High-selectivity flame-retardant covalent-organic framework diaphragm as well as preparation method and application thereof

Abstract

The invention provides a high-selectivity flame-retardant covalent-organic framework diaphragm as well as a preparation method and application thereof. The preparation method comprises the following steps: respectively dissolving an organic aldehyde monomer and a fluorine-containing amine monomer on a benzene ring in an organic phase and a water phase, and carrying out acid catalysis and interfacial polymerization to obtain the fluorine-containing COF membrane. In the initial stage, the monomer firstly generates a fibrous product to be deposited at the bottom, then the fibrous product is transversely connected and grows into a two-dimensional sheet shape, and finally a continuous and compact film is formed on the surface layer. The lower layer of the obtained diaphragm is fibrous, provides strength support for the diaphragm, enhances the wettability of an electrolyte, and promotes ion transmission. And the surface compact layer can improve ion selectivity, so that the battery performance is optimized. The fluorine-containing COF film prepared by the invention has relatively high thermal stability and flame retardance due to the C-F bond, and meanwhile, high-stability lithium plating / stripping behavior can also be realized. The invention is expected to provide an advanced diaphragm material for a high-performance lithium ion battery, so that the performance of the high-performance lithium ion battery is remarkably improved.

Description

Technical Field

[0001] The invention belongs to the field of battery separators, and in particular relates to a highly selective flame-retardant covalent-organic framework separator and a preparation method and application thereof. Background Art

[0002] Currently, the world is competing to research rechargeable battery systems with lower cost and higher energy density. Lithium-ion batteries have high operating voltage (3.6 ~ 3.9 V), high energy density (≥ 200Wh kg -1 ), low self-discharge rate and wide operating temperature range, it has been used as a power battery and applied to pure electric vehicles, becoming an emerging industry in my country. However, with the continuous improvement of battery energy density, frequent thermal runaway accidents have become one of the important issues restricting the development of lithium-ion batteries (LIBs). During the thermal runaway process of the battery, continuous overheating of the internal system will cause the electrolyte to volatilize, and a large amount of gas and heat will be released inside the battery cavity, forming high pressure, which will cause the battery to burn or explode. In order to solve these problems, researchers have been working hard to develop new lithium battery technologies to improve their safety performance. Researchers have realized that the design of the electrolyte and diaphragm inside the lithium battery can effectively reduce the risk of fire and explosion.

[0003] Covalent organic framework (COF) is an organic crystalline material composed entirely of lightweight elements (such as C, H, O, N or B atoms) connected by covalent bonds. It has the characteristics of large specific surface area, high porosity, strong thermal stability and easy functionalization, and is expected to play a role in regulating ion transport in batteries. Although there are many functionalized COF-based separators, few people have introduced flame retardant functions into COF, so it is still challenging to prepare flame retardant COF-based practical separators. The interfacial in situ self-assembly polymerization strategy stands out for its low cost, environmental friendliness and perfect compatibility with existing battery production processes. It can provide COF-based separators with high ion selectivity and flame retardant dual functions, which have excellent electrochemical performance when applied in the battery field. Summary of the invention

[0004] In order to solve the above technical problems, the object of the present invention is to provide a highly selective flame-retardant covalent-organic framework membrane and a preparation method and application thereof.

[0005] In order to achieve the above object, the present invention adopts the following technical solution: A method for preparing a highly selective flame-retardant covalent-organic framework membrane comprises the following steps: (1) Dissolving 1,3,5-trialdehyde phloroglucinol in an organic phase and adding octanoic acid as a catalyst; (2) dissolving phenylenediamine monomers with different substitutions on the benzene ring in an aqueous phase and adding acetic acid as a catalyst; (3) slowly dropping the solution obtained in step (2) onto the surface of the solution obtained in step (1), and reacting at room temperature for several days to obtain a diaphragm; (4) The membrane obtained in step (3) is peeled off from the substrate, washed with dichloromethane, acetone and methanol in sequence, and vacuum dried at 80° C. for 4 h to obtain the highly selective flame-retardant covalent-organic framework membrane.

[0006] Furthermore, the overall aldehyde groups and amine groups undergo a Schiff base condensation reaction at a molar ratio of 1:1.

[0007] Furthermore, the organic phase is dichloromethane, and the volume ratio of the organic phase to the aqueous phase is 1:1.

[0008] Furthermore, in step (1), the concentration of octanoic acid is 3.0 M, and the ratio of octanoic acid to organic phase is 1:25.

[0009] Furthermore, in step (2), the phenylenediamine monomer is one of 1,4-phenylenediamine, 2-fluorobenzene-1,4-diamine, 2,3,5,6-tetrafluoro-1,4-phenylenediamine, 2-trifluoromethyl-1,4-phenylenediamine and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl; and DMF or methanol needs to be added when the 2-fluorobenzene-1,4-diamine, 2,3,5,6-tetrafluoro-1,4-phenylenediamine, 2-trifluoromethyl-1,4-phenylenediamine and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl are dissolved in the aqueous phase.

[0010] Furthermore, in step (2), the volume ratio of water to DMF / methanol is 3:2.

[0011] Furthermore, in step (2), the concentration of acetic acid is 3 M, and the ratio of acetic acid to aqueous phase is 1:25.

[0012] The present invention also provides a highly selective flame-retardant covalent-organic framework diaphragm obtained by the above preparation method.

[0013] The invention discloses an application of a highly selective flame-retardant covalent-organic framework separator in a lithium-ion battery.

[0014] The beneficial effects of the present invention are: This study developed a continuous, uniform and defect-free fluorinated COF membrane. The lower layer of the COF separator is fibrous, which can provide strength support for the separator, while enhancing the wettability of the electrolyte and promoting ion transport. The surface layer presents a dense structure, which enables lithium ions to pass only through the intrinsic ordered micropores of the separator, thereby ensuring the uniformity of lithium ion flux and optimizing battery performance. The CF bond is one of the strongest single bonds, so it can significantly improve the thermal stability and flame retardant properties of the separator, reduce battery thermal runaway and safety risks, and thus extend the battery cycle life. At the same time, Li +Migration number (t Li+ ) is 0.86, which is in sharp contrast to that of the PP separator (0.27), indicating that Li + The transport of LiFePO4 is significantly improved, the migration of anions is restricted, and the functional separator can achieve highly stable lithium plating / stripping behavior. In addition, compared with PP separator, LIBs based on this functional separator (LiFePO4 4 / Li and LiNi 0.8 Co 0.1 Mn 0.1 / Li) (reference CN112209449A a preparation method of lithium ion battery positive electrode material NCM811 and CN119400951A a high-safety electrolyte and lithium ion battery) have been enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 (a) Schematic diagram of the synthesis of TF-COF; (b) Schematic diagram of the microporous crystal structure of TF-COF, in which the pore size of TF-COF is about 9.8 Å, which can allow lithium ions (about 7.6 Å) to pass through; (c) Schematic diagram of the interface growth mechanism of TF-COF membrane, and the enlarged image in the figure shows the different growth stages of TF-COF membrane.

[0016] Figure 2 SEM images of PD-COF membrane; (a) top view; (b) bottom view; (c) cross-sectional view.

[0017] Figure 3 SEM images of FB-COF membrane; (a) top view; (b) bottom view; (c) cross-sectional view.

[0018] Figure 4 SEM images of TF-COF membrane; (a) Top view, the inset shows the flexibility of TF-COF membrane; (b) Bottom view; (c) Cross-sectional view, the inset corresponds to the EDX element distribution map of C, N, O, and F elements; (d) Cross-sectional view shows an enlarged view of the cross-section of the surface dense layer.

[0019] Figure 5 SEM images of DF-COF membrane; (a) top view; (b) bottom view; (c) cross-sectional view.

[0020] Figure 6 SEM images of BF-COF membrane; (a) top view; (b) bottom view; (c) cross-sectional view.

[0021] Figure 7Results of thermal stability and flame retardancy of TF-COF membrane; (a) Pictures of PP membrane and TF-COF membrane at different temperatures for half an hour; (b) corresponding infrared thermal imaging pictures and (c) thermal shrinkage curves; pictures of flame ignition experiments of (d) PP membrane and (e) TF-COF membrane after wetting with 100 μL ester electrolyte.

[0022] Figure 8 Chronoamperometric curves of lithium-lithium symmetric cells based on (a) TF-COF separator and (b) PP separator at a polarization of 10 mV (inset: Nyquist plot before and after polarization).

[0023] Figure 9-11 The electrochemical performance of LIB batteries based on different separators; Fig. 9 (a) Li-Li symmetric cell with TF-COF separator and PP separator: 1 mA / cm 2 , 1 mAh / cm 2 ; (b) Li||Cu battery with TF-COF separator and PP separator: 0.5 mA / cm 2 , 0.5 mAh / cm 2 .

[0024] Fig.10 (a) Long-term cycling performance of NCM811 / separator / Li half-cells with TF-COF separator and PP separator at 0.5 C rate, (b) rate performance.

[0025] Fig.11 Long-term cycling performance of LFP / separator / Li half-cells with TF-COF separator and PP separator at 1 C rate. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. The exemplary implementation modes of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0027] Comparative Example 1 Preparation of PD-COF membrane First, 29.7 mg of 1,3,5-triformylphloroglucinol monomer (TP) was dissolved in 30 mL of dichloromethane, and 1.2 mL of octanoic acid (3.0 M) was added as a catalyst, and solution A was obtained by ultrasonication for 10 minutes. Subsequently, 26.83 mg of 1,4-phenylenediamine monomer was dissolved in 30 mL of aqueous solution, and 1.2 mL of acetic acid (3 M) was added thereto and ultrasonication was performed for 10 minutes to obtain solution B. Solution B was then carefully and slowly added dropwise to the surface of solution A. After reacting at room temperature for 7 days, the PD-COF film was collected at the interface and thoroughly washed with dichloromethane, acetone and methanol in turn. Finally, the film was cut into the required size and dried under vacuum at 80°C for 4 hours for testing.

[0028] Example 1 Preparation of FB-COF membrane First, 32.4 mg of 1,3,5-trialdehyde phloroglucinol monomer was dissolved in 30 mL of dichloromethane, and then 1.2 mL of octanoic acid (3.0 M) was added as a catalyst, and solution A was obtained by ultrasonication for 10 minutes. Then, 34.02 mg of 2-fluorobenzene-1,4-diamine monomer was dissolved in a mixed solution of 30 mL of water and DMF (wherein the volume ratio of water to DMF was 3:2), and 1.2 mL of acetic acid (3 M) was added thereto and ultrasonication was performed for 10 minutes to obtain solution B. Then, solution B was carefully and slowly added dropwise to the surface of solution A. After reacting at room temperature for 7 days, the FB-COF film was collected at the bottom of the culture dish, and was thoroughly washed with dichloromethane, acetone and methanol in turn, and dried under vacuum at 80°C for 4 hours for testing.

[0029] Example 2 Preparation of TF-COF membrane First, 30.375 mg of 1,3,5-trialdehyde phloroglucinol monomer was dissolved in 30 mL of dichloromethane, and then 1.2 mL of octanoic acid (3.0 M) was added as a catalyst, and ultrasonic treatment was performed for 10 minutes to obtain solution A. Subsequently, 42 mg of 2,3,5,6-tetrafluoro-1,4-phenylenediamine (TFDA) monomer was dissolved in 30 mL of a mixed solution of water and DMF (wherein the volume ratio of water to DMF was 3:2), and 1.2 mL of acetic acid (3 M) was added thereto and ultrasonic treatment was performed for 10 minutes to obtain solution B. Then, solution B was carefully and slowly added dropwise to the surface of solution A. After reacting at room temperature for 7 days, the self-supporting TF-COF film was collected at the bottom of the culture dish and thoroughly washed with dichloromethane, acetone and methanol in turn. Finally, the film was cut into the required size and dried under vacuum at 80°C for 4 hours, and then transferred to a glove box for electrochemical testing.

[0030] Example 3 Preparation of DF-COF membrane First, 32.4 mg of 1,3,5-trialdehyde phloroglucinol monomer was dissolved in 30 mL of dichloromethane, and then 1.2 mL of octanoic acid (3.0 M) was added as a catalyst, and ultrasonicated for 10 minutes to obtain solution A. Subsequently, 40.76 mg of 2-trifluoromethyl-1,4-phenylenediamine (2,5-diaminobenzotrifluoride) monomer was dissolved in a mixed solution of 30 mL of water and DMF (wherein the volume ratio of water to DMF was 3:2), and 1.2 mL of acetic acid (3 M) was added thereto and ultrasonicated for 10 minutes to obtain solution B. Then, solution B was carefully and slowly added dropwise to the surface of solution A. After reacting at room temperature for 7 days, the self-supporting DF-COF film was collected at the bottom of the culture dish, and was thoroughly washed with dichloromethane, acetone and methanol in turn, and dried under vacuum at 80°C for 4 hours for testing.

[0031] Example 4 Preparation of BF-COF membrane First, 32.4 mg of 1,3,5-trialdehyde phloroglucinol monomer was dissolved in 30 mL of dichloromethane, and then 1.2 mL of octanoic acid (3.0 M) was added as a catalyst, and ultrasonicated for 10 minutes to obtain solution A. Subsequently, 74.11 mg of 2,2'-bis(trifluoromethyl)benzidine monomer was dissolved in 30 mL of a mixed solution of water and methanol (wherein the volume ratio of water to methanol was 3:2), and 1.2 mL of acetic acid (3 M) was added thereto and ultrasonicated for 10 minutes to obtain solution B. Then, solution B was carefully and slowly added dropwise to the surface of solution A. After reacting at room temperature for 7 days, the self-supporting BF-COF film was collected at the bottom of the culture dish, and was thoroughly washed with dichloromethane, acetone and methanol in turn, and dried under vacuum at 80°C for 4 hours for testing.

[0032] The synthesis process of the fluorinated COF membrane obtained using this method is shown in Figure 1 c. In the initial reaction stage (1-3 days), the monomers are highly reactive, diffuse quickly, and the interface is unstable. Reactions occur immediately upon contact, generating fibrous products that are deposited at the bottom. In the second stage, as the monomers are consumed, the interface gradually stabilizes, and these fibers begin to connect laterally and grow into two-dimensional sheets (4-5 days), eventually forming a continuous and dense film on the surface (6-7 days).

[0033] The microstructure of the COF film was observed by scanning electron microscopy (SEM). Figure 2As shown in a and b, since 1,4-phenylenediamine does not contain CF bonds, the monomer reaction activity is not high, the interface is relatively stable, the reaction mainly proceeds in the interface area, the film does not have a layered structure, the film quality is high, and the surface is smooth and dense. Figure 2 c shows that the thickness of the PD-COF film is 1.75 μm. Subsequently, a CF bond was introduced into the benzene ring of 1,4-phenylenediamine to obtain the surface morphology of the FB-COF film. Figure 3 As shown in a and b, the reactivity of the monomer is improved, and the reaction process corresponds to Figure 1 c, The surface layer is rough and dense, and a layer of fiber membrane is deposited on the bottom. The thickness of the dense layer and the fiber layer are 375 nm and 14 μm, respectively ( Figure 3 c).

[0034] The microstructure of the TFCOF film with four CF bonds introduced into the benzene ring of 1,4-phenylenediamine was also observed. Figure 4 As shown in a, the surface of TF-COF film is flat and dense. Figure 4 Illustration a shows the flexibility of the TF-COF separator. After being bent and rolled with tweezers, the separator still maintains its intact shape, which has a positive impact on the safety performance during battery operation. The lower layer of the separator is fibrous, and the fiber network is cross-linked and connected. It has the characteristics of high porosity and large specific surface area, which can absorb a large amount of electrolyte and provide good ionic conductivity for the battery ( Figure 4 b). The cross-sectional SEM image clearly shows the double-layer structure of the separator ( Figure 4 c, d), where the fiber layer thickness is about 29 μm and the surface layer thickness is about 500 nm. The element distribution map shows the uniform distribution of C, N, O, and F elements on the surface of the TF-COF membrane.

[0035] In order to further verify that this method of preparing COF membranes grown on the interface has strong applicability and universality, we also tried to use trifluoromethyl to replace fluorine to grow COF membranes. The surface morphologies of the obtained DF-COF and BF-COF membranes are shown in Figure 2. Figure 5 , 6, both show a double-layer membrane structure with uniform and dense surface and fibrous bottom. The thickness of the dense layer and the fibrous layer are given in the cross-sectional diagram, respectively. This membrane preparation method is relatively low in cost and has good industrial potential.

[0036] The thermal stability of the diaphragm is closely related to the overall safety of the battery. Figure 7As shown in a and c, the PP diaphragm begins to shrink after being kept at 100°C for 30 minutes, and the shrinkage rate is 17.5% at 120°C. As the temperature rises, the shrinkage phenomenon intensifies. It is worth noting that its color changes from white to transparent at 150°C, which is caused by the thermal closure of the polypropylene material. When the temperature reaches the melting point of the PP diaphragm (165°C), the diaphragm melts and shrinks. At 200°C, the thermal shrinkage rate is as high as 91.8%. In contrast, the TF-COF diaphragm has no obvious changes in color and size in the temperature range of 100°C to 200°C, which is due to its high melting point. Infrared thermal imaging images show ( Figure 7 b) TF-COF separator has a significantly faster heat transfer and more uniform heat distribution. Even when the temperature reaches above 200°C, TF-COF separator does not deform. However, PP separator shrinks locally and then curls and melts severely after being heated. This shows that TF-COF separator has better thermal stability and can better ensure the safety performance of the battery. 6 After wetting in EC:DEC:EMC (V:V:V = 1:1:1), flame ignition experiments were performed on TF-COF and PP membranes. Figure 7 As shown in Figure d, the PP membrane burned violently after contacting the flame and melted rapidly within 1 second. The TF-COF membrane underwent four ignition tests ( Figure 7 e) When it first came into contact with the flame, the electrolyte burned on the surface of the TF-COF membrane, emitting a bright flame. The flame went out after 1.1 seconds, and the membrane remained intact, proving once again that the TF-COF membrane can still maintain dimensional stability at high temperatures. After the second combustion, the flame still went out after 1.1 seconds, which fully verified the flame retardant properties of the TF-COF membrane. The TF-COF membrane was ignited with a lighter, and the third flame continued to burn on the surface for 3 seconds, leaving burn marks on the surface of the TF-COF membrane, which was partially carbonized. After 0.3 seconds of the fourth combustion, the TF-COF membrane burned out. The flame retardancy of the TF-COF membrane is greatly improved compared to that of ordinary PP membranes, with obvious self-extinguishing phenomenon, and no dripping occurred during the entire process.

[0037] In order to study the ion transport properties of TF-COF membranes, the classic Bruce-Vincent method was used to measure t Li+ The results are shown in Table 1. Figure 8 As shown, the PP separator is in ester electrolyte (1M LiPF 6 in EC:DEC:EMC(V:V:V = 1:1:1)) shows a lower t Li+ value (about 0.27). The lithium ion migration number (t Li+) is about 0.86, which is about three times that of PP separator. + The interaction between the two and the screening effect of the natural uniform sub-nanometer channels on ions can significantly accelerate the Li + The rapid transfer of cations results in a higher cation selectivity.

[0038] Application Example 1 In order to study the effect of different separators on lithium deposition, a lithium-li symmetric battery was assembled to conduct an electrochemical lithium plating stripping experiment (refer to CN116130774A, an electrolyte additive, an electrolyte and a lithium-air battery or a lithium-li symmetric battery). Fig. 9 As shown in a, when the current density is 1 mA / cm 2 , the plating capacity is 1 mAh / cm 2 The lithium-lithium symmetric battery equipped with TF-COF separator has a more stable lithium plating and stripping process, and the polarization potential is stable at 50 mV after more than 1000 h, while the polarization potential of the lithium-lithium symmetric battery with PP separator exceeds 86 mV. After plating / stripping for more than 300 h, lithium dendrites pierce the separator and the battery short-circuits.

[0039] Application Example 2 In addition, to evaluate the Coulombic efficiency (CE) of Li plating / stripping in carbonate-based electrolytes, Li||Cu asymmetric cells with PP and TF-COF separators were also assembled, respectively. Fig. 9 As shown in b, the battery using TF-COF separator achieved a Coulombic efficiency of 96.4%, which is better than the Li||Cu battery using PP separator (90.2%).

[0040] Application Example 3 In order to study the cycle stability of the separator used in the battery, the TF-COF separator was further assembled into LiFePO 4 The cycle performance test of NCM811 battery was carried out. Fig.10 As shown in a. In the high voltage charge and discharge range of 3.0-4.5V, the initial discharge capacity of NCM811 / membrane / Li batteries using TF-COF and PP membranes at 0.5C is 197 mAh / g. After 200 cycles, the discharge capacity of the battery using TF-COF membrane is 171.5 mAh / g, and the capacity retention rate is 87.1%, which is much higher than that of the battery using PP membrane. After 200 cycles, the discharge capacity is 136 mAh / g, and the capacity retention rate is 69.0%. The rate performance of the two NCM811 batteries is shown in Fig.10As shown in Figure b, the rate performance of the battery equipped with TF-COF separator is significantly better than that of the battery equipped with PP separator. At a high rate of 5 C, the battery equipped with TF-COF separator not only has a smaller overpotential, but also significantly increases the specific capacity by 61.1 mAh / g compared with the battery equipped with PP separator. 4 ||Separator||The long-term cycling performance of Li batteries at 1 C is shown in Fig.11 As shown, the battery using TF-COF separator maintains 90.4% capacity retention after 1000 cycles, with an average CE as high as 99.69%. In sharp contrast, the battery using PP separator maintains 64.7% capacity retention after 1000 cycles, with an average CE of only 98.97%.

[0041] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing a highly selective flame-retardant covalent-organic framework membrane, characterized in that: The steps include: (1) Dissolving 1,3,5-trialdehyde phloroglucinol in an organic phase and adding octanoic acid as a catalyst; (2) dissolving phenylenediamine monomers with different substitutions on the benzene ring in an aqueous phase and adding acetic acid as a catalyst; (3) slowly adding the solution obtained in step (2) dropwise onto the surface of the solution obtained in step (1), and reacting at room temperature for 7 days to obtain a diaphragm; (4) The membrane obtained in step (3) is peeled off from the substrate, washed with dichloromethane, acetone and methanol in sequence, and vacuum dried at 80° C. for 4 h to obtain the highly selective flame-retardant covalent-organic framework membrane.

2. The preparation method according to claim 1, characterized in that: The overall aldehyde groups and amine groups undergo Schiff base condensation reaction at a molar ratio of 1:

1.

3. The preparation method according to claim 1, characterized in that: The organic phase was dichloromethane, and the volume ratio of the organic phase to the aqueous phase was 1:

1.

4. The preparation method according to claim 1, characterized in that: In step (1), the concentration of octanoic acid is 3.0 M, and the ratio of octanoic acid to organic phase is 1:

25.

5. The preparation method according to claim 1, characterized in that: The phenylenediamine monomer in step (2) is one of 1,4-phenylenediamine, 2-fluorobenzene-1,4-diamine, 2,3,5,6-tetrafluoro-1,4-phenylenediamine, 2-trifluoromethyl-1,4-phenylenediamine and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl; DMF or methanol needs to be added when the 2-fluorobenzene-1,4-diamine, 2,3,5,6-tetrafluoro-1,4-phenylenediamine, 2-trifluoromethyl-1,4-phenylenediamine and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl are dissolved in the aqueous phase.

6. The preparation method according to claim 5, characterized in that: The volume ratio of the aqueous phase to DMF or methanol was 3:

2.

7. The preparation method according to claim 1, characterized in that: The concentration of acetic acid in step (2) is 3 M, and the ratio of acetic acid to aqueous phase is 1:

25.

8. A highly selective flame-retardant covalent-organic framework membrane obtained by the preparation method according to any one of claims 1 to 7.

9. Use of the highly selective flame-retardant covalent-organic framework separator as claimed in claim 8 in lithium-ion batteries.

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