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HighlightsFundamental rationalisation for high-energy batteries.Newly emerging and the state-of-the-art high-energy batteries vs. incumbent lithium-ion batteries: performance, cost and safety.Closing the gap between academic research and commercialisation of emerging high-energy batteries, and examination of the remaining challenges.Rechargeable batteries of high energy density and overall performance are becoming a critically important technology in the rapidly changing society of the twenty-first century. While lithium-ion batteries have so far been the dominant choice, numerous emerging applications call for higher capacity, better safety and lower costs while maintaining sufficient cyclability. The design space for potentially better alternatives is extremely large, with numerous new chemistries and architectures being simultaneously explored. These include other insertion ions (e.g. sodium and numerous multivalent ions), conversion electrode materials (e.g. silicon, metallic anodes, halides and chalcogens) and aqueous and solid electrolytes. However, each of these potential “beyond lithium-ion” alternatives faces numerous challenges that often lead to very poor cyclability, especially at the commercial cell level, while lithium-ion batteries continue to improve in performance and decrease in cost. This review examines fundamental principles to rationalise these numerous developments, and in each case, a brief overview is given on the advantages, advances, remaining challenges preventing cell-level implementation and the state-of-the-art of the solutions to these challenges. Finally, research and development results obtained in academia are compared to emerging commercial examples, as a commentary on the current and near-future viability of these “beyond lithium-ion” alternatives.

Organic compounds are desirable for sustainable Li-ion batteries (LIBs), but the poor cycle stability and low power density limit their large-scale application. Here we report a family of organic compounds containing azo group (N=N) for reversible lithiation/delithiation. Azobenzene-4,4′-dicarboxylic acid lithium salt (ADALS) with an azo group in the center of the conjugated structure is used as a model azo compound to investigate the electrochemical behaviors and reaction mechanism of azo compounds. In LIBs, ADALS can provide a capacity of 190 mAh g−1 at 0.5 C (corresponding to current density of 95 mA g−1) and still retain 90%, 71%, and 56% of the capacity when the current density is increased to 2 C, 10 C, and 20 C, respectively. Moreover, ADALS retains 89% of initial capacity after 5,000 cycles at 20 C with a slow capacity decay rate of 0.0023% per cycle, representing one of the best performances in all organic compounds. Superior electrochemical behavior of ADALS is also observed in Na-ion batteries, demonstrating that azo compounds are universal electrode materials for alkali-ion batteries. The highly reversible redox chemistry of azo compounds to alkali ions was confirmed by density-functional theory (DFT) calculations. It provides opportunities for developing sustainable batteries.

Benefiting from the advantageous features of structural diversity and resource renewability, organic electroactive compounds are considered as attractive cathode materials for aqueous Zn‐ion batteries (ZIBs). In this review, we discuss the recent developments of organic electrode materials for aqueous ZIBs. Although the proton (H+) storage chemistry in aqueous Zn‐organic batteries has triggered an overwhelming literature surge in recent years, this topic remains controversial. Therefore, our review focuses on this significant issue and summarizes the reported electrochemical mechanisms, including pure Zn2+ intercalation, pure H+ storage, and H+/Zn2+ co‐storage. Moreover, the impact of H+ storage on the electrochemical performance of aqueous ZIBs is discussed systematically. Given the significance of H+ storage, we also highlight the relevant characterization methods employed. Finally, perspectives and directions on further understanding the charge storage mechanisms of organic materials are outlined. We hope that this review will stimulate more attention on the H+ storage chemistry of organic electrode materials to advance our understanding and further its application. This review focuses on proton (H+) storage chemistry in aqueous Zn‐organic batteries and summarizes the reported electrochemical mechanisms as well as the impact of H+ storage on the electrochemical performance.

Organic electrode materials take advantages of potentially sustainable production and structural tunability compared with present commercial inorganic electrode materials. However, their applications in traditional rechargeable batteries with nonaqueous electrolytes suffer from the premature failure and safety concerns. In comparison, aqueous rechargeable batteries based on organic electrode materials have received extensive attentions in recent years for low-cost and sustainable energy storage systems due to their inherent safety. This review aims to provide a comprehensive summary on the recent progress in advanced organic electrode materials for aqueous rechargeable batteries. We start from the overview of working principles and general design strategies of organic electrode materials in aqueous rechargeable batteries. Then the research advances of organic electrode materials in various aqueous rechargeable batteries are highlighted in terms of charge carriers (monovalent ions, multivalent ions, and anions). We emphasized the characteristics of organic electrode materials in various charge carriers. Finally, the critical challenges and future efforts of aqueous organic rechargeable batteries are discussed. More organic electrode materials with better electronic conductivity and fast reaction kinetics are still needed to build advanced aqueous batteries for commercial applications.

The application of flexible electronics in the field of communication has made the transition from rigid physical form to flexible physical form. Flexible electrode technology is the key to the wide application of flexible electronics. However, flexible electrodes will break when large deformation occurs, failing flexible electronics. It restricts the further development of flexible electronic technology. Flexible stretchable electrodes are a hot research topic to solve the problem that flexible electrodes cannot withstand large deformation. Flexible stretchable electrode materials have excellent electrical conductivity, while retaining excellent mechanical properties in case of large deformation. This paper summarizes the research results of flexible stretchable electrodes from three aspects: material, process, and structure, as well as the prospects for future development.

With the increasing scarcity of non-recyclable resources such as international fossil energy and coal industries, representatives from various countries at the Paris Summit advocated the strategy of “carbon neutrality, carbon peaking” and promoted the use of recyclable resources, namely electricity. There are many ways to generate electricity, such as solar power, hydroelectric power, thermal power, tidal power and wind power. However, the method of mobile storage of electricity is relatively single - using batteries. The most widely used mobile battery currently is lithium-ion battery. This research will focus on the different choices of cathode materials for lithium-ion batteries, analysing and comparing their performance, functional stability, lifespan, capacity, price, and working conditions under extreme temperatures. At the same time, in-depth analysis will be conducted on existing electrode material recycling methods. Finally, the development direction of electrode materials with economic benefits and promising development prospects, as well as recycling methods that are more in line with economic benefits and increase resource utilization, are proposed.

Small-molecule organic electrode materials (SMOEMs) have shown tremendous potential as cathodes or anodes for various rechargeable batteries including lithium and sodium batteries, due to their easy material availability, high structure designability, attractive theoretical capacity, and wide adaptability to counterions. However, they suffer from the severe dissolution problem and the subsequent shuttle effect in nonaqueous electrolytes, which cause the poor cycling stability and Coulombic efficiency. To satisfy the demands on the energy density and cycling stability simultaneously, the molecular structures of SMOEMs need to be rationally designed, and extrinsic approaches including electrode engineering and electrolyte optimizations can be further conducted. In this review, we summarize the fundamental knowledge about SMOEMs, including their working principles and applications, structure classifications, molecular structure design methods, and extrinsic optimization strategies. Moreover, we also provide some original insights aiming at guiding the research and development of SMOEMs in a more scientific and practical way. In brief, SMOEMs are facing huge opportunities and challenges as candidates to enable the next-generation of efficient, sustainable, and green rechargeable batteries.

For hybrid electric vehicles, supercapacitors are an attractive technology which, when used in conjunction with the batteries as a hybrid system, could solve the shortcomings of the battery. Supercapacitors would allow hybrid electric vehicles to achieve high efficiency and better power control. Supercapacitors possess very good power density. Besides this, their charge-discharge cycling stability and comparatively reasonable cost make them an incredible energy-storing device. The manufacturing strategy and the major parts like electrodes, current collector, binder, separator, and electrolyte define the performance of a supercapacitor. Among these, electrode materials play an important role when it comes to the performance of supercapacitors. They resolve the charge storage in the device and thus decide the capacitance. Porous carbon, conductive polymers, metal hydroxide, and metal oxides, which are some of the usual materials used for the electrodes in the supercapacitors, have some limits when it comes to energy density and stability. Major research in supercapacitors has focused on the design of stable, highly efficient electrodes with low cost. In this review, the most recent electrode materials used in supercapacitors are discussed. The challenges, current progress, and future development of supercapacitors are discussed as well. This study clearly shows that the performance of supercapacitors has increased considerably over the years and this has made them a promising alternative in the energy sector.

p‐Type redox‐active organic materials (ROMs) draw increasing attention as a promising alternative to conventional inorganic electrode materials in secondary batteries due to high redox voltage, fast rate capability, environment friendliness, and abundance. First, fundamental properties of the p‐type ROMs regarding the energy levels and the anion‐related chemistry are briefly introduced. Then, the development progress of the p‐type ROMs is outlined in this review by classifying them according to their redox centers. The molecular design strategies employed for improving their electrochemical performance are discussed to guide further research. Finally, a summary of the electrochemical performance achieved, regarding voltage, specific energy with power, and cycle stability, is provided with perspectives. Herein, p‐type redox‐active organic materials (ROMs) are focused on for high‐performance rechargeable batteries. Herein this review, the basic properties of p‐type ROMs are systematically discussed by presenting representative structures, their challenges, and future prospects. To design outperforming p‐type ROMs, the electrochemical properties, such as redox chemistry, energy levels, and intercalated counter anions, are considered comprehensively.

Organic material electrodes are regarded as promising candidates for next-generation rechargeable batteries due to their environmentally friendliness, low price, structure diversity, and flexible molecular structure design. However, limited reversible capacity, high solubility in the liquid organic electrolyte, low intrinsic ionic/electronic conductivity, and low output voltage are the main problems they face. A lot of research work has been carried out to explore comprehensive solutions to the above problems through molecular structure design, the introduction of specific functional groups and specific molecular frameworks, from small molecules to polymer molecules, metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and heterocyclic molecules; from simple organic materials to organic composites; from single functional groups to multi-functional groups; etc. The inevitable relationship between various molecular structure design and enhanced electrochemical properties has been illustrated in detail. This work also specifically discusses several approaches for the current application of organic compounds in batteries, including interfacial protective layer of inorganic metal oxide cathode, anode (metal lithium or silicon) and solid-state electrolyte, and host materials of sulfur cathode and redox media in lithium-sulfur batteries. This overview provides insight into a deep understanding of the molecular structure of organic electrode materials (OEMs) and electrochemical properties, broadens people’s research ideas, and inspires researchers to explore the advanced application of electroactive organic compounds in rechargeable batteries. Graphical Abstract