here, it is discussed, how they can be used outside the beaten paths of materials design for realizing systems with advanced and sometimes unprecedented properties. The versatility of the approach is demonstrated by applying electrostatic design not only to metal-organic interfaces and adsorbed (complex) monolayers, but also to inter-layer interfaces in van der Waals heterostructures, to polar metal-organic frameworks (MOFs), and to the cylindrical pores of covalent organic frameworks (COFs). The presented design ideas are straightforward to simulate and especially for metal-organic interfaces also their experimental implementation has been amply demonstrated. For van der Waals heterostructures, the needed building blocks are available, while the required assembly approaches are just being developed. Conversely, for MOFs the necessary growth techniques exist, but more work on advanced linker molecules is required. Finally, COF structures exist that contain pores decorated with polar groups, but the electrostatic impact of these groups has been largely ignored so far. All this suggest that the dawn of the age of electrostatic design is currently experienced with potential breakthroughs lying ahead.

Constructing Donor-Acceptor-Linked COFs Electrolytes to Regulate Electron Density and Accelerate the Li+ Migration in Quasi-Solid-State Battery

Regulation the electronic density of solid-state electrolyte by donor-acceptor (D-A) system can achieve highly-selective Li+ transportation and conduction in solid-state Li metal batteries. This study reports a high-performance solid-state electrolyte thorough D-A-linked covalent organic frameworks (COFs) based on intramolecular charge transfer interactions. Unlike other reported COF-based solid-state electrolyte, the developed concept with D-A-linked COFs not only achieves electronic modulation to promote highly-selective Li+ migration and inhibit Li dendrite, but also offers a crucial opportunity to understand the role of electronic density in solid-state Li metal batteries. The introduced strong electronegativity F-based ligand in COF electrolyte results in highly-selective Li+ (transference number 0.83), high ionic conductivity (6.7 × 10-4 S cm-1), excellent cyclic ability (1000 h) in Li metal symmetric cell and high-capacity retention in Li/LiFePO4 cell (90.8% for 300 cycles at 5C) than substituted C- and N-based ligands. This is ascribed to outstanding D-A interaction between donor porphyrin and acceptor F atoms, which effectively expedites electron transferring from porphyrin to F-based ligand and enhances Li+ kinetics. Consequently, we anticipate that this work creates insight into the strategy for accelerating Li+ conduction in high-performance solid-state Li metal batteries through D-A system.

Optimizing the Structure and Electrochemical Properties of Benzoquinone-Embedded COF via Heat Treatment for a High-Energy Organic Cathode

A benzoquinone-embedded aza-fused covalent organic framework (BQ COF) with the maximum loading of redox-active units per molecule was employed as a cathode for lithium-ion batteries (LIBs) to achieve high energy and power densities. The synthesis was optimized to obtain high crystallinity and improved electrochemical performance. Synthesis at moderate temperature followed by a solid-state reaction was found to be particularly useful for achieving good crystallinity and the activation of the COF. When used as a cathode for LIBs, very high discharge capacities of 513, 365, and 234 mAh g-1 were obtained at 0.1C, 1C, and 10C, respectively, showing a remarkable rate performance. More than 70% of the initial capacity was retained after 1000 cycles when the cathode was investigated for cyclic performance at 2.5C. We demonstrated that a straightforward heat treatment led to enhanced crystallinity, an optimized structure, and favorable morphology, resulting in enhanced electrode kinetics and an improved overall electrochemical behavior. A comparative study was conducted involving an aza-fused COF lacking carbonyl groups (TAB COF) and a small molecule containing phenazine and carbonyl (3BQ), providing useful insights into new material design. A full cell was assembled with graphite as the anode to assess the commercial feasibility of BQ COF, and a discharge capacity of 240 mAh g-1 was obtained at 0.5C. Furthermore, a pouch-type cell with a high discharge capacity and an excellent rate performance was assembled, demonstrating the practical applicability of our designed cathode. Considering the entire mass of the working electrode, a specific energy density of 492 Wh kg-1 and a power density of 492 W kg-1 were achieved at the high current density of 1C, which are comparable to those of commercially available cathodes. These results highlight the promise of organic electrode materials for next-generation lithium-ion batteries. Furthermore, this study provides a systematic approach for simultaneously designing organic materials with high power and energy densities.

Practical organic batteries: Concepts to realize high mass loading with high performance

Organic electrode materials (OEMs) have been well developed in recent years. However, the practical applications of OEMs have not been paid sufficient attention. The concept here focused on one of the essential aspects for practical applications, i. e., high mass loading of active materials. This paper summarizes the challenges posed by high-mass loading of active materials in organic batteries and discusses the possible solutions in terms of organic electrode materials, conductive additives, electrode structures, and electrolytes or battery systems. We hope this concept can stimulate more attention to practical applications of organic batteries towards industry from lab.

Organic electrodes based on redox-active covalent organic frameworks for lithium batteries

Electroactive organic materials have received much attention as alternative electrodes for metal-ion batteries due to their high theoretical capacity, resource availability, and environmental friendliness. In particular, redox-active covalent organic frameworks (COFs) have recently emerged as promising electrodes due to their tunable electrochemical properties, insolubility in electrolytes, and structural versatility. In this Highlight, we review some recent strategies to improve the energy density and power density of COF electrodes for lithium batteries from the perspective of molecular design and electrode optimisation. Some other aspects such as stability and scalability are also discussed. Finally, the main challenges to improve their performance and future prospects for COF-based organic batteries are highlighted.

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.

Iodine doping induced activation of covalent organic framework cathodes for Li-ion batteries

Covalent organic frameworks (COFs) are considered as promising candidate organic electrode materials for lithium-ion batteries (LIBs) because of their relatively high capacity, ordered nanopores, and limited solubility in electrolyte. However, the practical capacity of COF materials is mainly affected by their low electronic/ionic conductivity and the deep-buried active sites inside the COFs. Here, we synthesize an iodine doped β-ketoenamine-linked COF (2,6-diaminoanthraquinone and 1,3,5-triformylphloroglucinol, denoted as COF-I) by a facile one-pot solvothermal reaction. The introduction of iodine can make the COF more lithiophilic inside and exhibit high intrinsic ion/electron transport, ensuring more accessible active sites of the COFs. Consequently, when used as the cathode of LIBs, COF-I demonstrates a high initial discharge capacity of 140 mA h g^-1 at 0.2 A g^-1, and excellent cycling stability with 92% capacity retention after 1000 cycles. Furthermore, a reversible capacity of 95 mA h g^-1 at 1.0 A g^-1 is also achieved after 300 cycles. Our study provides a facile way to develop high-performance COF electrode materials for LIB applications.

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.

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.

A Self-Stratified Thermally Regenerative Battery Using Nanoprism Cu Covering Ni Electrodes for Low-Grade Waste Heat Recovery

Developing a low-cost and high-performance thermally regenerative battery (TRB) is significant for recovering low-grade waste heat. A self-stratified TRB induced by the density difference between electrolytes is proposed to remove the commercial anion exchange membrane (AEM) and avoid ammonia crossover. The simulation and experiment results show the uneven distribution of NH₃, verifying the feasibility of self-stratified electrolytes. For better power generation performance, nanoprism Cu covering Ni electrodes with a high specific surface area and a stable framework are adopted to provide more reaction active sites for fast charge transfer during discharge. A maximum power density (12.7 mW cm^-2) and a theoretical heat-to-electricity conversion efficiency of 2.4% (relative to Carnot efficiency of 27.5%) are obtained in the self-stratified TRB employing nanoprism Cu covering Ni electrodes. Moreover, the cost-effectiveness, simple structure, and sustainable discharge operation indicate that it will be a potential choice for energy conversion from low-grade heat.

A covalent organic framework/graphene aerogel electrocatalyst for enhanced overall water splitting

The rational design of covalent organic framework (COF) based hybrid materials is of paramount importance to address the fundamental challenges of COFs with respect to their poor electron mobilization and the limited number of accessible active sites. Herein, we propose a new strategy for the fabrication of covalently bonded COF grafted graphene aerogel hybrid materials for electrocatalytic application. An in situ step-growth polymerization approach was developed to achieve the hybridization of COFs along the surface of amino-functionalized graphene nanosheets. By taking advantage of the three-dimensional conductive networks and highly accessible active sites, the cobalt-incorporated COF/graphene hybrid aerogel shows high oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performances with an overpotential of 300 and 275 mV at 10 mA cm^-2, respectively, under alkaline conditions. When applied to an electrochemical water-splitting electrolyzer, it is able to produce hydrogen and oxygen at competitive rates of 1.14 and 0.58 μL s^-1, respectively, under ambient conditions, demonstrating its potential for practical applications.

One-Step Synthesis of Cationic Covalent Organic Frameworks

Ionic covalent organic frameworks (iCOFs) have attractive properties that make them suitable for use as ion transport materials, as energy storage media, and for metal sorption. However, the synthetic pathways to prepare iCOFs are limited. Herein, we prepare an iCOF via a single-step reaction. The synthesized materials were isolated as polycrystalline nanowires. The theoretical and experimental data reveal that the synthesized iCOFs are predominately assembled into staggered configurations. The materials exhibit an uptake capacity of 3.5 g·g^-1 for iodine. The ab initio calculations point to the role of bromide counterions, forming I₂Br^- as stable ions within the framework.

Solution-Processable Redox-Active Polymers of Intrinsic Microporosity for Electrochemical Energy Storage

Redox-active organic materials have emerged as promising alternatives to conventional inorganic electrode materials in electrochemical devices for energy storage. However, the deployment of redox-active organic materials in practical lithium-ion battery devices is hindered by their undesired solubility in electrolyte solvents, sluggish charge transfer and mass transport, as well as processing complexity. Here, we report a new molecular engineering approach to prepare redox-active polymers of intrinsic microporosity (PIMs) that possess an open network of subnanometer pores and abundant accessible carbonyl-based redox sites for fast lithium-ion transport and storage. Redox-active PIMs can be solution-processed into thin films and polymer-carbon composites with a homogeneously dispersed microstructure while remaining insoluble in electrolyte solvents. Solution-processed redox-active PIM electrodes demonstrate improved cycling performance in lithium-ion batteries with no apparent capacity decay. Redox-active PIMs with combined properties of intrinsic microporosity, reversible redox activity, and solution processability may have broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

Kinetic Analysis of Bio-oil Derived Hierarchically Porous Carbon for Superior Li+/Na+ Storage

Herein, an efficient biomass utilization is proposed to prepare bio-oil-derived carbon (BODPC) with hierarchical pores and certain H/O/N functionalities for superior Li⁺/Na⁺ storage. Kinetic analyses reveal that BODPC has similar behavior in the electrochemical Li⁺ and Na⁺ storage processes, in terms of physical adsorption (Stage I), chemical redox reactions with surface functionalities (Stage II), and insertion into the graphitic interlayer (Stage III). Promisingly, BODPC exhibits a high reversible specific capacity (1881.7 mAh g^-1 for Li⁺ and 461.0 mAh g^-1 for Na⁺ at 0.1 A g^-1), superior rate capability (674.1 mAh g^-1 for Li⁺ and 125.7 mAh g^-1 for Na⁺ at 5.0 A g^-1), and long-term cyclability. More notably, the BODPC with highly capacitive-dominant behavior would hold great promise for the applications of high-power, durable, and safe rechargeable batteries/capacitors.

Influence of Polymorphism on the Electrochemical Behavior of Dilithium (2,3-Dilithium-oxy)-terephthalate vs. Li

Organic electrode materials offer obvious opportunities to promote cost-effective and environmentally friendly rechargeable batteries. Over the last decade, tremendous progress has been made thanks to the use of molecular engineering focused on the tailoring of redox-active organic moieties. However, the electrochemical performance of organic host structures relies also on the crystal packing, like the inorganic counterparts, which calls for further efforts in terms of crystal chemistry to make a robust redox-active organic center electrochemically efficient in the solid state. Following our ongoing research aiming at elaborating lithiated organic cathode materials, we report herein on the impact of polymorphism on the electrochemical behavior of dilithium (2,3-dilithium-oxy-)terephthalate vs. Li. Having isolated dilithium (3-hydroxy-2-lithium-oxy)terephthalate through an incomplete acid-base neutralization reaction, its subsequent thermally induced decarboxylation mechanism led to the formation of a new polymorph of dilithium (2,3-dilithium-oxy-)terephthalate referred to as Li4-o-DHT (β-phase). This new phase is able to operate at 3.1 V vs. Li+/Li, which corresponds to a positive potential shift of +250 mV compared to the other polymorph formerly reported. Nevertheless, the overall electrochemical process characterized by a sluggish biphasic transition is impeded by a large polarization value limiting the recovered capacity upon cycling.

Storing Mg Ions in Polymers: A Perspective

The electrochemical performance of rechargeable Mg batteries (RMBs) is primarily determined by the cathodes. However, the strong interaction between highly polarized Mg²⁺ and the host lattice is a big challenge for inorganic cathode materials. While endowed with weak interaction with Mg²⁺ , organic polymers are capable of fast reaction kinetics. Besides, with the advantages of light weight, abundance, low cost, and recyclability, polymers are deemed as ideal cathode materials for RMBs. Although polymer cathodes have remarkably progressed in recent years, there are still significant challenges to overcome before reaching practical application. In this perspective, the challenges faced by polymer cathodes are critically focused, followed by the retrospection of efforts devoted to design polymers. Some feasible strategies are proposed to explore new structures and chemistries for the practical application of polymer cathodes in RMBs.