Recent Advances and Perspectives in Single-Ion COF-Based Solid Electrolytes

Abstract

The rapid growth of renewable energy sources and the expanding market for electric vehicles (EVs) have escalated the demand for safe lithium-ion batteries (LIBs) with excellent performance. But the limitations of safety issues and energy density for LIBs continue to be obstacles to their future use. Recently, single-ion covalent-organic-framework-based (COF-based) solid electrolytes have emerged as a promising avenue to address the limitations of traditional liquid electrolytes and enhance the performance of LIBs. COFs have a porous structure and abundant electron-donating groups, enabling the construction of an available ionic conductive network. So, COFs are the subject of extensive and in-depth investigation, especially in terms of the impacts their adjustable porous structure and tunable chemistry on the research of ionic transport thermodynamics and transport kinetics. In this perspective, we present a comprehensive and significant overview of the recent development progress of single-ion COF-based solid electrolytes, highlighting their rare performance and potential applications in solid lithium batteries. This review illustrates the merits of single-ion conducting solid electrolytes and single-ion COF conductor-based solid electrolytes. Furthermore, the properties of anionic, cationic, and hybrid single-ion COF-based conducting electrolytes are discussed, and their electrochemical performance is also compared when applied in Li-ion batteries. Finally, to solve challenges in COF-based Li-ion batteries, strategies are provided to obtain a high lifespan, rate performance, and stable and safe batteries. This work is promising to offer valuable insights for researchers and the energy storage industry.

1. Introduction

The booming market for sustainable energy and the widespread adoption of electric vehicles (EVs) have fueled the demand for high-performance lithium-ion batteries (LIBs) that can keep pace with the changing needs of today’s consumers. However, the safety of LIBs is a highly important and non-negligible issue, especially as more and more consumers turn to these cutting-edge power sources for their energy needs [1,2,3]. Solid electrolytes promise to significantly reduce the risk of thermal runaway and catastrophic failures [4,5,6], and they can enable the use of high-capacity electrode materials, thus enhancing the energy density and overall performance of batteries [7,8]. Additionally, polymer-based solid electrolytes offer design flexibility, allowing for the development of more compact and versatile battery designs for applications like wearable and flexible electronics [9,10]. Overall, solid electrolytes are a key innovation that can transform the energy storage landscape, paving the way for more sustainable and efficient energy solutions across various applications [5,11,12].

Currently, there are three main kinds of solid electrolytes, and each type of solid electrolyte has its own advantages and disadvantages [13], as shown in Figure 1. Inorganic solid electrolytes exhibit high ionic conductivity and excellent stability but suffer from poor mechanical properties and processing challenges [14,15,16]. Despite the significant advancements made in ionic conductivity, inorganic solid electrolytes still have issues such as grain boundary resistance, instability at electrode interfaces, and inflexibility [17,18]. Polymer electrolytes provide better processability but may have lower ionic conductivity and less stability [19,20,21,22], and they are prone to dendrite penetration [23,24]. Single-ion conducting polymer electrolytes have easy processability, physical flexibility, close contact with electrodes, adaptable chemistry, and high cation transportation capability. Thus, as a substitute for inorganic solid electrolytes and polymer electrolytes, there has been a recent surge in interest towards single-ion conducting polymer electrolytes [25,26]. Covalent organic frameworks (COFs) have a tunable chemical structure and high porosity, and COF-based solid electrolytes can achieve high ionic conductivity and tailored properties for specific applications [27,28]. Their high thermal and chemical stability, combined with low flammability and good mechanical properties, make them safer and more durable than other solid electrolytes [28,29,30]. Furthermore, COF-based solid electrolytes hold the potential for enhanced electrode–electrolyte interface compatibility, which can lead to improved overall battery performance [27,31,32]. The majority of these COFs are based on two-dimensional (2D) topologies, while the number of three-dimensional (3D) COFs does not exceed 100 so far. Three-dimensional COF materials have enhanced specific surface area, interconnected channels, well-exposed functional moieties, and highly tunable structures; however, their high void framework and absence of π-π stacking affect the crystallinity and chemical stability of 3D COFs. At present, single-ion conductive COFs with relatively stable 2D structures are more practical. However, it is possible that with the development of synthetic strategies, single-ion conductive COF materials with a stable 3D structure will also be the best choice for solid-state electrolytes [33]. The development of single-ion COF-based solid electrolytes is essential, as they have the potential to optimize methods of energy storage and energy usage, thus contributing to a more efficient and sustainable energy landscape in the future [27,34].

Some of the critical aspects and benefits of developing COF-based electrolyte materials include the following: (i) Enhanced safety: This feature is particularly crucial for large-scale battery systems, such as electric vehicles or grid energy storage [35,36]. (ii) High ionic conductivity: Single-ion COF-based solid electrolytes can provide high ionic conductivity, enabling efficient charge transfer within the battery [37]. (iii) Structural stability: COF-based solid electrolytes exhibit strong covalent bonding, leading to a stable framework that can withstand high voltages and temperatures [38,39,40]. (iv) Single-ion transport: Only allowing the transport of one type of ion (e.g., lithium ions) reduces the likelihood of undesirable side reactions, improving the overall energy efficiency and ensuring a stable voltage during operation [41,42]. (v) Design flexibility: COFs offer a high degree of tunability in their structure, porosity, and functionality, allowing researchers to design materials with specific properties tailored for different battery chemistries and applications [43,44]. (vi) Environmental benefits: COFs are not typically composed of heavy metal elements; for instance, the elements carbon, hydrogen, oxygen, and nitrogen make up the majority of COFs, making them more environmentally friendly than other types of solid-state materials [45,46]. Apart from COF compounds, materials such as metal–organic frameworks (MOFs) [47] and zeolite [48] structures also exhibit similarities in their structural characteristics. These materials possess high porosity and crystallinity, making them potential candidates for various applications, including solid-state electrolytes. COFs are connected by covalent bonds, their structure is more stable, there are no heavy atoms, their crystallinity is slightly poor, and their pores are mostly lipophilic, which is suitable for lithium battery electrolytes. MOFs have a similar structure to COFs, which are connected by ionic bonds and coordination bonds. Even though there are applications for MOF-based solid electrolytes [49], their stability is not good and most of the pores in their structures are hydrophilic as well as zeolite. COFs are still regarded as having more potential as electrolytes for LIBs compared to MOFs.

Overall, the use of a COF as a solid electrolyte for LIBs offers several advantages over other solid electrolytes. Very recently, there some detailed reviews have summarized the importance and development of COF-based materials and their practical application, indicating the cutting-edge development of this kind of material in LIBs [49,50,51]. Furthermore, their scalability, cost-effectiveness, and safety concerns were revealed, which require further exploration and optimization [52,53,54]. In our mini review paper, we presented an evaluation of single-ion COF-based solid electrolytes, elucidating their advantages and challenges, and identifying study directions and opportunities for the further development and application of single-ion COF-based solid electrolytes.

2. Single-Ion Conducting Solid Electrolyte

2.1. Single-Ion Conductor

Aside from the transportation of Li⁺ ions, the movement of anions is also a crucial component of battery function while in operation. The ion transference number that evaluates the migration ability of a certain ion is generally represented by the ratio of the quantity of these ions in the system to the total quantity of ions, as shown in Formula (1) [55]:

$${\mathrm{t}}_{+}=\frac{{\mathrm{Q}}_{+}}{\mathrm{Q}}; {\mathrm{t}}_{-}=\frac{{\mathrm{Q}}_{-}}{\mathrm{Q}}; {\mathrm{t}}_{+}+{\mathrm{t}}_{-}=1,$$

(1)

where ${\mathrm{t}}_{+}$ and ${\mathrm{t}}_{-}$ are the quantity of transferred cations and anions, respectively, while Q is the total charge, Q⁺ is the amount of positive ionic charge, and Q⁻ is the amount of negative ionic charge. At present, the electrolyte salt LiPF₆ is mainly used as the electrolyte in commercial LIBs. During the operating (i.e., charging/discharging) process of the battery, Li⁺ and PF₆⁻ will move to opposite directions. Solvated sheaths surround Li⁺ in the solution, moving with Li⁺ and limiting the migration speed. However, PF₆⁻ ions are hardly solvated and move relatively quickly, as shown on the left in Figure 2 [56]. Some studies show anions moving over four times faster than Li⁺ ions, and as a result, the transferred Li⁺ ions account for a low fraction (20%) of the total ion current [57]. This phenomenon presents a problem whereby excessive anions accumulate on the surface of the cathode, creating a concentration gradient between the cathode and the anode. The accumulation of anions at the electrolyte–electrode interface results in cell polarization and a shortened cycle life [13].

A single-ion conducting electrolyte is a system in which anions are completely or partially trapped, or removable anions do not exist (such as an inorganic solid electrolyte). In single-ion conducting electrolytes, the migration of cations is relatively much faster, as shown on the right in Figure 2. Single-ion conducting electrolytes tend to eliminate concentration gradients and allow for fast charging/discharging [58,59,60]. The closer Li⁺ ions transference number is to 1, the higher the migration ratio of Li⁺ in the electrolyte, resulting in higher charge transfer efficiency between the cathode and anode. As a result, the Li⁺ ions transference number can affect the cycling efficiency and capacity of the battery [61,62,63,64].

2.2. Advantages of Single-Ion Conductors

Single-ion conducting solid electrolytes are the ideal choice for next-generation batteries, as they offer rapid charging capabilities, exceptional safety, and a high energy density [65,66,67], This is graphically represented in Figure 3a. Obviously, single-ion conductors endow the electrolytes with high ionic selectivity, oxidation potential, and resistance to dendrite growth.

Single-ion conducting electrolytes can delay the development of lithium dendrites. Chazalviel demonstrated that dendrite formation can arise in the space charge layer that is formed in diluted solutions [68,69]. The study predicted that at high current densities, dendrites would be generated when the concentration of Li⁺ ions on the interface between the electrolyte and electrode decreased to zero. The initial time for dendrites’ growth is referred to as the Sand’s time, which is calculated by Formula (2):

$$\mathsf{\tau }=\mathsf{\pi }\mathrm{D}\frac{{\mathrm{e}\mathrm{C}}_0{\left({\mathsf{\mu }}_{\mathrm{a}+{\mathsf{\mu }}_{{\mathrm{L}\mathrm{i}}^{+}}}\right)}^2}{2\mathrm{J}{{\mathsf{\mu }}_{\mathrm{a}}}^2}$$

(2)

where D is the bipolar diffusion coefficient, e represents the charge, C₀ is the initial concentration of the electrolyte, j is the effective current density, and μ_a and μ_Li+ are the migration rates of the anions and Li⁺, respectively. As claimed by Chazalviel’s model, Sand’s time τ empirically depends on the transferred numbers of electrons and lithium ions. By consuming effective current density (j) or speeding up the migration rate of lithium ions (μ_Li+), Sand’s time is extended, which delays the production of lithium dendrites. Therefore, methods such as adjusting the anode structure by enhancing the specific surface area of the two-dimensional material and using electrolytes with a high quantity of transferred lithium ions can restrain the formation of dendrites [70,71]. If Li+ ions 5 transference number of the single-ion conductor is higher, the formation time of lithium dendrites would be longer. So, the utilization of a high Li⁺ migration number in single-ion conductors is one of the effective methods to inhibit lithium dendrites. In addition, Figure 3b indicates that the formation of the problematic and potentially dangerous soft Li dendrites, as well as their associated side reactions, is significantly inhibited in single-ion-conductor-based cells thanks to the material’s exceptional selectivity concerning Li-ion transport (μ_Li+ = 0.63). This is due to the fact that with single-ion-conductor-based cells, the anion concentrations never reach zero on the anode interface due to the immobilized anion present [72]. Single-ion conducting electrolytes can reduce battery polarization as well. As shown in Figure 3c, dual-ion conducting solid polymer electrolytes exhibit superior kinetic characteristics in conditions prior to the occurrence of concentration polarization, but at a higher current, only the method of utilizing a single-ion conducting solid polymer electrolyte is possible to avoid the concentration polarization within the battery system [73].

Figure 3. The advantages of single-ion conducting electrolytes for preventing lithium dendrites (a) [13], reducing concentration polarization [72] (b), and inhabiting Li dendrites (c) [73]. Reproduced with permission from the Royal Society of Chemistry, American Chemical Society, and Wiley.

Figure 3. The advantages of single-ion conducting electrolytes for preventing lithium dendrites (a) [13], reducing concentration polarization [72] (b), and inhabiting Li dendrites (c) [73]. Reproduced with permission from the Royal Society of Chemistry, American Chemical Society, and Wiley.

3. Single-Ion COF-Based Electrolytes

3.1. Li-Ion Transport Mechanisms in COFs

In ceramic solid electrolytes, the conduction pathways of Li⁺ ions move from one crystal site to another through vacancies in the crystal lattice [11,74]. So, these pathways are often referred to as “interstitial” or “vacancy” conduction. In polymer solid electrolytes, the conduction pathways of Li⁺ ions are through the polymer chains. The lithium ions move along the polymer chains by hopping from one site to another. These pathways are often referred to as “ionic hopping” conduction [75]. Both of these conduction pathways are essential for the effective functioning of solid-state lithium-ion batteries. They enable the transport of Li⁺ ions, which is critical for the storage and release of electrical energy in the battery. So far, there are two categories of COF electrolytes: (i) pure COFs mixed with Li salts [76,77], and (ii) semi-solid electrolytes, which are hybrids including COFs and liquid electrolytes. There are three forms of ion transport: (1) Li⁺ hopping on the same COF cluster between the outmost oxygen [78], (2) Li⁺ hopping between COF clusters, and (3) solvated Li⁺ diffusion in bulk solvent [79]. In the pure COF-based electrolytes, no Li⁺ diffuses in bulk solvent. However, in the COF-based semi-solid electrolytes, three forms of ion transport exist simultaneously. Compared with pure COF-based solid electrolytes, the hybrid COF electrolytes show better ionic conductivity. However, the quantity of transferred Li⁺ in hybrid COF electrolytes is lower than that of pure COF-based solid electrolytes [74,80].

The Li⁺ traffic in 2D-COF solid electrolytes combines the performance of liquid electrolytes and solid inorganic electrolytes. Specifically, the local environment of Li⁺, as shown in Figure 4b, similar to those in the liquid phase (Figure 4a), exhibits a flatter energy landscape compared to inorganic crystalline electrolytes (Figure 4c). However, the inflexible COF skeleton hinders the extensive diffusion of solvents and anionic species, resulting in preventing the development of clusters that affect ionic conductivity. A number of scientists propose that electrostatic attractions prompt the ability to transport anions during diffusion for Li⁺-containing COF tunnels [81]. The complex nature of COF-based electrolytes, with their multiple components of Li ions and anions (Figure 4d), or Li ions, anions, and solvents (Figure 4e), present a major challenge for achieving efficient Li-ion transport and preventing undesirable interfacial reactions. The development of single-Li-ion conductors (Figure 4f) addresses this challenge, because highly efficient Li-ion transport networks are crucial for the realization of safe and high-performance energy storage systems at room temperature [82].

3.2. Single-Ion COF-Based Solid Electrolytes

3.2.1. Anionic Single-Ion COF Conducting Electrolytes

Recently, ionic covalent organic frameworks (ICOFs) have developed rapidly and been recognized as promising candidates in the field of solid electrolytes. Anionic COFs have higher Li⁺ transference numbers than neutral COFs [83]. The skeleton structure of the anionic COF contains negatively charged atoms [84] or is grafted with negatively charged functional groups [85]. Such COFs are often combined with lithium ions to form a lithium-containing single-ion conductor.

In order to fix anions in the COF’s porous framework, two methods are commonly used: the coordination anion opens up other cationic sites, or the anion is inserted directly into the structure. The function of “cages” or “hosts” for nanopores in COFs facilitates the transport ability of Li ions, since the mobility of larger anions is restrained. As shown in Figure 5a, in the imidazolate-containing COFs, the immobilization of anionic functional groups results in a higher degree of control of the ion transport pathways, leading to improved electrochemical performance and stability. By harnessing the conductivity derived exclusively from Li⁺ transfer, single-ion conducting solid electrolytes achieve higher transference numbers and facilitate faster charging and discharging. Ultimately, these advancements pave the way for safer, more efficient, and longer-lasting energy storage systems [84]. Furthermore, as shown in Figure 5b, lithium sulfonated COFs (TpPa-SO₃Li) as solvent-free and single-lithium-ion conductors, represent a major breakthrough in this regard. The exceptional performance of TpPa-SO₃Li can be attributed to its strategically crafted functional ion channels, a high concentration of Li+ ions, and anion groups linked covalently. These factors combine to produce ionic conductivity of 2.7 × 10⁻⁵ S cm⁻¹, coupled with Li⁺ ions transference number reaching 0.9 at ambient temperature [86], which is higher than that (t_Li⁺ = 0.881) of Polymer electrolytes (PEs) [87]. As shown in Figure 5c, the hydrazone-based COF modified with a phenol group was synthesized through an innovative approach, which led to the achievement of two important milestones: Firstly, the attainment of ion conductivity (10⁻⁵ S cm⁻¹) even at extremely low temperatures of −40 °C. This remarkable level of conductivity was accompanied by a high quantity of transferred Li⁺ (0.92), further highlighting the excellent performance of this solid electrolyte [88]. As shown in Figure 5d, incorporating flexible chains of oligo(ethylene oxide) into the structure of the porous walls of COFs has also been reported. The TPB-DMTP-COF was accomplished by condensation of 1,3,5-tri(4-aminophenyl)benzene (TPB) with 2,5-dimethoxyterephthalaldehyde (DMTP) or 2,5-bis ((2-methoxyethoxy) methoxy)terephthalaldehyde (BMTP). Upon complexation with lithium ions, the oligo(ethylene oxide) chains form a polyelectrolyte interface within the nanochannels and provide a solution for the transportation of lithium ions [78]. The ion conductivity of these materials was boosted by more than 10³-fold, representing a significant breakthrough in materials science. The enhanced conductivity was attributed to the unique vehicle mechanism of the polyelectrolyte COFs, which facilitated the motion of ions across the nanochannels.

It is worth mentioning that the synthesis of the lithium salt PaSO₃Li for the first time via an acid–base reaction between a 2,5-diaminobenzenesulfonic acid monomer and lithium hydroxide (LiOH) represents a groundbreaking achievement, as shown in Figure 5e. Similar to this synthesis method, single Na⁺-COFs and K⁺-COFs were produced by utilizing their respective salts applied as monomers. Impressively, the resulting COFs exhibited an excellent ionic conductivity (1.6 × 10⁻³ S cm⁻¹) at 0 °C, showcasing the potential of this approach for producing solid electrolytes with exceptional performance [89]. As we know, the spiroborate linkage is particularly attractive as it can be readily formed through a variety of methods, including the condensation of polyols with alkaline traborate or boric acid, as well as through transesterification between borate and polyols in a thermodynamically favorable manner (Figure 5f). This versatility and ease of production make COFs linked by spiroborates an attractive option for the development of new and innovative materials for energy storage. By leveraging the unique properties of COFs and spiroborates, it is possible to create materials with exceptional conductivity, stability, and efficiency [90].

3.2.2. Cationic Single-Ion COF Conducting Electrolytes

A cationic COF contains negatively charged atoms in the skeleton structure or is grafted with positively charged functional groups. This kind of material is first allowed to be complex with anions, such as TFSI⁻ or ClO₄⁻, and then form a more complex system with Li⁺ ions. Recently, a study has shown that the ionic conductivity of polyethylene glycol (PEG)-Li⁺@EB-COF-ClO₄ (as shown in Figure 6a), which was measured at an impressive 1.78 × 10⁻³ S cm⁻¹ at 120 °C, with the t_Li+ of PEG-Li⁺@EB-COF-ClO₄ as high as 0.60. It is noteworthy that the stability performance of PEG-Li⁺ @EB-COF-ClO₄ is excellent, without any decay even after 48 h of continuous operation at a high temperature of 90 °C. These findings represent a significant advancement in energy storage, as they demonstrate the potential for developing more stable and efficient lithium-ion batteries that can resist high temperatures for extended periods of time [91]. Chen et al. have successfully combined a cationic skeleton with the COF structure, which enabled us to split the Li⁺ ion pairs in a framework with different properties (as shown in Figure 6b). This breakthrough discovery led to a drastic increase in the movable Li⁺ concentration, thereby significantly improving the Li⁺ conductivity of solvent-free electrolytes (reaching 2.09 × 10⁻⁴ S cm⁻¹ at 70 °C). This novel approach potentially revolutionizes the energy storage field by providing a more effective and efficient method for improving the performance of lithium-ion batteries [92]. Li et al. utilized innovative imidazolium-based monomers and Im-COF-Br in the construction of a novel cationic COF, first through a Schiff base reaction, followed by replacing the anion of Br- with bis(trifluoromethylsulfonyl)imide ion (TFSI⁻), and obtained Im-COF-TFSI-based electrolytes (as shown in Figure 6c), which exhibited high Li⁺-ion conductivities (up to 4.64 × 10⁻⁴ at 353 K and 4.04 × 10⁻³ S cm⁻¹ at 423 K), excellent thermostability, and a stable interface between electrode and electrolyte. Most impressively, the all-solid-state Li battery of Li/Im-COF-TFSI@Li/LiFePO₄ exhibited a good initial discharge capacity and cycling ability, reaching 123.3 mAh g⁻¹ and maintaining 91.6% in capacity following 100 cycles, even at 80 °C [93]. The introduction of a functional group (imidazolium) is also incorporated onto the porous structure of COFs through the Schiff base reaction, which has been sufficiently demonstrated to be a highly effective method for developing pure solid electrolytes for the conduction of Li⁺ ions. This approach has resulted in the successful creation of dCOF-ImBr-Xs- and dCOF-ImTFSI-Xs-based electrolytes (as shown in Figure 6d), which exhibit remarkable ion conductivity across a wide range of operating temperatures. Notably, the ionic conductivity of the dCOF-ImTFSI-60-based material has been found to reach an impressive 7.05 × 10⁻³ S cm⁻¹ at 423 K [94]. In addition, Li⁺ conductors were synthesized by incorporating ionic liquids (ILs) into the porous nanochannels of three kinds of crystalline thiophene-based imine-linked COFs. These COF–IL composite electrolytes were characterized and showed remarkable thermal stability (as high as 673 K °C) and impressive ionic conductivity (reaching 2.60 × 10⁻³ S cm⁻¹ at 393 K), which are the top values recorded for doped porous organic electrolytes. Furthermore, their specific capacity (140.8 mAh/g at 373 K) and stable cycling ability is remarkable in COF–IL composite electrolyte-based LiFePO₄–Li fuel cells. This COF–IL electrolytes outperform pure poly (ethylene oxide)-based electrolytes, suggesting their vast potential for applications at high temperatures [95].

3.2.3. Hybrid Single-Ion COF Conducting Electrolytes

Presently, practical requirements for real single-ion solid-state ion conduction remain unfulfilled. The tightly linked Li⁺ ions predominantly move along the surface of the pore walls of lithiated COFs, which implies that the large pore volume or porosity is not ideal for effective ionic mobility because it can “dilute” the total concentration of charge carriers [96,97]. A recent study has reported exciting developments in the field of solid-state batteries. It has been discovered that by incorporating flexible dimethylacetamide (DMA) chains into COF channels, the functional groups of these chains are capable of liberating Li+ ions from the stiff COF structure while decoupling the lithium salt (as shown in Figure 7 upper). The resultant altered pore environment leads to a dramatic acceleration of Li⁺ movement, in an ordered manner. This, in turn, leads to an excellent ionic conductivity of 1.7 × 10⁻⁴ S cm⁻¹ for the DMA@LiTFSI^-mediated COF (DLC) electrolyte, which is approximately 100 times higher than that of other COFs. Moreover, the DLC electrolyte exhibits a high t₊ of 0.85 at room temperature. It is worth noting that COF crystals can be made into ultrathin and highly flexible films, which opens up possibilities for the advancement of foldable solid-state batteries for practical demonstrations. These findings represent a significant advancement in the development of more efficient, durable, and practical energy storage solutions for the future [98]. Similarly, the COF-poly(vinyl ethylene carbonate) (PVEC) composite film was designed as a flexible electrolyte (as depicted at the bottom in Figure 7). The designed COF-PVEC electrolyte film enriched with lithium-philictriazine and carbon–carbon double bonds within the COF structure demonstrated a good ionic conductivity, reaching 1.11 × 10⁻⁴ S·cm⁻¹ at 313 K, as well as, high mechanical strength and dendrite growth resistance [99].

3.2.4. Performance of the Cells with Single-Ion COF-Based Solid Electrolytes

Currently, COF-based solid electrolytes are still in the early stages of development and require further research to overcome challenges related to large-scale synthesis, processing, and optimization. Single-ion COF-based solid electrolytes applied in Li/LiFePO₄ batteries are widely reported. Li et al. [94] constructed several 2D COFs with defect sites through introducing imidazole functional groups and the ion-exchange strategy of Br⁻ and TFSI⁻, reaching a conductivity of 7.05 × 10⁻³ S/cm at 423 K and applied in LiFePO₄-based Li-ion batteries, with a discharge capacity of 441.3 mAh/g and 98.3% capacity over 40 cycles. In addition, for high-voltage Li-ion batteries, Niu et al. [100] prepared a high-voltage stable quinoline aromatic ring COF network, which maintained electrochemical stability at a voltage of 5.6 V (compared with the voltage of Li/Li⁺). After being assembled into an NMC811 battery, the Coulombic efficiency was greater than 99%, and the capacity did not significantly decay after 400 cycles. Wang [101] incorporated plastic crystals into three-dimensional COF and applied the modified solid electrolyte to Li|LiCoO₂ batteries, achieving a capacity of 152 mAh/g and a Coulombic efficiency of 97%. Compared with some inorganic cathode materials, organic cathode materials generally have better cycle stability and can better resist volume changes and structural damage during the insertion/extraction of lithium ions.

An impressive performance of the quasi-solid-state C₆O₆|LiOOC-COF₃|Li battery was demonstrated by its exceptional rate behavior and its excellent cyclic stability, as depicted in Figure 8a. The obtained LiOOC-COF₃-based solid electrolyte effectively avoided the risk of disintegration of organic cathodes in the liquid electrolyte, maintaining a more stable and long-lasting battery system. This illustrates the potential of solid electrolytes for improving the performance and safety of next-generation batteries. The outstanding results of this study highlight the importance of developing new materials and approaches to span the limitations of conventional liquid electrolytes and pave the way for the advancement of high-performance and stable all-solid-state batteries [82]. The anthraquinone (AQ)|LiO₃S-COF₂|Li battery has demonstrated superior rate behaviors and cycling stability compared to the organic AQ|LiTFSI|Li battery, as evidenced by their respective performance curves in Figure 8b. The exceptional cycling stability of the AQ|LiO₃S-COF₂|Li battery is attributed to the restraining effect that it has on the dissolution of the AQ cathode in the liquid electrolyte. This highlights the potential of LiO₃S-COF₂ as a solid electrolyte for enhancing the stability and performance of lithium-ion batteries, ensuring that they can withstand prolonged usage and repeated charging cycles [102]. As shown in Figure 8c, a COF-based semi-solid battery was prepared, which matched the high-voltage system of graphite|LiNi_0.5Mn_1.5O₄. This study indicated that the cycling performance of the anionic COF-based semi-solid electrolyte is superior to that of the standard liquid electrolyte, with capacity retention of 83% after 100 cycles.

These findings suggest that the use of COF-based semi-solid electrolytes has potential for improving the electric output and stability performance of lithium-ion batteries [85]. In addition, the COF-COOH@PP separator helps to improve the CE value (Figure 8d) and discharge capacity, making it a promising candidate for future battery technology. The benefits of the fixed anions and enhanced desolvation, working in tandem, lead to more reliable and consistent lithium deposition, which ultimately translates to a longer cycle life and improved battery functionality. These findings highlight the significance of material design and selection in the development of high-performance batteries [103].

3.3. Other Organic Single-Ion Conducting Polymers

In other organic single-ion conducting polymers, anions form covalent bonds with the polymer main chain or are fixed by interacting with anion receptors, leading to a cation migration number approaching 1, thereby inhibiting the polarization of the anion concentration. At the same time, by designing and synthesizing anions with high negative charge density or introducing strong electron-withdrawing groups to increase the electron domain, the dissociation degree of lithium ions and anions is improved, increasing the ionic conductivity.

A series of lithium-conducting polymer electrolytes derived from singly ionized borate groups is shown in Figure 9a. The resulting methacrylic polymers were investigated for their ionic conductivity, with a focus on the impact of substituent groups bonded on the boron atom. As far as we are aware, the lithium- and borate-based polymers characterized by their flexible and electron-withdrawing substituents have the highest recorded ionic conductivity when applied as a lithium single-ion conducting homopolymer, noted at 1.65 × 10⁻⁴ S cm⁻¹ (at 333 K). Moreover, they have remarkable quantities of transferred lithium (t = 0.93) and a wide electrochemical stability window, reaching 4.2 V against Li⁰/Li⁺. The polymers show potential for use in lithium battery applications [104]. Porcarelli et al. [105] synthesized single-ion conducting polyurethanes (SIPUs) utilizing a uniquely structured ionic liquid monomer (bis-MPTFSI), along with the adaptable chemistry of polyurethanes, as shown in Figure 9b. The resulting SIPU-based gel polymer electrolytes performed extremely well in lithium metal batteries at 293 K, achieving nearly 100% efficiency over 80 cycles. These SIPUs represent a pioneering example of poly(ionic liquid)s based on polyurethane for use in battery science. Gao et al. [106] prepared an anionic polymer network featuring borate anions linked by branched ethylene glycol linkers and tethered into the polymer frame, offering highly selective cation transport and controlled cation conductivity through systematic engineering of segmental mobility (Figure 9c). Furthermore, Figure 9d shows that a lithium-sulfonamide-based functional surface (LiPNP) has been used as an effective electrolyte in the field of lithium metal batteries. When combined with lithium bis(trifluoromethane)sulfonimide and poly(ethylene oxide) (LiTFSI/PEO), the particles prompted a considerable effect, while the membranes managed to maintain good ionic conductivity, marked at 6.6 × 10⁻⁴ S cm⁻¹. This result suggests that all polymer-based nanoparticles can reasonably serve as novel foundational materials for applications in solid-state lithium batteries [107,108].

As we know, some gel electrolytes have already met commercial requirements at higher operating temperatures, but the ionic conductivity of this kind of solid-state electrolyte at room temperature is still low, which limits their practical applications. Also, the compatibility between polymer electrolytes and electrode materials—especially lithium metal—needs urgent optimization. In comparison, single-ion COF-based solid electrolytes have more advantages and future application potential.

Unlike single-ion COF conducting electrolytes, dual-ion COF conducting electrolytes do not contain certain functional groups that can fix cations or anions. Li+ migrates to the active sites of COFs and, simultaneously, anions are transferred to the opposite direction during the charge–discharge process. To increase ionic conductivity, structural optimization and chemical modification were proposed and applied. Zhang et al. [109] and Xu et al. [78] arched oligo(ethylene oxide) chains in COFs applied as solid electrolytes in Li-ion batteries. The COF provided a fast ionic conductive tunnel, and the grafted amorphous oligo(ethylene oxide) chains became a fast ionic conductor transporting Li+ due to its disordered state. The conductivity of this solid-state electrolyte reached 1.33 × 10⁻³ S/cm. Xie et al. [37] provided a method of inserting various organics or polymer chains in one-dimensional COF nanochannels for stability or high ionic conductivity. Xu et al. [110] provided a method for preparing fast ion-conductor transport pathways by tailoring the pore structure of COFs for special requirements and customization of electrolyte chains. Vazquez-Molina et al. [76] studied the effects of mechanically extruded COFs on stability and conductivity, and they prepared anisotropic crystals with high conductivity.

4. Conclusions and Perspectives

Although the basic research of single-ion conducting COF-based electrolytes has made significant progress, their application is still in its infancy. Researchers must develop innovative approaches for the design, synthesis, and characterization of single-ion COF-based solid electrolytes, as well as fostering interdisciplinary collaboration between materials scientists, electrochemists, and engineers to accelerate the development and deployment of these advanced materials. Developing single-ion conducting COF-based solid-state electrolytes poses various challenges that need to be addressed to pave the way for their widespread application in energy storage systems. Some of these challenges include the following:

Achieving high ionic conductivity: High ionic conductivity is essential for efficient charge transfer in electrochemical devices. Modifying the functional groups that form electrostatic interaction or bonding with Li⁺ can improve the ion-conductive sites. In addition, doping conductive polymers or carbon into COFs can open the electron transfer tunnels between different interlayers. Furthermore, inspired by the study predicting that short distance between anionic centers is a critical factor for Li⁺/K⁺ hopping [111], in terms of single-ion structure design, we should minimize the distance between anionic centers to increase the particle conductivity.

Interface compatibility: Ensuring compatibility between solid electrolytes and electrode materials is crucial for optimal device performance. This requires the development of COF-based solid electrolytes that facilitate efficient ion transport and minimize interfacial resistance while maintaining chemical and mechanical stability at the interface. It should be noted that organic cathode materials also have some challenges, such as capacity fading, short cycle life, and other issues. It is worth looking forward to the development of COF-based solid electrolytes and organic cathode materials that will complement one another in the future, alleviating the supply crisis of lithium-ion battery materials caused by the shortage of cobalt [112].

Scalable and cost-effective synthesis: Developing scalable and cost-effective fabrication methods for single-ion COF-based solid electrolytes is essential for their commercial viability. Many current synthesis strategies involve complex procedures and expensive precursors, which can limit their large-scale production and widespread adoption.