A Microporous Covalent-Organic Framework with Abundant Accessible Carbonyl Groups for Lithium-Ion Batteries

Emerging Lithiated Organic Cathode Materials for Lithium-Ion Full Batteries

Organic electrode materials have application potential in lithium batteries owing to their high capacity, abundant resources, and structural designability. However, most reported organic cathodes are at oxidized states (namely unlithiated compounds) and thus need to couple with Li-rich anodes. In contrast, lithiated organic cathode materials could act as a Li reservoir and match with Li-free anodes such as graphite, showing great promise for practical full-battery applications. Here we summarize the synthesis, stability, and battery applications of lithiated organic cathode materials, including synthetic methods, stability against O₂ and H₂ O in air, and strategies to improve comprehensive electrochemical performance. Future research should be focused on new redox chemistries and the construction of full batteries with lithiated organic cathodes and commercial anodes under practical conditions. This Minireview will encourage more efforts on lithiated organic cathode materials and finally promote their commercialization.

Advanced Covalent Organic Frameworks for Multi-Valent Metal Ion Batteries

Covalent organic frameworks (COFs) have received increased interest in recent years as an advanced class of materials. By virtue of the available monomers, multiple conformations and various linkages, COFs offer a wide range of opportunities for complex structural design and specific functional development of materials, which has facilitated the widespread application in many fields, including multi-valent metal ion batteries (MVMIBs), described as the attractive candidate replacing lithium-ion batteries (LIBs). With their robust skeletons, diverse pores, flexible structures and abundant functional groups, COFs are expected to help realize a high performance MVMIBs. In this review, we present an overview of COFs, describe advances in topology design and synthetic reactions, and study the application of COFs in MVMIBs, as well as discuss challenges and solutions in the preparation of COFs electrodes, in the hope of providing constructive insights into the future direction of COFs.

Mesoporous Polyimide-Linked Covalent Organic Framework with Multiple Redox-Active Sites for High-Performance Cathodic Li Storage

Covalent organic frameworks (COFs) are gaining increasing attention as renewable cathode materials for Li-ion batteries. However, COF electrodes reported so far still exhibit unsatisfying capacity due to their limited active site density and insufficient utilization. Herein, a new two-dimensional polyimide-linked COF, HATN-AQ-COF with multiple redox-active sites for storing Li⁺ ions, was designed and fabricated from a new module of 2,3,8,9,14,15-hexacarboxyl hexaazatrinaphthalene trianhydrides with a 2,6-diaminoanthraquinone linker. HATN-AQ-COF possessing excellent stability, good conductivity, and a large pore size of 3.8 nm enables the stable and fast ion transport. This, in combination with the abundant redox active sites, results in a high reversible capacity of 319 mAh g^-1 at 0.5 C (1 C=358 mA g^-1 ) for the HATN-AQ-COF electrode with a high active site utilization of 89 % and good cycle performance, representing one of the best performances among the reported COF electrodes.

Covalent Organic Framework with Highly Accessible Carbonyls and π-Cation Effect for Advanced Potassium-Ion Batteries

Covalent organic frameworks (COF) possess a robust and porous crystalline structure, making them an appealing candidate for energy storage. Herein, we report an exfoliated polyimide COF composite (P-COF@SWCNT) prepared by an in situ condensation of anhydride and amine on the single-walled carbon nanotubes as advanced anode for potassium-ion batteries (PIBs). Numerous active sites exposed on the exfoliated frameworks and the various open pathways promote the highly efficient ion diffusion in the P-COF@SWCNT while preventing irreversible dissolution in the electrolyte. During the charging/discharging process, K⁺ is engaged in the carbonyls of imide group and naphthalene rings through the enolization and π-K⁺ effect, which is demonstrated by the DFT calculation and XPS, ex-situ FTIR, Raman. As a result, the prepared P-COF@SWCNT anode enables an incredibly high reversible specific capacity of 438 mA h g^-1 at 0.05 A g^-1 and extended stability. The structural advantage of P-COF@SWCNT enables more insights into the design and versatility of COF as an electrode.

Imine-Induced Metal-Organic and Covalent Organic Coexisting Framework with Superior Li-Storage Properties and Activation Mechanism

Due to the adjustable structure and the broad application prospects in energy and other fields, the exploration of porous organic materials [metal-organic polymers (MOPs), covalent organic frameworks (COFs), etc.] has attracted extensive attention. In this work, an imine-induced metal-organic and covalent organic coexisting framework (Co-MOP@COF) hybrid was designed based on the combination between the amino units from the organic ligands of Co-MOP and the aldehyde groups from COF. The obtained Co-MOP@COF hybrid with layer-decorated microsphere morphology exhibited good electrochemical cycling performance (a large reversible capacity of 1020 mAh g^-1 after 150 cycles at 100 mA g^-1 and a reversible capacity of 396 mAh g^-1 at 500 mA g^-1 ) as the anode for Li-ion batteries. The coexisting framework structure endowed the Co-MOP@COF hybrid with more surface area exposed in the exfoliated COF structure, which provided rapid Li-ion diffusion, better electrolyte infiltration, and effective activation of functional groups. Therefore, the Co-MOP@COF hybrid material achieved an enhanced Li storage mechanism involving multi-electron redox reactions, related to the Co^II center and organic groups (C=C groups of benzene rings and C=N groups), and furthermore improved electrochemical performance.

Dual Electroactivity in a Covalent Organic Network with Mechanically Interlocked Pillar[5]arenes

The synthesis and characterization of a polyrotaxanated covalent organic network (CON) based on the association between the viologen and pillar[5]arene (P[5]OH) units are reported. The mechanical bond allows for the irreversible insertion of n-type redox centers (P[5]OH macrocycles) within a pristine structure based on p-type viologen redox centers. Both redox units are active on a narrow potential range and, in water, the presence of P[5]OH greatly increases the electroactivity of the material.

Insights into the Capacity and Rate Performance of Transition-Metal Coordination Compounds for Reversible Lithium Storage

Coordination compounds are well-known compounds that are being used as new materials for lithium storage because of their unique advantages, that is, designable structures, abundant active sites, and facile as well as mild synthetic routes. However, the electrode stability, low rate performance, and cycle life of coordination compounds are currently the main issues preventing their application as electrode materials, and the lithium-storage mechanism in coordination networks is not well understood. Herein, isostructural one-dimensional coordination compounds were synthesized to study their lithium-storage performance. Co-HIPA and Ni-HIPA showed superior electrolyte stability than other M-HIPAs, and Co-HIPA displayed a superior reversible capacity and cycle stability, excellent rate performance, and clear voltage platform. DFT calculations and kinetic analysis revealed the influence of the metal center with different electronic structures on the lithium-storage mechanism.

Crosslinked Polyimide and Reduced Graphene Oxide Composites as Long Cycle Life Positive Electrode for Lithium-Ion Cells

Conjugated polymers with electrochemically active redox groups are a promising class of positive electrode material for lithium-ion batteries. However, most polymers, such as polyimides, possess low intrinsic conductivity, which results in low utilization of redox-active sites during charge cycling and, consequently, poor electrochemical performance. Here, it was shown that this limitation can be overcome by synthesizing polyimide composites (PIX) with reduced graphene oxide (rGO) using an in situ polycondensation reaction. The polyimide composites showed increased charge-transfer performance and much larger specific capacities, with PI50, which contains 50 wt % of rGO, showing the largest specific capacity of 172 mAh g-1 at 500 mA g-1 . This corresponds to a high utilization of the redox active sites in the active polyimide (86 %), and this composite retained 80 % of its initial capacity (125 mAh g-1 ) after 9000 cycles at 2000 mA g-1 .

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

As the world moves toward electromobility, our daily lives are flooded with variety of lithium ion batteries (LIBs), and the concerns of cost, safety and environmental friendliness of LIBs spring up in the minds of scientists. Although organic electrodes have been considered as promising alternatives to their inorganic counterparts, some intrinsic weaknesses still plague scientists, such as high solubility, low conductivity and sluggish ion diffusion. The emergence of covalent organic frameworks (COFs) attracts our attention because of their robust networks and open pores that could facilitate the infiltration of electrolyte ions when used as electrodes for metal-ion batteries (MIBs). In this review, we summarized the recent progress of COFs as electrode materials, and the strategies toward enhancing electrochemical performance of COF-based electrode in MIBs are discussed. Hopefully, this review will provide a fundamental guidance for future development of COF-based electrodes.

Design Strategies for High-Performance Aqueous Zn/Organic Batteries

Organic electroactive compounds are attractive to serve as the cathode materials of aqueous zinc-ion batteries (ZIBs) because of their resource renewability, environmentally friendliness and structural diversity. Up to now, various organic electrode materials have been developed and different redox mechanisms are observed in aqueous Zn/organic battery systems. In this Minireview, we present the recent developments in the energy storage mechanisms and design of the organic electrode materials of aqueous ZIBs, including carbonyl compounds, imine compounds, conductive polymers, nitronyl nitroxides, organosulfur polymers and triphenylamine derivatives. Furthermore, we highlight the design strategies to improve their electrochemical performance in the aspects of specific capacity, output voltage, cycle life and rate capability. Finally, we discuss the challenges and future perspectives of aqueous Zn/organic batteries.

A Self-Polymerized Nitro-Substituted Conjugated Carbonyl Compound as High-Performance Cathode for Lithium-Organic Batteries

Conjugated carbonyl compounds have received much attention as cathode materials for developing green lithium-ion batteries (LIBs). However, their high dissolution and poor electronic conductivity in organic electrolyte restrict their further application. Herein, a self-polymerized nitro-substituted conjugated carbonyl compound (2,7-dinitropyrene-4,5,9,10-tetraone, PT-2 NO₂ ) is applied as a high-performance cathode material for LIBs. PT-2 NO₂ exhibits a high reversible capacity of 153.9 mAh g^-1 at 50 mA g^-1 after 120 cycles, which is higher than that of other substituted compounds. Detailed characterization and theoretical calculations have testified that PT-2 NO₂ is transformed into an azo polymer through an irreversible reductive coupling reaction in the first discharge process, and then carbonyl and azo groups reversibly react with Li ions in subsequent cycles. In addition, this azo polymer is also synthesized and applied as the electrode material, which shows similar electrochemical performance to PT-2 NO₂ but with higher initial coulombic efficiency. Thus, this work provides a simple but effectively way to construct organic cathode materials with multiple redox sites for green and high-performance LIBs.

Molecular Design Strategy for High-Redox-Potential and Poorly Soluble n-Type Phenazine Derivatives as Cathode Materials for Lithium Batteries

The n-type phenazine (PZ) derivatives represent an emerging class of cathode materials in lithium batteries for low-cost and sustainable energy storage. However, their low redox potential (<2 V) and high solubility hinder their application to battery systems. To explore and solve such problems in lithium batteries, we investigate the redox characteristics of 13 n-type PZ derivatives and their dissolution behavior in seven organic electrolytes systematically by using DFT calculations. Two decisive factors are observed to tune the redox potentials for these molecules: the first is the electron density around the N active sites and the second is the chelation on lithium by both the active N and the substituent group. Specific approaches that include the reduction of aromatic rings and the introduction of functional groups at β sites in n-type PZ derivatives can improve the redox potential to approximately 3 V. In addition, we develop a new index denoted as E_diff to investigate the solubility of n-type PZ derivatives. The most effective way to reduce the dissolution of electrodes in solvents is to improve intermolecular attraction between the electrode molecules by introducing π-π stacking and hydrogen bonds. Such all-around guidelines should promote the application of n-type PZ-based organic cathodes with a high redox potential and low electrode solubility for lithium batteries.

Benzoquinone- and Naphthoquinone-Bearing Polymers Synthesized by Ring-Opening Metathesis Polymerization as Cathode Materials for Lithium-Ion Batteries

Organic electrode materials have attracted great interest for next-generation lithium-ion batteries owing to their merits of low cost, resource sustainability, and environmental friendliness. Dissolution in organic electrolyte is one of critical factors that limit their development, and constructing corresponding polymers is an effective way to prevent it. Herein, the synthesis of benzoquinone- and naphthoquinone-bearing polymers by ring-opening metathesis polymerization of monomers with an exo-type four-membered ring between polymerizable norbornene and redox-active quinone units is reported. They exhibit significantly reduced solubility and clearly enhanced electrochemical performance. In particular, a high capacity (189.7 mAh g^-1 at 0.1 C, 1 C=216.1 mA g^-1 ), stable cycling (75.6 % capacity retention after 500 cycles at 2 C), and good rate capability (retaining 80.4 % from 0.1 to 2 C) were obtained for the naphthoquinone-bearing polymer, which stand out among naphthoquinone-bearing polymer electrode materials. This work offers rational molecular design and a new polymerization strategy to construct high-performance polymer electrode materials.

Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built-In Functions

A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high-order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built-in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.

An Environmentally Friendly and Flexible Aqueous Zinc Battery Using an Organic Cathode

Rechargeable batteries have been used to power various electric devices and store energy from renewables, but their toxic components (namely, electrode materials, electrolyte, and separator) generally cause serious environment issues when disused. Such toxicity characteristic makes them difficult to power future wearable electronic devices. Now an environmentally friendly and highly safe rechargeable battery, based on a pyrene-4,5,9,10-tetraone (PTO) cathode and zinc anode in mild aqueous electrolyte is presented. The PTO-cathode shows a high specific capacity (336 mAh g^-1 ) for Zn²⁺ storage with fast kinetics and high reversibility. Thus, the PTO//Zn full cell exhibits a high energy density (186.7 Wh kg^-1 ), supercapacitor-like power behavior and long-term lifespan (over 1000 cycles). Moreover, a belt-shaped PTO//Zn battery with robust mechanical durability and remarkable flexibility is first fabricated to clarify its potential application in wearable electronic devices.