Covalent Bonding of MXene/COF Heterojunction for Ultralong Cycling Li-Ion Battery Electrodes

Covalent organic frameworks (COFs) have emerged as promising renewable electrode materials for LIBs and gained significant attention, but their capacity has been limited by the densely packed 2D layer structures, low active site availability, and poor electronic conductivity. Combining COFs with high-conductivity MXenes is an effective strategy to enhance their electrochemical performance. Nevertheless, simply gluing them without conformal growth and covalent linkage restricts the number of redox-active sites and the structural stability of the composite. Therefore, in this study, a covalently assembled 3D COF on Ti3C2 MXenes (Ti3C2@COF) is synthesized and serves as an ultralong cycling electrode material for LIBs. Due to the covalent bonding between the COF and Ti3C2, the Ti3C2@COF composite exhibits excellent stability, good conductivity, and a unique 3D cavity structure that enables stable Li+ storage and rapid ion transport. As a result, the Ti3C2-supported 3D COF nanosheets deliver a high specific capacity of 490 mAh g−1 at 0.1 A g−1, along with an ultralong cyclability of 10,000 cycles at 1 A g−1. This work may inspire a wide range of 3D COF designs for high-performance electrode materials.


Introduction
In the past few decades, due to the excessive consumption of fossil fuels and the increasing frequency of human activities, the global environment has deteriorated [1][2][3].In response to this, global goals for peak carbon emissions and carbon neutrality have been established, and there is a strong emphasis on promoting the use of green energy such as wind, tidal, and solar power [4][5][6].However, due to the intermittent nature of renewable energy, efficient energy storage systems are needed to store and utilize it.Among the various energy storage systems, LIBs have dominated the market as a power supply system for smart devices, electric vehicles (EVs), and more [7][8][9].Nevertheless, with the rapid evolution of EVs and smart grids, there are higher demands on the performance indicators of LIBs, particularly in terms of energy density and power density [10][11][12].Therefore, there is an urgent need to explore new electrodes with high capacity to meet these increasing requirements [13,14].Currently, the most frequently used anode material for LIBs is graphite, but it has nearly achieved its theoretical specific capacity of 372 mAh g −1 [14,15].The limited specific capacity of the anode material poses a constraint on LIBs to achieve an energy density of 300 Wh kg −1 [16,17].This inability to meet the demands of the new energy market highlights the importance of designing and developing new anode materials that offer both high specific capacity and high-rate performance [18][19][20].
COFs are a new type of polymer constructed using modular organic building blocks linked by strong covalent bonds and have gained attention for their various applications in areas such as gas storage, semiconductors, catalysis, and drug delivery [21][22][23][24].Recently, COFs have shown excellent potential as electrode materials due to their excellent structure and thermal stability.For example, Huang et al. [25] have successfully synthesized a novel Janus dione-based COF with well-organized 2D crystalline structures and abundant redox-active sites which makes it highly suitable as an electrode material for LIBs.The COF-based cathode demonstrated a significant capacity of 338 mAh g −1 at 0.1 C. Another study by Lee et al. [26] involved the fabrication of redox-active COF materials capable of accommodating up to 30 Li + ions, resulting in a remarkable specific capacity of 764.1 mAh g −1 .Both these studies exemplify the potential of COFs as active materials for battery electrodes.However, it is worth noting that despite these advancements, there are still challenges to be addressed.The tightly packed two-dimensional layer structures of COFs often lead to the burial of interior accessible active sites, limiting their full utilization and resulting in reduced reversible capacity [27,28].Additionally, COFs face limitations in terms of electronic conductivity and sluggish lithium diffusion kinetics, which will hinder their broader applications in energy storage [29].Ti 3 C 2 MXene is a two-dimensional layered transition metal carbide material with a graphite-like structure [30][31][32].It is primarily produced through etching of the precursor material Ti 3 AlC 2 [33][34][35].The unique layered structure of Ti 3 C 2 gives it exceptional electrical conductivity, high surface area, and chemical stability, thanks to its specific elemental composition and surface functional groups [36,37].As a result, Ti 3 C 2 has found extensive applications as an active material in energy storage systems.Considering the outstanding properties of Ti 3 C 2 and the limitations of COFs, combining these two materials appears to be a promising approach for fabricating high-performance electrode materials [38,39].By leveraging the conductivity and large surface area of Ti 3 C 2 along with the unique features of COFs, it is possible to develop electrode materials with enhanced performance for various energy storage applications.However, simply gluing them without conformal growth and covalent linkage restricts the number of redox-active sites and the structural stability of the composite [40][41][42][43][44][45].
In this study, a covalently assembled 3D COF on Ti 3 C 2 MXene (Ti 3 C 2 @COF) is synthesized and serves as an ultralong cycling electrode material for LIBs.To achieve this, the COF and Ti 3 C 2 are connected through covalent linking between aldehydes and amino groups, resulting in the formation of a Ti 3 C 2 @COF composite.Due to the covalent bonding between the COF and Ti 3 C 2 , the Ti 3 C 2 @COF composite exhibits excellent stability, good conductivity, and a unique 3D cavity structure that enables stable Li + storage and rapid ion transport.The 3D COFs offer several advantages over their 2D counterparts, including higher specific surface areas and more pore structures.The unique structure of Ti 3 C 2 in the composite plays a key role in shortening the diffusion pathway for ions and electrons and providing a larger surface area for the combination with the 3D COF.Through DFT calculations, it is confirmed that the 3D COF can accommodate four Li + ions within its 3D pore channels and store an additional three Li + ions at the interface between the COF and Ti 3 C 2 .This exceptional Li + storage capability of the Ti 3 C 2 @COF anode is further demonstrated experimentally.The Ti 3 C 2 @COF anode exhibited a high capacity of 490 mAh g −1 at a rate of 0.1 A g −1 and demonstrated an extraordinarily long cycling life, maintaining a capacity of 180 mAh g −1 over 10,000 cycles at a rate of 1 A g −1 .This work expands the diversity of 3D COFs in the field of energy storage, showcasing the potential of 3D COFs as a high-performance electrode material for LIBs.

Results and Discussion
The fabrication of Ti 3 C 2 @COF composite is schematically shown in Figure 1.Briefly, Ti 3 C 2 T x nanosheets were exfoliated from the MAX phase and aminated using 3−aminopropyl triethoxysilane (APTES).Then, the 3D COF layer grew on the amino-grafted Ti 3 C 2 (Ti 3 C 2 -NH 2 ) surface via the covalent linking between the aldehydes and amino groups of the respective monomers to obtain a Ti 3 C 2 @COF nanoarchitecture.Specifically, pre-grafting homologous APTES onto the surface of Ti 3 C 2 nanosheets is crucial for controlling the growth of uniform crystal 3D COF materials.Due to the large number of terminal groups on the surface of Ti 3 C 2 , such as -F, -O, and -OH, hydrogen bonding interactions ensure that the APTES molecules tightly bind to Ti 3 C 2 , resulting in modified Ti 3 C 2 nanosheets with rich -NH 2 groups.The introduction of Ti 3 C 2 is aimed at enhancing the electronic conductivity of the obtained Ti 3 C 2 @COF composite.Notably, the unique 2D lamellar structure of Ti 3 C 2 can shorten the diffusion pathway of ions/electrons and offer more surface area to combine the 3D COF with high Li storage capacity.
The fabrication of Ti3C2@COF composite is schematically shown in Figure 1.Briefly, Ti3C2Tx nanosheets were exfoliated from the MAX phase and aminated using 3−aminopropyl triethoxysilane (APTES).Then, the 3D COF layer grew on the aminografted Ti3C2 (Ti3C2-NH2) surface via the covalent linking between the aldehydes and amino groups of the respective monomers to obtain a Ti3C2@COF nanoarchitecture.Specifically, pre-grafting homologous APTES onto the surface of Ti3C2 nanosheets is crucial for controlling the growth of uniform crystal 3D COF materials.Due to the large number of terminal groups on the surface of Ti3C2, such as -F, -O, and -OH, hydrogen bonding interactions ensure that the APTES molecules tightly bind to Ti3C2, resulting in modified Ti3C2 nanosheets with rich -NH2 groups.The introduction of Ti3C2 is aimed at enhancing the electronic conductivity of the obtained Ti3C2@COF composite.Notably, the unique 2D lamellar structure of Ti3C2 can shorten the diffusion pathway of ions/electrons and offer more surface area to combine the 3D COF with high Li storage capacity.To confirm the morphological changes during the synthesis process, SEM and TEM were conducted on the Ti3C2 and Ti3C2@COF composite, as shown in Figures 2a-d, S2 and S3. Figure 2a shows the morphology of Ti3C2 nanosheets, which reveal a 2D lamellar structure with a large surface area.Figure S2 shows the aminated lamellar Ti3C2 without any particles on the surface.In addition, the EDS elemental mapping of amino-grafted Ti3C2 nanosheets confirms the uniform distribution of C, N, Ti, and O elements, which demonstrates the successful amino groups grafted on the Ti3C2 surface (Figure S3).SEM image of Ti3C2@COF composite demonstrated the homogeneous and rough surface of COFs with granular morphology (Figure 2b).The assembly of 3D COFs on the Ti3C2 surface could be seen in TEM images (Figure 2c,d).Notably, the Ti3C2@COF composite still retains a 2D lamellar structure.This phenomenon demonstrates the grafted amino can firmly anchor 3D COF on the Ti3C2 surface.A high-resolution TEM image of the COF@Ti3C2 composite is shown in Figure 2d.The lattice spacing values of 0.23 and 0.35 nm correspond to the (104) surface of Ti3C2 and the (101) surface of TiO2, respectively.Furthermore, the (104) surface of TiO2 observed in TEM is attributed to the slight oxidation of the Ti3C2 material due to the high-temperature effect.To confirm the morphological changes during the synthesis process, SEM and TEM were conducted on the Ti 3 C 2 and Ti 3 C 2 @COF composite, as shown in Figure 2a-d, Figures S2 and S3. Figure 2a shows the morphology of Ti 3 C 2 nanosheets, which reveal a 2D lamellar structure with a large surface area.Figure S2 shows the aminated lamellar Ti 3 C 2 without any particles on the surface.In addition, the EDS elemental mapping of amino-grafted Ti 3 C 2 nanosheets confirms the uniform distribution of C, N, Ti, and O elements, which demonstrates the successful amino groups grafted on the Ti 3 C 2 surface (Figure S3).SEM image of Ti 3 C 2 @COF composite demonstrated the homogeneous and rough surface of COFs with granular morphology (Figure 2b).The assembly of 3D COFs on the Ti 3 C 2 surface could be seen in TEM images (Figure 2c,d).Notably, the Ti 3 C 2 @COF composite still retains a 2D lamellar structure.This phenomenon demonstrates the grafted amino can firmly anchor 3D COF on the Ti 3 C 2 surface.A high-resolution TEM image of the COF@Ti 3 C 2 composite is shown in Figure 2d.The lattice spacing values of 0.23 and 0.35 nm correspond to the (104) surface of Ti 3 C 2 and the (101) surface of TiO 2 , respectively.Furthermore, the (104) surface of TiO 2 observed in TEM is attributed to the slight oxidation of the Ti 3 C 2 material due to the high-temperature effect.
Figure 2e shows the XRD patterns of Ti 3 C 2 , Ti 3 C 2 -NH 2 , COF, and Ti 3 C 2 @COF composite.The characteristic peak of 9.0 • corresponds to the (002) plane of Ti 3 C 2 .Importantly, the (002) plane corresponding to Ti 3 C 2 in the Ti 3 C 2 -NH 2 composite can be detected in the XRD pattern, which proves that the amino-functionalization process does not affect the structure of Ti 3 C 2 nanosheets.After the hydrothermal reaction, new peaks appeared at 25.3 • , 37.9 • , 47.8 • , 53.8 • , 54.8 • , and 62.5 • corresponding to the (101), (004), ( 200), ( 105), (211), and (304) planes of anatase TiO 2 , respectively.Moreover, the peaks at 36 • and 42 • corresponding to Ti 3 C 2 in the magnified XRD spectra remained, confirming the formation of the COF@Ti 3 C 2 hybrid structure.The surface area and pore volume play a critical role in the electrochemical performance of an electrode.Therefore, the COF and Ti 3 C 2 's specific surface area and pore properties were analyzed using N 2 adsorption-desorption plots, and the obtained data are presented in Figures 2g and S4.The 3D COF shows a high surface area of 546.0 m 2 g −1 and pore volume of 1.0 cm 3 g −1 , but the Ti 3 C 2 shows a low surface area of 6.0 m 2 g −1 and pore volume of 0.03 cm 3 g −1 .Figure 2e shows the XRD patterns of Ti3C2, Ti3C2-NH2, COF, and Ti3C2@COF composite.The characteristic peak of 9.0° corresponds to the (002) plane of Ti3C2.Importantly, the (002) plane corresponding to Ti3C2 in the Ti3C2-NH2 composite can be detected in the XRD pattern, which proves that the amino-functionalization process does not affect the structure of Ti3C2 nanosheets.After the hydrothermal reaction, new peaks appeared at 25.3°, 37.9°, 47.8°, 53.8°, 54.8°, and 62.5° corresponding to the (101), (004), (200), ( 105), (211), and (304) planes of anatase TiO2, respectively.Moreover, the peaks at 36° and 42° corresponding to Ti3C2 in the magnified XRD spectra remained, confirming the formation of the COF@Ti3C2 hybrid structure.The surface area and pore volume play a critical role in the electrochemical performance of an electrode.Therefore, the COF and Ti3C2's specific surface area and pore properties were analyzed using N2 adsorptiondesorption plots, and the obtained data are presented in Figures 2g and S4.The 3D COF shows a high surface area of 546.0 m 2 g −1 and pore volume of 1.0 cm 3 g −1 , but the Ti3C2 shows a low surface area of 6.0 m 2 g −1 and pore volume of 0.03 cm 3 g −1 .
To demonstrate the successful functionalization of MXenes and the successful composite of 3D COF materials, the surface group evolution was examined through ATRFTIR and XPS.The covalent amination of MXene was confirmed by the presence of the NH2 band in FTIR.As depicted in Figure 2f, the appearance of peaks at 1040 and 980 cm −1 corresponding to the Si-O-Si and Ti-O-Si bonds in the Ti3C2 nanosheets confirms the covalent reaction between APTES and MXene.Additionally, the peak at 1617 cm −1 , which is attributed to the bending mode of the N-H bond of the primary amine units, confirms the successful amino-functionalization of the MXene nanosheet.The infrared spectra of 3D COF and Ti3C2@COF are shown in Figure S5.A peak at 1550 cm −1 in the FTIR spectra is associated with the triazine ring structure within the COF material.As shown in Figure S5, the FTIR spectra of the COF and Ti3C2@COF composite exhibit a peak at 1550 cm −1 , indicating the successful synthesis of the COF and Ti3C2@COF composite.To demonstrate the successful functionalization of MXenes and the successful composite of 3D COF materials, the surface group evolution was examined through ATRFTIR and XPS.The covalent amination of MXene was confirmed by the presence of the NH 2 band in FTIR.As depicted in Figure 2f, the appearance of peaks at 1040 and 980 cm −1 corresponding to the Si-O-Si and Ti-O-Si bonds in the Ti 3 C 2 nanosheets confirms the covalent reaction between APTES and MXene.Additionally, the peak at 1617 cm −1 , which is attributed to the bending mode of the N-H bond of the primary amine units, confirms the successful amino-functionalization of the MXene nanosheet.The infrared spectra of 3D COF and Ti 3 C 2 @COF are shown in Figure S5.A peak at 1550 cm −1 in the FTIR spectra is associated with the triazine ring structure within the COF material.As shown in Figure S5, the FTIR spectra of the COF and Ti 3 C 2 @COF composite exhibit a peak at 1550 cm −1 , indicating the successful synthesis of the COF and Ti 3 C 2 @COF composite.Figure 2h  To better understand the redox mechanism of the Ti 3 C 2 @COF electrode, lithiated models of COF with four Li + ions and Ti 3 C 2 @COF with seven Li + ions were first simulated, and the adsorption energy, free energy, and total energy of Li + ions in these models were calculated by DFT calculation, as shown in Figure 3.According to the calculated results, Li + ions most prefer bonds with N or C atoms and insert into the 3D cavity structure of COF material, forming COF + 4Li.The structural evolution of COF with different numbers of Li + ions during the lithiation procedure is shown in Figure 3a.The Ti 3 C 2 @COF composite could receive another 3Li + ions at the interface of COF and Ti 3 C 2 materials, and the structural evolution of Ti 3 C 2 @COF composite during the lithiation procedure is shown in Figure 3c.These results prove that the combination of 3D COF and Ti 3 C 2 materials could offer more active sites to store Li + ions.As shown in Figure 3b, the Ti 3 C 2 @COF shows smaller adsorption energy than that of pure COF.This reveals that the combination of COF and Ti 3 C 2 could speed up Li adsorption in the COF, which will contribute to better rate performance.The free energy of the COF and Ti 3 C 2 @COF electrode with different insert numbers of Li + ions was calculated and shown in Figure S6.During the discharge process, the Gibbs free energy of the COF electrode increases during the initial insertion of the first Li + , but subsequently decreases as more lithium ions are inserted.This indicates that the adsorption of Li + ions on the active site of the COF is initially unstable, but becomes more stable with an increasing number of lithium ion insertions (Figure S6).It is surprising that the Gibbs free energy of the Ti 3 C 2 @COF electrode continuously decreases as Li + ions are inserted during the discharge process.This indicates that Li + ions can stably adsorb onto the composite material.This result demonstrates that the introduction of Ti 3 C 2 enhances the stability of COF in Li + storage (Figure S6c).The total energy for Ti 3 C 2 @COF is −1.263 × 10 5 eV.The energies of the lithiation process for the 3D COF cavity and the heterojunction of COF and Ti 3 C 2 decreased to −1.269 × 10 5 and −1.277 × 10 5 eV after inserting four and three Li + ions (Figure 3d).The total energy shows a downward trend in the lithiation process for the Ti 3 C 2 @COF configuration indicating that Li adsorption on all these steps is thermodynamically favorable.Furthermore, the energy barrier for Li + ion diffusion was calculated, as shown in Figure 3e.The calculated results confirm the low Li + ions diffusion energy barrier of 0.46 eV on the interface of COF and Ti 3 C 2 , compared with a fresh COF electrode (1.37 eV).In summary, DFT calculation confirms that the Ti 3 C 2 can enhance the lithium storage stability of COF electrodes, increase the lithium storage capacity of composite electrodes, and promote the rapid transfer of Li + ions.Notably, the 3D COF layer grew on the amino-grafted Ti 3 C 2 (Ti 3 C 2 -NH 2 ) surface via the covalent linking between the aldehydes and amino groups of the respective monomers to obtain a Ti 3 C 2 @COF nanoarchitecture.Therefore, the COF and Ti 3 C 2 are connected by an amino group, which means there is a spacing for the amino group between COF and Ti 3 C 2 composite.In the process of building the model, to simplify the configuration and accelerate the analysis process, we omitted the amino group and retained a gap between COF and Ti 3 C 2 , which is about 7 Å.In actual situations, there are some amino groups in the spacing between COF and Ti 3 C 2 , which would act as a barrier toward Li diffusion and affect the corresponding adsorption energies to some extent.However, the simplified configuration of COF and Ti 3 C 2 shows a clear trend of promoting Li + transport and enhancing lithium storage capacity, which is consistent with the electrochemical performance testing results of the Ti 3 C 2 @COF nanoarchitecture.Therefore, simplified configurations can to some extent reflect the actual situations of materials.
To illustrate the electrochemical performances of Ti 3 C 2 @COF composite, the EIS, CV, rate, and long-term cycle performance were investigated, as shown in Figure 4.The EIS results show that the Ti 3 C 2 @COF composite has lower charge transfer resistance than the pure Ti 3 C 2 and COF electrodes, which is attributed to the combination of better electroconductivity of Ti 3 C 2 nanosheets and the unique 3D pore structure of COF (Figure 4a). Figure 4b and Figure S7 show the CV test of the Ti 3 C 2 @COF and COF, Ti 3 C 2 electrode at 0.1 mV s −1 .The Ti 3 C 2 @COF electrode shows distinct redox peaks at 1.80 and 2.1 V which are assigned to the reversible lithiation/delithiation reactions.In particular, a prominent peak at 1.8 V during the discharge process of Ti 3 C 2 @COF is attributed to the lithium reaction with the C=N/C-N groups, while the peak at 0.7 V assigned to the insertion of Li + ions into the heterojunction of Ti 3 C 2 and COF materials.Figure S7a shows the CV curves of pure COF anode, the two peaks at 1.5 and 0.6 V ascribed to the reduction reaction of C=N and C-N with Li + .The CV curves of Ti 3 C 2 display two peaks in the first cycle, which is ascribed to an irreversible reaction of the formation of a solid electrolyte interface (SEI) film on the electrode surface (Figure S7b).Compared with the Ti 3 C 2 @COF electrode, the COF electrode shows a weak CV-responsive signal and the Ti 3 C 2 anode has too much irreversible capacity.To illustrate the electrochemical performances of Ti3C2@COF composite, the EIS, CV, rate, and long-term cycle performance were investigated, as shown in Figure 4.The EIS results show that the Ti3C2@COF composite has lower charge transfer resistance than the pure Ti3C2 and COF electrodes, which is attributed to the combination of better electroconductivity of Ti3C2 nanosheets and the unique 3D pore structure of COF (Figure 4a).Figures 4b and S7 show the CV test of the Ti3C2@COF and COF, Ti3C2 electrode at 0.1 mV s −1 .The Ti3C2@COF electrode shows distinct redox peaks at 1.80 and 2.1 V which are assigned to the reversible lithiation/delithiation reactions.In particular, a prominent peak at 1.8 V during the discharge process of Ti3C2@COF is attributed to the lithium reaction with the C=N/C-N groups, while the peak at 0.7 V assigned to the insertion of Li + ions into the heterojunction of Ti3C2 and COF materials.Figure S7a shows the CV curves of pure COF anode, the two peaks at 1.5 and 0.6 V ascribed to the reduction reaction of C=N and C-N with Li + .The CV curves of Ti3C2 display two peaks in the first cycle, which is ascribed to an irreversible reaction of the formation of a solid electrolyte interface (SEI) film on the electrode surface (Figure S7b).Compared with the Ti3C2@COF electrode, the COF electrode shows a weak CV-responsive signal and the Ti3C2 anode has too much irreversible capacity.The specific capacities of prepared electrodes were tested using galvanostatic chargedischarge curves at 1 A g −1 , as demonstrated in Figure 4c.The 3D COF electrode only delivers a low capacity of 24 mAh g −1 , which is attributed to the low electrical conductivity and dense structure of the COF itself.The specific capacity of the Ti3C2 electrode is only 98 mAh g −1 .Such a low specific capacity can be attributed to the low-capacity nature of The specific capacities of prepared electrodes were tested using galvanostatic chargedischarge curves at 1 A g −1 , as demonstrated in Figure 4c.The 3D COF electrode only delivers a low capacity of 24 mAh g −1 , which is attributed to the low electrical conductivity and dense structure of the COF itself.The specific capacity of the Ti 3 C 2 electrode is only 98 mAh g −1 .Such a low specific capacity can be attributed to the low-capacity nature of Ti 3 C 2 .Notably, the Ti 3 C 2 @COF anode delivers a high capacity of 266 mAh g −1 at the same current density, indicating that more redox active sites of Ti 3 C 2 @COF are utilized due to the enhanced conductivity and more exposed 3D cavity structure.Then, the rate capability performance of COF, Ti 3 C 2 , and the Ti 3 C 2 @COF electrodes were investigated over the range from 0.1 to 1 A g −1 (Figure 4d).For the Ti 3 C 2 @COF, average capacities of 490, 380, 312, and 266 mAh g −1 can be obtained under 0.1, 0.2, 0.5, and 1 A g −1 , respectively.In addition, the capacity recovers to nearly 465 mAh g −1 after the current density returns to 0.1 A g −1 , which suggests good electrochemical reversibility of Ti 3 C 2 @COF anode.However, the COF and Ti 3 C 2 electrodes show low capacity and inferior rate performance.Further evidence for the electrochemical stability of Ti 3 C 2 @COF anodes is provided by the similarity in the shape of the charge/discharge curves.This indicates the absence of significant polarization and indirectly demonstrates the excellent transport properties of both Li ions and electrons within the Ti 3 C 2 @COF anodes, as shown in Figure S8.The long-term cycling performance of Ti 3 C 2 @COF anodes was further evaluated at 0.1 C, as shown in Figure 4d.It is worth noting that Ti 3 C 2 @COF anodes still have a high capacity of 355 mAh g −1 even after 1000 cycles.Nevertheless, as can be seen in Figure 4e, the excellent cycling stability of Ti 3 C 2 @COF anodes is also verified by electrochemical measurements at high rates of 1 A g −1 , showing a high capacity of 180 mAh g −1 even after 10,000 cycles, representing one of the best COF-based electrodes in LIBs in terms of both long cycling stability and high capacity.
The CV test at 0.1-0.5 mV s −1 was conducted to investigate the kinetic behavior of Li + storage in the Ti 3 C 2 @COF anodes, as shown in Figure 5. Equations ( 1) and ( 2) were used to express the relationship between the peak current and the sweep rate.Here, i represents the peak current, v stands for the sweep rate, and a and b denote adjustable parameters.
Molecules 2024, 29, x FOR PEER REVIEW 8 of 12 to express the relationship between the peak current and the sweep rate.Here, i represents the peak current, v stands for the sweep rate, and a and b denote adjustable parameters.
We further utilize Cook's analytical method to ascertain the collective impact of pseudo-capacitors across different sweep rates.The total current at a constant potential is segmented into a pseudocapacitive mechanism (k1v) and an ion diffusion process (k2v 1/2 ).The calculation of the pseudocapacitive contribution is determined by the following Equation (4).
As illustrated in Figure 5b-f, the proportion of the capacitive contribution increases as the scan rate increases.This suggests that the insertion/extraction of Li + in the Ti3C2@COF electrode relied on a rapidly kinetic pseudocapacitive process, which improved the rate performance of the Ti3C2@COF electrode.

Preparation of Ti3C2 MXenes
To synthesize Ti3C2Tx MXene, the selective etching of aluminum from Ti3AlC2 is carried out.Firstly, 1 g of LiF is dissolved in 9 mol L −1 hydrochloric acid.Then, 1 g of We further utilize Cook's analytical method to ascertain the collective impact of pseudocapacitors across different sweep rates.The total current at a constant potential is segmented into a pseudocapacitive mechanism (k 1 v) and an ion diffusion process (k 2 v 1/2 ).The calculation of the pseudocapacitive contribution is determined by the following Equation (4).
As illustrated in Figure 5b-f, the proportion of the capacitive contribution increases as the scan rate increases.This suggests that the insertion/extraction of Li + in the Ti 3 C 2 @COF electrode relied on a rapidly kinetic pseudocapacitive process, which improved the rate performance of the Ti 3 C 2 @COF electrode.

Preparation of Ti 3 C 2 MXenes
To synthesize Ti 3 C 2 T x MXene, the selective etching of aluminum from Ti 3 AlC 2 is carried out.Firstly, 1 g of LiF is dissolved in 9 mol L −1 hydrochloric acid.Then, 1 g of Ti 3 AlC 2 is gradually added to the solution, and the mixture is stirred at 35 • C for 24 h.Afterward, the acidic suspension is washed with deionized water through centrifugation at 3500 rpm until the pH of the solution exceeds 6.Following the washing step, the suspension is subjected to ultrasonic dispersion for 2 h, ensuring a uniform dispersion.The mixture is then shaken and subjected to centrifugation, and the supernatant is collected for further use.

Synthesis of COF and Ti 3 C 2 @COF Composite
The next step involves adding 3−aminopropyl triethoxysilane (APTES) to the obtained suspension and stirring it for 12 h.The resulting precipitate is then washed several times with ethanol and ultrapure water.Subsequently, it undergoes freeze-drying to obtain amino−grafted Ti 3 C 2 .The functionalized Ti 3 C 2 is then combined with a solution consisting of melamine (3.5 mg) and terephthalaldehyde (5.5 mg) dissolved in dimethyl sulfoxide (3 mL).This mixture is placed in a 20 mL autoclave and heated to 180 • C for 10 h.After cooling down to room temperature, the Ti 3 C 2 @COF composite is filtered and washed using acetone, tetrahydrofuran, and dichloromethane.The resulting powder, which appears white, is then dried overnight at a temperature of 45 • C in a vacuum oven.Furthermore, pure COF (Schiff base networks (SNW)) was synthesized using the same procedure but without Ti 3 C 2 supernatant.Briefly, melamine (3.5 mg) and terephthalaldehyde (5.5 mg) were dissolved in dimethyl sulfoxide (3 mL) and placed in a 20 mL autoclave.Then, the mixture was heated to 180 • C for 10 h.The synthesis route and chemical structure of COF are shown in Figure S1 in the Supporting Information.

Materials Characterization
The surface morphology of the prepared samples was analyzed using scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) analysis.For this analysis, a FE-SEM (JSM-7001F, JEOL Ltd., Tokyo, Japan) was utilized.Additionally, the samples were examined using transmission electron microscopy (TEM) analysis with a JEM-2100 TEM instrument (JEOL Ltd., Tokyo, Japan).To investigate the structural properties of the samples, X-ray diffraction (XRD) analysis was performed using a D8-ADVANCE instrument (Bruker, Billerica, MA, USA) with Cu Kα radiation.Fourier-transform infrared spectroscopy (FT-IR) spectra were recorded using a JASCO model FT IR-6100 infrared spectrometer (JASCO, Oklahoma City, OK, USA).For further characterization of the samples, X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo Scientific K-Alpha energy spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).The nitrogen adsorption and desorption isotherms were analyzed using an ASAP 2460 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA) at a temperature of 77 K.

Electrochemical Characterization
The electrochemical properties were conducted by a CR2016 coin-type cell in a glove box.The working electrodes were prepared by mixing 80 wt% active materials, 10 wt% Super P, and 10 wt% poly(vinylidene fluoride) (PVDF).Coin-type test cells were assembled in an argon-filled glove box with Li foil as the counter electrode, Celgard 2400 as the separator, and 1 M LiPF 6 in ethylene carbonate/dimethyl carbonate (EC/DMC vol 1:1) as the electrolyte.Cyclic voltammetry (CV) (0.01-3 V, 0.1 mV s −1 ) and electrochemical impedance spectroscopy (EIS) (0.01-100,000 Hz, 5 mV) were conducted using a CHI660E electrochemical workstation.

DFT Calculations
All DFT calculations employing spin polarization were carried out using the Vienna Ab Initio Simulation Package (VASP 6.1.0)[46].The Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA) was employed to represent electron exchange-correlation, and the projector augmented wave (PAW) method was used to define the pseudo-potentials [47].The structure model of COF is built from crystalline structures in the Inorganic Crystal Structure Database (ICSD) [48].A 5 × 5 × 1 supercell of the Ti 3 C 2 O 2 (001) facet was utilized.The Brillouin zones were sampled using the Monkhorst-Pack scheme with a 3 × 3 × 1 k-point grid.The DFT-D3 correction with the Grimme scheme was applied to consider the dispersion interaction [49].The energy profiles of the barrier for Li + ion diffusion on the interface of COF and Ti 3 C 2 and the surface of COF were simulated by the climbing-image nudged elastic band (CI-NEB) method implemented in the VASP [50].In this work, the Ti 3 C 2 is obtained by selectively etching Al element from the precursor Ti 3 AlC 2 in a hydrofluoric acid-containing solution.This method inevitably introduces various content-uncontrollable terminations on the surface of Ti 3 C 2 , such as -O, -OH, and -F.Therefore, choosing Ti 3 C 2 O 2 as the structure model to simulate the lithiation procedures and Li + diffusion in the Ti 3 C 2 @COF electrode is closer to the real environment, a more realistic reflection of the transport and embedding process of Li + in electrodes.
The Li adsorption energy (E ads ) is calculated by the following Equation ( 5): where E tb and E ta are the total energies of COF (or Ti 3 C 2 @COF) and a Li-adsorbed COF (or Ti 3 C 2 @COF), respectively.E isolated Li is the energies of an isolated Li atom and n is the number of adsorbed Li atoms.

Conclusions
In conclusion, a Ti 3 C 2 @COF electrode for LIBs was prepared by covalent bonding which possesses high surface areas and abundant pore channels, making it well-suited for storing and transferring Li ions.The unique structural advantages of 3D COFs, combined with the robust skeleton of Ti 3 C 2 , allow a Ti 3 C 2 @COF electrode to obtain high capacities, excellent rate performance, high utilization of the active site, and good cyclability.This represents one of the best overall performances among the COF-based electrodes reported thus far.The pathway for the storage of Li + in the Ti 3 C 2 @COF electrode has been elucidated through a series of electrochemical analyses and DFT calculations.The Ti 3 C 2 -supported 3D COF nanosheets deliver a high specific capacity of 490 mAh g −1 at 0.1 A g −1 between 0.01-3 V, which could offer a much higher energy density than the commercial graphite anode.However, the preparation procedure of the Ti 3 C 2 @COF anode needs to be further improved and simplified, due to the complex preparation process of Ti 3 C 2 nanosheets will increase the cost of large-scale production.This work would inspire a wide range of 3D COF designs for high-performance electrode materials.These results not only present a new 3D COF anode with better electrochemical performances but also provide a platform for designing and manufacturing high-performance electrode materials for energy storage.

Figure 1 .
Figure 1.Schematic illustration of the synthesis of Ti3C2@COF composite.

Figure 1 .
Figure 1.Schematic illustration of the synthesis of Ti 3 C 2 @COF composite.
shows the XPS wide spectra of Ti 3 C 2 -NH 2 and Ti 3 C 2 , which confirm the appearance of N and Si elements in the Ti 3 C 2 -NH 2 .This result is consistent with FTIR spectra, confirming the success of the amination process.The C1s core level spectra resolving results reveal the C-Ti bond from Ti 3 C 2 nanosheets and the C-F and C-O bonds which derive from the etching process.The -NH 3 , -NH/NH 2 , and N-C bonds in N 1s and C1s correspond to the amino group grafted on the COF surface.The N=C bond in N 1s and C 1s proves the 3D COF and Ti 3 C 2 incorporated successfully.The above evidence indicates the successful production of Ti 3 C 2 @COF composite.

Molecules 2024 , 12 Figure 3 .
Figure 3. Structural evolution of (a) COF and (c) Ti3C2@COF electrodes during the lithiation procedures.(b) The Li adsorption energies in the lithiation process for the COF and Ti3C2@COF.(d) Proposed lithiation pathway for the Ti3C2@COF electrode.The right axis shows the redox potential versus Li + /Li, and the left axis shows the total energy of various lithiated Ti3C2@COF structures.(e) Energy profiles of the barrier for Li + ion diffusion on the interface of COF and Ti3C2 and the surface of COF.

Figure 3 . 12 Figure 4 .
Figure 3. Structural evolution of (a) COF and (c) Ti 3 C 2 @COF electrodes during the lithiation procedures.(b) The Li adsorption energies in the lithiation process for the COF and Ti 3 C 2 @COF.(d) Proposed lithiation pathway for the Ti 3 C 2 @COF electrode.The right axis shows the redox potential versus Li + /Li, and the left axis shows the total energy of various lithiated Ti 3 C 2 @COF structures.(e) Energy profiles of the barrier for Li + ion diffusion on the interface of COF and Ti 3 C 2 and the surface of COF.Molecules 2024, 29, x FOR PEER REVIEW 7 of 12

Figure 5 .
Figure 5. (a) CV curves of Ti3C2@COF anode at different scan rates.(b-f) The calculated proportion of diffusion and capacitive contribution of Ti3C2@COF anode at different scan rates.

Figure 5 .
Figure 5. (a) CV curves of Ti 3 C 2 @COF anode at different scan rates.(b-f) The calculated proportion of diffusion and capacitive contribution of Ti 3 C 2 @COF anode at different scan rates.