Amphiphilic Carborane-Based Covalent Organic Frameworks as Efficient Polysulfide Nano-Trappers for Lithium-Sulfur Batteries

Porous Organic Framework Materials (MOF, COF, and HOF) as the Multifunctional Separator for Rechargeable Lithium Metal Batteries

The separator is an important component in batteries, with the primary function of separating the positive and negative electrodes and allowing the free passage of ions. Porous organic framework materials have a stable connection structure, large specific surface area, and ordered pores, which are natural places to store electrolytes. And these materials with specific functions can be designed according to the needs of researchers. The performance of porous organic framework-based separators used in rechargeable lithium metal batteries is much better than that of polyethylene/propylene separators. In this paper, the three most classic organic framework materials (MOF, COF, and HOF) are analyzed and summarized. The applications of MOF, COF, and HOF separators in lithium-sulfur batteries, lithium metal anode, and solid electrolytes are reviewed. Meanwhile, the research progress of these three materials in different fields is discussed based on time. Finally, in the conclusion, the problems encountered by MOF, COF, and HOF in different fields as well as their future research priorities are presented. This review will provide theoretical guidance for the design of porous framework materials with specific functions and further stimulate researchers to conduct research on porous framework materials.

Recent Advances in Carborane-Based Crystalline Porous Materials

The field of carborane research has witnessed continuous development, leading to the construction and development of a diverse range of crystalline porous materials for various applications. Moreover, innovative synthetic approaches are expanding in this field. Since the first report of carborane-based crystalline porous materials (CCPMs) in 2007, the synthesis of carborane ligands, particularly through innovative methods, has consistently posed a significant challenge in discovering new structures of CCPMs. This paper provides a comprehensive summary of recent advances in various synthetic approaches for CCPMs, along with their applications in different domains. The primary challenges and future opportunities are expected to stimulate further multidisciplinary development in the field of CCPMs.

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.

The Role of Transition Metal Versus Coordination Mode in Single-Atom Catalyst for Electrocatalytic Sulfur Reduction Reaction

Electrocatalytic sulfur reduction reaction (SRR) is emerging as an effective strategy to combat the polysulfide shuttling effect, which remains a critical factor impeding the practical application of the Li-S battery. Single-atom catalyst (SAC), one of the most studied catalytic materials, has shown considerable potential in addressing the polysulfide shuttling effect in a Li-S battery. However, the role played by transition metal vs coordination mode in electrocatalytic SRR is trial-and-error, and the general understanding that guides the synthesis of the specific SAC with desired property remains elusive. Herein, we use first-principles calculations and machine learning to screen a comprehensive data set of graphene-based SACs with different transition metals, heteroatom doping, and coordination modes. The results reveal that the type of transition metal plays the decisive role in SAC for electrocatalytic SRR, rather than the coordination mode. Specifically, the 3d transition metals exhibit admirable electrocatalytic SRR activity for all of the coordination modes. Compared with the reported N3C1 and N4 coordinated graphene-based SACs covering 3d, 4d, and 5d transition metals, the proposed para-MnO2C2 and para-FeN2C2 possess significant advantages on the electrocatalytic SRR, including a considerably low overpotential down to 1 mV and reduced Li2S decomposition energy barrier, both suggesting an accelerated conversion process among the polysulfides. This study may clarify some understanding of the role played by transition metal vs coordination mode for SAC materials with specific structure and desired catalytic properties toward electrocatalytic SRR and beyond.

Size Effect for Inhibiting Polysulfides Shuttle in Lithium-Sulfur Batteries

It is undeniable that the dissolution of polysulfides is beneficial in speeding up the conversion rate of sulfur in electrochemical reactions. But it also brings the bothersome "shuttle effect". Therefore, if polysulfides can be retained on the cathode side, the efficient utilization of the polysulfides can be guaranteed to achieve the excellent performance of lithium-sulfur batteries. Based on this idea, considerable methods have been developed to inhibit the shuttling of polysulfides. It is necessary to emphasize that no matter which method is used, the solvation mechanism, and existence forms of polysulfides are essential to analyze. Especially, it is important to clarify the sizes of different forms of polysulfides when using the size effect to inhibit the shuttling of polysulfides. In this review, a comprehensive summary and in-depth discussion of the solvation mechanism, the existing forms of polysulfides, and the influencing factors affecting polysulfides species are presented. Meanwhile, the size of diverse polysulfide species is sorted out for the first time. Depending on the size of polysulfides, tactics of using size effect in cathode, separator, and interlayer parts are elaborated. Finally, a design idea of materials pore size is proposed to satisfy the use of size effect to inhibit polysulfides shuttle.

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.

Fluorinated Benzimidazole-Linked Highly Conjugated Polymer Enabling Covalent Polysulfide Anchoring for Stable Sulfur Batteries

Sulfur is one of the most abundant and economical elements in the p-block family and highly redox active, potentially utilizable as a charge-storing electrode with high theoretical capacities. However, its inherent good solubility in many electrolytes inhibits its accessibility as an electrode material in typical metal-sulfur batteries. In this work, the synthetically designed fluorinated porous polymer, when treated with elemental sulfur through a well-known nucleophilic aromatic substitution mechanism (SN Ar), allows for the covalent integration of polysulfides into a highly conjugated benzimidazole polymer by replacing the fluorine atoms. Chemically robust benzimidazole linkages allow such harsh post-synthetic treatment and facilitate the electronic activation of the anchored polysulfides for redox reactions under applied potential. The electrode amalgamated with sulfurized polymer mitigates the so-called polysulfide shuttle effect in the lithium-sulfur (Li-S) battery and also enables a reversible, more environmentally friendly, and more economical aluminum-sulfur (Al-S) battery that is configured with mostly p-block elements as cathode, anode, and electrolytes. The improved cycling stabilities and reduction of the overpotential in both cases pave the way for future sustainable energy storage solutions.

Carborane-Based Three-Dimensional Covalent Organic Frameworks

The predesignable porous structure and high structural flexibility of covalent organic frameworks (COFs) render this material desirable as a platform for addressing various cutting-edge issues. Precise control over their composition, topological structure, porosity, and stability to realize tailor-made functionality still remains a great challenge. In this work, we developed a new kind of three-dimensional (3D) carborane-based COF with a 7-fold interpenetrating dia topological diagram. The resulting COFs exhibited high crystallinity, exceptional porosity, and strong robustness. The slightly lower electronegativity of boron (2.04) than that of hydrogen (2.20) can lead to the polarization of the B-H bond into a Bδ+-Hδ- mode, which renders these COFs as high-performance materials for the adsorption and separation of hexane isomers through the B-Hδ-···Hδ+-C interaction. Significantly, the carborane content of obtained COFs reached up to 54.2 wt %, which gets the highest rank among all the reported porous materials. Combining high surface area, strong robustness, and high content of carborane, the obtained COFs can work as efficient adsorbents for the separation of the five hexane isomers with high separation factors. This work not only enhances the diversity of 3D functional COFs but also constitutes a further step toward the efficient separation of alkane isomers.

Brominated flame retardants coated separators for high-safety lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) have become highly promising next-generation secondary lithium batteries owing to their high theoretical energy density and abundance of sulfur. Nevertheless, the large-scale application of LSBs is still restricted by the shuttle effect of lithium polysulfide (LiPSs) and the potential fire hazard caused by flammable electrolytes. Herein, three electrolyte-insoluble brominated flame retardants (BFRs) are selected and coated on both sides of commercial polypropylene separators by a facile slurry coating method. The effects of the three BFRs on the safety and electrochemical properties of LSBs are characterized and compared. The coating modification separators greatly improves the flame retardancy of LSBs through radical elimination mechanism. The self-extinguishing time of the electrolyte is reduced from 0.66 s/mg to 0.20 s/mg. Moreover, it is demonstrated that the oxygen (O)-containing BFRs exert a significant adsorption capacity and are more advantageous than O-free BFRs in LSBs. In addition, octabromoether (BDDP) coated separator is more effective in trapping LiPSs than decabromodiphenyl ether (DBDPO) due to higher O content, which can mitigate the shuttle effect and enhance the cycle and rate performance of LSBs. This simple coating strategy for separators with BFRs offers a strongly competitive option for the large-scale production of high-safety LSBs.

Highly Connected Three-Dimensional Covalent Organic Framework with Flu Topology for High-Performance Li-S Batteries

Lithium-sulfur batteries (LSBs) have been considered as a promising candidate for next-generation energy storage devices, which however still suffer from the shuttle effect of the intermediate lithium polysulfides (LiPSs). Covalent-organic frameworks (COFs) have exhibited great potential as sulfur hosts for LSBs to solve such a problem. Herein, a pentiptycene-based D_2h symmetrical octatopic polyaldehyde, 6,13-dimethoxy-2,3,9,10,18,19,24,25-octa(4'-formylphenyl)pentiptycene (DMOPTP), was prepared and utilized as a building block toward preparing COFs. Condensation of DMOPTP with 4-connected tetrakis(4-aminophenyl)methane affords an expanded [8 + 4] connected network 3D-flu-COF, with a flu topology. The non-interpenetrated nature of the flu topology endows 3D-flu-COF with a high Brunauer-Emmett-Teller surface area of 2860 m² g^-1, large octahedral cavities, and cross-linked tunnels in the framework, enabling a high loading capacity of sulfur (∼70 wt %), strong LiPS adsorption capability, and facile ion diffusion. Remarkably, when used as a sulfur host for LSBs, 3D-flu-COF delivers a high capacity of 1249 mA h g^-1 at 0.2 C (1.0 C = 1675 mA g^-1), outstanding rate capability (764 mA h g^-1 at 5.0 C), and excellent stability, representing one of the best results among the thus far reported COF-based sulfur host materials for LSBs and being competitive with the state-of-the-art inorganic host materials.

Preparation of Carborane-Tailored Covalent Organic Frameworks by a Postsynthetic Modification Strategy as a Barrier to Polysulfide in Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have attracted much attention due to their high energy density and theoretical specific capacity. However, the "shuttle effect" of polysulfides limits their application. Herein, we propose a postsynthetic modification (PSM) strategy to synthesize a fibrous carborane-tailored covalent organic framework (PMCB-COF). Benefiting from its amphiphilicity, strong adsorption ability, high specific surface area, and accessible Li⁺ transport channels, PMCB-COF could serve as a barrier to polysulfide to inhibit the shuttle effect. The cell assembled with PMCB-COF exhibits a high initial capacity of 926 mAh g^-1 at 1 C. A Coulombic efficiency of 98% and a fading rate of only 0.039% per cycle are exhibited even after 1500 cycles. So far as we know, PMCB-COF is one of the best materials as a separator of LSBs. This work provides a safe and efficient avenue for tailoring COFs with carborane and might help promote the development of secure, low-cost, and durable rechargeable batteries.

Covalent Organic Frameworks for Separator Modification of Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are regarded as one of the promising energy storage systems. However, rapid capacity attenuation caused by shuttle effect of soluble polysulfides is major challenge in practical application. The separator modification is regarded as one countermeasure besides the construction of sulfur host materials. Covalent organic frameworks (COFs) are one type of outstanding candidates for suppressing shuttle effect of polysulfides. Herein, recent advances of COFs in the application as commercial separator modifiers are summarized. COFs serve as ionic sieves, the importance of porous size and surface environments in inhibiting soluble polysulfides shuttling and promoting lithium ions conduction is highlighted. The superiority of charge-neutral COFs, ionic COFs, and the composites of COFs with conductive materials for improving reversible capacity and cycling stability is demonstrated. Some new strategies for the design of COF-based separator modifiers are proposed to achieving high energy density. The review provides new perspectives for future development of high-performance Li-S batteries.

The application of covalent organic frameworks in Lithium-Sulfur batteries: A mini review for current research progress

Recently, how to enhance the energy density of rechargeable batteries dramatically is becoming a driving force in the field of energy storage research. Among the current energy storage technologies, the lithium-sulfur (Li-S) batteries are one of the most promising candidates for achieving high-capacity and commercial batteries. The theoretical energy density of Li-S batteries reaches to 2,600 Wh kg−1 with the theoretical capacity of 1,675 mA h g−1. Therefore, Li-S batteries are considered as the great potential for developing future energy storage technology. However, some of problems such as Li dendrites growth, the shuttle effect of sulfides and the electronic insulation feature of sulfur, have brought obstacles to the development of Li-S batteries. The covalent organic frameworks (COFs) are a series of porous materials with different topological structures, which show the versatile characteristics of high specific surface area, permanent pores, ordered porous channels and tunable internal structure. Potentially, their ordered channels and extended conjugated frameworks could facilitate rapid Li-ion diffusion and electron transport for the remarkable rate capability. On the basis of these merits, the COFs become the potential electrode materials to solve the above serious problems of Li-S batteries. In this mini review, we summarize the research progress of COFs utilized as electrode materials in the Li-S batteries, including the cathode, separator and anode materials. Accordingly, the outlook of COFs as electrodes for future development in Li-S batteries is also given.

A Facile Immobilization Strategy for Soluble Phosphazene to Actualize Stable and Safe Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have attracted extensive attention owing to their high energy density and abundant sulfur resources. However, LSBs are still restricted by the unsatisfactory electrochemical performance resulting from the shuttle effect of lithium polysulfide (LiPSs), and the potential fire hazard caused by inflammable ether electrolytes and polyolefin separators. Herein, a facile immobilization strategy for hexachlorocyclotriphosphazene (HCCP) is creatively applied to address the above issues simultaneously. Insoluble HCCP cross-linked microspheres (H-CMP) are firstly obtained at ambient temperature using tannic acid (TA) as a cross-linking agent and then a multifunctional separator coating is constructed based on H-CMP. The released phosphorus-related radicals from H-CMP in wide temperatures effectively prevent the combustion of electrolytes and separators, and hence improve the fire safety of the Li-S pouch cell. Furthermore, H-CMP availably chemisorbs LiPSs to interdict the shuttle effect, thereby dramatically improving the electrochemical performance of LSBs. The effectiveness of this strategy is also verified in high sulfur loading (6.38 mg cm^-2 ), high temperature (50 °C), and Li-S pouch cells. More importantly, H-CMP exhibits sufficient stability for Li metal and suppression of Li dendrites. This facile immobilization strategy for multifunctional phosphazenes provides a competitive option for the large-scale fabrication of high-safety and high-performance LSBs.

Porous Dithiine-Linked Covalent Organic Framework as a Dynamic Platform for Covalent Polysulfide Anchoring in Lithium-Sulfur Battery Cathodes

Dithiine linkage formation via a dynamic and self-correcting nucleophilic aromatic substitution reaction enables the de novo synthesis of a porous thianthrene-based two-dimensional covalent organic framework (COF). For the first time, this organo-sulfur moiety is integrated as a structural building block into a crystalline layered COF. The structure of the new material deviates from the typical planar interlayer π-stacking of the COF to form undulated layers caused by bending along the C-S-C bridge, without loss of aromaticity and crystallinity of the overall COF structure. Comprehensive experimental and theoretical investigations of the COF and a model compound, featuring the thianthrene moiety, suggest partial delocalization of sulfur lone pair electrons over the aromatic backbone of the COF decreasing the band gap and promoting redox activity. Postsynthetic sulfurization allows for direct covalent attachment of polysulfides to the carbon backbone of the framework to afford a molecular-designed cathode material for lithium-sulfur (Li-S) batteries with a minimized polysulfide shuttle. The fabricated coin cell delivers nearly 77% of the initial capacity even after 500 charge-discharge cycles at 500 mA/g current density. This novel sulfur linkage in COF chemistry is an ideal structural motif for designing model materials for studying advanced electrode materials for Li-S batteries on a molecular level.