Covalent–Organic Frameworks: Advanced Organic Electrode Materials for Rechargeable Batteries

Covalent–organic frameworks (COFs), featuring structural diversity, framework tunability and functional versatility, have emerged as promising organic electrode materials for rechargeable batteries and garnered tremendous attention in recent years. The adjustable pore configuration, coupled with the functionalization of frameworks through pre‐ and post‐synthesis strategies, enables a precise customization of COFs, which provides a novel perspective to deepen the understanding of the fundamental problems of organic electrode materials. In this review, a summary of the recent research into COFs electrode materials for rechargeable batteries including lithium‐ion batteries, sodium‐ion batteries, potassium‐ion batteries, and aqueous zinc batteries is provided. In addition, this review will also cover the working principles, advantages and challenges, strategies to improve electrochemical performance, and applications of COFs in rechargeable batteries.     

Metal‐Organic Framework‐Derived Carbons for Battery Applications

The applications of carbon and carbon‐based materials with high porosity, high surface area, and functionalities based on metal‐organic framework precursors and/or templates have attracted significant research interest in recent years, particularly in the field of batteries. The chemical and physical properties of carbon and carbon‐based materials obtained by the heat treatment of various metal‐organic framework precursors or templates are improved to a certain extent. In this comprehensive review, the synthetic methods and electrochemical performance of carbon materials derived from metal‐organic frameworks (metal/carbon, metal oxide/carbon, nitrogen‐doped carbon, porous carbon, etc.) along with their applications in batteries are outlined.     

Carbonyls: Powerful Organic Materials for Secondary Batteries

The application of organic carbonyl compounds as high performance electrode materials in secondary batteries enables access to metal‐free, low‐cost, environmental friendly, flexible, and functional rechargeable energy storage systems. Organic compounds have so far not received much attention as potential active materials in batteries, mainly because of the success of inorganic materials in both research and commercial applications. However, new requirements in secondary batteries such as flexibility accompanied with low production costs and environmental friendliness, in particular for portable devices, reach the limit of inorganic electrode materials. Organic carbonyl compounds represent the most promising materials to satisfy these needs. Here, recent efforts of the research in the field of organic carbonyl materials for secondary batteries are summarized, and the working principle and the structural design of different groups of carbonyl material is presented. Finally, the influence of conductive additives and binders on the cell performance is closely evaluated for each class of materials.     

Stable Li–Organic Batteries with Nafion‐Based Sandwich‐Type Separators

Similar to Li–S batteries, Li–organic batteries have also been plagued by the dissolution of active materials and the resulting shuttle effect for many years. An effective strategy to eliminate the shuttle effect is adopting solid electrolytes or Li–ion permselective separators to prohibit the dissolved electroactive species from migrating to the Li anode. A polypropylene/Nafion/polypropylene (PNP) sandwich‐type separator is reported with many advantages in comparison with previously reported LISICON, polymer electrolyte, and other Nafion utilization forms. The physical and chemical properties of PNP separators are studied in detail by cross‐section scanning electron microscopy (SEM), infrared spectroscopy (IR), and electrochemical impedance spectroscopy. 1,1′‐Iminodianthraquinone (IDAQ), a novel organic cathode, is taken as an example to quantitatively investigate the function of PNP separators. In the presence of PNP5 with the most appropriate Nafion loading of 0.5 mg cm^–2, IDAQ is able to achieve dramatically improved cycling stability with capacity retention of 76% after 400 cycles and Coulombic efficiency above 99.6%, which reaches the highest level for reported soluble organic electrode materials. Besides Li–organic batteries, such kind of Nafion‐based sandwich‐type separators are also promising for Li–S batteries and other new battery designs involving dissolved electroactive species.     

Recent Progress of the Solid‐State Electrolytes for High‐Energy Metal‐Based Batteries

Secondary batteries based on metal anodes (e.g., Li, Na, Mg, Zn, and Al) are among the most sought‐after candidates for next‐generation mobile and stationary storage systems because they are able to store a larger amount of energy per unit mass or volume. However, unstable electrodeposition and uncontrolled interfacial reactions occuring in liquid electrolytes cause unsatisfying cell performance and potential safety concerns for the commercial application of these metal anodes. Solid‐state electrolytes (SSEs) having a higher modulus are considered capable of inhibiting difficulties associated with the anodes and may enable building of safe all‐solid‐state metal batteries, yet several challenges, such as insufficient room‐temperature ionic conductivity and poor interfacial stability between the electrode and the electrolyte, hinder the large‐scale development of such batteries. Here, research and development of SSEs including inorganic ceramics, organic solid polymers, and organic–inorganic hybrid/composite materials for metal‐based batteries are reviewed. The comparison of different types of electrolytes is discussed in detail, in the context of electrochemical energy storage applications. Then, the focus of this study is on recent advances in a range of attractive and innovative battery chemistries and technologies that are enabled by SSEs. Finally, the challenges and future perspectives are outlined to foresee the development of SSEs.     

Trends in Aluminium‐Based Intercalation Batteries

Over the last decade, optimizing energy storage has become significantly important in the field of energy conversion and sustainability. As a result of immense progress in the field, cost‐effective and high performance batteries are imperative to meeting the future demand of sustainability. Currently, the best performing batteries are lithium‐ion based, but limited lithium (Li) resources make research into alternatives essential. In recent years, the performance of aluminium‐ion batteries has improved remarkably in all battery‐relevant metrics, which renders them a promising alternative. Compared with monovalent Li‐ion batteries, aluminium (Al) cations can carry three positive charges, which could result in higher energy densities. This review describes recent developments in Al‐based cathode materials. The major goal of this review is to highlight strengths and weaknesses of various different approaches and provide guidelines for future research.     

Tattooing Dye as a Green Electrode Material for Lithium Batteries

Lawsone (2‐hydroxy‐1,4‐naphthoquinone), a naturally derived red‐orange dye, is investigated as a promising cathode material for next‐generation lithium batteries. Lithium cells based on lawsone cathode display a high discharge capacity of 280 mA h g^?1 (99% theoretical capacity), a high energy density of 664 W h kg^?1, and long life of 1000 cycles at 0.5 C along with good rate performance up to 5 C. These results represent significant improvements from previously reported organic cathode materials, and surpass those of conventional lithium batteries based on LiCoO₂ cathodes (140 mA h g^?1 and 520 W h kg^?1, respectively). Its success stems from the unique 2D planar packing of lawsone molecules, with maximized overlap of adjacent p orbitals for redox active sites. The result is the simultaneous enhancement of electrical and ionic conductivities that are an order of magnitude higher than those of other synthetic quinones. Given that lawsone is derived from the henna plant and has long been used as a dye for human hair and skin, this work may open a new chapter in the design of future green batteries.     

Routes to High Energy Cathodes of Sodium‐Ion Batteries

Sodium‐ion batteries (SIBs) are now being actively developed as low cost and sustainable alternatives to lithium‐ion batteries (LIBs) for large‐scale electric energy storage applications. In recent years, various inorganic and organic Na compounds, mostly mimicked from their Li counterparts, have been synthesized and tested for SIBs, and some of them indeed demonstrate comparable specific capacity to the presently developed LIB electrodes. However, the lack of suitable cathode materials is still a major obstacle to the commercial development of SIBs. Here, we present a brief review on the recent developments of SIB cathodes, with a focus on low cost and high energy density materials (> 450 Wh kg^?1 vs Na) together with discussion of their Na‐storage mechanisms. The considerable differences in the structural requirements for Li‐ and Na‐storage reactions mean that it is not sufficient to design SIB cathode materials by simply mimicking LIB materials, and therefore great efforts are needed to discover new materials and reaction mechanisms to further develop variable cathodes for advanced SIB technology. Some directions for future research and possible strategies for building advanced cathode materials are also proposed here.     

Zwitterions for Organic/Perovskite Solar Cells,Light‐Emitting Devices,and Lithium Ion Batteries: Recent Progress and Perspectives

Zwitterions, a class of materials that contain covalently bonded cations and anions, have been extensively studied in the past decades owing to their special features, such as excellent solubility in polar solvents, for solution processing and dipole formation for the transfer of carriers and ions. Recently, zwitterions have been developed as electrode modifiers for organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light‐emitting devices (OLEDs), as well as electrolyte additives for lithium ion batteries (LIBs). With the rapid advances of zwitterionic materials, high‐performance devices have been constructed with enhanced efficiencies by introducing them as interface layers and electrolyte additives. In this review, recent progress in OSCs, PVSCs, OLEDs, and LIBs by using zwitterions is highlighted. The authors also elaborate the role of various zwitterionic materials as interfacial layers and additives for highly efficient OSCs, PVSCs, OLEDs, and LIBs. This article presents an overview of device performance of zwitterionic materials. The structure–property relationship is also discussed. Finally, the prospects of zwitterion materials are also addressed.     

A Safe Organic Oxygen Battery Built with Li‐Based Liquid Anode and MOFs Separator

Safe rechargeable batteries of improved energy density and high power performance are urgently needed for the development of large electric devices. Herein, an Li‐based organic liquid anode is proposed, and an organic oxygen battery with a metal organic framework membrane separator is realized, which is able to conduct Li ions and separate other large species in the system. Equipped with the dual redox mediator strategy, the organic oxygen battery exhibits superior rate performance with long cycling life and low overpotential. A “solid electrolyte interface”‐like layer is observed between the organic liquid anode and the ion conducting separator. This work not only introduces a new type of anode for Li‐based batteries, but also provides fundamental insights for the better application of biphenyl‐based liquid anodes.     

All‐Organic Battery Composed of Thianthrene‐ and TCAQ‐Based Polymers

An all‐organic battery consisting of two redox‐polymers, namely poly(2‐vinylthianthrene) and poly(2‐methacrylamide‐TCAQ) is assembled. This all‐organic battery shows excellent performance characteristics, namely flat discharge plateaus, an output voltage exceeding 1.3 V, and theoretical capacities of both electrodes higher than 100 mA h g^?1. Both organic electrode materials are synthesized in two respective three synthetic steps using the free‐radical polymerization technique. Li‐organic batteries manufactured from these polymers prove their suitability as organic electrode materials. The cathode material poly(2‐vinylthianthrene) (3) displays a discharging plateau at 3.95 V versus Li⁺/Li and a discharge capacity of 105 mA h g^?1, corresponding to a specific energy of about 415 mW h g^?1. The anode material poly(2‐methacrylamide‐TCAQ) (7) exhibits an initial discharge capacity of 130 mA h g^?1, corresponding to 94% material activity. The combination of both materials results in an all‐organic battery with a discharge voltage of 1.35 V and an initial discharge capacity of 105 mA h g^?1 (95% material activity).     

Exfoliated Triazine‐Based Covalent Organic Nanosheets with Multielectron Redox for High‐Performance Lithium Organic Batteries

The development of the next‐generation lithium ion battery requires environmental‐friendly electrode materials with long cycle life and high energy density. Organic compounds are a promising potential source of electrode materials for lithium ion batteries due to their advantages of chemical richness at the molecular level, cost benefit, and environmental friendliness, but they suffer from low capacity and dissatisfactory cycle life mainly due to hydrophobic dissolution in organic electrolytes and poor electronic conductivity. In this work, two types of triazine‐based covalent organic nanosheets (CONs) are exfoliated and composited with carbon nanotubes. The thin‐layered 2D structure for the exfoliated CONs can activate more functional groups for lithium storage and boost the utilization efficiency of redox sites compared to its bulk counterpart. Large reversible capacities of above 1000 mAh g^?1 can be achieved after 250 cycles, which is comparable to high‐capacity inorganic electrodes. Moreover, the lithium‐storage mechanism is determined to be an intriguing 11 and 16 electron redox reaction, associated with the organic groups (unusual triazine ring, piperazine ring, and benzene ring, and common C?N, ? NH? groups).     

Hybrid and Aqueous Lithium‐Air Batteries

Lithium‐air (Li‐air) batteries have become attractive because of their extremely high theoretical energy density. However, conventional Li‐air cells operating with non‐aqueous electrolytes suffer from poor cycle life and low practical energy density due to the clogging of the porous air cathode by insoluble discharge products, contamination of the organic electrolyte and lithium metal anode by moist air, and decomposition of the electrolyte during cycling. These difficulties may be overcome by adopting a cell configuration that consists of a lithium‐metal anode protected from air by a Li⁺‐ion solid electrolyte and an air electrode in an aqueous catholyte. In this type of configuration, a Li⁺‐ion conducting “buffer” layer between the lithium‐metal anode and the solid electrolyte is often necessary due to the instability of many solid electrolytes in contact with lithium metal. Based on the type of buffer layer, two different battery configurations are possible: “hybrid” Li‐air batteries and “aqueous” Li‐air batteries. The hybrid and aqueous Li‐air batteries utilize the same battery chemistry and face similar challenges that limit the cell performance. Here, an overview of recent developments in hybrid and aqueous Li‐air batteries is provided and the factors that influence their performance and impede their practical applications, followed by future directions are discussed.     

Transition‐Metal (Fe,Co, Ni) Based Metal‐Organic Frameworks for Electrochemical Energy Storage

Transition‐metal (Fe, Co, Ni) based metal‐organic framework materials with controllable structures, large surface areas and adjustable pore sizes have attracted wide research interest for use in next‐generation electrochemical energy‐storage devices. This review introduces the synthesis of transition‐metal (Fe, Co, Ni) based metal‐organic frameworks and their derivatives with the focus on their application in supercapacitors and batteries.     

Utilizing Latent Multi‐Redox Activity of p‐Type Organic Cathode Materials toward High Energy Density Lithium‐Organic Batteries

Organic electrode materials hold great potential due to their cost‐efficiency, eco‐friendliness, and possibly high theoretical capacity. Nevertheless, most organic cathode materials exhibit a trade‐off relationship between the specific capacity and the voltage, failing to deliver high energy density. Herein, it is shown that the trade‐off can be mitigated by utilizing the multi‐redox capability of p‐type electrodes, which can significantly increase the specific capacity within a high‐voltage region. The molecular structure of 5,10‐dihydro‐5,10‐dimethylphenazine is modified to yield a series of phenoxazine and phenothiazine derivatives with elevated redox potentials by substitutions. Subsequently, the feasibility of the multi‐redox capability is scrutinized for these high‐voltage p‐type organic cathodes, achieving one of the highest energy densities. It is revealed that the seemingly impractical second redox reaction is indeed dependent on the choice of the electrolyte and can be reversibly realized by tailoring the donor number and the salt concentration of the electrolyte, which places the voltage of the multi‐redox reaction within the electrochemical stability window. The results demonstrate that high‐energy‐density organic cathodes can be practically achieved by rational design of multi‐redox p‐type organic electrode materials and the compatibility consideration of the electrolyte, opening up a new avenue toward advanced organic rechargeable batteries.     

Improvements to the Overpotential of All‐Solid‐State Lithium‐Ion Batteries during the Past Ten Years

After the research that shows that Li₁₀GeP₂S₁₂ (LGPS)‐type sulfide solid electrolytes can reach the high ionic conductivity at the room temperature, sulfide solid electrolytes have been intensively developed with regard to ionic conductivity and mechanical properties. As a result, an increasing volume of research has been conducted to employ all‐solid‐state lithium batteries in electric automobiles within the next five years. To achieve this goal, it is important to review the research over the past decade, and understand the requirements for future research necessary to realize the practical applications of all‐solid‐state lithium batteries. To date, research on all‐solid‐state lithium batteries has focused on achieving overpotential properties similar to those of conventional liquid‐lithium‐ion batteries by increasing the ionic conductivity of the solid electrolytes. However, the increase in the ionic conductivity should be accompanied by improvements of the electronic conductivity within the electrode to enable practical applications. This essay provides a critical overview of the recent progress and future research directions of the all‐solid‐state lithium batteries for practical applications.     

Nonfullerene n‐Type Organic Semiconductors for Perovskite Solar Cells

Compared to inorganic semiconductors and/or fullerene derivatives, nonfullerene n‐type organic semiconductors present some advantages, such as low‐temperature processing, flexibility, and molecule structure diversity, and have been widely used in perovskite solar cells (PSCs). In this research news article, the recent advances in nonfullerene n‐type organic semiconductors which function as electron‐transporting, interface‐modifying, additive, and light‐harvesting materials in PSCs are summarized. The remaining challenges and promising future directions of nonfullerene‐based PSCs are also discussed.     

Rational Construction of Nitrogen‐Doped Hierarchical Dual‐Carbon for Advanced Potassium‐Ion Hybrid Capacitors

Potassium‐ion hybrid capacitors (PIHCs), elaborately integrate the advantages of high output power as well as long lifespan of supercapacitors and the high energy density of batteries, and exhibit great possibilities for the future generations of energy storage devices. The critical next step for future implementation lies in exploring a high‐rate battery‐type anode with an ultra‐stable structure to match the capacitor‐type cathode. Herein, a “dual‐carbon” is constructed, in which a three‐dimensional nitrogen‐doped microporous carbon polyhedron (NMCP) derived from metal‐organic frameworks is tightly wrapped by two‐dimensional reduced graphene oxide (NMCP@rGO). Benefiting from the synergistic effect of the inner NMCP and outer rGO, the NMCP@rGO exhibits a superior K‐ion storage capability with a high reversible capacity of 386 mAh g^?1 at 0.05 A g^?1 and ultra‐long cycle stability with a capacity of 151.4 mAh g^?1 after 6000 cycles at 5.0 A g^?1. As expected, the as‐assembled PIHCs with a working voltage as high as 4.2 V present a high energy/power density (63.6 Wh kg^?1 at 19 091 W kg^?1) and excellent capacity retention of 84.7% after 12 000 cycles. This rational construction of advanced PIHCs with excellent performance opens a new avenue for further application and development.     

Advanced Phosphorus‐Based Materials for Lithium/Sodium‐Ion Batteries: Recent Developments and Future Perspectives

High‐performance and lost‐cost lithium‐ion and sodium‐ion batteries are highly desirable for a wide range of applications including portable electronic devices, transportation (e.g., electric vehicles, hybrid vehicles, etc.), and renewable energy storage systems. Great research efforts have been devoted to developing alternative anode materials with superior electrochemical properties since the anode materials used are closely related to the capacity and safety characteristics of the batteries. With the theoretical capacity of 2596 mA h g^?1, phosphorus is considered to be the highest capacity anode material for sodium‐ion batteries and one of the most attractive anode materials for lithium‐ion batteries. This work provides a comprehensive study on the most recent advancements in the rational design of phosphorus‐based anode materials for both lithium‐ion and sodium‐ion batteries. The currently available approaches to phosphorus‐based composites along with their merits and challenges are summarized and discussed. Furthermore, some present underpinning issues and future prospects for the further development of advanced phosphorus‐based materials for energy storage/conversion systems are discussed.     

Nonfullerene Acceptors for Semitransparent Organic Solar Cells

Semitransparent organic solar cells (ST‐OSCs) have appealing features, such as flexibility, transparency, and color in addition to generating clean energy, and therefore show potential applications in building integrated photovoltaics and photovoltaic vehicles. Concerted efforts in materials synthesis (particularly low‐band‐gap polymer donors and nonfullerene acceptors) and device optimization (particularly incorporating transparent electrodes) have raised the efficiencies of ST‐OSCs to >10%, with average visible transparency of >30%. In this Research News article, the recent progress in nonfullerene‐based ST‐OSCs is summarized and discussed. The future perspectives and research directions for the ST‐OSCs field are proposed.