Covalent triazine frameworks (CTFs) drive innovative advances in rechargeable metal-ion batteries: a review
Abstract
In the field of energy storage technology, the organic electrodes, separators, and electrolytes have unique advantages over inorganic materials, such as low cost, environmental friendliness, and a wide range of applications. Due to the advantages of organics such as light elements, abundant reserves, and recyclability, they have become favorable candidate materials for solving the energy storage problems caused by the fossil energy crisis. In recent years, as a high-performance branch of covalent organic frameworks, covalent triazine structures (CTFs) have attracted great interest due to their applications in electrochemical energy storage. CTFs have gradually become excellent organic materials for metal-ion batteries applications due to their large specific surface area, nitrogen richness, customizable structural features, and electron donor-acceptor/conductive parts. However, the relatively poor conductivity of the triazine ring in the main structure and the harsh polycondensation conditions limit its commercial application. To overcome these challenges, many effective strategies have emerged in terms of structural optimization, functional construction, and triazine-based composites. This review summarizes in detail the synthesis methods and applications of CTFs cathodes, electrolytes, and separators in the past decade. It is found that for CTFs, large-scale synthesis methods and performance regulation strategies have reached a bottleneck. It is hoped that the systematic summary of this review will provide strategic screening and prospects for the further expansion of CTFs research in next-generation batteries.
Keywords
INTRODUCTION
Recently, the burgeoning energy crisis has caused the prices of crude oil and other commodities to rise continuously, in which the extent of its ecological damage and economic impact are worth pondering. As a result, the formidable challenge of energy scarcity has emerged as a pivotal concern for future scientific inquiry[1-4]. Currently, the most widely used pumped hydro energy storage method has the shortcomings due to its over-reliance on geographical location[5-7]. However, the energy storage batteries have become the best alternative due to their strong scalability, long lifetime, and high flexibility[8]. Within the realm of batteries, metal-ion batteries are distinguished by their substantial energy density and superior conversion efficacy, which exhibit strategic significance in mitigating the emissions of carbon dioxide (CO2) and fortifying the foundations of energy security. In the 1970s, M.S. Whittingham harnessed titanium sulfide as the cathode and metallic lithium as the anode, thereby inventing the first lithium battery. Subsequently, in 1991, Sony commercialized the first lithium-ion batteries (LIBs) product[9-13]. With the continued strong demand in the new energy market and the release of production capacity in the industry, new batteries such as zinc ions[14], sodium ions[15], and lithium-sulfur[16] have appeared dramatically. During this period, scientific researchers conducted continuous research on the battery efficiency and found that it largely depended on the characteristics of the cathode materials[17].
At present, the cathode and anode materials of commercialized metal-ion batteries are mainly concentrated on inorganic materials. Among them, the cathode materials can be divided into hexagonal layered transition metal oxides (LiTMO2, TM is one or more of Ni, Co, Mn), spinel compounds (LiMn2O4, LiNi0.5Mn1.5O4,
Among the most of COFs, the CTFs are characterized by using aromatic rigid triazine (C3N3) as the building block to construct an organic framework by forming Schiff bases[34,35]. Through integrating heteroatoms such as oxygen, fluorine, sulfur, and carbon to forge bonds such as C-O, C-F, thiophene, and pyridine linkages, one can engender a plethora of functional groups, encompassing carboxyl (C=O), imine (C=N), and even azo (N=N) groups. Within a CTF possessing an array of these groups, the energy storage mechanism converts double bonds into single bonds through the incorporation of lithium ions (Li+), triggering electron translocation within the molecular scaffold[36]. CTFs have no fragile linkages other than aromatic linkages, giving them excellent thermal and chemical stability. This inherent robustness enables them to undergo one or more electron reductions throughout charge and discharge cycles, thereby endowing them with expansive realms of application and promising prospects[37]. However, traditional CTFs materials are generally prepared through methods such as ion heat or strong acid catalysis with harsh reaction conditions. So, most CTFs are produced in powder or film form[35], which limits production capacity and structural diversity design. Nevertheless, cathode materials predicated on CTFs are not without their deficiencies, such as the suboptimal exploitation of active sites and a diminished energy density, where these constraints impede the capacity of batteries to attain high-rate energy storage. To develop a universal CTFs synthesis method to provide a promising selection path for organic materials in metal-ion batteries[38], this review systematically summarizes the synthetic methods and cutting-edge applications of CTFs, to facilitate subsequent research on CTF-based cathode, electrolyte and separator materials.
SYNTHESIS METHODOLOGIES
In 2008, Kuhn and Thomas reported the first CTFs for the first time[39], where they used 1,4-dicyanobenzene (DCB) as the basic structural unit, and ZnCl2 salt as the solvent and catalyst. This method requires a high-temperature process of more than 300 °C. After the emergence of this method, many strategies for synthesizing CTFs with various two- (2D) or three-dimensional (3D) structures were developed, such as super acid-catalyzed polymerization[40] and aromatic amide condensation[41]. Based upon the necessity of elevated thermal conditions, the synthesis methodologies can be broadly categorized into high- and low-temperature techniques: the ion thermal generation used by Kuhn et al. and the subsequent phosphorus pentoxide (P2O5)-involved methods are both high-temperature methods (> 300 °C)[39], while the condensation polymerization of aldehyde monomers and amidine monomers and the catalytic method under strong acid conditions are all carried out under low-temperature conditions (< 200 °C). This section will systematically discuss the preparation method of CTFs, including ionothermal trimerization at high temperature, P2O5 catalyzed at high temperature, polyphosphoric acid-catalyzed polymerization, super acid-catalyzed polymerization, amidine-based polycondensation and Friedel-Crafts reaction, as presented in Figure 1.
Figure 1. Summary of the synthesis methodologies of CTFs.
Ionothermal trimerization
Through the ionization heat synthesis route utilized in preparing CTFs, Kuhn et al. first synthesized
Some harsh conditions, including elevated heating rates and temperatures, will inevitably cause carbonization of the product. Therefore, Lan et al. developed a low-temperature method by using a ternary NaCl-KCl-ZnCl2 mixture as a eutectic salt to prepare CTFs[43]. The melting point of the mixture hovers around 200 °C, significantly lower than the melting threshold of pure ZnCl2 (318 °C). The low temperature overcomes the carbonation process associated with the final product, where the resulting material (CTF-ES200) exhibits superior photocatalytic efficacy relative to its counterpart CTF-1 by avoiding carbonization of the main polymer chain during the polycondensation process.
P2O5 catalyzed at high temperature
Even though the synthesis method using ZnCl2 as a solvent is simple and feasible, the solvent itself is challenging to remove completely, and the presence of Zn ions may affect the further use of CTFs. Recently, Yu et al. proposed a method using P2O5 as a catalyst instead of ZnCl2[41]. Meanwhile, aromatic amides such as benzamide were used to replace the precursors of nitrile benzene, and the triazine ring was prepared in two steps, as shown in Figure 2. Preliminarily, under the catalysis of P2O5, the aromatic amide group was converted into a nitrile group, and then a uniform triazine ring was formed through condensation. Surprisingly, the synthesis of the triazine derivative (pCTF-1) through this methodology yielded a heightened surface area (2,034 m2 g-1) and enhanced crystallinity. In addition, excess catalyst of P2O5 can be removed by simply mixing the product with water. This method makes it easier to remove the catalyst and is, therefore, more environmentally friendly than the classic ZnCl2-based synthetic route. However, it still requires a higher temperature (> 400 °C), and the energy consumption is as high as ion thermal trimerization.
Figure 2. (A) Schematic diagram of the synthesis of pCTF-1. (B) Top view of the spatial structure of pCTF-1. (C) Side view of the spatial structure of pCTF-1. C: gray; N: blue; H: white[41]. Copyright 2018 John Wiley and Sons.
Polyphosphoric acid (H6P4O13) catalyzed polymerization
In addition to the problem of high-temperature carbonization, both the traditional ionothermal and P2O5-catalyzed methods possess difficulties, such as low yield and poor crystallinity. To this end, Sun et al. creatively discovered that the catalytic employment of H6P4O13 in the polymerization of CTFs presents a promising road[44]. The reaction mechanism included three steps: nucleophilic addition, cyclic addition, and acid separation. This method used aromatic nitriles as monomers to synthesize a series of high crystallinity CTFs at 300 °C. The microporous structure of these CTFs, with a high surface area from 794 to 1,335 m2 g-1, is much higher than most previously reported crystalline CTFs. Under the same conditions, only semi-crystalline or even amorphous CTFs can be obtained using H3PO4 and P2O5 as catalysts. This methodology exhibits a commendable level of operational simplicity, and H6P4O13 is also cost-effective. However, this approach is scalable to the CTFs preparation on a kilogram scale, and the output is much higher than previously reported methods, providing the possibility for an industrial-scale application.
Super acid-catalyzed polymerization
It is well-known that the nitriles can be trimerized under the catalysis of strong Bronsted acids[40,45,46]. As early as 1966, Anderson et al. endeavored to employ chlorosulfonic acid as a catalyst for the trimerization of aromatic nitriles, aiming to synthesize s-triazine polymers[45]. In an effort to mitigate the carbonization attributed to the high-temperature synthesis, Ren et al. pioneered a technique operable at ambient temperature, utilizing a Bronsted acid to act as a catalyst[40]. This method utilized trifluoromethanesulfonic acid (TFMS) to trimerize aromatic nitrile monomers at room temperature into a series of CTFs (yield: 86%). The obtained product was no longer a black material (carbonized), but a free and amorphous light yellow fluorescent powder. However, the reaction time was long at room temperature, so they used the microwaves to heat the reaction vessel to 110 °C. Under microwave assistance, the reaction time was shortened from more than 4 h to less than 1 h, and the yield was significantly improved (yield: 96%). However, the crystallinity was still poor, and only some materials showed limited crystallinity. The surface area of these materials varied depending on the monomers (from 2 to 1,152 m2 g-1). This method successfully realizes the possibility of synthesizing CFT at low temperatures to avoid high energy consumption and carbonization. Subsequently, this approach has undergone refinement and optimization, leading to notable advancements, in which the synthetic strategies for non-carbonized CTFs have emerged for various applications, such as TFMS vapor-assisted[47,48] and interfacial polymerization methods[49-53]. In 2016,
Figure 3. Schematic diagram of TFMS vapor-assisted synthesis of CTFs[47]. Copyright 2016 The Royal Society of Chemistry.
It is noted that the super acid polymerization overcomes the disadvantages in high reaction temperature and long reaction time of the ionothermal synthesis method, which is beneficial to the synthetic operation. Moreover, to solve the problems of carbonization and decomposition of CTFs, low-temperature conditions are more conducive to directly obtaining film-typed materials. In addition, the safety hazards caused by the high toxicity of ZnCl2 under high temperatures can also be eliminated. However, the catalyst used in this method is highly corrosive, and the large demand for solvents results in high costs. Therefore, it is still unclear whether this method can be transferred from laboratory milligram-scale synthesis to ton-scale production.
Amidine-based polycondensation
To further augment the synthesis scope whilst circumventing elevated thermal conditions or the employment of exceedingly strong acids, Wang et al. reported in 2017 a novel amidine-aldehyde polycondensation strategy to prepare various CTFs[55]. This methodology encompasses reversible Schiff base formation[56-59], Michael addition reaction[59-61], and irreversible processes of cyclization and dehydrogenation. Within the process of the Schiff base reaction, the aldehyde and amino groups undergo dehydration and condensation to forge an imine linkage. In the Michael addition reaction, the amino and the imine groups are deaminated and condensed to form a C=N double bond, and finally, a triazine unit is formed. This method uses dimethyl sulfoxide (DMSO) as the solvent, attributed to its weak oxidizing properties and high boiling point. In contrast, cesium carbonate (Cs2CO3) is used as the base due to its appropriate alkalinity. Ultimately, two kinds of monomers, including 1,4-benzene-dialdehye, 4,4′-biphenyl-dialdehyde, tris(4-formylphenyl)-amine, and tris(4-formylbiphenyl)-amine, could react at 120 °C to get CTFs with bright colors, amorphous and layered structures, and high surface areas. Importantly, this method shows potential for scale-up synthesis since it only requires one-pot polymerization under mild conditions and does not involve strong acid or specialized equipment. Unfortunately, the crystallinity of the obtained material is still insufficient.
To solve the crystallinity problem of CTF, Liu et al. further developed a method to synthesize CTFs with higher crystallinity using an in-situ oxidation strategy in the following year[62]. In DMSO and alkaline solutions, alcohols can be slowly oxidized to aldehydes with air[63,64]. According to this strategy, the crystallinity of CTFs is greatly improved, allowing them to obtain higher thermal stability and layer structure. Meanwhile, the increase in crystallinity enables the materials to realize better charge transport capabilities and wider light absorption, which significantly improves the photocatalytic performance.
Friedel-crafts reaction
It is widely acknowledged that the classic Friedel-Crafts reaction has found extensive utility in the synthetic CTFs due to its low cost, mild conditions, effortless control, and reproducibility[65-68]. It involves cyanuric chloride and aromatic structural units as substrates, and under the action of catalysts such as anhydrous aluminum chloride and methanesulfonic acid (CH3SO3H), porous CTFs materials with favorable thermal and chemical stability can be realized[69-73].
One complex and deeply ingrained concept of the synthetic methods mentioned above is that the solvent is essential for the reaction, where the solvent promotes the interaction among the monomers involved in the reaction[74]. Therefore, mechanochemical method will be a time-saving, cost-effective and scalable alternative synthetic route[75,76]. In 2023, Krusenbaum et al. used mechanochemistry with the Friedel-Crafts alkylation method to eliminate the use of dangerous solutions and successfully synthesized triazine-based polymers with a specific surface area of more than 1,500 m2 g-1 in 1 min[77]. Significantly, the grinding of materials is critical to mechanochemical synthesis, which accelerates the direct progression of reactions. Moreover, the solvent-free allows the reaction to be freely observed, which provides a time-saving and scalable green solution for the synthesis of CTFs. However, the substitution of aromatic monomer sites in the Friedel-Crafts alkylation reaction is not the only selectivity, so the CTFs with the desired structure cannot be accurately constructed, and the structure obtained is often disordered.
In summary, the main methods for synthesizing CTFs can be roughly divided into high- and low-temperature methods. The high-temperature method has stringent requirements on equipment and cost, but it can produce CTFs with a clear structure and high specific surface area. The low-temperature method is more flexible and more scalable in industrial-level production, but due to the long reaction time at low temperatures and the dependence on solvents, the yield and crystallinity are always low. The specific method used for the synthesis of CTFs is mainly to be adjusted according to the physical and chemical properties such as the pore size, crystallinity, specific surface area, conductivity, etc. In addition, it is necessary to consider the basic properties of the monomers to avoid the loss of monomers during the reaction. Table 1 summarizes and compares the advantages and disadvantages of six common CTFs synthesis methods.
Comparison of the advantages and disadvantages of CTFs synthesis methods
| Synthesis methods | Advantages | Disadvantages |
| Ionothermal trimerization at high temperature | Economical catalysts, abundantly sourced, and excellent reactive properties | Partial carbonization of CTFs, generation of toxic gases, and residual Zn in the product |
| P2O5 catalyzed at high temperature | High specific surface area, high yield, and easier solvent removal | Partial carbonization of CTFs |
| H6P4O13 catalyzed polymerization | Low cost, high crystallinity, and industrial production potential | High pollution and complex steps |
| Super acid-catalyzed polymerization | Mild conditions and short reaction time | Amorphous products, expensive equipment, low surface area and porosity |
| Amidine-based polycondensation | Mild conditions | Amorphous products |
| Friedel-Crafts reaction | Mild conditions, high specific surface area, and no hazardous solutions | Disordered structure and imprecise products |
APPLICATION OF CTFS AS CATHODE MATERIALS
In contrast to traditional polymers, porous organic frameworks exhibit exceptional and distinct attributes. Owing to their robust networks, they enable the facile ingress of electrolyte ions. As an avant-garde class of porous polymers, CTFs are distinguished by their chemical stability, large specific surface area, and small particle size. As a prospective material for cathode, CTFs proffer several advantages:
(1) The high conjugation of the polymer skeleton promotes electron transfer.
(2) The meticulously arranged nanopores afford a conducive environment for the efficient storage and controlled release of ions.
(3) The higher nitrogen content of the azine group provides a large amount of reaction sites.
The above advantages have led to many studies focusing on the application of composite materials in recent years. This section aims to encapsulate the substantial strides made in exploring the applications of CTFs cathodes across two key dimensions, corresponding to the energy storage materials of alkaline ion batteries, lithium-sulfur batteries (LSBs).
Alkaline-ion batteries
The alkaline-ion batteries with two active electrodes exhibit the disparate redox potentials to facilitate reversible redox reactions, and thereby accomplish charge and discharge cycles[78]. Throughout the discharge cycle, positive ions move from the anode to the cathode, a process that occurs simultaneously with the release of electrons. In contrast, during the charging phase, the directional flow of the reaction is reversed[79-81]. By selecting appropriate electrode materials and further optimizing their physical and chemical properties, the efficiency and cycling lifetime can be improved, thus promoting the further development of electrochemical energy storage technology[82,83]. The ion transport rate of the cathode material has also become one of the factors limiting the battery performance. Hence, the researchers are committed to developing efficient cathode materials with high ion transport rates and excellent electrochemical properties to improve the energy and power densities of batteries[84-88]. To understand the properties of metal-based batteries distinctly, the device performance is summarized in Table 2.
Electrochemical performances of representative CTF-based metal-ion batteries
| Main body | Types of batteries | Voltage range (V) | Current density (A g-1) | Cycles (retention rate) | Specific capacity (mA h g-1) | Ref. |
| Azo-CTF | LIBs | 1.2-3.0 | 0.1 | 5,000 (89.1%) | 205.6 | [33] |
| CTF-P | LIBs | 1.0-4.3 | 1.0 | 2,000 (78%) | 78 | [36] |
| CTF-P | LIBs | 1.0-4.3 | 0.1 | 50 (79%) | 135 | [36] |
| CTF-P | LIBs | 1.0-4.3 | 0.02 | 247 | [36] | |
| MPT-CTF@CNT | LIBs | 1.5-4.2 | 0.4 | 60 (93%) | 297 | [38] |
| G-PPF-p-400-600 | LIBs | 1.5-4.5 | 5.0 | 5,100 (95%) | 395 | [91] |
| G-PPF-p-600 | LIBs | 1.5-4.5 | 0.4 | 400 | 255 | [91] |
| G-PPF-p-600 | LIBs | 1.5-4.5 | 3.2 | 400 | 165 | [91] |
| G-PPF-p-400 | LIBs | 1.5-4.5 | 0.4 | 400 | 155 | [91] |
| G-PPF-p-400 | LIBs | 1.5-4.5 | 3.2 | 400 | 100 | [91] |
| G-PPF-m-400 | LIBs | 1.5-4.5 | 0.4 | 400 | 125 | [91] |
| G-PPF-p-400 | LIBs | 1.5-4.5 | 3.2 | 400 | 75 | [91] |
| G-PPF-o-400 | LIBs | 1.5-4.5 | 0.4 | 400 | 55 | [91] |
| G-PPF-p-400 | LIBs | 1.5-4.5 | 3.2 | 400 | 25 | [91] |
| FCTF | LIBs | 1.5-4.5 | 0.2 | 400 (88%) | 106.3 | [110] |
| FCTF | LIBs | 1.5-4.5 | 0.1 | 200 | 125.6 | [110] |
| BDMI-CTF | LIBs | 1.5-4.5 | 1.0 | 2,000 (95%) | 186.5 | [114] |
| BDMI-CTF | LIBs | 1.5-4.5 | 2 | 107.5 | [114] | |
| CTF-based-Mg | MIBs | 0.8-2.4 | 5 C | 3,000 (41.2%) | 72 | [122] |
| 2D-NT-COF | AIBs | 0.5-2.8 | 0.1 | 4,000 (97%) | 132 | [129] |
| TTPQ | ZIBs | 0.1-1.4 | 0.3 | 250 (94%) | 404 | [133] |
| F-CQN-1-600 | LIBs | 1.5-4.5 | 2.0 | 2,000 (95.8%) | 120 | [183] |
| F-CQN-1-600 | LIBs | 1.5-4.5 | 5 | 105 | [183] | |
| F-CQN-1-600 | LIBs | 1.5-4.5 | 0.1 | 250 | [183] | |
| CTF-Az-400/600 | LIBs | 1.5-4.5 | 0.2 | 5,000 (95%) | 118.5 | [184] |
| CTF-800 | LIBs | 1.5-4.2 | 0.1 | 400 (88%) | 116 | [185] |
Lithium-ion batteries
Regarding the cathodes of LIBs, the commercialization of inorganic cathode materials (lithium manganate, lithium cobalt oxide (LiCoO2), nickel-cobalt-manganese ternary composite, etc.) has been relatively mature. Both of high energy density and more than 30 years of market exploration have enabled it to dominate major markets such as electric vehicles and electronic equipment. However, current LIBs have almost reached the limit of energy density (based on volume and mass) of 750 Wh L-1. Tesla's high-tension cylindrical 18,650 battery has reached about 600-650 Wh L-1 (20% reduction in the bag and prismatic battery configurations)[89]. The finite reserves of lithium confront boundless market demand, a challenge compounded by the scarcity of elemental resources such as cobalt and nickel. This dilemma is further exacerbated by the laborious and energy-intensive synthesis processes necessitated at elevated temperatures, alongside the environmental toxicity posed by alkali metals[90]. Hence, CTFs, as one of the extensively investigated organic porous materials, boast attributes of environmental friendliness, operational safety, and an absence of recycling dilemmas, offering a substitute for conventional cathode materials.
However, CTF-based cathode materials need to be developed in practical applications, such as low-rate performance, low energy density, and insufficient utilization of active sites. These issues limit the performance of LIBs, where the solutions need to be found to overcome these challenges. To this end, researchers have developed multiple strategies, including composite carbon-based materials, structure modification, and conductivity tuning. In 2014, Su et al. combined 2D graphene with CTFs for the first time and proposed a strategy to fix CTFs with customized pore structure through covalent bonding, achieving the optimal pore diameter (2.1 to 7.3 nm) and specific surface area (651 to 1,683 m2 g-1)[91]. The resulting 2D coupled graphene-porous polyaryltriazine-derived framework (G-PPF) successfully gave the outstanding cycling stability as a cathode for LIBs, exhibiting a rate performance of 395 mAh g-1 at 5 A g-1. After over 5,000 cycles, it can still show an excellent rate performance of 135 mAh g-1 at a high current density
Figure 4. (A) TEM image of CTF-rGO composite cathode. (B) SEM image of CTF-rGO composite cathode. (C) SEM image of CTF-GA (Graphene aerogel) composite cathode. (D) Discharge capacity comparison at 0.1 A g-1 after 80 cycles[97]. Copyright 2018 Molecular Diversity Preservation International.
In the same year, Lei et al. used a mechanical exfoliation method to polish two high-temperature synthesized CTFs (CIN-1 and SNW-1) and carbon nanotubes (CNTs) under solvent-free conditions at room temperature[21]. These two exfoliated covalent organic CNT composites (E-CIN-1/CNT and
Figure 5. (A and B) Schematic diagram of the synthesis of CIN-1 and SNW-1. (C) Mechanical exfoliating process of CIN-1/CNT and SNW-1/CNT[21]. Copyright © 2019 John Wiley & Sons.
To enhance the conductivity and overall efficacy of CTFs composites, carbonaceous materials such as CNTs and graphene are integrated into their matrix, which give the composite excellent electrical properties[97,101-103]. Mechanical exfoliation can cause changes in the stacking of 2D materials, thereby transforming bulk crystalline materials into single-layer 2D materials[24,104,105]. However, only one method cannot solve the problem of few active sites in CTF. Zhang et al. ever designed a multi-active CTF to improve anode performance in 2019[24]; however, the anode strategy was also difficult to implement in the cathode during synthesis. Until 2021, Zhao et al. obtained a CTFs (E-TP-COF) cathode material with C-O and C-N dual active centers by mechanically exfoliating polyimide and triazine-based few-layer COFs[106]. This method fully uses the insoluble characteristics of CTFs to avoid the dissolution threat caused by electrolytes, showing the excellent cycling stability. The dual active centers of C-N and C-O groups enable it to obtain a higher capacity than the pristine CTFs cathode with a single active center. To further verify the performance difference caused by structural changes, the exfoliated multi-layer nanosheet CTFs
Importantly, the molecular design is also the main starting point for changing the electrochemical properties of CTFs. The reactive functional groups, such as C=O, C=N, and N=N, could react reversibly with Li+, triggering electron transfer through the reduction and transformation between double bonds and single bonds[107-109]. Wu et al. prepared an azo-linked CTF (Azo-CTF) by utilizing 4,4′,4″-(1,3,5-triazine-2,4,6-triyl) triphenylamine monomer (Tta) with undergoing an oxidative self-coupling reaction under the copper catalyst[33]. The Azo-CTF cathode used in LIBs presents a capacity retention rate of 89.1% after
Figure 6. (A and B) Structural diagram of DCPY-CTF, TCNQ-CTF and BDMI-CTF. (C and D) Schematic diagram of the synthesis of TCNQ-CTF and BDMI-CTF[114]. Copyright 2024 Elsevier B.V.
Sodium-ion batteries
Due to the abundant reserves of sodium on earth and its similar chemical properties to lithium, sodium-ion batteries (SIBs) are considered as the mainstream product that is expected to replace LIBs in the next generation of batteries[115-117]. Because the atomic mass of sodium is higher than that of lithium, the energy density of SIBs is lower, implying that the weight and volume of SIBs are much larger than LIBs under the same energy density[118,119]. Till now, the CTFs are not widely used as cathode materials in SIBs, possibly resulting from low energy density of SIBs, but the characteristics of CTFs could stabilize the performance of SIBs potentially. In 2013, the application of CTFs in SIBs was first reported by Sakaushi by synthesizing a CTFs (BPOE) cathode composite material with a honeycomb skeleton using 2D aromatic and triazine rings[115]. The BPOE is a bipolar material that exhibits dual characteristics of n-type/p-type doping. At low current densities, a specific capacity of up to 200 mAh g-1 was demonstrated. Although the capacity is not high, the material is extremely stable. After 7,000 cycles at a high current density of 1.0 A g-1, the capacity still retains 80% of the initial value. Li et al. used a simple polycondensation reaction to link 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPT) with 1,4,5,8-naphthalenetracarboxylic dianhydride (NTCDA) and 1,2,4,5-benzenete-tracarboxylic anhydride (PMDA) to synthesize two different CTFs,
Figure 7. (A) Morphology control method of TAPA-COF. (B) Schematic diagram of the preparation, vulcanization and electrochemical mechanism of TAPA/TAPT-COF[121]. Copyright 2023 Elsevier B.V.
Other metal-ion batteries (Mg2+, Al3+, Zn+)
Recently, the magnesium, aluminum and zinc ions-based aqueous batteries have emerged as new metal-ion batteries. The anode of these metal-ion batteries is immutable, so the task of improving the performance of batteries falls on finding a suitable cathode urgently. The unique coordination mechanism of organic cathode materials can easily insert/deintercalate metal cations, but the problem of redox intermediate dissolution will cause rapid capacity decay. The polymeric rigid framework in CTFs is highly resistant to dissolution, and its structure and molecular design are more conducive to adjusting ion dynamics. Sun et al. first attempted to use CTFs as a cathode material for magnesium-ion batteries (MIBs), where the resulting material was able to accommodate 9 Mg2+[122]. They intended to use the highly ordered porous structure and large surface area of CTFs to increase the magnesium-ion diffusion rate and fully adapt to volume changes. The initial capacity of 102 mA h g-1 (at a current rate of 0.5 C) is not high, but its capacity only decays by 0.0196% after 3,000 cycles at a high rate of 5 C. This cycle stability is the best among reported organic MIBs[123,124], opening up the possibility for the application of porous polymers as high-performance OEMs in MIBs. Moreover, the aluminum metal batteries (AIBs) are considered as green and safe metal-ion batteries due to their non-flammable properties, high reserves, and nontoxic ionic liquids[125-127]. However, their low energy density and the complex dynamics of aluminum-based ions (Al3+, AlCl2+, AlCl2+, AlCl4- and
In current commercial LIBs, the primary cathode materials employed are LiCoO2, lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4). The LiCoO2, with a layered structure, serves as a cathode material and possesses a theoretical capacity of 274 mAh g-1, displaying a practical highest capacity of up to 155 mAh g-1. The LiMn2O4 features a spinel structure and offers a theoretical capacity of 148 mAh g-1, with actual capacities ranging from 90 to 120 mAh g-1. Meanwhile, LiFePO4 has a theoretical capacity of
Lithium-sulfur batteries
Because sulfur is a cheap and abundant non-metallic cathode active material, LSBs have garnered substantial attention and engendered widespread scholarly inquiry in recent years. LSBs generally consist of sulfur positive electrodes, lithium negative electrodes, electrolytes, and separators. During the discharge process, insoluble sulfur elements first combine with Li to form Li2S8 and then are further converted into soluble polysulfides (LixS) with different components. In this process, LixS shuttles the electrolyte, causing the battery volume to expand and subsequently pierce the membrane[135-139]. In addition, once LixS is produced by diffusion from the cathode, it is difficult to restore again in the form of lithium sulfide. So, the insoluble products often precipitate in the cathode and electrolyte, resulting in an irreversible reduction in battery cycle life[140,141]. To solve these problems, researchers have long been committed to developing technologies that can alleviate the shuttle effect in the past few years, including anode protection[142], separator strengthening[143], and electrolyte modification[144]. Compared with the traditional LIBs, sulfur has poor conductivity and requires carbon materials to improve its conductivity at the cathode. Therefore, another effective strategy is to sequester sulfur in carbon skeletons, especially porous carbon materials represented by mesoporous carbon[145], hollow carbon[146], and carbon nanospheres[147]. Owing to their abundant pores, adjustable pore size, high specific surface area and designable functional groups, CTFs are also considered as an excellent host material for sulfur cathodes. They can anchor LixS in the middle by forming chemical solid bonds to prevent the shuttle effect. During the discharge process, it is combined with sulfur through covalent bonds, avoiding the dissolution and deposition of LixS. In addition, there are many optional monomers and strategies for constructing CTFs, which provides considerable flexibility for designing CTFs with different pore sizes and functional groups[148].
In 2014, Liao demonstrated the loading capacity of CTFs for sulfur cathodes for the first time[149]. They used a conventional ionothermal trimerization strategy to synthesize CTF-1 with a pore size of 1.23 nm from DCB under ZnCl2 catalysis at 400 °C. After CTF-1 and sulfur were mixed at a mass ratio of 3:2, they were heated at a high temperature of 155 °C for 15 h through a melt diffusion strategy to obtain the composite material CTF-1/S@155 °C. At a rate of 0.1 C, a discharge capacity of 1,497 mA h g-1 was demonstrated in the first cycle. The capacity can remain at 762 mA h g-1 after 50 cycles (0.98% attenuation rate per cycle). To further compare and confirm the basal effect of CTF, they also prepared a simple mixture of CTF-1 and sulfur in the same ratio, CTF-1/S@RT. In contrast, CTF-1/S@RT exhibited poor initial capacity
Figure 8. Schematic diagram of the synthesis of SF-CTF-1 and the SNAr reaction process[152]. Copyright 2017 John Wiley & Sons.
Even though the research progress has been made on how to increase the sulfur loading, there are few reports on the incorporation of heteroatoms into CTFs or the intentional functional design to improve the electrochemical performance of LSBs. Introducing functional groups with specific atom affinity into the structure of COFs is an effective method to weaken the shuttle effect caused by LixS, which could improve the performance of LSBs dramatically[157-160]. Inspired by this, Xiao et al. constructed a new boroxane-based structure that was both lithiophilic and sulfiphilic to stabilize the adsorption kinetics of LixS[161]. The boroxane unit can capture LixS through sulfophilic interactions, while the triazine ring can chemically absorb Li+ through a large number of nitrogen atoms provided by C=N bonds. However, forming two reversible covalent bonds simultaneously was difficult due to mutual interference, and constructing bifunctional CTFs from a single monomer presents a challenge. Xiao et al. synthesized a CTFs derivative (TB-COF) containing triazine and boroxine units by using 4-cyanophenylboronic acid for self-condensation[161]. After 800 cycles, the TB-COF/S electrode exhibited a reversible capacity of 663 mAh g-1 at a charge-discharge rate of 1 C, and the average capacity decay rate per cycle was only 0.023%. With a similar idea, Liang et al. prepared a CTF (COF-Tr-BA) with a pore diameter of 2.61 nm and a specific surface area of 650 m2 g-1 by introducing the boronic acid group (4-ethynylphenyl) into the COF-Tr through the Diels-Alder cycloaddition reaction [Figure 9][162,163]. The interaction between the N atoms of the quinoline and triazine moieties on the main chain alleviates the dissolution of LixS and reduces the shuttle effect. The first discharge capacity is as high as 1,349 mAh g-1, and the discharge capacity can still be maintained at 46% after 200 cycles. Zhang et al. also used a heteroatom doping strategy to construct a CTF containing N and O structural units (NO-CTF)[164]. The NO-CTF material exhibits a large surface area and pore volume, enabling the loading of a high sulfur content (4.8 mg cm-2). Additionally, it provides expedited pathways for both electron and Li+ transport, thereby enhancing the rate capability of the system, as evidenced by a capacity of 678 mAh g-1 at a rate of 2 C. Moreover, the uniform distribution of N and O heteroatoms in NO-CTF facilitates a strong interaction with lithium LixS, thereby accelerating their conversion and improving cycling stability and capacity, respectively.
Figure 9. Schematic diagram of the synthetic route of COF-Tr-BA[162]. Copyright 2023 Elsevier B.V.
The integration of conductive materials, including CNTs, graphene, and conductive polymers, into CTF-based electrode structures, represents a promising avenue for enhancing electron conductivity and optimizing electrochemical performance. The introduction of these conductive additives facilitates efficient electron transport and promotes the utilization of active sulfur species during the charge-discharge processes. Moreover, the utilization of metal-modified CTFs and carbon composite architectures has emerged as a compelling solution for mitigating the notorious "shuttle effect" phenomenon, referring to the undesirable migration of LixS intermediates in LSBs. This innovative approach exhibits pronounced potential in suppressing LixS dissolution, inhibiting shuttle processes, and consequently, improving the cycling stability, coulombic efficiency, and overall energy storage performance of LSBs systems. Gomes and Bhattacharyya used molecular engineering to synthesize new CTFs composite nanosheets (CNT-CON) by functionalizing multi-walled CNT with triazine and crystallizing COF[165]. Benefiting from the synergistic effect of CNT and CON trapping mechanisms, non-polar sulfur is confined by CNTs. In contrast, polar LixS is trapped by the ‘chemical traps’ in the CON framework. The cycle performance of CNT-CON/S is much better than that of sulfur-containing bare COF/S and sulfur-loaded bare CNT/S, achieving a first discharge capacity of up to 1,353 mAh g-1. In 2020, Guan et al. grew N and O atom-rich diazinone-based CTFs in situ onto rGO through sulfur-mediated cyclization of dinitrile monomers, forming a ternary composite electrode (S/P-CTF@rGO)[103]. Owing to the nanopore structure, polar groups in nazinone and triazine, and conductive rGO, the specific capacity of the S/P-CTFs@rGO cathode can initially reach 1,130 mAh g-1, where it can remain at 920 mAh g-1 (81.4%) after 500 cycles. Compared with the S/P-CTFs cathode without rGO, the S/P-CTFs@rGO cathode can provide higher capacity at high current rates. This impressive initial specific capacity of 1,130 mAh g-1 indicates that the efficient charge transfer and substantial utilization of active sulfur species are carried out sufficiently. Furthermore, even at a current rate of 0.5 C, the
Figure 10. The structure, mechanism of action and electrochemical performance of cPpy-S-CTF[16]. Copyright 2020 American Chemical Society.
Summary of the electrochemical performance of representative CTF-based LSBs
| Main body | Sulfur loading (wt%) | Voltage range (V) | Current density (C) | Cycles (retention rate) | Specific capacity (mA h g-1) | Ref. |
| cPpy-S-CTF | 83 | 1.8-2.8 | 0.05 | 500 (86.8%) | 1,203.4 | [16] |
| S/P-CTF@rGO | 70% | 1.7-2.8 | 0.5 | 500 (81.4%) | 1,130 | [103] |
| CTF-1/S@155 °C | 34 | 1.1-3.0 | 0.1 | 50 (64%) | 1,197 | [149] |
| S-CTF-1 | 62 | 1.7-2.7 | 1 | 300 (85.8%) | 562 | [150] |
| SF-CTF | 86 | 1.8-2.7 | 0.05 | 300 (81.6%) | 1,138.2 | [152] |
| FMCTF-S | 77 | 1.6-3.0 | 1 | 400 (62.6%) | 681 | [153] |
| ART-COF/S | 62 | 1.8-2.7 | 0.2 | 100 (96%) | 1,270 | [156] |
| ART-COF/S | 62 | 1.8-2.7 | 1 | 500 (82%) | 993 | [156] |
| TB-COF | 40 | 1.5-3.0 | 0.1 | 150 (71%) | 1,044 | [161] |
| COF-Tr-BA@S | 40 | 1.7-2.8 | 0.5 | 200 (46%) | 1,349 | [162] |
| NO-CTF-1/S | 71 | 1.5-3.0 | 0.5 | 300 (92%) | 1,250 | [164] |
| CNT-CON/S | 78 | 1.0-3.0 | 0.5 | 50 (75%) | 1,353 | [165] |
| GPF-S-3 | 63 | 1.7-2.8 | 2 | 120 (66%) | 1,461 | [186] |
| FCTF-S | 53% | 1.7-2.8 | 0.1 | 150 (73.7%) | 1,130 | [187] |
| CTF-Celgard | 60 | 1.7-2.8 | 1 | 800 (58.4%) | 1,672 | [180] |
| S/FCTF-400 | 62 | 1.7-2.8 | 0.5 | 200 (66.7%) | 741 | [188] |
| FLC | 2.2 mg cm-2 | 1.8-3.0 | 1 | 1,000 (63%) | 792 | [189] |
| OLC | 2.2 mg cm-2 | 1.8-3.0 | 1 | 1,000 (44%) | 723 | [189] |
| S@CTF-Mono | 22 | 1.2-3.0 | 0.1 | 200 (41%) | 2,582 | [190] |
| S@CTF-Bi | 26 | 1.2-3.0 | 0.1 | 200 (34%) | 1,053 | [190] |
| S-NC | 1.8 mg cm-2 | 1.7-2.8 | 1 | 80 (99.12%) | 931.8 | [191] |
| S@EB-COF-PS | 71.70% | 1.6-3.0 | 4 | 300 (41%) | 1,136 | [192] |
| S@CTF/TNS | 76 | 1.5-2.8 | 1 | 1,000 (86%) | 748 | [193] |
| COF-SQ-Ph-S | 80 | 1.7-2.8 | 0.5 | 150 (46%) | 1,331 | [194] |
| S@CTFO | 67 | 1.7-2.8 | 1 | 300 (65%) | 791 | [195] |
| Co-CMP | 16 | 1.7-2.8 | 0.5 | 1,000 (55%) | 1,404 | [196] |
| POP@F/S | 71.5 | 1.8-2.7 | 0.5 | 500 (90.5%) | 956.6 | [197] |
| STP | 5 mg cm-2 | 1.6-2.8 | 0.5 | 150 (79%) | 588 | [198] |
| CoS2-HUT-8/S | 61 | 1.6-2.8 | 1 | 500 (77%) | 757 | [199] |
| CMP-M | 1.7-2.8 | 0.5 | 1,000 (55%) | 916 | [200] | |
| S@THZ-DMTD | 82 | 1.6-2.8 | 1 | 200 (77.8%) | 1,149 | [201] |
| i-SCTF | 60.53% | 1.5-3.0 | 0.1 | 1,000 (89%) | 1,146 | [202] |
Performance improvement strategies
Based on the above research results, the performance enhancement strategies for CTFs in cathode materials focus on the following aspects:
(1) Composite with carbon-based materials: The nitrogen-rich properties of CTFs provide considerable chemical properties, but their inherent 2D structure, structural overlap and low conductivity limit the performance improvement. To solve the problem of low conductivity, an efficient way is to composite with carbon-based materials, such as graphene, rGO, CNTs and other materials. This allows the one-dimensional microporous structure to develop into a multi-level hierarchical micro-mesoporous structure, reducing the micropore ratio while increasing the micropore ratio. Moreover, the micropores enhance the double-layer capacitance, and the porous structure increases the number of ion channels. However, adding conductive carbon materials to CTFs composites will reduce the total energy density of the original CTFs.
(2) Structure functionalization: The redox reaction of metal-ion batteries during the charge and discharge process is the process of reversible cation/proton insertion and extraction of positive and negative electrodes and electron migration in the external circuit. CTFs have abundant redox active sites that can interact with metal cations and generate capacity through cation/proton insertion mechanisms during the redox reaction. The introduction of active functional groups into CTFs is an effective strategy to regulate ion transport efficiency, and improve ion conductivity and cation transfer number. So far, there are two main methods to introduce active functional groups into CTFs: Firstly, introducing target functional groups such as C=O, C=N, and N=N during the design of monomers, and then assembling the functional monomers into target CTFs through covalent linkage. This design strategy can precisely control the number of active functional groups and accurately design the structure of CTFs for specific targets. Secondly, synthesizing target CTFs through the groups in existing monomer materials (such as -OH, -CHO, -NH, -CN, etc.) and other functional side chains. This post-synthetic modification strategy can also cause structural defects, exposing more active sites and further enhancing performance. Especially for LSBs, specific groups (such as -OH) can improve the conversion rate of LixS and limit the shuttle effect.
(3) Exfoliation: Generally, CTFs with high crystallinity have stronger π-π interactions and tighter interlayer arrangement. However, this also leads to a long ion transmission distance and a large transmission resistance. So, it is difficult for ions to penetrate through the channels of CTFs to the active sites inside the skeleton, which greatly affects the electrochemical performance of CTFs-based electrodes. Interestingly, the interlayer exfoliation can reduce the interlayer stacking density, realize the thin layers of CTFs with small thickness, and shorten the ion diffusion distance, which is an effective strategy for regulating ion diffusion. The current exfoliation strategies are mainly divided into two types of exfoliation methods: physical and chemical.
In short, the above strategies are effective whether it is hybridized with conductive graphene materials, introducing functional groups for customization or regulating the electrochemical properties of CTFs through interface engineering. Within LIBs, the aromatic triazine linkages and robust covalent bonds in CTFs provide stable redox sites for extended cycling, while their porous structure facilitates the diffusion of electrolytes. Beyond the triazine and benzene rings in CTFs, additional nitrogen sites such as secondary and tertiary amines offer further redox sites, enhancing the energy density. In addressing the volumetric expansion issues of SIBs, the rigid and porous architecture of CTFs helps mitigate safety concerns caused by expansion. Furthermore, the insolubility of CTFs enhances the safety of SIBs. Regarding LSBs, the orderly and stable porous structure of CTFs allows for higher sulfur loading, promotes uniform sulfur distribution, and restricts the dissolution of LixS, which is crucial in suppressing the "shuttle effect". Additionally, the extra electron pairs in nitrogen and oxygen within CTFs can interact with the strong Lewis acidic sites of terminal lithium atoms in LixS, thereby improving electrochemical performance significantly. The introduction of electron-donating or electron-withdrawing groups by screening monomers and the increase of redox active sites can increase the concentration of the active parts of CTF, thereby improving the theoretical specific capacity. In addition to the structural advantages of COFs, the triazine ring in the skeleton also gives CTFs high structural stability and rich nitrogen content. The stability of the structure is conducive to practical applications under extreme conditions, and the rich nitrogen content gives CTFs materials excellent heteroatom effects (HAE) and provides them with rich active centers. In addition, the fully conjugated network of CTFs can promote the transport of electrons and substances. Although considerable progress has been made in the development of positive electrode materials for metal-ion batteries based on CTFs, it still faces some limitations such as low tap density and difficulties in large-scale production.
APPLICATION OF CTFS AS ELECTROLYTES IN SOLID-STATE BATTERIES
According to the classification of solid-state electrolytes (SSEs), solid-state batteries (SSBs) have three mainstream technical routes: polymer[168], oxide[169], and sulfide SSBs[170]. Among them, polymer electrolytes belong to organic electrolytes, while the oxide and sulfide electrolytes belong to inorganic electrolytes. The ideal solid electrolyte materials should have high ionic conductivity, excellent chemical and electrochemical stability to alkali metals, strong inhibition ability of the generation of lithium dendrites and low cost[171]. However, these three major technical routes currently have both the positive and negative effects. None of them can meet the above requirements at the same time, and there are still certain difficulties in technological breakthroughs. Therefore, the electrolyte chemistry has become an important part for SSBs research.
The CTFs are one of promising candidate solutions as organic SSEs due to their tunable structure, functional design, and relatively large Li+ migration number. Under normal conditions, due to strong Coulomb interactions, the Li+ components tend to be tightly bound in the CTFs channels in the form of ion pairs or aggregates. Their migration efficiency mainly depends on the concentration of active sites enabling Li+ to jump and the distance between active sites in pure CTFs electrolyte, namely the Grotthuss mechanism[172]. Hou et al. constructed vinyl and triazine-based bifunctional CTFs (V-COF) materials by room temperature synthesis, and obtained poly(ethylene glycol) dimethacrylate/V-CTF (PDM/V-COF) by covalent polymerization with ether segments [Figure 11][173]. The covalent bond connection method avoids the formation of two-phase interface. The regular one-dimensional pores and triazine-rich structure of
Figure 11. (A) Schematic diagram of Li+ transport mechanism. (B) Preparation and structure of Vinyl-functionalized CTFs (V-COFs) and PDM/V-COFs[173]. Copyright 2022 John Wiley and Sons.
To enhance the ionic conductivity of CTF-based electrolytes, solvents that promote ion migration can be added. In 2020, Shi et al. first proposed to improve the safety of lithium metal batteries by preparing quasi-solid-state electrolytes via simple mechanical blending of functionalized CTFs and ionic liquids[174]. The quasi-solid-state electrolyte exhibits high strength and stability due to its strong CTFs skeleton, and effectively inhibits the growth of lithium dendrites. Its ordered pore structure can load a large amount of ionic liquids and construct 3D continuous ion channels to achieve rapid Li+ conduction and stable deposition of lithium. The cyano functionalization on its surface can effectively promote the dissociation of lithium salts and enhance the stability of ionic liquids. Also, the quasi-solid-state electrolyte is used in lithium metal batteries to exhibit high room temperature ionic conductivity (1.33 mS cm-1), high Li+ migration number (0.648), excellent electrochemical/cycling stability, good rate specific capacity and effective inhibition of lithium dendrite growth. Although this study has opened up a new and effective method for the research and application of quasi-solid-state batteries, it is not suitable for solving the interface contact and interface stability of CTFs in all-solid-state batteries (ASSBs).
In 2022, Cheng et al. successfully synthesized a Novel single-ion conductive nitrogen-hybridized conjugated framework (NCS) electrolyte with a "donor-acceptor" structural unit, whose framework is composed of an electron-withdrawing triazine ring and an electron-donating piperazine ring [Figure 12][175]. The results show that NCS has more outstanding thermal stability and higher ionic conductivity compared with the control sample COFs without triazine structure (TAL). The triazine ring has a strong coordination effect with Li+, and the strong electrostatic force formed by the piperazine ring. The density functional theory (DFT) simulation results show that COF with an amphoteric structure can effectively dissociate lithium bis(trifluoromethane)sulfonamide (LiTSFI), and the Li+ generated after the dissociation of the lithium salt is coordinated and complexed with the conjugated electrolyte. Therefore, without the use of solvents, the room-temperature Li+ conductivity of NCS can reach 1.49 mS·cm-1, and the Li+ migration number is as high as 0.84. It also shows excellent electrochemical performance when applied to all-solid-state lithium metal batteries, and the capacity retention is 82% after 100 cycles at 0.5 C. This is due to the stronger acidity in the triazine core, which enables stronger coordination with Li+ and forms more loose ion pairs. In addition, the solid electrolyte prepared in this study has excellent flame retardancy and thermal stability, thus ensuring the safe use of ASSBs.
Figure 12. (A) Synthesis of NCS and TAl, (B) Top view of the AA stacking pattern of NCS-Li, (C) PXRD patterns of NCS and NCS-Li, (D) Thermogravimetric images of NCS-Li and TAL-Li, (E) HRTEM image of NCS, and (F-J) Flame retardancy of NCS-Li[175]. Copyright © 2023 Springer Publishing Company.
The insolubility and porous nature of CTFs render them exceptionally suitable as carrier substrates for ionic liquids, thereby facilitating their use as quasi-solid electrolytes. Moreover, the triazine rings within CTFs exhibit a strong coordination interaction with Li+, which can lower the energy barrier for ion migration, hence achieving noteworthy ionic conductivity. However, despite their commendable performance, most SSEs based on CTFs are currently fabricated through a powder pressing method, resulting in potential brittleness and interfacial issues with electrodes, thus limiting their practical application in flexible and wearable batteries. Therefore, the development of CTF-based SSEs possesses high flexibility.
APPLICATION OF CTFS AS SEPARATOR
Separators are key components in water electrolyzers, fuel cells, metal-ion batteries, electrodialysis and other related processes[176,177]. The efficiency of ion transfer in the separator depends on the energy barrier of ion transfer across the separator. Therefore, constructing efficient ion channels in the separator and reducing the energy barrier of ion transfer across the separator are the key to developing high-performance ion separators. "Microphase Separation" ion separators represented by Nafion membranes have wide ion channels that can efficiently conduct ions, but the ion channels are prone to swelling after absorbing water, resulting in a decrease in the mechanical strength of the separator and a decrease in selectivity and barrier properties[178].
Currently, the widely used separators in metal-ion batteries are polyolefin materials (pore size > 100 nm) and glass fibers (pore size > 450 nm)[179]. These traditional separators are all macroporous materials, which are not conducive to the precise screening of ions. CTFs can easily adjust the geometric size and shape of the pores by selecting different monomers. It can also use molecules with positive or negative charges or no charge to control the number of conductive ions to obtain cationic or anionic conductive membranes.
Figure 13. EIS plots of Celgard and CTF-Celgard separators[180]. Copyright © 2019 Elsevier B.V.
In 2021, Shi et al. advanced their efforts by employing the electrostatic layer-by-layer self-assembly technique to fabricate an ultralight functional separator with an ordered structure, known as LBL-f separator[181]. This innovative membrane comprises CTFs (CTF@PDDA), enveloped by positively charged poly(diallyldimethylammonium chloride) (PDDA), and negatively charged poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), as presented in Figure 14. The CTF@PDDA layers exhibit robust LixS anchoring capability through a combination of physical and chemical interactions, and their substantial specific surface area coupled with a porous configuration enables the enhanced electrolyte absorption. Furthermore, the PEDOT:PSS layer functions as a conductive layer that promotes electron transfer and serves as an exceptional interface stabilizer and adhesive. Consequently, when the proposed LBL-f separators are employed in conjunction with standard sulfur cathodes and lithium-metal anodes, they exhibit remarkable cycling stability (capacity decay rate per cycle of 0.052% over 1,000 cycles at 1.0 C), and elevated sulfur utilization rates (90.7% at 0.1 C and 59.2% at 2.0 C). In recent research,
Figure 14. (A-D) Schematic diagram of flexible, semi-rigid, and rigid polymer membrane ion channels, (E) CTF preparation using superacid-catalyzed organic sol-gel reaction, and (F) Free-standing CTF membrane with a diameter of more than 10 cm[182]. Copyright © 2023 Springer Nature.
Figure 15. Characterization of negatively charged CTFs membranes (SCTF)[182]. Copyright © 2023 Springer Nature.
The inherent microporous structure of CTFs can enhance the ability of commercial separators to capture
CONCLUSIONS AND OUTLOOKS
The richness of structural units in CTFs and the diversity of ligands render them a veritable "database" for the development of excellent materials in metal-ion batteries. Their high degree of designability and unique, stable structural characteristics endow them superior ion loading capacity, uniform distribution, rapid transport, and selective affinity. These contributions have been demonstrated in exemplary applications within electrodes, separators, and electrolytes. For instance, CTFs can effectively confine the dissolution of LixS, thereby fundamentally enhancing the cycle lifetime of LSBs. However, the sulfur adsorption capacity of conventionally designed CTFs is constrained by their surface area and pore size distribution. Additionally, with the operation of energy storage devices, the CTFs cathodes undergo gradual, irreversible structural disintegration, leading to the detachment of anchored cathode materials from the CTFs matrix, resulting in a decline in the battery performance. Despite the increasing importance of CTFs in the energy storage sector due to the versatility and precision of their synthesis methods, their application in metal-ion batteries remains fraught with challenges [Figure 16].
Figure 16. Challenges and opportunities of CTFs.
(1) Small Scale. Although there are synthetic methods for CTFs, such as high-temperature polycondensation and strong acid catalysis, most of them require high temperature, a large amount of catalyst and solvent. Such methods require a strict environment, and the synthesis time is relatively long. Therefore, these synthetic methods remain at the laboratory scale which are not universal. Therefore, it is an inevitable trend to explore and optimize low-cost, operable, and high-yield CTFs synthesis strategies for industrial-scale production and commercialization in the CTFs field.
(2) Low Crystallinity. The high stability of the triazine ring will negatively affect the association and separation of bonds during the polymerization reaction, thereby reducing the order of the target materials. Therefore, it is still an urgent issue to find a more gentle method to construct a higher-crystalline CTF.
(3) Side Reactions of Functionalization. Under the chemical functionalization and customization of CTFs, some side reactions are inevitably produced due to the combined effects of different monomers and high temperatures, resulting in a large number of by-products. This side reaction leads to a decrease in the purity of the target CTFs. In addition, the types of active functional groups embedded in the CTFs skeleton are limited.
(4) Low Conductivity. The low conductivity of CTFs is the main factor limiting the development of CTFs as electrodes. Although the ordered pores in CTFs can promote the diffusion of ions, the diffusion rate in COFs is slow due to the long ion transmission path. Secondly, the structures of CTFs monomers are usually composed of aromatic and triazine rings. This structure determines that the chemical bonds of COFs are long, which also limits ion transmission efficiency. Improving the conjugated structure design of the material and introducing conductive media are the main methods at present. However, such approaches are only temporary solutions and cannot improve the energy density of the material itself.
(5) Electrochemical Mechanism. Although the ion transmission and sealing mechanism of CTFs has been widely discussed academically, the difference in electrochemical performance caused by pore size is still unclear. Meanwhile, the complex mechanism between different pore sizes and ion channels still needs to be explored.
Although CTFs face some challenges, their application research is still under the development. The insolubility of CTFs combined with their considerable ion selectivity has the potential for application in quasi-solid-state and solid-state batteries. However, it is still necessary to combine interface engineering and defect engineering to solve the problems of interface contact and exposure of active sites. In addition, there are no application scenarios for flexible CTFs in metal-ion batteries. Therefore, the CTFs with flexibility and high electrochemical performance may be the potential application in extreme scenarios and wearable mobile devices. Moreover, due to the limitations of current synthesis methods and functional group selection, it is urgent to develop large-scale synthesis methods for CTFs materials. At last, the computational simulation plays an important role in promoting the development of reproducible and scalable synthesis methods for industrial-level production and application.
DECLARATIONS
Authors’ contributions
Conceived the idea of this manuscript: Zhang B
Wrote the manuscript: Wang Z
Assisted in the collection of literature and scientific research drawings: Zou X
Supervised and guided the academic expression of this work: Lv M
Further revised the logic, grammar, and professionalism of the manuscript: Wang Z, Lv M, Zhang B
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (52373175), High-level Innovative Talents Foundation of Guizhou Province (Grant No. QKHPTRC-GCC[2023]024), Science and Technology Innovation Team of Natural Science Foundation of Guizhou Province (Grant No. CXTD[2023]005), Science and Technology Innovation Team of Higher Education Department of Guizhou Province (Grant No. QJJ[2023]053), and Natural Science Foundation of Guizhou University (GZUTGH[2023]12).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
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