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

Zn-based batteries for sustainable energy storage: strategies and mechanisms

Batteries play a pivotal role in various electrochemical energy storage systems, functioning as essential components to enhance energy utilization efficiency and expedite the realization of energy and environmental sustainability. Zn-based batteries have attracted increasing attention as a promising alternative to lithium-ion batteries owing to their cost effectiveness, enhanced intrinsic safety, and favorable electrochemical performance. In this context, substantial endeavors have been dedicated to crafting and advancing high-performance Zn-based batteries. However, some challenges, including limited discharging capacity, low operating voltage, low energy density, short cycle life, and complicated energy storage mechanism, need to be addressed in order to render large-scale practical applications. In this review, we comprehensively present recent advances in designing high-performance Zn-based batteries and in elucidating energy storage mechanisms. First, various redox mechanisms in Zn-based batteries are systematically summarized, including insertion-type, conversion-type, coordination-type, and catalysis-type mechanisms. Subsequently, the design strategies aiming at enhancing the electrochemical performance of Zn-based batteries are underscored, focusing on several aspects, including output voltage, capacity, energy density, and cycle life. Finally, challenges and future prospects of Zn-based batteries are discussed.

High Sodium Ion Storage by Multifunctional Covalent Organic Frameworks for Sustainable Sodium Batteries

Rechargeable sodium batteries hold great promise for circumventing the increasing demand for lithium-ion batteries (LIBs) and the limited supply of lithium. However, efficient sodium ion storage remains a great impediment in this field. In this study, we report the designed synthesis of a multifunctional two-dimensional covalent organic framework featuring hexaazatrinaphthalene cores linked by imidazole moieties and demonstrate its effective performance in sodium ion storage. Benzimidazole-linked covalent organic framework (BCOF-1) was synthesized by a condensation reaction between hexaazatrinaphthalenehexamine (HATNHA) and terephthalaldehyde (TA) and exhibited a high theoretical specific capacity of 392 mA h g-1. BCOF-1 crystallizes, forming eclipsed AA stacking and mesoporous hexagonal one-dimensional channels with high surface area (840 m2 g-1), facilitating fast ionic mobility and charge transfer and enabling high-rate capability at high current rates. BCOF-1 exhibits pseudocapacitive-like behavior with a high specific capacity of 387 mA h g-1, an energy density of 302 W h kg-1 at 0.1 C, and a power density of 682 W kg-1 at 5 C. Our results demonstrate that redox-active COFs have the desired structural and electronic merits to advance the use of organic electrodes in sodium-ion storage toward sustainable and efficient batteries.

A covalent organic framework as a dual-active-center cathode for a high-performance aqueous zinc-ion battery

Organic electrode materials have shown significant potential for aqueous Zn ion batteries (AZIBs) due to their flexible structure designability and cost advantage. However, sluggish ionic diffusion, high solubility, and low capacities limit their practical application. Here, we designed a covalent organic framework (TA-PTO-COF) generated by covalently bonding tris(4-formylbiphenyl)amine (TA) and 2,7-diaminopyrene-4,5,9,10-tetraone (PTO-NH2). The highly conjugated skeleton inside enhances its electron delocalization and intermolecular interaction, leading to high electronic conductivity and limited solubility. The open channel within the TA-PTO-COF provides ionic diffusion pathways for fast reaction kinetics. In addition, the abundant active sites (C[double bond, length as m-dash]N and C[double bond, length as m-dash]O) endow the TA-PTO-COF with a large reversible capacity. As a result, the well-designed TA-PTO-COF cathode delivers exceptional capacity (255 mA h g-1 at 0.1 A g-1), excellent cycling stability, and a superior rate capacity of 186 mA h g-1 at 10 A g-1. Additionally, the co-insertion mechanism of Zn2+/H+ within the TA-PTO-COF cathode is revealed in depth by ex situ spectroscopy. This study presents an effective strategy for developing high-performance organic cathodes for advanced AZIBs.

Six polyoxotungstate-based transition metal compounds for electrochemical capacitor application and a comparative analysis of factors affecting capacitances

Six different polyoxotungstate-based transition metal complexes were synthesized, namely [Cu5(2,2'-bpy)5(μ2-Cl)2(PO4)2(H2O)2][HPW12O40]·2H2O (1), [Cu1.5(2,2'-bpy)1.5(inic)2(H2O)1.5]3[H1.5PW12O40]2·16.25H2O (2), [Cu(2,2'-bpy)2]2[SiW12O40]·10H2O (3), [Zn(phen)3]2[PWVWVI11O40]·5H2O (4), [Zn(phen)2(H2O)]2[SiW12O40]·2H2O (5), and [Zn(2,2'-bpy)2]2[SiW12O40] (6) (2,2'-bpy = 2,2'-bipyridine, inic = isonicotinic acid, phen = 1,10-phenanthroline). Compound 1 is based on [HPW12O40]2- anions, which are accommodated within the open channels of a supramolecular network formed by novel Cu-P-Cl coordination clusters. Compound 2 is constructed from [H1.5PW12O40]1.5- and novel [Cu1.5(2,2'-bpy)1.5(inic)2(H2O)1.5]+ coordination fragments, and polyoxoanions are encapsulated within the pores created by the copper coordination fragments, resulting in a unique three-dimensional supramolecular architecture. Compound 3 is a two-dimensional structure formed through the covalent linkage between [SiW12O40]4- and [Cu(2,2'-bpy)2]2+. Compound 4 is a supramolecular architecture formed by [PWVWVI11O40]4- and [Zn(phen)3]2+ coordination fragments, while compound 5 is a supramolecular structure based on POM bi-supported Zn coordination complexes. Compound 6 is a two-dimensional framework structure constituted by [SiW12O40]4- and [Zn(2,2'-bpy)2]2+via covalent interactions. In addition, electrochemical measurement results show that the copper-based tungstate compounds 1-3 and zinc-based tungstate compounds 4-6 exhibit different performances and durabilities as electrochemical capacitors (compound 1 shows the highest specific capacitance of 94.0 F g-1 at 1.5 A g-1, whereas compound 6 maintains the best cycling stability with the capacity retention of 80.7% after 1000 cycles at 4 A g-1.). This study contributes to the development of POM-based transition metal complexes with high capacitance by providing insights into the design and synthesis process.

Anchoring π-d Conjugated Metal-Organic Frameworks with Dual-Active Centers on Carbon Nanotubes for Advanced Potassium-Ion Batteries

Potassium-ion batteries (PIBs) are gradually gaining attention owing to their natural abundance, excellent security, and high energy density. However, developing excellent organic cathode materials for PIBs to overcome the poor cycling stability and slow kinetics caused by the large radii of K+ ions is challenging. This study demonstrates for the first time the application of a hexaazanonaphthalene (HATN)-based 2D π-d conjugated metal-organic framework (2D c-MOF) with dual-active centers (Cu-HATNH) and integrates Cu-HATNH with carbon nanotubes (Cu-HATNH@CNT) as the cathode material for PIBs. Owing to this systematic module integration and more exposed active sites with high utilization, Cu-HATNH@CNT exhibits a high initial capacity (317.5 mA h g-1 at 0.1 A g-1 ), excellent long-term cycling stability (capacity retention of 96.8% at 5 A g-1 after 2200 cycles), and outstanding rate capacity (147.1 mA h g-1 at 10 A g-1 ). The reaction mechanism and performance are determined by combining experimental characterization and density functional theory calculations. This contribution provides new opportunities for designing high-performance 2D c-MOF cathodes with multiple active sites for PIBs.

Heteroatoms-Doped Mesoporous Carbon Nanosheets with Dual Diffusion Pathways for Highly Efficient Potassium Ion Storage

The high potassization/depotassization energy barriers and lack of efficient ion diffusion pathways are two serious obstacles for carbon-based materials to achieve satisfactory potassium ion storage performance. Herein, a facile and controllable one-step exfoliation-doping-etching strategy is proposed to construct heteroatoms (N, O, and S)-doped mesoporous few-layer carbon nanosheets (NOS-C). The mixed molten salts of KCl/K2 SO4 are innovatively used as the exfoliators, dopants, and etching agents, which enable NOS-C with expanded interlayer spacing and uniformly distributed mesopores with the adjusted electronic structure of surrounding carbon atoms, contributing efficient dual (vertical and horizontal) K-ion diffusion pathways, low potassization/depotassization energy barriers and abundant active sites. Thus, the NOS anodes achieve a high reversible capacity of 516.8 mAh g-1 at 0.05 A g-1 , superior rate capability of 202.8 mAh g-1 at 5 A g-1 and excellent long-term cyclic stability, and their practical application potential is demonstrated by the assembled potassium-ion full batteries. Moreover, a surface-interlayer synergetic K+ storage mechanism is revealed by a combined theoretical and experimental approach including in situ EIS, in situ Raman, ex situ XPS, and SEM analysis. The proposed K+ storage mechanism and unique structural engineering provide a new pathway for potassium-ion storage devices and even beyond.

Covalent Organic Frameworks: Their Composites and Derivatives for Rechargeable Metal-Ion Batteries

Covalent organic frameworks (COFs) have attracted considerable interest in the field of rechargeable batteries owing to their three-dimensional (3D) varied pore sizes, inerratic porous structures, abundant redox-active sites, and customizable structure-adjustable frameworks. In the context of metal-ion batteries, these materials play a vital role in electrode materials, effectively addressing critical issues such as low ionic conductivity, limited specific capacity, and unstable structural integrity. However, the electrochemical characteristics of the developed COFs still fall short of practical battery requirements due to inherent issues such as low electronic conductivity, the tradeoff between capacity and redox potential, and unfavorable micromorphology. This review provides a comprehensive overview of the recent advancements in the application of COFs, COF-based composites, and their derivatives in rechargeable metal-ion batteries, including lithium-ion, lithium-sulfur, sodium-ion, sodium-sulfur, potassium-ion, zinc-ion, and other multivalent metal-ion batteries. The operational mechanisms of COFs, COF-based composites, and their derivatives in rechargeable batteries are elucidated, along with the strategies implemented to enhance the electrochemical properties and broaden the range of their applications.

Single Ru Sites on Covalent Organic Framework-Coated Carbon Nanotubes for Highly Efficient Electrocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) with precisely controllable structures and highly ordered porosity possess great potential as electrocatalysts for hydrogen evolution reaction (HER). However, the catalytic performance of pristine COFs is limited by the poor active sites and low electron transfer. Herein, to address these issues, the conductive carbon nanotubes (CNTs) are coated by a defined structure RuBpy(H2 O)(OH)Cl2 in bipyridine-based COF (TpBpy). And this composite with single site Ru incorporated can be used as HER electrocatalyst in alkaline conditions. A series of crucial issues are carefully discussed through experiments and density functional theory (DFT) calculations, such as the coordination structure of the atomically dispersion Ru ions, the catalytic mechanism of the embedded catalytic site, and the effect of COF and CNTs on the electrocatalytic properties. According to DFT calculations, the embedded single sites Ru act as catalytic sites for H2 generation. Benefitting from increasing the catalyst conductivity and the charge transfer, the as-prepared c-CNT-0.68@TpBpy-Ru shows an excellent HER overpotential of 112 mV at 10 mA cm-2 under alkaline conditions as well as an excellent durability up to 12 h, which is superior to that of most of the reported COFs electrocatalysts in alkaline solution.

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.

Stable organic polymer anode for high rate and fast charge sodium based dual-ion battery

Considering the extensive resources, flexible structural designability, and abundant active sites, organic electrodes have been considered as the ideal sodium storage materials. However, organic materials generally face the limitations of unstable and dissolved characteristic, leading to a poor cyclic stability. In this work, we proposed a carbon nanotube (CNT) modified polyimide as the anode for sodium-based dual-ion battery (SDIB). The polyimide remains well the structure and morphology of monomer with a stable conjugated structure and high degree of crystallinity, effectively enhancing the electrochemical performance of the SDIBs. Also, the cooperation with CNT particularly improves the ion conductivity of the anode and advances the rate performance. Combined with an ionic liquid electrolyte, the constructed dual-ion battery exhibits excellent rate capability, high specific discharge capacity and stable cycling performance. It delivers a specific discharge capacity of 119.3 mA h g-1 at 0.2 C (1 C=100 mA g-1 ) and still has a specific discharge capacity of 82.3 mA h g-1 even after 1000 cycles at 10 C. Besides, the system displays a low self-discharge rate and stable fast charging performance, which is expected to be applied in the large-scale electrochemical energy storage devices and inspire the future development of SDIBs.

Ultrathin Cobalt-Based Prussian Blue Analogue Nanosheet-Assembled Nanoboxes Interpenetrated with Carbon Nanotubes as a Fast Electron/Potassium-Ion Conductor for Superior Potassium Storage

Rechargeable potassium-ion batteries (PIBs) are regarded as potential substitutes for industrial lithium-ion batteries in large scale energy storage systems due to the world's abundant potassium supplies. Althogh cobalt hexacyanocobaltate (CoHCC) exhibits broad potential as a PIB anode material, its performance is currently unsatisfactory. Herein, novel 5 nm scale ultrathin CoHCC nanosheet-assembled nanoboxes with interspersed carbon nanotubes (CNTs/CoHCC nanoboxes) are fabricated to realize a highly reactive PIB anode. The ultrathin CoHCC layers substantially accelerate electron conduction and provide numerous active sites, while the connected CNTs provide fast axial electron transport. Consequently, the optimized anode exhibits a remarkable discharge capacity of 580.9 mAh g-1 at 0.1 A g-1 and long-term stability with 71.3% retention over 1000 cycles. In situ and ex situ characterizations and density functional theory calculations are further employed to elucidate the K+ storage process and the reason for the enhanced performance of the CNTs/CoHCC nanoboxes.

A phthalocyanine-based porous organic polymer for a lithium-ion battery anode

Porous organic polymers (POPs) are a novel class of polymeric materials with high flexibility and designability for building structures. Herein, a phthalocyanine-based porous organic polymer (PcPOP) was constructed in situ on copper foil from H2Pc(ethynyl)4 [Pc(ethynyl)4 = 2(3),9(10),16(17),23(24)-tetra(ethynyl)phthalocyanine] by the coupling reaction. Benefiting from the uniformly distributed electron-rich nitrogen atoms in the Pc structure and the sp-hybridized carbons in the acetylenic linkage, Li intercalation in the porous organic polymer would be improved and stabilized. As a result, PcPOP showed remarkable electrochemical performance in lithium-ion batteries as the anode, including high specific capacity (a charge capacity of 1172 mA h g-1 at a current density of 150 mA g-1) and long cycling stability (a reversible capacity of 960.1 mA h g-1 can be achieved even after 600 cycles at a current density of 1500 mA g-1). The result indicates that the intrinsic doping of electron-rich sites of the building molecules is beneficial for the electrochemical performance of the porous organic polymer.

Poly(p-phenylenediamine)-Coated Metal-Organic Frameworks for High-Performance Sodium-Ion Batteries: The Balance of Capacity and Stability

Metal-organic frameworks (MOFs) with well-defined porous structures and highly active frameworks are considered as promising electrode materials for sodium-ion batteries (SIBs). However, the structure pulverization upon sodiation/desodiation impacts on their practical application in SIBs. To address this issue, poly(p-phenylenediamine) (PPA) was uniformly coated onto the surface of MIL-88A, a typical Fe-based MOF through in situ polymerization initiated by the metal ions (Fe3+) of MIL-88A. Used as an anode material for SIBs, the PPA-coated MIL-88A, denoted as PPA@MIL-88A, showed significantly improved electrochemical performance. A reversible capacity as high as 230 mAh g-1 was achieved at 0.2 A g-1 even after 500 cycles. MIL-88A constructed with electrochemically active Fe3+ and fumaric acid ligands guarantees the high specific capacity, while the PPA polymer coating effectively inhibits the pulverization of MIL-88A. This work provides an efficient strategy for improving the structure and cycling stability of MOFs-based electrode materials.

Magnetic Field-Enhanced Performance of Superparamagnetic LiMn2 O4 -Based Composite Slurry Electrode for Semisolid Flow Battery

Semisolid flow batteries are expected to be applied to large-scale energy storage fields due to the combination of the high energy density of rechargeable batteries and the flexible design of flow batteries. However, electronic conductivity, specific capacity, and viscosity of slurry electrodes are generally mutually restrictive. Here, a new concept of semisolid flow batteries based on magnetic modification slurry electrode is proposed and the electrochemical performance of the semisolid electrode is expected to be improved by close contact and enhanced electronic conductivity between the active particles with the aid of external magnetic field. This concept is further demonstrated using superparamagnetic LiMn2 O4 -Fe3 O4 -carbon nanotube composite as semisolid cathode. It achieves a capacity of 113.7 mAh g-1 at a current density of 0.5 mA cm-2 with the aid of external magnetic field (about 0.4 T), which is about 21% higher than that without external magnetic field. Simulation study also reveals this improvement mainly resulted from the increase of the conductive paths of electrons after the rearrangement of the active particles under the external magnetic field. It is believed that this strategy gives a new and effective method for controlling the viscosity and electronic conductivity of the slurry electrodes and related flowable electrochemical energy storage systems.

Short-Range Graphitic Nanodomains in Hypocrystalline Carbon Nanotubes Realize Fast Potassium Ion Migration and Multidirection Stress Release

Defect-rich carbon materials are considered as one of the most promising anodes for potassium-ion batteries due to their enormous adsorption sites of K+ , while the realization of both rate capability and cycling stability is still greatly limited by unstable electrochemical kinetics and inevitable structure degradation. Herein, an Fe3+ -induced hydrothermal-pyrolysis strategy is reported to construct well-tailored hybrid carbon nanotubes network architecture (PP-CNT), in which the short-range graphitic nanodomains are in-situ localized in the pea pod shape hypocrystalline carbon. The N,O codoped hypocrystalline carbon region contributes to abundant defect sites for potassium ion storage, ensuring high reversible capacity. Meanwhile, the short-range graphitic nanodomains with expanded interlayer spacing facilitate stable K+ migration and fast electron transfer. Furthermore, the finite element analysis confirms the volume expansion caused by K+ intercalation can be availably buffered due to the multidirection stress release effect of the unique porous pea pod shape, endowing carbon nanotubes with superior structural integrity. Consequently, the PP-CNT anode exhibits superior potassium-storage performance, including high reversible capacity, exceptional rate capability, and ultralong cycling stability. This work opens a new avenue for the fabrication of advanced carbon materials for achieving durable and fast potassium storage.

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

Organic materials are promising for cation storage in calcium ion batteries (CIBs). However, the high solubility of organic materials in an electrolyte and low electronic conductivity remain the key challenges for high-performance CIBs. Herein, a nitrogen-rich covalent organic framework with multiple carbonyls (TB-COF) is designed as an aqueous anode to address those obstacles. TB-COF demonstrates a high reversible capacity of 253 mAh g-1 at 1.0 A g-1 and long cycle life (0.01% capacity decay per cycle at 5 A g-1 after 3000 cycles). The redox mechanism of Ca2+/H+ co-intercalated in COF and chelating with C═O and C═N active sites is validated. In addition, a novel C═C active site was identified for Ca2+ ion storage. Both computational and empirical results reveal that per TB-COF repetitive unit, up to nine Ca2+ ions are stored after three staggered intercalation steps, involving three distinct Ca2+ ion storage sites. Finally, the evolution process of radical intermediates further elucidates the C═C reaction mechanism.

Covalent Organic Frameworks as Model Materials for Fundamental and Mechanistic Understanding of Organic Battery Design Principles

Redox-active covalent organic frameworks (COFs) have recently emerged as advanced electrodes in polymer batteries. COFs provide ideal molecular precision for understanding redox mechanisms and increasing the theoretical charge-storage capacities. Furthermore, the functional groups on the pore surface of COFs provide highly ordered and easily accessible interaction sites, which can be modeled to establish a synergy between ex situ/in situ mechanism studies and computational methods, permitting the creation of predesigned structure-property relationships. This perspective integrates and categorizes the redox functionalities of COFs, providing a deeper understanding of the mechanistic investigation of guest ion interactions in batteries. Additionally, it highlights the tunable electronic and structural properties that influence the activation of redox reactions in this promising organic electrode material.

One-Dimensional Covalent Organic Framework as High-Performance Cathode Materials for Lithium-Ion Batteries

Covalent organic frameworks (COFs) have emerged as a new class of cathode materials for energy storage in recent years. However, they are limited to two-dimensional (2D) or three-dimensional (3D) framework structures. Herein, this work reports designed synthesis of a redox-active one-dimensional (1D) COF and its composites with 1D carbon nanotubes (CNTs) via in situ growth. Used as cathode materials for Li-ion batteries, the 1D COF@CNT composites with unique dendritic core-shell structure can provide abundant and easily accessible redox-active sites, which contribute to improve diffusion rate of lithium ions and the corresponding specific capacity. This synergistic structural design enables excellent electrochemical performance of the cathodes, giving rise to 95% utilization of redox-active sites, high rate capability (81% capacity retention at 10 C), and long cycling stability (86% retention after 600 cycles at 5 C). As the first example to explore the application of 1D COFs in the field of energy storage, this study demonstrates the great potential of this novel type of linear crystalline porous polymers in battery technologies.

Spacer-Engineered Ionic Channels in Covalent Organic Framework Membranes toward Ultrafast Proton Transport

Side-chain engineering of covalent organic frameworks as advanced ion conductors is a critical issue to be explored. Herein, ionic covalent organic framework membranes (iCOFMs) with spacer-engineered ionic channel are de novo designed and prepared. The ionic channels are decorated with side chains comprising spacers having different carbon chain lengths and the -SO₃ H groups at the end. Attributed to the synergistic contribution from the spacers and the -SO₃ H groups, the iCOFM with moderate-length spacer exhibit the highest through-plane proton conductivity of 889 mS cm^-1 at 90 °C.

Ion Co-storage in Porous Organic Frameworks through On-site Coulomb Interactions for High Energy and Power Density Batteries

Fast and continuous ion insertion is blocked in the common electrodes operating with widely accepted single-ion storage mechanism, primarily due to Coulomb repulsion between the same ions. It results in an irreconcilable conflict between capacity and rate performance. Herein, we designed a porous organic framework with novel multiple-ion co-storage modes, including PF₆ ^- /Li⁺ , OTF^- /Mg²⁺ , and OTF^- /Zn²⁺ co-storage. The Coulomb interactions between cationic and anionic carriers in the framework can significantly promote electrode kinetics, by rejuvenating fast ion carrier migration toward framework interior. Consequently, the framework via PF₆ ^- /Li⁺ co-storage mode shows a high energy density of 878 Wh kg^-1 cycled more than 20 000 cycles, with an excellent power density of 28 kW kg^-1 that is already comparable to commercial supercapacitors. The both greatly improved energy and power densities via the co-storage mode may pave a way for exploring new electrodes that are not available from common single-ion electrodes.

Multiple Accessible Redox-Active Sites in a Robust Covalent Organic Framework for High-Performance Potassium Storage

Covalent organic framework (COF) materials with porous character and robust structure have significant applied implications for K-ion battery (KIB) anodes, but they are limited by the low reversible capacity and inferior rate capability. Here, based on theoretical calculations, we identified that a porous bulk COF featuring numerous pyrazines and carbonyls in the π-conjugated periodic skeleton could provide multiple accessible redox-active sites for high-performance potassium storage. Its porous structure with a surface-dominated storage mechanism enabled the fast and stable storage of K-ions. Its insolubility in organic electrolytes and small volumetric change after potassiation ensured a robust electrode for stable cycling. As a KIB anode, this bulk COF demonstrated an unprecedentedly outstanding combination of reversible capacity (423 mAh g^-1 at 0.1 C), rate capability (185 mAh g^-1 at 10 C), and cyclability. The theoretical simulation and comprehensive characterizations confirmed the active sites are contributed by C═O, C═N, and the cation-π effect.

Covalent Organic Frameworks: Recent Progress in Biomedical Applications

Covalent organic frameworks (COFs) are a type of crystalline organic porous material with specific features and interesting structures, including porosity, large surface area, and biocompatibility. These features enable COFs to be considered as excellent candidates for applications in various fields. Recently, COFs have been widely demonstrated as promising materials for biomedical applications because of their excellent physicochemical properties and ultrathin structures. In this review, we cover the recent progress of COF materials for applications in photodynamic therapy, gene delivery, photothermal therapy, drug delivery, bioimaging, biosensing, and combined therapies. Moreover, the critical challenges and further perspectives with regards to COFs for future biology-facing applications are also discussed.

Confine, Defect, and Interface Manipulation of Fe3 Se4 /3D Graphene Targeting Fast and Stable Potassium-Ion Storage

The fast electrochemical kinetics behavior and long cycling life have been the goals in developing anode materials for potassium ion batteries (PIBs). On account of high electron conductivity and theoretical capacity, transition metal selenides have been deemed as one of the promising anode materials for PIBs. Herein, a systematic structural manipulation strategy, pertaining to the confine of Fe₃ Se₄ particles by 3D graphene and the dual phosphorus (P) doping to the Fe₃ Se₄ /3DG (DP-Fe₃ Se₄ /3DG), has been proposed to fulfill the efficient potassium-ion (K-ion) evolution kinetics and thus boost the K-ion storage performance. The theoretical calculation results demonstrate that the well-designed dual P doping interface can effectively promote K-ion adsorption behavior and provide a low energy barrier for K-ion diffusion. The insertion-conversion and adsorption mechanism for multi potassium storage behavior in DP-Fe₃ Se₄ /3DG composite has been also deciphered by combining the in situ/ex situ X-ray diffraction and operando Raman spectra evidences. As expected, the DP-Fe₃ Se₄ /3DG anode exhibits superior rate capability (120.2 mA h g^-1 at 10 A g^-1 ) and outstanding cycling performance (157.9 mA h g^-1 after 1000 cycles at 5 A g^-1 ).

Ionothermal Synthesis of Fully Conjugated Covalent Organic Frameworks for High-Capacity and Ultrastable Potassium-Ion Batteries

Fully aromatic conjugated covalent organic frameworks (FAC-COFs) with excellent physicochemical stability have been emerging as active semiconductors for diverse potential applications. Developing efficient synthesis methods for fabricating FAC-COFs will significantly facilitate the exploration over their material and photonic/electronic functionalities. Herein, a facile solvent-free strategy is developed for the synthesis of 2D phthalocyanine-based FAC-COFs (FAC-Pc-COFs). Cyclopolymerization of benzo[1,2-b:4,5-b']bis[1,4]benzodioxin-2,3,9,10-tetracarbonitrile (BBTC) and quinoxalino[2',3':9,10]phenanthro[4,5-abc]phenazine-6,7,15,16-tetracarbonitrile (QPPTC) in ZnCl₂  leads to the fast formation and isolation of BB-FAC-Pc-COF and QPP-FAC-Pc-COF, respectively. Powder X-ray diffraction and electron microscopy analysis reveal their crystalline nature with sql topology and AA stacking configuration. Thermogravimetric analysis and immersion experiment indicate their excellent stability. The conductivity test demonstrates their high conductivity of 0.93-1.94 × 10^-4  S cm^-1  owing to the fully π-conjugated electronic structural nature. In particular, the as-prepared FAC-Pc-COFs show high-performance K⁺ storage in potassium-ion batteries due to their excellent conductivity, highly ordered and robust structure, and N/O-rich framework nature. Impressively, QPP-FAC-Pc-COF shows a large reversible capacity of 424 mA h g^-1  after 100 cycles at 50 mA g^-1  and a capacity retention of nearly 100% at 2000 mA g^-1  for over 10 000 cycles.

In Situ Growth of Covalent Organic Framework Nanosheets on Graphene as the Cathode for Long-Life High-Capacity Lithium-Ion Batteries

The poor electronic and ionic conductivities of covalent organic frameworks (COFs) severely restrict the development of COF-based electrodes for practical rechargeable batteries, therefore inspiring more research interest from the direction of both material synthesis and technology. Herein, a dual-porous COF, USTB-6, with good crystallinity and rich redox-active sites is conceived and fabricated by the polymerization of 2,3,8,9,14,15-hexa(4-formylphenyl)diquinoxalino [2,3-a:2',3'-c]phenazine and 2,7-diaminopyrene-4,5,9,10-tetraone. In particular, the heterogeneous polymerization of the same starting materials in the presence of graphene affords uniformly dispersed COF nanosheets with a thickness of 8.3 nm on a conductive carbon substrate, effectively enhancing the electronic conductivity of the COF-based electrode. Such a graphene-supported USTB-6 nanosheets cathode when used in a lithium-ion battery exhibits a specific capacity of 285 mA h g^-1 at a current density of 0.2 C and excellent rate performance with a prominent capacity of 188 mA h g^-1 at 10 C. More importantly, a capacity of 170 mA h g^-1 is retained by using the USTB-6 nanosheets cathode after 6000 cycles charge and discharge measurement at 5 C.

Achieving Sustainable and Stable Potassium-Ion Batteries by Leaf-Bioinspired Nanofluidic Flow

In nature, living systems have evolved integrated structures, matching optimized nanofluidics to adapt to external conditions. In rechargeable batteries, high-capacity electrodes are often plagued by the crucial and universal bottleneck of dissolution and shuttle of active substance into electrolyte, posing obstacles of inevitable capacity degradation. Introducing the concept of intelligent nanofluidics to electrodes, a leaf-bioinspired electrode configuration with hierarchical architecture to tackle this problem is proposed. This integrated structure with fine-tuned surface pores and unobstructed interior porous media, can spatially control the anisotropic nanofluidic flux, in an efficient and self-protectable way: tailoring the outflow across the electrode's surface and free transport in interior, to ensure speedy and stable energy conversion. As proofs of concept, applications of sustainable electrodes rejuvenated from fallen leaf and spent commercial batteries, are designed with leaf-bioinspired architecture. Both KCoS₂ and KS battery systems show advanced steady cycling with effectively mitigated shuttle issues in this smart architecture (0.15% and 0.21% capacity decay per cycle), even at high areal mass loading, when compared with open porous structure (0.60% and 0.39%). This work may pave a new way from a biomimetic view to integrated electrode engineering with regulated surface shielding to conquer the universal dissolution-shuttle problems facing high-capacity materials.

Flat-Zigzag Interface Design of Chalcogenide Heterostructure toward Ultralow Volume Expansion for High-Performance Potassium Storage

Heterostructure construction of layered metal chalcogenides can boost their alkali-metal storage performance, where the charge transfer kinetics can be promoted by the built-in electric fields. However, these heterostructures usually undergo interface separation due to severe layer expansion, especially for large-size potassium accommodation, resulting in the deconstruction of heterostructures and battery performance fading. Herein, first a stable interface design strategy where two metal chalcogenides with totally different layer-morphologies are stacked to form large K⁺ transport channels, rendering ultralow interlayer expansion, is presented. As a proof of concept, the flat-zigzag MoS₂ /Bi₂ S₃ heterostructures stacked with zigzag-morphology Bi₂ S₃ and flat-morphology MoS₂ present an ultralow expansion ratio (1.98%) versus MoS₂ (9.66%) and Bi₂ S₃ (9.61%), which deliver an ultrahigh potassium storage capacity of above 600 mAh g^-1 and capacity retention of 76% after 500 cycles, together with the built-in electric field of heterostructures. Once the heterostructures are used as an anode for potassium-based dual-ion batteries (K-DIBs), it achieves a superior full-cell capacity of ≈166 mAh g^-1 with a capacity retention of 71% after 400 cycles, which is an outstanding performance among the reported K-DIBs. This proposed interface stacking strategy may offer a new way toward stable heterostructure design for metal ions storage and transport applications.

Triple Conductive Wiring by Electron Doping, Chelation Coating and Electrochemical Conversion in Fluffy Nb2 O5 Anodes for Fast-Charging Li-Ion Batteries

High-rate anode material is the kernel of developing fast-charging lithium ion batteries (LIBs). T-Nb₂ O₅ , well-known for its "room and pillar" structure and bulk pseudocapacitive effect, is expected to enable the fast lithium (de)intercalation. But this property is still limited by the low electronic conductivity or insufficient wiring manner. Herein, a strategy of triple conductive wiring through electron doping, chelation coating, and electrochemical conversion inside the microsized porous spheres consisting of dendrite-like T-Nb₂ O₅ primary particles is proposed to achieve the fast-charging and durable anodes for LIBs. The penetrative implanting of conformal carbon coating (derivative from polydopamine chelate) and NbO domains (induced by excess discharging) reinforces the global supply of electronically conductive wires, apart from those from Co/Mn heteroatom or O vacancy doping. The polydopamine etching on T-Nb₂ O₅ spheres promotes their evolution into fluffy morphology with better electrolyte infiltration. The synergic electron and ion wiring at different scales endow the modified T-Nb₂ O₅ anode with ultralong cycling life (143 mAh g^-1 at 1 A g^-1 after 8500 cycles) and high-rate performance (144.1 mAh g^-1 at 10.0 A g^-1 ). The permeation of multiple electron wires also enables a high mass loading of T-Nb₂ O₅ (4.5 mg cm^-2 ) with a high areal capacity of 0.668 mAh cm^-2 even after 150 cycles.

Flower-Shaped Hollow VOOH Spheres Wrapped by Carbon Nanotubes as the Cathode Electrocatalyst Enable Ultrafast and Long-Lasting Li-S Batteries

Lithium-sulfur batteries (LSBs) have been considered promising candidates for next-generation energy storage devices owing to their high energy density, low price, and environment-friendly characteristics. However, their commercialization has been hindered by the "shuttle effect", which occurs during the charge/discharge cycles and leads to poor cycling performance and low coulombic efficiency. Here, we synthesized flower-shaped hollow VOOH spheres on the carbon nanotube (CNT) network, which were used as the multifunctional sulfur host materials for the first time in LSBs. These VOOH spheres can chemically and physically confine polysulfides as well as catalyze their redox conversion; additionally, their hollow structure can effectively accommodate the volume change during cycling. Moreover, the CNTs among spheres can improve the conductivity of the host material and increase the number of active sites for interfacial reactions. Accordingly, when used as a cathode material, VOOH@CNTs/S composites exhibited a large specific discharge capacity of 1414.63 mAh/g at 0.1 C and excellent cycling stability. At a low current density of 0.5 C, VOOH@CNTs/S exhibited a capacity decay of 0.044% per cycle after 100 cycles. Importantly, at an ultrahigh current density of 5 C, a specific capacity as high as 455.09 mAh/g could be still be delivered after 1000 cycles, corresponding to a superior capacity retention of 90.46% and an ultralow capacity decay of 0.009% per cycle. These findings open up a new material for the practical application of LSBs with ultrafast charge/discharge property and long-lasting cyclic stability.

MOFs-Derived Flower-Like Hierarchically Porous Zn-Mn-Se/C Composite for Extraordinary Rate Performance and Durable Anode of Sodium-Ion and Potassium-Ion Batteries

The slow kinetics and poor structural stability prevent transition metal selenides from being widely used in sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Herein, the "flower-like" porous carbon anchored by Zn-Mn binary selenides (ZMS@FC) composites are fabricated by selenizing the modified hierarchically metal-organic frameworks. The 2D conductive hierarchically flakes' abundant pore structure and multiple active sites shorten the ion diffusion length and promote conductivity, while the synergistic effect of the binary metals and intrinsic large pseudocapacitive contribution effectively improve capacity and rate performance. ZMS@FC composites exhibit impressive rate capability of 294.4 mA h g^-1 at 10 A g^-1 and excellent cyclic stability with 369.6 mA h g^-1 specific capacity retention at 2 A g^-1 after 1000 cycling in SIBs. It is noted that 156.9 mA h g^-1 can be retained at 5 A g^-1 and 227.0 mA h g^-1 is remained after 500 cycles at 2 A g^-1 in PIBs. The ex situ X-ray diffraction patterns and transmission electron microscopy pictures are used to confirm the conversion reaction processes of the Zn-Mn-Se. Designing high-performance energy storage materials may benefit greatly from the universal synthesis technology of bimetallic sulfide anodes for enhanced SIBs and PIBs.

Dual-Ion Stabilized Layered Structure of OVO for Zero-Strain Potassium Insertion and Extraction

Potassium-ion batteries (KIB) have similar energy storage mechanism with lithium-ion battery, but the potassium (K) resource is rich, which shows great potential for large-scale energy storage system. Recently, the anode materials of KIB studied mainly include carbon materials, transition metal oxides, and alloy materials. The amorphous hard carbon shows the best comprehensive performance, but its intercalation potential is close to 0 V (versus K⁺ /K), which is easy to cause K dendrite and brings security risks. The oxide materials have high capacity but high intercalation potential, low first cycle efficiency, and unstable cycle. Here, based on the understanding of the K intercalation mechanism of vanadium oxides, a novel zero strain anode material with layered structure of dual-ions (Na⁺ /K⁺ ) is designed (NaK(VO₃ )₂ V₂ O₅ ). The introduction of Na/K ion contributed to the transmission and further stabilized the structure. It has an excellent rate performance (10 A g^-1 , up to 25 000th cycle), and its special K storage mechanism and zero-strain characteristics are revealed for the first time by ex situ scanning electron microscope, X-ray powder diffraction, X-ray photoelectron spectroscopy, and other test methods. Considering the excellent performance endowed by these unique inherent properties, NaK(VO₃ )₂ V₂ O₅ shows great potential for commercial anode materials and may promote the innovation of KIB.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

Covalent organic frameworks (COFs) are a class of organic crystalline porous materials discovered in the early 21st century that have become an attractive class of emerging materials due to their high crystallinity, intrinsic porosity, structural regularity, diverse functionality, design flexibility, and outstanding stability. However, many chemical and physical properties strongly depend on the presence of metal ions in materials for advanced applications, but metal-free COFs do not have these properties and are therefore excluded from such applications. Metalated COFs formed by combining COFs with metal ions, while retaining the advantages of COFs, have additional intriguing properties and applications, and have attracted considerable attention over the past decade. This review presents all aspects of metalated COFs, from synthetic strategies to various applications, in the hope of promoting the continued development of this young field.

Solid Carbon Spheres with Interconnected Open Pore Channels Enabling High-Efficient Polysulfide Conversion for High-Rate Lithium-Sulfur Batteries

Hollow carbon spheres or core-sheath porous carbon spheres have been widely used in the S cathode of lithium-sulfur batteries. However, the sphere shells or the pore walls may block the free transport of active species to a certain extent and may have a negative influence on the effective accommodation of elemental sulfur. Herein, solid but porous carbon spheres (PNCS) with large porosity and high specific surface area are developed, which enable high sulfur loading and ample cathode/electrolyte contact area, and the interconnected open pore channels significantly shorten the ion/electron transport pathways. Together with high-conducting nitrogen-doped graphene (NG), facilitated polysulfide conversion kinetics is realized in the as-assembled Li-S batteries, which deliver a high initial discharge capacity of 1445 mAh g^-1 at 0.2 C, excellent rate capability of 872 mAh g^-1 at 4 C, and low capacity decay of 0.047% per cycle for 500 cycles at 1 C. Even under high sulfur loading of 5.5 mg cm^-2 and low electrolyte/sulfur (E/S) ratio of 5 μL mg^-1, the Li-S batteries still display high specific capacities of 896 mAh g^-1 and 4.96 mAh cm^-2. The real application of PNCS/NG is also demonstrated by the corresponding Li-S pouch cells showing high discharging capacity and stable open circuit voltage. This work exhibits the promising application of the solid carbon spheres as the S host for effectively addressing the polysulfide shuttle and propelling the development of high-performance Li-S batteries.

Construction of a Few-Layered COF@CNT Composite as an Ultrahigh Rate Cathode for Low-Cost K-Ion Batteries

Potassium-ion batteries (PIBs) are attracting great interest for large-scale energy storage owing to the abundant resources and low redox potential of K⁺/K. However, the large volume changes and slow kinetics caused by the larger ionic radius of K⁺ for cathode materials remain a critical challenge for PIBs. Herein, we construct few-layered covalent organic frameworks integrated with carboxylated carbon nanotubes (DAAQ-COF@CNT) as cathode materials for PIBs. The synthesized DAAQ-COF@CNT features numerous active sites, a stable conductive framework, and an appropriate surface area with nanopores, which can render high electrical conductivity, shorten the ion/electron diffusion distance, and accelerate K⁺ diffusion. In consequence, the DAAQ-COF@CNT delivers a high reversible capacity of 157.7 mAh g^-1 at 0.1 A g^-1, an excellent rate capability of 111.2 mAh g^-1 at 1 A g^-1, and a long cycling stability of 77.6% capacity retention after 500 cycles at 0.5 A g^-1. The integrated characterization of ex situ X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and theoretical simulation discloses that the storage mechanism of DAAQ-COF@CNT is based on the reversible reaction between electroactive C═O groups and K⁺ during two successive steps. This work provides a promising high-performance cathode material for PIBs and encourages the development of new types of covalent organic frameworks for PIBs.

Unusual Size Effect in Ion and Charge Transport in Micron-Sized Particulate Aluminum Anodes of Lithium-Ion Batteries

Aluminum is a promising anode material for lithium-ion batteries owing to its high theoretical capacity, excellent conductivity, and natural abundance. An anomalous size effect was observed for micron-sized aluminum powder electrodes in this work. Experimental and theoretical investigations reveal that the insulating oxide surface layer is the underlying cause, which leads to poor electrical conductivity and limited capacity utilization when the particle is too small. Additionally, poor electrolyte wettability also accounts for the hindered reaction kinetics due to the weak polarity feature of the oxide layer. Surface grafting of polar amino groups was demonstrated to be an effective strategy to improve electrolyte wettability. The present work revealed the critical limitations and underlying mechanisms for the aluminum anode, which is crucial for its practical application. Our results are also valuable for other metallic anodes with similar issues.

Advanced Lithium Primary Batteries: Key Materials, Research Progresses and Challenges

In recent years, with the vigorous development and gradual deployment of new energy vehicles, more attention has been paid to the research on lithium-ion batteries (LIBs). Compared with the booming LIBs, lithium primary batteries (LPBs) own superiority in specific energy and self-discharge rate and are usually applied in special fields such as medical implantation, aerospace, and military. Widespread application in special fields also means more stringent requirements for LPBs in terms of energy density, working temperature range and shelf life. Therefore, how to obtain LPBs with high energy density, wide operational temperature range and long storage life is of great importance in future development. In view of the above, this paper reviews the latest research on LPBs in cathode, anode and electrolyte over the years, and puts forward relevant insights for LPBs, along with the intention to explore avenues for the design of LPBs components in the coming decades and promote further development in this field.

Advanced Anode Materials for Sodium-Ion Batteries: Confining Polyoxometalates in Flexible Metal-Organic Frameworks by the "Breathing Effect"

Polyoxometalates (POMs) have shown great potential in sodium-ion batteries (SIBs) due to their reversible multielectron redox property and high ionic conductivity. Currently, POM-based SIBs suffer from the irreversible trapping and sluggish transmission kinetics of Na⁺. Herein, a series of POMs/metal-organic frameworks (MOFs)/graphene oxide (GO) (MOFs = MIL-101, MIL-53, and MIL-88B; POM = [PMo₁₂O₄₀]^3-, denoted as PMo₁₂) composites are developed as SIB anode materials for the first time. Unlike MIL-101 with large pore structures, the pores in flexible MIL-53 and MIL-88B swell spontaneously upon the accommodation of PMo₁₂. Particularly, the PMo₁₂/MIL-88B/GO composites deliver an excellent specific capacity of 214.2 mAh g^-1 for 600 cycles at 2.0 A g^-1, with a high initial Coulombic efficiency (ICE) of 51.0%. The so-called "breathing effect" of flexible MOFs leads to the relatively tight confinement space for PMo₁₂, which greatly modulates its electronic structure, affects the adsorption energy of Na⁺, and eventually reduces the trapping of sodium ions. Additionally, the straight and multidimensional channels in MIL-88B significantly accelerate ion diffusion, inducing favored energetic kinetics and thus generating high-rate performance.

An Advanced High-Entropy Fluorophosphate Cathode for Sodium-Ion Batteries with Increased Working Voltage and Energy Density

Impossible voltage plateau regulation for the cathode materials with fixed active elemental center is a pressing issue hindering the development of Na-superionic-conductor (NASICON)-type Na₃ V₂ (PO₄ )₂ F₃ (NVPF) cathodes in sodium-ion batteries (SIBs). Herein, a high-entropy substitution strategy, to alter the detailed crystal structure of NVPF without changing the central active V atom, is pioneeringly utilized, achieving simultaneous electronic conductivity enhancement and diffusion barrier reduction for Na⁺ , according to theoretical calculations. The as-prepared carbon-free high-entropy Na₃ V_1.9 (Ca,Mg,Al,Cr,Mn)_0.1 (PO₄ )₂ F₃ (HE-NVPF) cathode can deliver higher mean voltage of 3.81 V and more advantageous energy density up to 445.5 Wh kg^-1 , which is attributed by the diverse transition-metal elemental substitution in high-entropy crystalline. More importantly, high-entropy introduction can help realize disordered rearrangement of Na⁺ at Na(2) active sites, thereby to refrain from unfavorable discharging behaviors at low-voltage region, further lifting up the mean working voltage to realize a full Na-ion storage at the high voltage plateau. Coupling with a hard carbon (HC) anode, HE-NVPF//HC SIB full cells can deliver high specific energy density of 326.8 Wh kg^-1 at 5 C with the power density of 2178.9 W kg^-1 . This route means the unlikely potential regulation in NASICON-type crystal with unchangeable active center becomes possible, inspiring new ideas on elevating the mean working voltage for SIB cathodes.

One-Pot Synthesis of Porous Crystal Structured Nanosponge-Like Pristine Copper Metal-Organic Framework for Hybrid Supercapacitor Application

Hybrid supercapacitors are promising energy storage devices where high-power delivery is needed in continuous supply (like e-vehicles, cranes, industrialized machines, etc.). Metal-Organic Frameworks (MOFs) are the leading porous materials, and...