Covalent Organic Framework with Multiple Redox Active Sites for High-Performance Aqueous Calcium Ion Batteries

Tailorable Dual-Redox Polymer with Molecular Flexibility for Enhanced Electrochemical Desalination and Water Purification

The global water crisis demands sustainable desalination innovations, with capacitive deionization (CDI) emerging as an energy-efficient electrochemical alternative. While organic materials demonstrate pseudocapacitive ion capture through ion coordination for CDI electrodes, their effectiveness remains constrained by molecular chain packing and deficient redox-active sites. This work introduces a novel biocompatible dual-redox polymer (PNDBI) with high molecular flexibility to address these limitations. In-situ characterization and theoretical analyses unveil that the concerted interaction between C═O and C═N bifunctional groups enhances Na+ capture. Concurrently, the polymer's pliable backbone and narrow HOMO-LUMO gap ensure active site accessibility and facilitate electron mobility, which endow the PNDBI polymer with substantial pseudocapacitive capacitance and remarkable stability for 4Na+ capture. The CDI device employing the PNDBI electrode demonstrates outstanding desalination performance, achieving a remarkable salt removal capacity of 112.1 mg g-1 and a rapid removal rate of 3.7 mg g-1 min-1. Impressively, the CDI device exhibits excellent electrochemical regeneration stability, retaining 92.0% efficiency over 200 cycles, placing it among the state-of-the-art CDI devices reported. Beyond desalination, the PNDBI-based CDI device showcases significant multifunctionality, enabling efficient water purification through the removal of hard water ions and cationic dyes, thereby offering a versatile and sustainable solution for environmental remediation.

Synthesis and Sodium-Ion Storage of Triazole-Substituted Graphdiyne

Sodium-ion batteries (SIBs) have developed rapidly in recent years, confronting low capacity and poor cycling stability issues for anode material. Herein, triazole-substituted graphdiyne (TzlGDY) was designed to tune the sodium-ion insertion sequence, and an effective diyne-radical Na-storage mechanism was discovered. The distinctive diyne-ditriazole architecture actualizes a preferential Na+-N complexation, then π-bond homolysis of diyne is induced by Na+ to generate two radicals at two end carbons of diyne, and thereby two radicals capture two additional Na+ by Na+-radical coupling. This Na+-N complexation followed by the Na+-radical coupling mechanism more effectively enhances capacity compared with the reported cation-π mechanism. Furthermore, other ditriazole-N atoms chelate two more Na+. The triazole-filled nanopores and full-carbon backbone in TzlGDY effectively stabilize diyne radicals and enhance the Na+-transport kinetics. As a result, TzlGDY's anode presented almost no capacity decay over 12,000 cycles at 5 A g-1 with a final capacity of 251.7 mAh g-1. Moreover, the TzlGDY||NVP full cell delivered a high specific capacity of 114 mAh g-1 at 0.2C with a capacity retention of 81.8% and an average CE of 99.6% after 150 cycles. Our results demonstrate the diyne-radical mechanism is a new concept of energy storage and open up a new route for efficiently regulating anode materials in SIBs.

Environmentally Benign and Long Cycling Mn-Ion Full Batteries Enabled by Hydrated Eutectic Electrolytes and Polycarbonyl Conjugated Organic Anodes

Aqueous rechargeable manganese (Mn)-ion batteries have recently emerged as a promising candidate for multivalent ion rechargeable batteries. However, challenges remain, particularly in expanding the electrolyte's voltage window and identifying compatible anode materials. Herein, we introduce a Mn-ion full battery comprising a nickel hexacyanoferrate (NiHCF) cathode, a perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) anode, and a novel hydrated eutectic electrolyte formulated from Mn(ClO4)2·6H2O and acetamide. This electrolyte composition, optimized for molar ratio, provides a stable solvation structure that suppresses water reactivity and supports high ionic conductivity, as confirmed by spectroscopic and molecular dynamics analyses. The PTCDI anode facilitates highly reversible Mn2+ storage via a unique enolization redox reaction, delivering exceptional rate capability and cycling stability. As a result, the NiHCF||PTCDI full battery achieves a 1.2 V plateau, excellent rate performance (up to 5.0 A g-1), and long cycling life with 95.6% capacity retention over 1200 cycles at 1.0 A g-1. This study proposes a feasible strategy for the construction of environment-friendly, long-life and low-cost aqueous Mn-ion full batteries, offering a sustainable and high-performance solution for future energy storage applications.

Precision design of covalent organic frameworks for cathode applications

The urgent demand for sustainable energy storage solutions has positioned covalent organic frameworks (COFs) as promising alternatives to conventional inorganic cathodes. With their programmable architectures, high theoretical capacities, and elemental sustainability, COFs hold transformative potential for next-generation energy storage devices. Despite their promise, the practical implementation of COFs has been impeded by limitations such as low conductivity and lower-than-anticipated practical capacities. This review explores recent advances in molecular and structural engineering strategies designed to overcome these challenges. The discussion encompasses ion-storage mechanisms, innovative chemical design strategies, and composite material synergies that enhance the performance of COF cathodes (COFCs). Looking to the future, breakthroughs in multi-electron redox chemistry, scalable synthesis, and advances in in situ characterization techniques will be critical to unlocking the full potential of COFCs. This review aims to provide valuable insights and guidance for the design of novel COFC materials, thereby advancing the development of next-generation high-performance secondary batteries.

Structural codes of organic electrode materials for rechargeable multivalent metal batteries

Rechargeable multivalent metal batteries (MMBs) are considered as promising alternatives to Li-ion and Pb-acid batteries for grid-scale energy storage applications due to the multi-electron redox capability of metal anodes. However, the conventional inorganic cathodes used in MMBs face challenges with the sluggish diffusivity and poor storage of charge-dense multivalent cations in their crystal lattice. Organic electrode materials (OEMs), on the other hand, offer several advantages as MMB cathodes, including flexible structural designability, high resource availability, sustainability, and a unique ion-coordination storage mechanism. This review explores the intrinsic connection between the structural features of OEMs and their charge storage performance, aiming to unveil key design principles for organic molecules used in various MMB applications. We begin with an overview of the fundamental aspects of different MMBs (i.e., Zn/Mg/Ca/Al batteries), covering electrolyte selection, metal stripping/plating electrochemistry, and the fundamentals of cathode operation. From a theoretical understanding of redox activities, we summarize the properties of different redox sites and correlate the electrochemical properties of OEMs with various structural factors. This analysis further leads to the introduction of critical design considerations for different types of OEMs. We then critically review a wide range of organic compounds for MMBs, from small organic molecules to redox-active polymers and covalent-organic frameworks, focusing on their structure-property relationships, key electrochemical parameters, and strengths and shortcomings for multivalent ion storage. Finally, we discuss the existing challenges and propose potential solutions for further advancing OEMs in MMBs.

A Comprehensive Review of Anode Materials in Rechargeable Calcium-Ion Batteries

Recently, rechargeable calcium-ion batteries (CIB) have been investigated extensively due to their safety, environmental friendliness, abundant storage capacity, low cost, and their energy density and power density characteristics. This paper presents a systematic review of recent research advancements in anode materials, including alloys, organic materials, carbon-based materials, and transition metal oxide. First, the basic properties of CIBs and the challenges associated with anode materials are described. Then, the reaction mechanism and improvement strategies of anode materials for CIBs are highlighted. Finally, the advantages and disadvantages of the identified anode materials are summarized, and potential future research directions are highlighted.

Two-Dimensional π-d Conjugated Conductive Metal-Organic Framework with Triple Active Centers as High-Performance Cathodes for Flexible Zinc Batteries

Conductive metal-organic frameworks (C-MOFs) have received extensive interest in high-performance zinc-ion batteries (ZIBs) owing to multi-redox sites and high electrical conductivity. Here, we present a π-d C-MOF by coordinating 2,3,5,6-tetraaminobenzoquinone (TABQ) ligands with Cu2+ ions (2D Cu-TABQ) acting as cathodes for ZIBs. Benefiting from a triple active center (Cu2+, C=O, and C=N), 2D Cu-TABQ shows an ultra-high reversible capacity of 297.7 mAh g-1 at 0.2 A g-1. Meanwhile, 2D Cu-TABQ also has superior cycle stability with a capacity of up to 98.2 mAh g-1 after 1000 times at 2.0 A g-1. Considering the instability of the ligand bonds of C-MOFs in aqueous electrolytes, this work uses gel electrolytes to reduce the dissolution of organic ligands into the electrolyte, thus suppressing the shuttle effect, significantly improving the cycling stability of 2D Cu-TABQ. The flexible battery assembled by 2D Cu-TABQ shows excellent capacity retention (64.4 %) after 50 times at 0.2 A g-1, which is significantly better than 36.4 % in the common electrolyte, as well as outstanding bending resistance and electrochemical properties at different folding angles. This investigation will highlight the electrochemical application of C-MOFs in flexible zinc ion batteries and offer novel ideas for the structural design of cathodes with multiple active centers.

Dual Active Site Covalent Organic Framework Anode Enables Stable Aqueous Proton Batteries

Aqueous proton batteries (APBs) have attracted increasing interest owing to their potential for grid-scale energy storage with extraordinary sustainability and excellent rate abilities. However, there are limited anode materials and it remains a great challenge to effectively balance capacity and cycling performance. Here, we report a covalent organic framework containing C=O and C=N dual active sites (TABQ-COF) as a high-capacity and long-cycle anode for proton batteries. The proton storage ability of the dual active sites and the up to nine proton redox chemistry mechanisms for each repetitive unit have been demonstrated by experiments and density functional theory calculations. The insoluble TABQ-COF electrode displayed a remarkably high specific capacity of 401 mAh g-1, outstanding cycling stability (100 % capacity retention after 7500 cycles) and high rate performance (90 mAh g-1 at 50 A g-1). When coupling with a MnO2 cathode, the constructed TABQ-COF//MnO2 battery achieves a reversible capacity of 247 mAh g-1 at 5 A g-1, with a remarkable capacity retention of 100 % over 10000 cycles. Furthermore, the TABQ-COF//MnO2 battery can operate well and shows high capacity and cycle stability in a frozen electrolyte at -40°C, implying great potential for energy storage at extreme temperatures.

A Fast and Highly Stable Aqueous Calcium-Ion Battery for Sustainable Energy Storage

Aqueous alkali-ion batteries are gaining traction as a low-cost, sustainable alternative to conventional organic lithium-ion batteries. However, the rapid degradation of commonly used electrode materials, such as Prussian Blue Analogs and carbonyl-based organic compounds, continues to challenge the economic viability of these devices. While stability issues can be addressed by employing highly concentrated water-in-salt electrolytes, this approach often requires expensive and, in many cases, fluorinated salts. Here, we show that replacing monovalent K+ ions with divalent Ca2+ ions in the electrolyte significantly enhances the stability of both a copper hexacyanoferrate cathode and a polyimide anode. These findings have direct implications for developing an optimized aqueous Ca-ion battery that demonstrates exceptional fast-charging capabilities and ultra-long cycle life and points toward applying Ca-based batteries for large-scale energy storage.

Enabling high-performance multivalent metal-ion batteries: current advances and future prospects

The battery market is primarily dominated by lithium technology, which faces severe challenges because of the low abundance and high cost of lithium metal. In this regard, multivalent metal-ion batteries (MVIBs) enabled by multivalent metal ions (e.g. Zn2+, Mg2+, Ca2+, Al3+, etc.) have received great attention as an alternative to traditional lithium-ion batteries (Li-ion batteries) due to the high abundance and low cost of multivalent metals, high safety and higher volumetric capacities. However, the successful application of these battery chemistries requires careful control over electrode and electrolyte chemistries due to the higher charge density and slower kinetics of multivalent metal ions, structural instability of the electrode materials, and interfacial resistance, etc. This review comprehensively explores the recent advancements in electrode and electrolyte materials as well as separators for MVIBs, highlighting the potential of MVIBs to outperform Li-ion batteries regarding cost, energy density and safety. The review first summarizes the recent progress and fundamental charge storage mechanism in several MVIB chemistries, followed by a summary of major challenges. Then, a thorough account of the recently proposed methodologies is given including progress in anode/cathode design, electrolyte modifications, transition to semi-solid- and solid-state electrolytes (SSEs), modifications in separators as well as a description of advanced characterization tools towards understanding the charge storage mechanism. The review also accounts for the recent trend of using artificial intelligence in battery technology. The review concludes with a discussion on prospects, emphasizing the importance of material innovation and sustainability. Overall, this review provides a detailed overview of the current state and future directions of MVIB technology, underscoring its significance in advancing next-generation energy storage solutions.

Flux Synthesis of Robust Polyimide Covalent Organic Frameworks with High-Density Redox Sites for Efficient Proton Batteries

Aqueous proton batteries are attracting increasing attention in the large-scale next-generation energy storage field. However, the electrode materials for proton batteries often suffer from low specific capacity and unsatisfactory cycle durability. Herein, we synthesize two highly crystalline and robust polyimide covalent organic frameworks (COFs) through a solvent-free flux synthesis approach with benzoic acid as a flux and catalyst. The as-synthesized COFs possess enriched redox-active sites for proton storage and intrinsic Grotthuss proton conduction, rendering them ideal candidates for proton electrode materials. The optimal COF electrodes achieve a high specific capacity of 180 mAh/g at 0.1 A/g, among the highest COF-based proton batteries, and exhibit an outstanding rate capability of up to 100 A/g and long-term cycling stability with capacity retention of 99 % after 5000 cycles at 5 A/g. The assembled full cells deliver a specific capacity of 150 mAh/g at 0.2 A/g with a maximum energy density of 72 Wh/kg and a maximum supercapacitor-level power density of 64 kW/kg, surpassing all reported COF-based systems. This work paves a new avenue for the design of electrode materials for aqueous proton batteries with high energy density, power density, rate capability and long-term cycling stability.

Calcium Chemistry as A New Member of Post-Lithium Battery Family: What Can We Learn from Sodium and Magnesium Systems

The development of next-generation battery technologies needs to consider their environmental impact throughout the whole cycle life, which has brought new chemistries based on earth-abundant elements into the spotlight. Rechargeable calcium batteries are such an emerging technology, which shows the potential to provide high cell voltage and high energy density close to lithium-ion batteries. Additionally, the use of Ca2+ as a charge carrier renders significant sustainable values. Although pioneering work on the electrochemistry of Ca has been carried out for more than half a century, demonstration of reversible Ca0/Ca2+ redox chemistry in non-aqueous media was only achieved within the past decade. In this review, we will present recent development of rechargeable calcium batteries, focusing on mainly the similarities but also differences between Ca chemistry and other post-lithium chemistry. According to the periodic nature of elements, magnesium (an alkaline earth element as Ca) and sodium (a diagonally adjacent element to Ca) have similar chemical properties to Ca in various aspects. We shall elaborate on how the solution chemistry, metal behaviors and transport mechanisms of Ca-ions can be better understood in light of the established principles in the respective Mg/Na systems. We hope the discussion will inspire synergetic development between Ca batteries and other post-lithium systems.

Enhancing Electrochemical CO2 Reduction via Redox Non-Innocent Spheres in Copper-Coordinated Covalent Organic Frameworks

Significant efforts have been dedicated to the development of highly efficient electrocatalysts for electrochemical CO2 reduction reactions (eCO2RR). The outer coordination spheres of catalytic centers may play a pivotal role in the reaction pathway and kinetics for eCO2RR. Herein, three single copper sites coordinated Aza-fused conjugated organic frameworks (Aza-COFs-Cu) with different outer coordination spheres around Cu sites are designed. Experiment and density functional theory (DFT) calculation results reveal that the redox non-innocent outer spheres around Cu sites significantly influence the catalytic performance of Aza-COFs-Cu for eCO2RR. When adjacent redox non-innocent groups of uncoordinated aromatic-N and quinone around the Cu centers act as the symmetry-breaking sites, the energy-consuming activation process of CO2 molecules can be accelerated via the H+/e- transfer process to form *COOH intermediates, which will significantly improve the performance for eCO2RR. This study provides a new perspective on the design of more advanced electrocatalysts for eCO2RR through redox non-innocent spheres engineering.

Molecular Engineering to Construct MoS2 with Expanded Interlayer Spacing and Enriched 1T Phase for "Rocking-Chair" Aqueous Calcium-Ion Pouch Cells

The moderate working voltage and high capacity of transition metal dichalcogenides (TMDs) make them promising anode materials for aqueous calcium-ion batteries (ACIBs). However, the large radius and two charges of Ca2+ cause TMDs to exhibit poor performance in ACIBs. Therefore, effective regulation strategies are crucial for enabling the application of TMDs in ACIBs. Herein, MoS2 with expanded interlayer spacing and an enriched 1T phase (ES-1T-MoS2) is constructed by molecular engineering and reported as an anode material for ACIBs. Molecular engineering increases the capacity of MoS2 from 29.4 to 91.2 mAh g-1 and improves its rate performance from 20 to 76.1 mAh g-1 at 2.0 A g-1. ES-1T-MoS2 also shows a -20 to 50 °C wide temperature working capability. Furthermore, the capacity improvement reasons and the calcium storage mechanism of ES-1T-MoS2 are revealed through density functional theory calculations and in situ/ex situ characterizations. Finally, a "rocking-chair" aqueous calcium-ion pouch cell with a Prussian blue analogue cathode and ES-1T-MoS2 anode is assembled. The pouch cell exhibits a life of 150 cycles with over 90.8% capacity retention at 0 and 25 °C. This work demonstrates that molecular engineering is an effective strategy to improve the calcium storage performance of TMDs and promotes the advancement of ACIBs.

Growing large single crystals of two- or three-dimensional covalent organic polymers through unconventional Te-O-P linkages

Understanding precise structures of two-/three- dimensional (2D/3D) covalent organic polymers (COPs) through single-crystal X-ray diffraction (SCXRD) analysis is important. However, how to grow high-quality single crystals for 2D/3D COPs is of challenge due to poor reversibility and difficult self-correction of covalent bonds. In addition, the success of introducing tellurium into the backbone to construct 2D/3D COPs and obtaining their single crystals is rare. Here, utilizing the strategy that a heavy element (e.g., tellurium) can form dynamic linkages with a self-correction function, we develop a fast and universal method for growing large-sized single crystals (up to 500 µm) for 2D/3D COPs, especially for 2D COPs. Three 2D COPs and one 3D COP are harvested through dynamic -Te-O-P- bonds in two days, with structures clearly uncovered via the SCXRD analysis. These 2D/3D COPs also show promising photocatalytic activities (nearly 100% selectivity and 100% yield) in superoxide anion radical-mediated coupling of (arylmethyl)amines.

Fully sp2 Carbon-Conjugated Covalent Organic Frameworks with Multiple Active Sites for Advanced Lithium-Ion Battery Cathodes

Covalent organic frameworks (COFs) hold great promise for rechargeable batteries. However, the synthesis of COFs with abundant active sites, excellent stability, and increased conductivity remains a challenge. Here, chemically stable fully sp2 carbon-conjugated COFs (sp2c-COFs) with multiple active sites are designed by the polymerization of benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8-tricarbaldehyde) (BTT) and s-indacene-1,3,5,7(2H,6H)-tetrone (ICTO) (denoted as BTT-ICTO). The morphology and structure of the COF are precisely regulated from "butterfly-shaped" to "cable-like" through an in situ controllable growth strategy, significantly promoting the exposure and utilization of active sites. When the unique "cable-like" BTT-ICTO@CNT is employed as lithium-ion batteries (LIBs) cathode, it exhibits exceptional capacity (396 mAh g-1 at 0.1 A g-1 with 97.9 % active sites utilization rate), superb rate capacity (227 mAh g-1 at 5.0 A g-1), and excellent cycling performance (184 mAh g-1 over 8000 cycles at 2.0 A g-1 with 0.00365 % decay rate per cycle). The lithium storage mechanism of BTT-ICTO is exhaustively revealed by in situ Fourier transform infrared, in situ Raman, and density functional theory calculations. This work provides in-depth insights into fully sp2c-COFs with multiple active sites for high-performance LIBs.

Ultraviolet Nonlinear Optical Single Crystals of A Three-Dimensional Chiral Covalent Framework Containing Te-O-B-O Bonds

Extending covalent organic frameworks (COFs) into crystalline carbon-free covalent backbones is an important strategy to endow these materials with more exotic functions. Integrating metal-free inorganic and organic components into one covalent framework is an effective way to address the issue of poor thermal/solvent stability in the field of nonlinear optics (NLO). However, constructing such structures is very challenging. Here, we linked 3-connected nods (BO3) and 2-connected organic building blocks (Te(Ph)2) together to produce colorless single crystals (size up to 400 μm) of a three-dimensional (3D) chiral covalent framework (CityU-22). The single-crystal X-ray diffraction (SCXRD) analysis reveals that CityU-22 has a non-carbon Te-O-B-O bond-based network with the srs topology. The chiral CityU-22 displays good stability under the treatment of different common solvents or heat (the decomposition temperature above 300 °C). Due to its non-π-conjugated backbone (-Te-O-B-O-), CityU-22 shows an ultraviolet NLO behavior with a second-harmonic generation (SHG) response similar to KH2PO4 (KDP).

Aqueous Multivalent Metal-ion Batteries: Toward 3D-printed Architectures

Energy storage has become increasingly crucial, necessitating alternatives to lithium-ion batteries due to critical supply constraints. Aqueous multivalent metal-ion batteries (AMVIBs) offer significant potential for large-scale energy storage, leveraging the high abundance and environmentally benign nature of elements like zinc, magnesium, calcium, and aluminum in the Earth's crust. However, the slow ion diffusion kinetics and stability issues of cathode materials pose significant technical challenges, raising concerns about the future viability of AMVIB technologies. Recent research has focused on nanoengineering cathodes to address these issues, but practical implementation is limited by low mass-loading. Therefore, developing effective engineering strategies for cathode materials is essential. This review introduces the 3D printing-enabled structural design of cathodes as a transformative strategy for advancing AMVIBs. It begins by summarizing recent developments and common challenges in cathode materials for AMVIBs and then illustrates various 3D-printed cathode structural designs aimed at overcoming the limitations of conventional cathode materials, highlighting pioneering work in this field. Finally, the review discusses the necessary technological advancements in 3D printing processes to further develop advanced 3D-printed AMVIBs. The reader will receive new fresh perspective on multivalent metal-ion batteries and the potential of additive technologies in this field.

Ladder-Type Redox-Active Polymer Achieves Ultra-Stable and Fast Proton Storage in Aqueous Proton Batteries

A ladder-type rigid-coplanar polymer with highly ordered molecular arrangement has been designed via a covalent cycloconjugation conformational strategy. Benefitting from the extended π-electron delocalization in the highly aromatic ladder-type polymeric backbone, the prepared polymer exhibits fast intra-chain charge transport along the polymeric chain, realizing extraordinary proton-storage capability in aqueous proton batteries.Affordable and safe aqueous proton batteries (APBs) with unique "Grotthuss mechanism," are very significant for advancing carbon neutrality initiatives. While organic polymers offer a robust and adaptable framework that is well-suited for APB electrodes, the limited proton-storage redox capacity has constrained their broader application. Herein, a ladder-type polymer (PNMZ) has been designed via a covalent cycloconjugation conformational strategy that exhibits optimized electronic structure and fast intra-chain charge transport within the high-aromaticity polymeric skeleton. As a result, the polymer exhibits exceptional proton-storage redox kinetics, which are evidenced by in-operando monitoring techniques and theoretical calculations. It achieves a remarkable proton-storage capacity of 189 mAh g-1 at 2 A g-1 and excellent long-term cycling stability, with approximately 97.8 % capacity retention over 10,000 cycles. Finally, a high-performance all-polymer APB device has been successfully constructed with a desirable capacity retention of 99.7 % after 6,000 cycles and high energy density of 56.3 Wh kg-1.

Fully Conjugated Covalent Triazine Framework Integrating Hexaazatrinaphthylene Unit as Anode Material for High-Performance Hybrid Lithium-Ion Capacitors

As a high-performance energy storage device consisting of a battery-type anode and a capacitor-type cathode, hybrid lithium-ion capacitors (HLICs) combine the advantages of high energy density of batteries and high power density of capacitors. However, the imbalance in electrochemical kinetics between the battery-type anode and the capacitor-type cathode hinders the further development of HLICs. Fully conjugated covalent organic frameworks have great potential as electrode materials for HLICs due to the designability of their structure. Herein, a fully conjugated covalent triazine framework (PT-CTF) integrating the hexaazatrinaphthylene unit was constructed, which provides abundant active sites (C═N and C═C groups) as the pseudocapacitive anode material for HLICs. And the connection of the triazine unit of PT-CTF improves the molecular conjugate degree, facilitating the transport of electrons. The fabricated PT-CTF||AC HLICs exhibit a high energy density (164.9 Wh kg-1 at 100 mA g-1), large power density (13.1 kW kg-1 at 4 A g-1), and excellent cycling capability (72% after 10 000 cycles at 2 A g-1).

Cathode materials for non-aqueous calcium rechargeable batteries

Calcium rechargeable batteries based on divalent charge carriers have the potential to meet the future demands for large-scale energy storage applications, due to the crustal abundance of Ca element and the high capacity and high safety of Ca metal anodes. The discernible progress in electrolyte and anode materials has put calcium battery technology a step closer to practice. However, the pursuit of high-voltage, high-capacity and stable cathode materials had been formidable because of the sluggish ion migration kinetics and the instability of host lattices during Ca2+ insertion and extraction. Unlocking the potential of Ca rechargeable batteries particularly hinges on the strategic identification of high-performance cathode materials. Herein, this review summarizes the representative strategies to develop novel cathode materials that allow reversible accommodation of Ca2+ ions for high energy output. The cathode materials can be classified into intercalation-type (layered structure, polyanionic compounds, and Prussian blue analogues) and conversion-type (organic materials, sulfur, and oxygen). The scrutinization of their performances and drawbacks sheds light on the current stage of cathode material advancement and provides informative suggestions for future studies to develop advanced calcium rechargeable batteries with competitive performance.

Strategically Modulating Proton Activity and Electric Double Layer Adsorption for Innovative All-Vanadium Aqueous Mn2+/Proton Hybrid Batteries

Aqueous Mn-ion batteries (MIBs) exhibit a promising development potential due to their cost-effectiveness, high safety, and potential for high energy density. However, the development of MIBs is hindered by the lack of electrode materials capable of storing Mn2+ ions due to acidic manganese salt electrolytes and large ion radius. Herein, the tunnel-type structure of monoclinic VO2 nanorods to effectively store Mn2+ ions via a reversible (de)insertion chemistry for the first time is reported. Utilizing exhaustive in situ/ex situ multi-scale characterization techniques and theoretical calculations, the co-insertion process of Mn2+/proton is revealed, elucidating the capacity decay mechanism wherein high proton activity leads to irreversible dissolution loss of vanadium species. Further, the Grotthuss transfer mechanism of protons is broken via a hydrogen bond reconstruction strategy while achieving the modulation of the electric double-layer structure, which effectively suppresses the electrode interface proton activity. Consequently, the VO2 demonstrates excellent electrochemical performance at both ambient temperatures and -20 °C, especially maintaining a high capacity of 162 mAh g-1 at 5 A g-1 after a record-breaking 20 000 cycles. Notably, the all-vanadium symmetric pouch cells are successfully assembled for the first time based on the "rocking-chair" Mn2+/proton hybrid mechanism, demonstrating the practical application potential.

Insight on photocatalytic synchronous oxidation and reduction for pollutant removal: Chemical energy conversion between macromolecular organic pollutants and heavy metal

Collaborative treatment of pollutants is a promising approach for wastewater treatment. In this work, a covalent organic framework material (COFs) with an imine structure was synthesised by the Schiff base reaction, and photochemical tests showed good photochemical effects. It was used to explore the photocatalytic treatment of co-existing pollutants (heavy metal ions and antibiotics) and the performance of treating co-existing wastewater was investigated. The degradation performance of levofloxacin (LVX) and Cr(VI) was improved in the coexisting pollutants system, with the LVX degradation being 4.2 times more effective than that of the LVX solitary system. Moreover, this phenomenon was also observed in LVX/Ag(I), LVX/Fe(III), sulfadiazine/Cr(VI), norfloxacin/Cr(VI) and tetracycline/Cr(VI) systems. The analysis of active species suggesting that the synergistic promotion of photocatalytic oxidation-reduction systems was not only promoting from the improvement of simple charge separation, but it was also found that high-valent metal species can act directly in the oxidative decomposition of antibiotics. The interaction of pollutants and intermediates were rationally exploited and confirmed by control experiments and theoretical calculation. This conclusion helps us to re-examine the underlying mechanisms of photocatalytic synchronous oxidation and reduction reactions, simultaneously beneficial for the development of mixed pollutant control processes.

Design and Synthesis of Electrocatalysts Base on Catalysis-Unit Engineering

It is a pressing need to develop new energy materials to address the existing energy crisis. However, screening optimal targets out of thousands of material candidates remains a great challenge. Herein, an alternative concept for highly effective materials screening based on dual-atom salphen catalysis units, is proposed and validated. Such an approach simplifies the design of catalytic materials and reforms the trial-and-error experimental model into a building-blocks-assembly like process. First, density functional theory (DFT) calculations are performed on a series of potential catalysis units that are possible to synthesize. Then, machine learning (ML) is employed to define the structure-performance relationship and acquire chemical insights. Afterward, the projected catalysis units are integrated into covalent organic frameworks (COFs) to validate the concept Electrochemical tests confirming that Ni-SalphenCOF and Co-SalphenCOF are promising conductive agent-free oxygen evolution reaction (OER) catalysts. This work provides a fast-tracked strategy for the design and development of functional materials, which serves as a potentially workable framework for seamlessly integrating DFT calculations, ML, and experimental approaches.

Ferrocene-Based Polymer Organic Cathode for Extreme Fast Charging Lithium-Ion Batteries with Ultralong Lifespans

To meet the growing demand for energy storage, lithium-ion batteries (LIBs) with fast charging capabilities has emerged as a critical technology. The electrode materials affect the rate performance significantly. Organic electrodes with structural flexibility support fast lithium-ion transport and are considered promising candidates for fast-charging LIBs. However, it is a challenge to create organic electrodes that can cycle steadily and reach high energy density in a few minutes. To solve this issue, accelerating the transport of electrons and lithium ions in the electrode is the key. Here, it is demonstrated that a ferrocene-based polymer electrode (Fc-SO3Li) can be used as a fast-charging organic electrode for LIBs. Thanks to its molecular architecture, LIBs with Fc-SO3Li show exceptional cycling stability (99.99% capacity retention after 10 000 cycles) and reach an energy density of 183 Wh kg-1 in 72 seconds. Moreover, the composite material through in situ polymerization with Fc-SO3Li and 50 wt % carbon nanotube (denoted as Fc-SO3Li-CNT50) achieved optimized electron and ion transport pathways. After 10 000 cycles at a high current density of 50C, it delivered a high energy density of 304 Wh kg-1. This study provides valuable insights into designing cathode materials for LIBs that combine high power and ultralong cycle life.

Intermolecular Hydrogen Bonding Networks Stabilized Organic Supramolecular Cathode for Ultra-High Capacity and Ultra-Long Cycle Life Rechargeable Aluminum Batteries

Rechargeable aluminum batteries (RABs) are a promising candidate for large-scale energy storage, attributing to the abundant reserves, low cost, intrinsic safety, and high theoretical capacity of Al. However, the cathode materials reported thus far still face challenges such as limited capacity, sluggish kinetics, and undesirable cycle life. Herein, we propose an organic cathode benzo[i] benzo[6,7] quinoxalino [2,3-a] benzo [6,7] quinoxalino [2,3-c] phenazine-5,8,13,16,21,24-hexaone (BQQPH) for RABs. The six C=O and six C=N redox active sites in each molecule enable BQQPH to deliver a record ultra-high capacity of 413 mAh g-1 at 0.2 A g-1. Encouragingly, the intermolecular hydrogen bonding network and π-π stacking interactions endow BQQPH with robust structural stability and minimal solubility, enabling an ultra-long lifetime of 100,000 cycles. Moreover, the electron-withdrawing carbonyl group induces a reduction in the energy level of the lowest unoccupied molecular orbital and expands the π-conjugated system, which considerably enhances both the discharge voltage and redox kinetics of BQQPH. In situ and ex situ characterizations combined with theoretical calculations unveil that the charge storage mechanism is reversible coordination/dissociation of AlCl2 + with the N and O sites in BQQPH accompanied by 12-electron transfer. This work provides valuable insights into the design of high-performance organic cathode materials for RABs.

Covalent organic frameworks and their composites for rechargeable batteries

Covalent organic frameworks (COFs), characterized by well-ordered pores, large specific surface area, good stability, high precision, and flexible design, are a promising material for batteries and have received extensive attention from researchers in recent years. Compared with inorganic materials, COFs can construct elastic frameworks with better structural stability, and their chemical compositions and structures can be precisely adjusted and functionalized at the molecular level, providing an open pathway for the convenient transfer of ions. In this review, the energy storage mechanism and unique superiority of COFs and COF composites as electrodes, separators and electrolytes for rechargeable batteries are discussed in detail. Special emphasis is placed on the relationship between the establishment of COF structures and their electrochemical performance in different batteries. Finally, this review summarizes the challenges and prospects of COFs and COF composites in battery equipment.

Construction of a Bifunctional Redox-Site Conjugated Covalent-Organic Framework for Photoinduced Precision Trapping of Uranyl Ions

The performance of covalent-organic frameworks (COFs) for the photocatalytic extraction of uranium is greatly limited by the number of adsorption sites. Herein, inspired by electronegative redox reactions, we designed a nitrogen-oxygen rich pyrazine connected COF (TQY-COF) with multiple redox sites as a platform for extracting uranium via combining superaffinity and enhanced photoinduction. The preorganized bisnitrogen-bisoxygen donor configuration on TQY-COF is entirely matched with the typical geometric coordination of hexavalent uranyl ions, which demonstrates high affinity (tetra-coordination). In addition, the presence of the carbonyl group and pyrazine ring effectively stores and controls electron flow, which efficaciously facilitates the separation of e-/h+ and enhances photocatalytic performance. The experimental results show that TQY-COF removes up to 99.8% of uranyl ions from actual uranium mine wastewater under the light conditions without a sacrificial agent, and the separation coefficient reaches 1.73 × 106 mL g-1 in the presence of multiple metal ions, which realizes the precise separation in the complex environment. Importantly, DFT calculations further elucidate the coordination mechanism of uranium and demonstrate the necessity of the presence of N/O atoms in the photocatalytic adsorption of uranium.

Engineering Covalent Organic Frameworks Toward Advanced Zinc-Based Batteries

Zinc-based batteries (ZBBs) have demonstrated considerable potential among secondary batteries, attributing to their advantages including good safety, environmental friendliness, and high energy density. However, ZBBs still suffer from issues such as the formation of zinc dendrites, occurrence of side reactions, retardation of reaction kinetics, and shuttle effects, posing a great challenge for practical applications. As promising porous materials, covalent organic frameworks (COFs) and their derivatives have rigid skeletons, ordered structures, and permanent porosity, which endow them with great potential for application in ZBBs. This review, therefore, provides a systematic overview detailing on COFs structure pertaining to electrochemical performance of ZBBs, following an in depth discussion of the challenges faced by ZBBs, which includes dendrites and side reactions at the anode, as well as dissolution, structural change, slow kinetics, and shuttle effect at the cathode. Then, the structural advantages of COF-correlated materials and their roles in various ZBBs are highlighted. Finally, the challenges of COF-correlated materials in ZBBs are outlined and an outlook on the future development of COF-correlated materials for ZBBs is provided. The review would serve as a valuable reference for further research into the utilization of COF-correlated materials in ZBBs.

Constructing Carbon Nanotube-Enhanced Ultra-Thin Organic Compounds with Multi-Redox Sites for "All-Temperature" Potassium-Ion Battery Anode and its Step-Wise K-Storage Mechanism

Organic compounds are regarded as important candidates for potassium-ion batteries (KIBs) due to their light elements, controllable polymerization, and tunable functional groups. However, intrinsic drawbacks largely restrict their application, including possible solubility in electrolytes, poor conductivity, and low diffusion coefficients. To address these issues, an ultrathin layered pyrazine/carbonyl-rich material (CT) is synthesized via an acid-catalyzed solvothermal reaction and homogeneously grown on carbon nanotubes (CNTs), marked as CT@CNT. Such materials have shown good features of exposing functional groups to guest ions and good electron transport paths, exhibiting high reversible capacity and remarkable rate capability over a wide temperature range. Two typical electrolytes are compared, demonstrating that the electrolyte of LX-146 is more suitable to maximize the electrochemical performances of electrodes at different temperatures. A stepwise reaction mechanism of K-chelating with C═O and C═N functional groups is proposed, verified by in/ex situ spectroscopic techniques and theoretical calculations, illustrating that pyrazines and carbonyls play the main roles in reacting with K+ cations, and CNTs promote conductivity and restrain electrode dissolution. This study provides new insights to understand the K-storage behaviors of organic compounds and their "all-temperature" application.

Conductive Metal-Organic Framework with Superior Redox Activity as a Stable High-Capacity Anode for High-Temperature K-Ion Batteries

High-temperature rechargeable batteries are essential for energy storage in elevated-temperature situations. Due to the resource abundance of potassium, high-temperature K-ion batteries are drawing increasing research interest. However, raising the working temperature would aggravate the chemical and mechanical instability of the KIB anode, resulting in very fast capacity fading, especially when high capacity is pursued. Here, we demonstrated that a porous conductive metal-organic framework (MOF), which is constructed by N-rich aromatic molecules and CuO4 units via π-d conjugation, could provide multiple accessible redox-active sites and promised robust structure stability for efficient potassium storage at high temperatures. Even working at 60 °C, this MOF anode could deliver high initial capacity (455 mAh g-1), impressive rate, and extraordinary cyclability (96.7% capacity retention for 1600 cycles), which is much better than those of reported high-temperature KIB anodes. The mechanistic study revealed that C═N groups and CuO4 units contributed abundant redox-active sites; the synergistic effect of π-d conjugated character and reticular porous architecture facilitated the K+/e- transport and ensured an insoluble electrode with small volume deformation, thus achieving stable high-capacity potassium storage.

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

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

Application of New COF Materials in Secondary Battery Anode Materials

Covalent organic framework materials (COFs), as a new type of organic porous material, not only have the characteristics of flexible structure, abundant resources, environmental friendliness, etc., but also have the characteristics of a regular structure and uniform pore channels, so they have broad application prospects in secondary batteries. Their functional group structure, type, and number of active sites play a crucial role in the performance of different kinds of batteries. Therefore, this article starts from these aspects, summarizes the application and research progress of the COF anode materials used in lithium-ion batteries, sodium-ion batteries, and potassium-ion batteries in recent years, discusses the energy storage mechanism of COF materials, and expounds the application prospects of COF electrodes in the field of energy storage.