An Insoluble Benzoquinone‐Based Organic Cathode for Use in Rechargeable Lithium‐Ion Batteries

Carbonyl Chemistry for Advanced Electrochemical Energy Storage Systems

On the basis of the sustainable concept, organic compounds and carbon materials both mainly composed of light C element have been regarded as powerful candidates for advanced electrochemical energy storage (EES) systems, due to theie merits of low cost, eco-friendliness, renewability, and structural versatility. It is investigated that the carbonyl functionality as the most common constituent part serves a crucial role, which manifests respective different mechanisms in the various aspects of EES systems. Notably, a systematical review about the concept and progress for carbonyl chemistry is beneficial for ensuring in-depth comprehending of carbonyl functionality. Hence, a comprehensive review about carbonyl chemistry has been summarized based on state-of-the-art developments. Moreover, the working principles and fundamental properties of the carbonyl unit have been discussed, which has been generalized in three aspects, including redox activity, the interaction effect, and compensation characteristic. Meanwhile, the pivotal characterization technologies have also been illustrated for purposefully studying the related structure, redox mechanism, and electrochemical performance to profitably understand the carbonyl chemistry. Finally, the current challenges and promising directions are concluded, aiming to afford significant guidance for the optimal utilization of carbonyl moiety and propel practicality in EES systems.

A Long-Life and Excellent Rate-Capability Aqueous Zn-Benzoquinone Battery Enabled by Iodine-Catalyzed Cathode

Benzoquinone (BQ) is considered to be a desirable cathode material for aqueous zinc-based batteries. The major limitations of BQ electrode are the severe sublimation and poor electrical conductivity, which results in serious mass loss during electrode preparation and inferior rate performance. In this study, iodine (I2) species are utilized as an efficient catalyst for the highly reversible conversion of BQ/BQ2- couple in the Zn-BQ battery system, wherein N-doped porous carbon is employed as a host material for anchoring the BQ molecule. In the combination electrode (denoted as BQ-I@NPC) with 1wt% I2 additive where I2 can serve as a carrier to accelerates the Zn2+ transmission, and reduce the voltage hysteresis of the electrode. As a result, the BQ-I@NPC cathode delivers a high specific capacity of ≈482 mAh g-1 at 0.25 A g-1, realizing a high energy density of 545 Wh kg-1 (based on BQ), which is the highest values among reported organic cathode materials for aqueous Zn-based batteries. Also, a high BQ loading (8 mg cm-2) can be attained, and achieving a superior cycling stability with a capacity retention of ≈80% after 20,000 times at 10 C. The work proposes an effective approach toward high performance organic electrode materials.

Fe doping mechanism of Na0.44MnO2 tunnel phase cathode electrode in sodium-ion batteries

Due to its stability and low cost, the tunnel-style sodium-manganese oxide (Na0.44MnO2) material is deemed a popular cathode choice for sodium-ion rechargeable batteries. However, the Jahn-Teller effect caused by Mn3+ in the material results in poor capacity and cycling stability. The purpose of this experimental study is to partially replace Mn3+ with Fe3+, in order to reduce the Jahn-Teller effect of the material during charging and discharging process. The results of Raman spectroscopy and X-ray photoelectron spectroscopy confirmed that the content of Mn3+ decreased after Fe3+ doping. Electrochemical studies show that the Na0.44Mn0.994Fe0.006O2 cathode has better rate performance (exhibits a reversible capacity of 87.9 mAh/g at 2 C) and cycle stability in sodium-ion batteries. The diffusion coefficient of sodium ions increases by Fe3+ doping. The excellent rate performance and capacity improvement are verified by density functional theory (DFT) calculation. After doping, the band gap decreases significantly, and the results show that the state density of O 2p increases near the Fermi level, which promotes the oxidation-reduction of oxygen. This work provides a straightforward approach to enhance the performance of Na0.44MnO2 nanorods, and this performance improvement has guiding significance for the design of other materials in the energy storage domain.

A Layered Organic Cathode for High-Energy, Fast-Charging, and Long-Lasting Li-Ion Batteries

Eliminating the use of critical metals in cathode materials can accelerate global adoption of rechargeable lithium-ion batteries. Organic cathode materials, derived entirely from earth-abundant elements, are in principle ideal alternatives but have not yet challenged inorganic cathodes due to poor conductivity, low practical storage capacity, or poor cyclability. Here, we describe a layered organic electrode material whose high electrical conductivity, high storage capacity, and complete insolubility enable reversible intercalation of Li+ ions, allowing it to compete at the electrode level, in all relevant metrics, with inorganic-based lithium-ion battery cathodes. Our optimized cathode stores 306 mAh g-1cathode, delivers an energy density of 765 Wh kg-1cathode, higher than most cobalt-based cathodes, and can charge-discharge in as little as 6 min. These results demonstrate the operational competitiveness of sustainable organic electrode materials in practical batteries.

Selective electrocatalytic transformation of highly toxic phenols in wastewater to para-benzoquinone at ambient conditions

The selective transformation of organics from wastewater to value-added chemicals is considered an upcycling process beneficial for carbon neutrality. Herein, we present an innovative electrocatalytic oxidation (ECO) system aimed at achieving the selective conversion of phenols in wastewater to para-benzoquinone (p-BQ), a valuable chemical widely utilized in the manufacturing and chemical industries. Notably, 96.4% of phenol abatement and 78.9% of p-BQ yield are synchronously obtained over a preferred carbon cloth-supported ruthenium nanoparticles (Ru/C) anode. Such unprecedented results stem from the weak Ru-O bond between the Ru active sites and generated p-BQ, which facilitates the desorption of p-BQ from the anode surface. This property not only prevents the excessive oxidation of the generated p-BQ but also reinstates the Ru active sites essential for the rapid ECO of phenol. Furthermore, this ECO system operates at ambient conditions and obviates the need for potent chemical oxidants, establishing a sustainable avenue for p-BQ production. Importantly, the system efficacy can be adaptable in actual phenol-containing coking wastewater, highlighting its potential practical application prospect. As a proof of concept, we construct an electrified Ru/C membrane for ECO of phenol, attaining phenol removal of 95.8% coupled with p-BQ selectivity of 73.1%, which demonstrates the feasibility of the ECO system in a scalable flow-through operation mode. This work provides a promising ECO strategy for realizing both phenols removal and valuable organics recovery from phenolic wastewater.

Sun Light Responsive 2D Covalent-Organic Frameworks Platform as a Catalysts Boost C-H Bond Arylation and Dopamine Regeneration

Photocatalysis is one of the most promising methods for producing organic compounds with a renewable source of energy. Two-dimensional covalent organic frameworks (2D COFs) are a type of polymer that has developed as a potential light-harvesting catalyst for artificial photosynthesis with a design-controllable platform that might be developed into a new type of cost-effective and metal-free photocatalyst. Here, we present a two-dimensional covalent organic framework synthesis technique as a low-cost and highly efficient visible light active flexible photocatalyst for C-H bond activation and dopamine regeneration. 2D COF were synthesized from tetramino-benzoquinone (TABQ) and terapthaloyl chloride monomer through condensation polymerization reaction and the resultant photocatalyst have remarkable performance due to its visible light-harvesting capacity, appropriate band gap, and highly organized π-electron channels. The synthesized photocatalyst is capable to convert dopamine into leucodopaminechrome with a higher yield (77.08%) and also capable to activate the C-H bond between 4-nitrobenzenediazonium tetrafluoroborate and pyrrole.

Trinitroaromatic Salts as High-Energy-Density Organic Cathode Materials for Li-Ion Batteries

Even though organic molecules with designed structures can be assembled into high-capacity electrode materials, only limited functional groups such as -C═O and -C═N- could be designed as high-voltage cathode materials with enough high capacity. Here, we propose a common chemical raw material, trinitroaromatic salt, to have promising potential to develop organic cathode materials with high discharge voltage and capacity through a strong delocalization effect between -NO₂ and aromatic ring. Our first-principles calculations show that electrochemical reactions of trinitroaromatic potassium salt C₆H₂(NO₂)₃OK are a 6-electron charge-transfer process, providing a high discharge capacity of 606 mAh g^-1 and two voltage plateaus of 2.40 and 1.97 V. Electronic structure analysis indicates that the discharge process from C₆H₂(NO₂)₃OK to C₆H₂(NO₂Li₂)₃OK stabilizes oxidized [C₆]ⁿ⁺ to achieve a stable conjugated structure through electron delocalization from -NO₂ to [C₆]ⁿ⁺. The ordered layer structure C₆H₂(NO₂)₃OK can provide large spatial pore channels for Li-ion transport, achieving a high ion diffusion coefficient of 3.41 × 10^-6 cm² s^-1.

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.

Graphical abstract

Enhanced cycling stability and rate capability of a graphene-supported commercialized Vat Blue 4 anode for advanced Li-ion batteries

Commercialized Vat Blue 4 (VB4) has attracted more attention as a promising anode for large-scale applications in Li-ion batteries (LIBs) due to its high electrochemical activity, low price, and large-scale production. However, its moderate solubility results in severe capacity decay and low utilization of active components. Herein, we present a graphene-supported VB4 composite (VB4/rGO) prepared by a facile sonication and hydrothermal process for long cycling stability and high-rate capability. This design can significantly enhance the Li-storage properties, including high capacity (1045 mA h g-1 at 0.1 A g-1), long cycling stability (537 mA h g-1 even over 1000 cycles at 1 A g-1), and rate capability (315 mA h g-1 at 5 A g-1). Strong π-π interaction derived from the aromatic rings within the π-conjugated system (graphene and VB4) and spatial confinement in-between graphene sheets both can suppress the high solubility of VB4 for superior capacity retention. Moreover, conductive graphene and channels in-between nanosheets can simultaneously facilitate the electron and Li+ transfer. This work demonstrates a simple and effective method to improve the electrochemical performance of commercialized Vat dyes and provides a low-cost and large-scale strategy to develop their practical application in the energy storage field.

Synthetic Control of Electronic Property and Porosity in Anthraquinone-Based Conjugated Polymer Cathodes for High-Rate and Long-Cycle-Life Na-Organic Batteries

Redox-active carbonyl-containing compounds have received extensive attention as cathode materials for sodium-ion batteries (SIBs) because of their excellent attributes, including elemental sustainability, high theoretical capacity, diverse structures, and tunable properties. However, the storage of Na⁺ in most carbonyl-based cathode materials is plagued by the low capacity, unsatisfying rate performance, and short cycling life. Herein, we develop a series of anthraquinone-based conjugated polymer cathodes consisting of anthraquinone and benzene with different linking patterns. It reveals that the linkage sites on benzene ring could affect the electronic structures of the resulting polymers and thus their charge-storage capabilities. The 1,2,4,5-linkage on benzene leads to a high surface area, a narrow band gap, and the lowest unoccupied molecular orbital for the resulting polymer PBAQ-3. As a cathode for SIBs, it delivers a high capacity of around 200 mAh g^-1 and excellent rate performance (105 mAh g^-1 at 200 C) as well as stable cycling with a capacity retention of 95.8% after 1000 cycles at 0.05 A g^-1 and 83.1% after 40000 cycles at 3 A g^-1. Our findings highlight the influence of linking patterns of the building blocks on the electrochemical performance of organic electrodes.

Self-Sacrificing Template of the POMs-Based Composite for the High-Performance Organic-Inorganic Hybrid Cathode of Lithium-Ion Batteries

The high theoretical capacity of vanadium oxides makes them promising cathode candidates for the rechargeable lithium-ion batteries (LIBs). Nevertheless, the relatively poor electrical conductivity and capacity retention hinder the practical application and have to be overcome urgently for the increasing demand for storage technologies. Herein, a new BRG system composed of bimetallic oxide/rhodamine B (RB)/reduced graphene oxide (RGO) was prepared through the facile self-sacrificing template of the precursor polyoxometalate (POM) composites POMs/RB/RGO (PRG). RB not only acts as a cationic mediator to facilitate the loading of POMs on graphene for conversion to oxides but also promotes the formation of uniform nanorods on the RGO. The prepared composite FeV₃O₈-RB/RGO-1 as the cathode exhibits superior cycling stability (specific capacity of 225 mA h g^-1 at 100 mA g^-1) and elastic rate capabilities for LIBs. What is more, the new PRG precursor provides versatile possibilities for the design of oxide composites from the self-sacrificing template of POMs-based composites with abundant architectural designs and compositions for the energy storage system.

High-Potential Cathodes with Nitrogen Active Centres for Quasi-Solid Proton-Ion Batteries

Although proton-ion batteries have received considerable attention owing to their reliability, safety, toxin-free nature, and low cost, their development remains in the early stages because of lacking proper electrolytes and cathodes for facilitating a high output voltage and stable cycle performance. We present a novel cathode based on active nitrogen centre, which provides a flat discharge plateau at 1 V with a capacity of 115 mAh g^-1 and excellent stability. Moreover, a quasi-solid electrolyte was developed to overcome the issue of corrosion, broaden the potential window of the electrolyte, and prevent the active material from dissolving. While using the unique as-developed electrolyte, the newly designed cathode retained 89.67 % of its original capacity after 2000 cycles. Finally, we demonstrated the excellent cycle performance of the as-developed metal-free, flexible, soft-packed battery. Notably, even when a portion of the battery was cut off, it continued to function normally.

Quinone-based conjugated polymer cathodes synthesized via direct arylation for high performance Li-organic batteries

Direct arylation cross-coupling reaction was employed to prepare quinone-based conjugated polymer cathodes, which realize a high reversible capacity of 200 mA h g^-1 at 0.05 A g^-1, an excellent rate capability of 111 mA h g^-1 at 30 A g^-1 (150C), and a stable cycling performance for more than 3000 cycles.

Cross-Linked PVA/HNT Composite Separator Enables Stable Lithium-Organic Batteries under Elevated Temperature

Li-organic batteries (LOBs) are promising advanced battery systems because of their unique advantages in capacity, cost, and sustainability. However, the shuttling effect of soluble organic redox intermediates and the intrinsic dissolution of small-molecular electrodes have hindered the practical application of these cells, especially under high operating temperatures. Herein, a cross-linked membrane with abundant negative charge for high-temperature LOBs is prepared via electrospinning of poly(vinyl alcohol) containing halloysite nanotubes (HNTs). The translocation of negatively charged organic intermediates can be suppressed by the electronic repulsion and the cross-linked network while the positively charged Li⁺ are maintained, which is attributed to the intrinsic electronegativity of HNTs and their well-organized and homogeneous distribution in the PVA matrix. A battery using a PVA/HNT composite separator (EPH-10) and an anthraquinone (AQ) cathode exhibits a high initial discharge capacity of 231.6 mAh g^-1 and an excellent cycling performance (91.4% capacity retention, 300 cycles) at 25 °C. Even at high temperatures (60 and 80 °C), its capacity retention is more than 89.2 and 80.4% after 100 cycles, respectively. Our approach demonstrates the potential of the EPH-10 composite membrane as a separator for high-temperature LOB applications.

Soluble Organic Cathodes Enable Long Cycle Life, High Rate, and Wide-Temperature Lithium-Ion Batteries

Organic electrode materials free of rare transition metal elements are promising for sustainable, cost-effective, and environmentally benign battery chemistries. However, severe shuttling effect caused by the dissolution of active materials in liquid electrolytes results in fast capacity decay, limiting their practical applications. Here, using a gel polymer electrolyte (GPE) that is in situ formed on Nafion-coated separators, the shuttle reaction of organic electrodes is eliminated while maintaining the electrochemical performance. The synergy of physical confinement by GPE with tunable polymer structure and charge repulsion of the Nafion-coated separator substantially prevents the soluble organic electrode materials with different molecular sizes from shuttling. A soluble small-molecule organic electrode material of 1,3,5-tri(9,10-anthraquinonyl)benzene demonstrates exceptional electrochemical performance with an ultra-long cycle life of 10 000 cycles, excellent rate capability of 203 mAh g^-1 at 100 C, and a wide working temperature range from -70 to 100 °C based on the solid-liquid conversion chemistry, which outperforms all previously reported organic cathode materials. The shielding capability of GPE can be designed and tailored toward organic electrodes with different molecular sizes, thus providing a universal resolution to the shuttling effect that all soluble electrode materials suffer.

Pyridine-based benzoquinone derivatives as organic cathode materials for sodium ion batteries

2,5-Bis(p-benzoquinonyl) pyridine (QPQ-2) was developed as the cathode for sodium-ion batteries.

Secondary Bonding Channel Design Induces Intercalation Pseudocapacitance toward Ultrahigh-Capacity and High-Rate Organic Electrodes

Organic electrode materials have shown extraordinary promise for green and sustainable electrochemical energy storage devices, but usually suffer from low specific capacity and poor rate capability, which is largely caused by inactive components and diffusion-controlled Li⁺ intercalation. Herein, high-rate Li⁺ intercalation pseudocapacitance in organic molecular crystals is achieved through introducing weak secondary bonding channels, far exceeding their theoretical capacity based on redox chemistry at functional groups. The authors' combined experimentally electrochemical characterization with first-principles calculations show that the heterocyclic organic molecule 2,2'-bipyridine-4,4'-dicarboxylic acid (BPDCA) crystal permits a four-electron redox reaction at conventional CO and CN groups and a six-electron intercalation pseudocapacitance along conjugated alkene hydrogen bonding channels (H₂ NC₅ H⋯OC(OH)) and heterocyclic aromatic stacking channels (C₅ H₃ N⋯NH₃ C₅ ). The BPDCA electrode delivers an ultrahigh reversible capacity of 1206 mAh g^-1 at 0.5 A g^-1 and an exceptional rate capability. A 4.8 V high-energy/power-density BPDCA anode-based hybrid Li-ion capacitor is thus realized. This work opens a new avenue for developing organic intercalation pseudocapacitive materials via secondary bonding structure design.

Conjugated microporous polyarylimides immobilization on carbon nanotubes with improved utilization of carbonyls as cathode materials for lithium/sodium-ion batteries

Aromatic polyimide (PI)-based compounds have been widely studied for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to their higher specific energy density, economical, environmentally friendly and adjustable redox potential window. However, their solubility in aprotic electrolytes, inherently poor conductivity and low active site utilization limit their application in large-scale energy storage system (ESS). Here, we synthesized two aromatic PI-based conjugated microporous polymers (CMPs) and integrated them with multi-walled carbon nanotubes (CNT) (TAPT-NTCDA@CNT and TAPT-PMDA@CNT) for using as cathode materials for LIBs and SIBs. The aromatic PI-based CMP can effectively utilize the redox activity site due to its abundant π-conjugated redox active units, stable imide bond, high specific surface area and clear pore structure. As expected, the optimum TAPT-NTCDA@CNT exhibits good rate performance (89.7 mAh g^-1 at 2000 mA g^-1) and long cycle stability (87.3% capacity retention after 500 cycles) in LIBs. Also, TAPT-NTCDA@CNT can provide a higher initial capacity of 91.1 mAh g^-1 in SIBs at 30 mA g^-1. This work provides key insights for the further development of other new organic electrodes for other advanced rechargeable batteries.

Molecular Engineering of Aromatic Imides for Organic Secondary Batteries

Aromatic imides are a class of attractive organic materials with inherently electroactive groups and large π electron-deficient scaffolds, which hold potential as electrode materials for organic secondary batteries (OSBs). However, the undecorated aromatic imides are usually plagued by low capacity, high solubility in electrolyte, and poor electronic/ionic conductivity. Molecular engineering has been demonstrated to be an effective strategy to address unsatisfying characteristics of the aromatic imides, thereby expanding their scope for applications in OSBs. In this review, the recent research progress in modulation of the capacity, dissolution, and electronic/ionic conductivity of aromatic imides for organic lithium batteries, organic sodium batteries, and redox flow batteries are summarized. In addition, the challenge and prospective of aromatic imides in organic secondary battery applications are also discussed.

High Rate and Long Lifespan Sodium-Organic Batteries Using Pseudocapacitive Porphyrin Complexes-Based Cathode

HIGHLIGHTS

Functionalized porphyrin complexes are proposed as new pseudocapacitive cathodes for SIBs based on four-electron transfer. The presence of copper(II) ion partially contributes the charge storage and significantly stabilizes the structure of porphyrin complex for electrochemical energy storage. The electrochemical polymerization of porphyrin complex through the ethynyl groups in self-stabilization process contributes to high rate capability and excellent cycling stability.

ABSTRACT

Sodium-organic batteries utilizing natural abundance of sodium element and renewable active materials gain great attentions for grid-scale applications. However, the development is still limited by lack of suitable organic cathode materials with high electronic conductivity that can be operated stably in liquid electrolyte. Herein, we present 5,15-bis(ethynyl)-10,20-diphenylporphyrin (DEPP) and [5,15-bis(ethynyl)-10,20-diphenylporphinato]copper(II) (CuDEPP) as new cathodes for extremely stable sodium-organic batteries. The copper(II) ion partially contributes the charge storage and significantly stabilizes the structure of porphyrin complex for electrochemical energy storage. In situ electrochemical stabilization of organic cathode with a lower charging current density was identified which enables both improved high energy density and power density. An excellent long-term cycling stability up to 600 cycles and an extremely high power density of 28 kW kg⁻¹ were achieved for porphyrin-based cathode. This observation would open new pathway for developing highly stable sodium-organic cathode for electrochemical energy storage. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version of this article (10.1007/s40820-021-00593-8) contains supplementary material, which is available to authorized users.

Oxygen-Functionalized Polyacrylonitrile Nanofibers with Enhanced Performance for Lithium-Ion Storage

Functionalization and morphological construction can promote lithium-ion storage performance of organic polymers. In this contribution, exceptional lithium ion storage performance is empowered to porous polyacrylonitrile (PAN) nanofibers via the integration of template-assisted electrospinning technology and thermal treatment. It is found that the atmosphere adopted during the annealing process controls the storage behaviors of Li⁺. Impressively, the samples annealed in air present competitive capacities, rate capabilities, and a stable lifetime, compared with other counterparts (PAN powders and PAN fibers treated in N₂). Such enhancement in performance is attributed to the enriched oxygen-based functionalities (mainly C=O group) which guarantee a high specific capacity and the porous structure which facilitates the transportation of Li⁺ and electrons to improve the rate capability. It is envisioned that such morphology control and surface functionalization open up new horizons in the development of organic electrode materials with enhanced lithium-ion storage performances.

Electronic Conductive Inorganic Cathodes Promising High-Energy Organic Batteries

The electrochemical utilization of organic electrode materials (OEMs) is highly dependent on an excess amount of inactive carbon at the expense of low packing density and energy density. In this work, the challenges by substituting inactive carbon with electronic conductive inorganic cathode (ECIC) materials, which are endowed with high electronic conductivity to transport electrons for redox reactions of the whole electrodes, high ion-storage capacity to act as secondary active materials, and strong affinity with OEMs to inhibit their dissolution, are addressed. Combining representative ECICs (TiS₂ and Mo₆ S₈ ) with organic electrode materials (perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and hexaazatrinaphthalene (HATN)) simultaneously achieves high capacity, low porosity, lean electrolyte, and thus high energy density. High gravimetric and volumetric energy densities of 153 Wh kg^-1 and 200 Wh L^-1 are delivered with superior cycling stability in a 30 mA h-level Li/PTCDA-TiS₂ pouch cell. The proof-of-concept of organic-ECIC electrodes is also successfully demonstrated in monovalent Na, divalent Mg, and trivalent Al batteries, indicating their feasibility and generalizability. With the discovery of more ECIC materials and OEMs, it is anticipated that the proposed organic-ECIC system can result in further improvements at cell level to compete with transition metal-based Li-ion batteries.

The Progress and Prospect of Tunable Organic Molecules for Organic Lithium-Ion Batteries

Compared to inorganic electrodes, organic materials are regarded as promising electrodes for lithium-ion batteries (LIBs) due to the attractive advantages of light elements, molecular-level structural design, fast electron/ion transferring, favorable environmental impacts, and flexible feature, etc. Not only specific capacities but also working potentials of organic electrodes are reasonably tuned by polymerization, electron-donating/withdrawing groups, and multifunctional groups as well as conductive additives, which have attracted intensive attention. However, organic LIBs (OLIBs) are also facing challenges on capacity loss, side reactions, electrode dissolution, low electronic conductivity, and short cycle life, etc. Many strategies have been applied to tackle those challenges, and many inspiring results have been achieved in the last few decades. In this review, we have introduced the basic concepts of LIBs and OLIBs, followed by the typical cathode and anode materials with various physicochemical properties, redox reaction mechanisms, and evolutions of functional groups. Typical charge-discharge behaviors and molecular structures of organic electrodes are displayed. Moreover, effective strategies on addressing problems of organic electrodes are summarized to give some guidance on the synthesis of optimized organic electrodes for practical applications of OLIBs.

Recent advances of covalent organic frameworks in lithium ion batteries

This review divides the active sites of COFs into four categories: carbonyl, phenyl, imine bonds and other groups, and introduces their applications in LIBs.

Material Design of Aqueous Redox Flow Batteries: Fundamental Challenges and Mitigation Strategies

Redox flow batteries (RFBs) are critical enablers for next-generation grid-scale energy-storage systems, due to their scalability and flexibility in decoupling power and energy. Aqueous RFBs (ARFBs) using nonflammable electrolytes are intrinsically safe. However, their development has been limited by their low energy density and high cost. Developing ARFBs with higher energy density, lower cost, and longer lifespan than the current standard is of significant interest to academic and industrial research communities. Here, a critical review of the latest progress on advanced electrolyte material designs of ARFBs is presented, including a fundamental overview of their physicochemical properties, major challenges, and design strategies. Assessment methodologies and metrics for the evaluation of RFB stability are discussed. Finally, future directions for material design to realize practical applications and achieve the commercialization of ARFB energy-storage systems are highlighted.

Structural Insight into the Abnormal Capacity of a Co-Substituted Tunnel-Type Na0.44MnO2 Cathode for Sodium-Ion Batteries

Tunnel-type (T-type) Na_0.44MnO₂ (NMO) is a promising cathode material for sodium-ion batteries (SIBs) owing to its high rate performance and cycling stability compared to manganese-based layered oxides. However, the low specific capacity still restricts its practical applications. Herein, a Co-doped T-type NMO is synthesized through a facile solid-state reaction method and utilized as a cathode material for SIBs. A T-type Na_0.44Mn_0.9925Co_0.0075O₂ (NMO-3) electrode can deliver a high reversible capacity of 138 mAh g^-1 at 0.1C, a superior rate capability (133, 130, 121, 106, and 93 mAh g^-1 at 0.5, 1, 2, 5, and 10C, respectively), and excellent cycling stability (85.2% at 10C after 500 cycles). The substitution of Co³⁺ by Mn³⁺ leads to the enlargement of small and S-shaped tunnel spaces, which facilitates the insertion/deinsertion of Na⁺ into/from NMO-3 and greatly enhances its rate capability and cycling stability. Moreover, the reduced energy barriers for Na⁺ diffusion in small tunnels make the inactive Na⁺ easier to be deintercalated, which should be responsible for its high specific capacity that exceeds the theoretical capacity of T-type NMO.

Conductive Polyacrylic Acid-Polyaniline as a Multifunctional Binder for Stable Organic Quinone Electrodes of Lithium-Ion Batteries

High solubility in aprotic organic electrolytes and poor electrical conductivity are the main restrictions of organic electrodes in practical application. Conductive binder contributes to the high-performance electrodes as it enables both mechanical and electronic integrity of the electrode, which have been scarcely explored for organic electrodes. Herein, a conductive interpenetrating polymeric network is synthesized through in situ polymerization of polyaniline with poly(acrylic acid) (denoted PAA-PANi), which served as a novel conductive binder for organic 2-aminoanthraquinone (AAQ) materials. The conductive PANi component enhances the electrical conductivity of the electrode. Meanwhile, the PAA component serves as the binding matrix to condense with the amino groups (-NH₂) of AAQ, which therefore effectively inhibits their dissolution and maintains electrode integrity during cycling. As expected, the conductive binder exhibits both excellent electrical conductivity (10^-3 S cm^-1) and strong mechanical adhesion. The AAQ/reduced graphene oxide (AAQ@rGO) composite electrode prepared with the as-synthesized PAA-PANi binder delivers a high specific capacity of 126.1 mAh g^-1 at 0.1 A g^-1, superior rate capability (71.3 mAh g ^-1 at 3 A g^-1), and outstanding cycling stability (2000 cycles at 1 A g^-1), which greatly rivals polyvinylidene fluoride and PAA binder-based electrodes. Such a strategy points the way for the design and synthesis of conductive polymeric binders for organic electrodes, whose electrical conductivity and dissolution are massive issues.

A novel π-conjugated poly(biphenyl diimide) with full utilization of carbonyls as a highly stable organic electrode for Li-ion batteries

Organic carbonyl redox polymers, especially conjugated polyimides with multiple reversible redox centers have attracted considerable attention as electrode materials for organic Li-ion batteries. However, the low utilization of carbonyls hindered their potential applications in energy storage. Herein, a novel π-conjugated polyimide (PBPI) based on biphenyl diimide (BPI) containing two seven-membered imide rings is developed. PBPI is used as an anode material for organic Li-ion batteries, which show high conductivity and insolubility in the electrolyte and enable intercalation of four Li-ions per BPI unit, thus contributing to a reversible capacity of 136 mA h g⁻¹ at 100 mA g⁻¹ with coulombic efficiency close to 100%. Moreover, the battery based on PBPI manifested superior high-rate performance (65 mA h g⁻¹ at 2000 mA g⁻¹) as well as significant cycling stability (over 1600 cycles at 100 mA g⁻¹). Remarkably, the full redox-active site (C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O) utilization of an aromatic diimide core to achieve its full potential applications is reported for the first time. This work provides a new strategy for developing redox π-conjugated polyimides and accommodation of more alkaline ions for high performance battery systems.

A novel π-conjugated polyimide based on the two seven-membered imide rings-containing BPI was reported, which be used as a highly stable anode electrode material with full utilization of carbonyls for the application organic Li-ion batteries.

Progress of Organic Electrodes in Aqueous Electrolyte for Energy Storage and Conversion

Aqueous batteries using inorganic compounds as electrode materials are considered a promising solution for grid-scale energy storage, while wide application is limited by the short life and/or high cost of electrodes. Organics with carbonyl groups are being investigated as the alternative to inorganic electrode materials because they offer the advantages of tunable structures, renewability, and they are environmentally benign. Furthermore, the wide internal space of such organic materials enables flexible storage of various charged ions (for example, H⁺ , Li⁺ , Na⁺ , K⁺ , Zn²⁺ , Mg²⁺ , and Ca²⁺ , and so on). We offer a comprehensive overview of the progress of organics containing carbonyls for energy storage and conversion in aqueous electrolytes, including applications in aqueous batteries as solid-state electrodes, in flow batteries as soluble redox species, and in water electrolysis as redox buffer electrodes. The advantages of organic electrodes are summarized, with a discussion of the challenges remaining for their practical application.

Improvement of the Battery Performance of Indigo, an Organic Electrode Material, Using PEDOT/PSS with d-Sorbitol

Rare-metal-free and high-performance secondary batteries are necessary for improving the efficiency of renewable energy systems. Organic compounds are attractive candidates for the active material of such batteries. Many studies have reported organic active materials that show high energy density per active material weight. However, organic active materials, most of which exhibit low conductivity and low specific density, typically require a large amount of a conductive additive (>50 wt %) to obtain a high utilization rate. Therefore, organic active materials rarely display high energy density per electrode weight. High energy densities per electrode weight can be obtained using high weight fractions of active materials and low weight fractions of conductive additives. Herein, we report that a low-conductivity organic active material, indigo, showed improved net discharge capacity density when even a small amount of a conductive polymer composite, poly(3,4-ethylenedioxythiophene)/polystyrene sulfonic acid (PEDOT/PSS) with d-sorbitol, was used as both a binder and conductive additive. The cycle life was also improved by coating one side of the separator with the composite, which probably hindered the dissolution of the active material. A discharge capacity of 96% of the theoretical capacity of indigo and an improved cycle life were achieved with an electrode containing 80 wt % indigo and with a PEDOT/PSS-coated separator. The optimal fraction of the conductive binder was examined, and the mechanism of conductivity enhancement was discussed. The present scheme allows us to replace the dispersion solvent of the slurry, N-methylpyrrolidone, with water, which can reduce the environmental load during battery manufacturing processes.

2D Coordination Network of Trioxotriangulene with Multiple Redox Abilities and Its Rechargeable Battery Performance

A three-fold symmetric trioxotriangulene derivative with three pyridyl groups as coordinating sites was designed and synthesized. In a cyclic voltammetry measurement, the trioxotriangulene skeleton exhibited a multi-stage redox ability from neutral radical to radical tetra-anion species. In the zinc complex of monoanion species, three pyridyl groups coordinated to the zinc ion to build up a two-dimensional coordination network with a cavity larger than 12 Å in diameter. This complex was utilized as a cathode active material of a lithium ion battery, and it exhibited a capacity of ca. 60 mAh g^-1 per the weight of the active material with a stable cycling performance up to 1000 cycles. This work shows that the coordination network formed by the trioxotriangulene-based ligand was effective in the improvement of cycle performance of the organic rechargeable battery.

A Quinone-Based Electrode for High-Performance Rechargeable Aluminum-Ion Batteries with a Low-Cost AlCl3/Urea Ionic Liquid Electrolyte

Intensive energy demand urges state-of-the-art rechargeable batteries. Rechargeable aluminum-ion batteries (AIBs) are promising candidates with suitable cathode materials. Owing to high abundance of carbon, hydrogen, and oxygen and rich chemistry of organics (structural diversity and flexibility), small organic molecules are good choices as the electrode materials for AIB. Herein, a series of small-molecule quinone derivatives (SMQD) as cathode materials for AIB were investigated. Nonetheless, dissolution of small organic molecules into liquid electrolytes remains a fundamental challenge. To nullify the dissolution problem effectively, 1,4-benzoquinone was integrated with four bulky phthalimide groups to form 2,3,5,6-tetraphthalimido-1,4-benzoquinone (TPB) as the cathode materials and assembled to be the AI/TPB cell. As a result, the Al/TPB cell delivered capacity as high as 175 mA h/g over 250 cycles in the urea electrolyte system. Theoretical studies have also been carried out to reveal and understand the storage mechanism of the TPB electrode.

An Overview and Future Perspectives of Rechargeable Zinc Batteries

Aqueous rechargeable zinc-based batteries have sparked a lot of enthusiasm in the energy storage field recently due to their inherent safety, low cost, and environmental friendliness. Although remarkable progress has been made in the exploration of performance so far, there are still many challenges such as low working voltage and dissolution of electrode materials at the material and system level. Herein, the central tenet is to establish a systematic summary for the construction and mechanism of different aqueous zinc-based batteries. Details for three major zinc-based battery systems, including alkaline rechargeable Zn-based batteries (ARZBs), aqueous Zn ion batteries (AZIBs), and dual-ion hybrid Zn batteries (DHZBs) are given. First, the electrode materials and energy storage mechanism of the three types of zinc-based batteries are discussed to provide universal guidance for these batteries. Then, the electrode behavior of zinc anodes and strategies to deal with problems such as dendrite and passivation are recommended. Finally, some challenge-oriented solutions are provided to facilitate the next development of zinc-based batteries. Combining the characteristics of zinc-based batteries with good use of concepts and ideas from other disciplines will surely pave the way for its commercialization.

An Insoluble Anthraquinone Dimer with Near-Plane Structure as a Cathode Material for Lithium-Ion Batteries

Conjugated carbonyl-based organic electrode materials for lithium-ion batteries have gained increasing interests owing to their many advantages such as resource abundance and sustainable development. However, serious dissolution in organic liquid electrolytes is often encountered, resulting in inferior electrochemical performance such as poor cycling stability. Herein, a new molecular design strategy was developed to address the dissolution issue of 9,10-anthraquinone (AQ). An AQ dimer with near-plane molecular structure, 1,4-bis(9,10-anthraquinonyl)benzene (BAQB), was facilely synthesized. The near-plane structure was proved by DFT calculations. It was found that the obtained BAQB was insoluble in ether electrolyte. Compared to AQ, BAQB displayed remarkably enhanced cycling stability. After 100 cycles at 0.2 C, a high capacity retention of 91.6 % was achieved (195 mAh g^-1 ). BAQB also exhibited excellent rate performance (138 mAh g^-1 at 10 C). The results demonstrate the effectiveness of the near-plane molecular design concept. This work provides a new idea for rational molecular design to inhibit the dissolution of conjugated carbonyl-based organic electrode materials.

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.

Carbonyl Bridge-Based p-π Conjugated Polymers as High-Performance Electrodes of Organic Lithium-Ion Batteries

Organic redox compounds have shown promising potential as electrode materials for lithium-ion batteries. Polymerization is an effective and feasible method to prevent rapid capacity decay. However, present conjugated polymers and nonconjugated polymers have their own limitations to constructing stable and high-performance electrodes. Herein, we report a novel polyimide NDI-O, which is connected by carbonyl bridges. The NDI-O is a p-π conjugated polymer that exhibits a high gravimetric energy density of 542 W h kg^-1 and an ultrahigh power density of 14,000 W kg^-1 due to its intriguing electronic properties. The combination of molecular electrostatic potential calculations and ex situ technologies reveals the lithium-ion storage mechanism during the charge and discharge processes. The orbital distribution calculations and electrochemical impedance spectroscopy tests have been shown to verify the excellent kinetic properties of NDI-O. This work expands the scope of polymers applied for LIBs and provides new methods to construct high-performance electrode materials for sustainable batteries.