Covalent Organic Framework as an Efficient Protection Layer for a Stable Lithium-Metal Anode

Recent Progress in Using Covalent Organic Frameworks to Stabilize Metal Anodes for Highly-Efficient Rechargeable Batteries

Alkali metals (e.g. Li, Na, and K) and multivalent metals (e.g. Zn, Mg, Ca, and Al) have become star anodes for developing high-energy-density rechargeable batteries due to their high theoretical capacity and excellent conductivity. However, the inevitable dendrites and unstable interfaces of metal anodes pose challenges to the safety and stability of batteries. To address these issues, covalent organic frameworks (COFs), as emerging materials, have been widely investigated due to their regular porous structure, flexible molecular design, and high specific surface area. In this minireview, we summarize the research progress of COFs in stabilizing metal anodes. First, we present the research origins of metal anodes and delve into their advantages and challenges as anodes based on the physical/chemical properties of alkali and multivalent metals. Then, special attention has been paid to the application of COFs in the host design of metal anodes, artificial solid electrolyte interfaces, electrolyte additives, solid-state electrolytes, and separator modifications. Finally, a new perspective is provided for the research of metal anodes from the molecular design, pore modulation, and synthesis of COFs.

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.

Rechargeable Metal-Sulfur Batteries: Key Materials to Mechanisms

Rechargeable metal-sulfur batteries are considered promising candidates for energy storage due to their high energy density along with high natural abundance and low cost of raw materials. However, they could not yet be practically implemented due to several key challenges: (i) poor conductivity of sulfur and the discharge product metal sulfide, causing sluggish redox kinetics, (ii) polysulfide shuttling, and (iii) parasitic side reactions between the electrolyte and the metal anode. To overcome these obstacles, numerous strategies have been explored, including modifications to the cathode, anode, electrolyte, and binder. In this review, the fundamental principles and challenges of metal-sulfur batteries are first discussed. Second, the latest research on metal-sulfur batteries is presented and discussed, covering their material design, synthesis methods, and electrochemical performances. Third, emerging advanced characterization techniques that reveal the working mechanisms of metal-sulfur batteries are highlighted. Finally, the possible future research directions for the practical applications of metal-sulfur batteries are discussed. This comprehensive review aims to provide experimental strategies and theoretical guidance for designing and understanding the intricacies of metal-sulfur batteries; thus, it can illuminate promising pathways for progressing high-energy-density metal-sulfur battery systems.

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

sp-Carbon-Conjugated Organic Polymer as Multifunctional Interfacial Layers for Ultra-Long Dendrite-Free Lithium Metal Batteries

Fatal issues in lithium metal anodes (LMA), such as detrimental lithium dendrites growth and fragile solid-electrolyte interphase (SEI) during the Li plating/stripping process, often hinder the practical application of Li metal batteries (LMBs). Herein, cobalt-coordinated sp-carbon-conjugated organic polymer (Co-spc-COP) is constructed as the protective layer for regulating the interface stability of LMA. The unique synergistic beneficial effect of organic functional groups (C≡C linkage, C=N units and aromatic rings) and Co sites not only regulate the Li+ coordination environment and rearrange Li+ concentration to facilitate its transport by optimizing the electronic density, enhancing the compatibility with electrolyte interface and supplying "external magnetic driving strategy", but also strengthens the interfacial stiffness with high Young's modulus to better withstand the mechanical stress. These beneficial effects and relative underlying working mode and mechanism of uniform Li plating and rapid Li+ migration on the Co-spc-COP are also revealed by various in situ/ex situ experimental technologies and theory calculation. The Co-spc-COP-based cell delivers an extraordinary lifespan of 6600 h and ultrahigh capacity retention of 78.3 % (111.9 mAh g-1) after 1000 cycles at 1 C. This demonstrated synergistic strategy in Co-coordinated organic polymer may gain new insights to regulate the uniform and non-dendritic deposition/dissolution behaviors for highly stable LMBs.

Lithiophilic Covalent Organic Framework as Anode Coating for High-Performance Lithium Metal Batteries

The growth of disorganized lithium dendrites and weak solid electrolyte interphase greatly impede the practical application of lithium metal batteries. Herein, we designed and synthesized a new kind of stable polyimide covalent organic frameworks (COFs), which have a high density of well-aligned lithiophilic quinoxaline and phthalimide units anchored within the uniform one-dimensional channels. The COFs can serve as an artificial solid electrolyte interphase on lithium metal anode, effectively guiding the uniform deposition of lithium ions and inhibiting the growth of lithium dendrites. The unsymmetrical Li||COF-Cu battery exhibits a Coulombic efficiency of 99 % at a current density of 0.5 mA cm-2 , which can be well retained up to 400 cycles. Meanwhile, the Li-COF||LFP full cell shows a Coulombic efficiency over 99 % at a charge of 0.3 C. And its capacity can be well maintained up to 91 % even after 150 cycles. Therefore, the significant electrochemical cycling stability illustrates the feasibility of employing COFs in solving the disordered deposition of lithium ions in lithium metal batteries.

A 3D Lithiophilic Host for Dendrite-Free Lithium Metal Anode via One-Step Carbonization of an Energetic Metal-Organic Framework

Low Coulombic efficiency (CE) and safety issues are huge problems that hinder the practical application of Li metal anodes. Constructing Li host structures decorated with functional species can restrain the growth of Li dendrites and alleviate the great volume change. Here, a 3D porous carbonaceous skeleton modified with rich lithiophilic groups (Zn, ZnO, and Zn(CN)2 ) is synthesized as a Li host via one-step carbonization of a triazole-containing metal-organic framework. The nano lithiophilic groups serve as preferred sites for Li nucleation and growth, regulating a uniform Li+ flux and uniform current density distribution. In addition, the 3D porous network functions as a Li reservoir that provides rich internal space to store Li, thus alleviating the volumetric expansion during Li plating/stripping process. Thanks to these component and structural merits, an ultra-low overpotential for Li deposition is achieved, together with high CE of over 99.5% for more than 500 cycles at 1 mA cm-2 and 1 mAh cm-2 in half cells. The symmetric cells exhibit a prolonged cycling of 900 h at 1 mA cm-2 . The full cells by coupling Zn/ZnO/Zn(CN)2 @C-Li anode with LiFePO4 cathode deliver a high capacity retention of 94.3% after 200 cycles at 1 C.

A Review on Covalent Organic Frameworks as Artificial Interface Layers for Li and Zn Metal Anodes in Rechargeable Batteries

Li and Zn metals are considered promising negative electrode materials for the next generation of rechargeable metal batteries because of their non-toxicity and high theoretical capacity. However, the uneven deposition of metal ions (Li+ , Zn2+ ) and the uncontrolled growth of dendrites result in poor electrochemical stability, unsatisfactory cycle life, and rapid capacity decay of batteries assembled with Li and Zn electrodes. Owing to the unique internal directional channels and abundant redox active sites of covalent organic frameworks (COFs), they can be used to promote uniform deposition of metal ions during stripping/electroplating through interface modification strategies, thereby inhibiting dendrite growth. COFs provide a new perspective in addressing the challenges faced by the anodes of Li metal batteries and Zn ion batteries. This article discusses the stability and types of COFs, and summarizes some novel COF synthesis methods. Additionally, it reviews the latest progress and optimization methods of using COFs for metal anodes to improve battery performance. Finally, the main challenges faced in these areas are discussed. This review will inspire future research on metal anodes in rechargeable batteries.

"Fast-Charging" Anode Materials for Lithium-Ion Batteries from Perspective of Ion Diffusion in Crystal Structure

"Fast-charging" lithium-ion batteries have gained a multitude of attention in recent years since they could be applied to energy storage areas like electric vehicles, grids, and subsea operations. Unfortunately, the excellent energy density could fail to sustain optimally while lithium-ion batteries are exposed to fast-charging conditions. In actuality, the crystal structure of electrode materials represents the critical factor for influencing the electrode performance. Accordingly, employing anode materials with low diffusion barrier could improve the "fast-charging" performance of the lithium-ion battery. In this Review, first, the "fast-charging" principle of lithium-ion battery and ion diffusion path in the crystal are briefly outlined. Next, the application prospects of "fast-charging" anode materials with various crystal structures are evaluated to search "fast-charging" anode materials with stable, safe, and long lifespan, solving the remaining challenges associated with high power and high safety. Finally, summarizing recent research advances for typical "fast-charging" anode materials, including preparation methods for advanced morphologies and the latest techniques for ameliorating performance. Furthermore, an outlook is given on the ongoing breakthroughs for "fast-charging" anode materials of lithium-ion batteries. Intercalated materials (niobium-based, carbon-based, titanium-based, vanadium-based) with favorable cycling stability are predominantly limited by undesired electronic conductivity and theoretical specific capacity. Accordingly, addressing the electrical conductivity of these materials constitutes an effective trend for realizing fast-charging. The conversion-type transition metal oxide and phosphorus-based materials with high theoretical specific capacity typically undergoes significant volume variation during charging and discharging. Consequently, alleviating the volume expansion could significantly fulfill the application of these materials in fast-charging batteries.

Lithium-Ion Charged Polymer Channels Flattening Lithium Metal Anode

The concentration difference in the near-surface region of lithium metal is the main cause of lithium dendrite growth. Resolving this issue will be key to achieving high-performance lithium metal batteries (LMBs). Herein, we construct a lithium nitrate (LiNO3)-implanted electroactive β phase polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) crystalline polymorph layer (PHL). The electronegatively charged polymer chains attain lithium ions on the surface to form lithium-ion charged channels. These channels act as reservoirs to sustainably release Li ions to recompense the ionic flux of electrolytes, decreasing the growth of lithium dendrites. The stretched molecular channels can also accelerate the transport of Li ions. The combined effects enable a high Coulombic efficiency of 97.0% for 250 cycles in lithium (Li)||copper (Cu) cell and a stable symmetric plating/stripping behavior over 2000 h at 3 mA cm-2 with ultrahigh Li utilization of 50%. Furthermore, the full cell coupled with PHL-Cu@Li anode and LiFePO4 cathode exhibits long-term cycle stability with high-capacity retention of 95.9% after 900 cycles. Impressively, the full cell paired with LiNi0.87Co0.1Mn0.03O2 maintains a discharge capacity of 170.0 mAh g-1 with a capacity retention of 84.3% after 100 cycles even under harsh condition of ultralow N/P ratio of 0.83. This facile strategy will widen the potential application of LiNO3 in ester-based electrolyte for practical high-voltage LMBs.

Work-Function-Induced Interfacial Electron/Ion Transport in Carbon Hosts toward Dendrite-Free Lithium Metal Anodes

Coupled electron/ion transport is a decisive feature of Li plating/stripping, wherein the compatibility of electron/ion transport rates determines the morphology of deposited Li. Local Li+ hotspots form due to inhomogeneous interfacial charge transfer and lead to uncontrolled Li deposition, which decreases the Li utilization rate and safety of Li metal anodes. Herein, we report a method to obtain dendrite-free Li metal anodes by driving electron pumping and accumulating and boosting Li ion diffusion by tuning the work function of a carbon host using cobalt-containing catalysts. The results reveal that increasing the work function provides an electron deviation from C to Co, and electron-rich Co shows favorable binding to Li+ . The Co catalysts boost Li+ diffusion on the carbon fiber scaffolds without local aggregation by reducing the Li+ migration barrier. The as-obtained dendrite-free Li metal anode exhibits a Coulombic efficiency of 99.0 %, a cycle life of over 2000 h, a Li utilization rate of 50 %, and a capacity retention of 83.4 % after 130 cycles in pouch cells at a negative/positive capacity ratio of 2.5. These findings provide a novel strategy to stabilize Li metal by regulating the work function of materials using electrocatalysts.

Ant-Nest-like Lithiophilic Host for Long-Life Lithium Metal Anodes

The rational design of confined host to tutor Li nucleation and deposition behavior remains a key challenge for the long stability of lithium metal anodes (LMAs), while the scalability and feasibility of the method need to be taken into concern. Herein, a biomimic strategy is designed for tutoring in-depth nucleation and bottom-up Li deposition by composing ant-nest-like lithiophilic hosts for LMAs with light-weight flexible and conductive carbon nanotubes (CNTs) as the framework, table salt (sodium chloride, NaCl) as the washable porous creator, and homogeneously dispersed nano-Si as the nucleation site. It possesses similar optimized structure as ant nests in nature and can provide large and conductive inner volume for Li storage. Combining with the interconnected passways can ensure effective ion compensation like food transport channels for ants, and the well-designed host can take effect as an individual Li anode (5 mA h cm-2 area Li loading for demonstration) and the record-long stable LMA host can be achieved for over a 2200 h lifespan with minimum volume expansion. Therefore, this biomimic strategy is developed with all commercialized battery materials, and all industry compatible production methods can provide a feasible technical path for the stable, long-cyclability, and reliable host design for LMAs.

Recent Progress for Concurrent Realization of Shuttle-Inhibition and Dendrite-Free Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have become one of the most promising new-generation energy storage systems owing to their ultrahigh energy density (2600 Wh kg-1 ), cost-effectiveness, and environmental friendliness. Nevertheless, their practical applications are seriously impeded by the shuttle effect of soluble lithium polysulfides (LiPSs), and the uncontrolled dendrite growth of metallic Li, which result in rapid capacity fading and battery safety problems. A systematic and comprehensive review of the cooperative combination effect and tackling the fundamental problems in terms of cathode and anode synchronously is still lacking. Herein, for the first time, the strategies for inhibiting shuttle behavior and dendrite-free Li-S batteries simultaneously are summarized and classified into three parts, including "two-in-one" S-cathode and Li-anode host materials toward Li-S full cell, "two birds with one stone" modified functional separators, and tailoring electrolyte for stabilizing sulfur and lithium electrodes. This review also emphasizes the fundamental Li-S chemistry mechanism and catalyst principles for improving electrochemical performance; advanced characterization technologies to monitor real-time LiPS evolution are also discussed in detail. The problems, perspectives, and challenges with respect to inhibiting the shuttle effect and dendrite growth issues as well as the practical application of Li-S batteries are also proposed.

A Permselective Coating Protects Lithium Anode toward a Practical Lithium-Sulfur Battery

Lithium metal is a desirable anode for high-energy density lithium-sulfur (Li-S) batteries. However, its reliability is severely limited by dendrite growth and side reactions with polysulfides, which are yet challenging to solve simultaneously. Herein, we report a protective layer that works the same way as the ion-permselective cell membrane, yielding a corrosion-resistant and dendrite-free Li metal anode specially for Li-S batteries. A self-limited assembly of octadecylamine together with Al³⁺ ions on a Li metal anode surface produces a dense, stable yet thin layer with ionic conductive Al-Li alloy uniformly embedded in it, which prevents the passage of polysulfides but regulates the penetrated Li ion flux for uniform Li deposition. As a result, the assembled batteries show excellent cycling stability even with a high sulfur-loaded cathode, suggesting a straightforward but promising strategy to stabilize highly active anodes for practical applications.

One-Step In Situ Polymerization: A Facile Design Strategy for Block Copolymer Electrolytes

To optimize the rapid transport of lithium ions (Li⁺ ) inside lithium metal batteries (LMBs), block copolymer electrolytes (BCPEs) have been fabricated in situ in LMBs via a one-step method combining reversible addition-fragmentation chain transfer (RAFT) polymerization and carboxylic acid-catalyzed ring-opening polymerization (ROP). The BCPEs balanced the Li⁺ coordination characteristics of the polyether- and polyester-based electrolytes to achieve a rapid Li⁺ migration in the SPEs. The carboxylic acid played a dual role since it both catalyzed the ROP and stabilized the interface. Furthermore, the in situ assembly of LMBs did effectively enable an efficient intercalation/de-intercalation of Li⁺ at the electrode/electrolyte interface. The in situ assembled Li/BCPE4/LFP exhibited high-capacity retention of 92 % after 400 cycles at 1 C. The one-step in situ fabrication of BCPEs provides a new direction for the design of polymer electrolytes.

Spatially Confined LiF Nanoparticles in an Aligned Polymer Matrix as the Artificial SEI Layer for Lithium Metal Anodes

The uncontrollable growth of lithium (Li) dendrites and the instability of the Li/electrolyte interface hinder the development of next-generation rechargeable lithium metal batteries. The combination of inorganic nanoparticles and polymers as the artificial SEI layer shows great potential in regulating lithium-ion flux. Here, we design spatially confined LiF nanoparticles in an aligned polymer matrix as the artificial SEI layer. A high dielectric polymer matrix homogenizes the electric field near the surface of lithium metal. Aligned pores with LiF nanoparticles promote the lithium-ion transport across the artificial SEI layer. The synergistic effect of the highly polar β-phase PVDF and LiF nanoparticles provides high stability over 900 h for the Li//Li symmetrical cell. Besides, a Li//LFP full battery equipped with this artificial layer shows good performance in the commercial carbonate electrolyte, demonstrating the great potential of this protective film in lithium metal batteries.

A New In Situ Prepared MOF-Natural Polymer Composite Electrolyte for Solid Lithium Metal Batteries with Superior High- Rate Capability and Long-Term Cycling Stability at Ultrahigh Current Density

Lithium metal batteries hold promise for energy storage applications but suffer from uncontrolled lithium dendrites. In this study, a new composite membrane based on modified natural polymer and ZIF-67 is designed and prepared by the in situ composite method for the first time. Among them, a modified natural polymer composed of lithium alginate (LA) and polyacrylamide (PAM) can be obtained by electrospinning. Importantly, the polar functional groups of natural polymers can interact by hydrogen bonding and MOFs can construct lithium-ion transport channels. Consequently, compared with LA-PAM electrolyte without MOF, the electrochemical stability window of ZIF-67-LA-PAM electrolyte becomes wider from 4.5 to 5.2 V, and the lithium-ion transference number (t_Li+ ) enhances from 0.326 to 0.627 at 30°C. It is worth noting that the symmetric cells with ZIF-67-LA-PAM have superior stable cycling performance at 40 and 100 mA cm^-2 , and a high rate at 10C and 20C for LFP cells. Besides, the cell with NCM811 high-voltage cathode can run stably for 400 cycles with an initial discharge capacity of 136.1 mAh g^-1 at 0.5C. This work provides an effective method for designing and preparing MOF-natural polymer composite electrolytes and exhibits an excellent application prospect in high-energy-density lithium metal batteries.

Revisiting the Role of Physical Confinement and Chemical Regulation of 3D Hosts for Dendrite-Free Li Metal Anode

Lithium metal anode has been demonstrated as the most promising anode for lithium batteries because of its high theoretical capacity, but infinite volume change and dendritic growth during Li electrodeposition have prevented its practical applications. Both physical morphology confinement and chemical adsorption/diffusion regulation are two crucial approaches to designing lithiophilic materials to alleviate dendrite of Li metal anode. However, their roles in suppressing dendrite growth for long-life Li anode are not fully understood yet. Herein, three different Ni-based nanosheet arrays (NiO-NS, Ni₃N-NS, and Ni₅P₄-NS) on carbon cloth as proof-of-concept lithiophilic frameworks are proposed for Li metal anodes. The two-dimensional nanoarray is more promising to facilitate uniform Li⁺ flow and electric field. Compared with the NiO-NS and the Ni₅P₄-NS, the Ni₃N-NS on carbon cloth after reacting with molten Li (Li-Ni/Li₃N-NS@CC) can afford the strongest adsorption to Li⁺ and the most rapid Li⁺ diffusion path. Therefore, the Li-Ni/Li₃N-NS@CC electrode realizes the lowest overpotential and the most excellent electrochemical performance (60 mA cm^-2 and 60 mAh cm^-2 for 1000 h). Furthermore, a remarkable full battery (LiFePO₄||Li-Ni/Li₃N-NS@CC) reaches 300 cycles at 2C. This research provides valuable insight into designing dendrite-free alkali metal batteries.