A thermosetting high-conductivity flexible graphite composite bipolar plate and its preparation method

Abstract

The present invention belongs to the field of liquid flow batteries, and specifically discloses a thermosetting high-conductivity flexible graphite composite bipolar plate and a preparation method thereof. The present invention solves the technical problems of increased contact resistance with a diffusion layer caused by resin precipitation to the surface of the plate during hot pressing, and local foaming or expansion caused by different thermal effects of the two during the hot pressing process by using an adhesion promoter that exhibits high-viscoelastic worm-like micelles at high temperature, thereby obtaining a high-conductivity composite bipolar plate; and also obtains a high-conductivity composite bipolar plate by performing an amination treatment on natural flake graphite so that it can be better combined with a chemical intercalation agent, thereby reducing the heat consumption of a kiln, improving production efficiency, and reducing costs.

Description

Technical Field

[0001] The present invention belongs to the field of flow batteries, and particularly relates to a thermosetting high-conductivity flexible graphite composite bipolar plate and a preparation method thereof. Background Art

[0002] For flow batteries, the bipolar plate mainly plays the roles of current collection and conduction, as well as isolating the positive and negative electrolyte solutions and providing mechanical support for the electrodes. Bipolar plates are divided into rigid graphite bipolar plates, carbon-plastic composite bipolar plates, metal bipolar plates, etc. On the one hand, rigid graphite plates have good thermal conductivity, electrical conductivity and corrosion resistance; on the other hand, they also have disadvantages such as high curing temperature, easy shrinkage and bending deformation, high production cost, and easy breakage during assembly. Although carbon-plastic composite plates have the advantage of low production cost, their electrical conductivity is only 10-15 S / CM. Ordinary metal bipolar plates have poor corrosion resistance, while those with good corrosion resistance such as gold, platinum, and titanium are relatively expensive. Compared with rigid graphite materials, carbon-plastic composite plates and various metal materials, flexible graphite composite bipolar plates not only retain the good thermal conductivity and electrical conductivity of rigid graphite plates, but also overcome the brittleness of rigid graphite. They also have outstanding advantages in terms of processing difficulty and cost. The preparation process of flexible graphite composite bipolar plates usually involves mixing conductive carbon materials (such as graphite, carbon black, carbon fiber, etc.) with polymer polymers (such as polyethylene PE, polyvinylidene fluoride PVDF, polypropylene PP, etc.), drying, and rolling into shape, so that they have both electrical conductivity and good mechanical properties.

[0003] Since the raw materials of graphite composite bipolar plates are mainly divided into graphite and thermosetting resins, in order to improve the electrical conductivity of the bipolar plate, it is necessary to increase the graphite content in the bipolar plate; while good airtightness and bending and compressive strength indicators require the bipolar plate to increase the resin content, which leads to a contradiction in the content ratio of graphite and resin. Those skilled in the art usually improve the electrical conductivity of graphite composite bipolar plates by adjusting the composition and formula of graphite and resin, or adding other conductive materials. Conductive materials generally include carbon black, graphite particles, and carbon nanotubes, etc. Because they have excellent electrical conductivity themselves, they can optimize the overall performance of the composite material to the greatest extent.

[0004] As disclosed in Patent CN116444940A, a preparation method of expanded graphite worm and resin prepolymer material is provided. Natural flake graphite is subjected to oxidation intercalation and high-temperature expansion to obtain expanded graphite worms. After mixing, the resin powder exists in the expanded graphite worms in a granular state. The expanded graphite used has a highly active surface, high chemical stability, high thermal and electrical conductivity, and its structural stability can better limit structural changes such as the volume expansion of the electrode material during the cycling process, thereby effectively improving the cycle life of the battery. Although this method has a friendly operating environment and will not cause secondary environmental pollution due to solvents, since no organic solvents are used in this method, the resin may overflow to the surface during hot pressing, affecting the conductivity of the finished product. Summary of the Invention

[0005] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a thermosetting highly conductive flexible graphite composite bipolar plate and its preparation method. Through air convection, uniform mixing of carbon materials and thermosetting resin is achieved, and the problem of resin overflowing to the surface during hot pressing is solved by a special adhesive, improving the conductivity and facilitating the demolding of the bipolar plate.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] On the one hand, the present invention provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate, comprising the following steps: mixing a carbon-based mixture, a nano-scale thermosetting resin, and an adhesive by air convection to obtain a mixture; spreading the powder and pre-pressing to obtain a blank plate; then molding the blank plate by die pressing to obtain the thermosetting highly conductive flexible graphite composite bipolar plate.

[0008] Wherein, the carbon-based mixture includes modified graphite worms, carbon fibers, and multi-walled carbon nanotubes.

[0009] In some embodiments, the specific operation of the die pressing is as follows: first, hot press at a hot plate temperature of 240 - 280 °C and a pressure of 8 - 14 MPa for 3 - 5 min; then hot press at a hot plate temperature of 180 - 220 °C and a pressure of 15 - 22 MPa for 2 - 4 min; then, at this temperature, mold the flow channels; finally, cool to room temperature and demold.

[0010] In some embodiments, the mass ratio of the carbon-based mixture, the nano-scale thermosetting resin, and the adhesive is 1:(0.2 - 0.3):(0.07 - 0.12).

[0011] In some embodiments, the mass ratio of the modified graphite worms, carbon fibers, and multi-walled carbon nanotubes is 1:(0.1 - 0.2):(0.05 - 0.15).

[0012] In some embodiments, the preparation steps of the modified graphite worm are as follows:

[0013] S1. Disperse natural flake graphite in dimethylformamide, add 1-ethyl-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, stir at 0-8 °C for 1-2 h, then add diethylenetriamine, and continue to stir at 160-200 °C for 16-20 h. After stirring, wash, filter by suction, and vacuum freeze-dry to obtain aminated flake graphite;

[0014] S2. Mix the aminated flake graphite obtained in step S1 with a chemical intercalating agent, stir and react, then hydrolyze, wash, and dry to obtain intercalated graphite. Heat-treat at 850-950 °C for 10-15 s to obtain the modified graphite worm.

[0015] In some preparation methods, in step S2, the chemical intercalating agent includes KMnO 4 and 75 wt% sulfuric acid.

[0016] Preferably, the mass ratio of the aminated flake graphite, KMnO 4 and 75 wt% sulfuric acid is 1:(0.15-0.25):(5-6).

[0017] The present invention prepares graphite worms by an intercalation and high-temperature expansion method, that is, mixing a chemical intercalating agent and flake graphite, and obtaining graphite worms after high-temperature heat treatment. This method can be used for industrial continuous large-scale production with a special expansion kiln, but the energy consumption is high. In order to reduce costs, the present invention conducts amination treatment on natural flake graphite before heat treatment. The aminated flake graphite is grafted with diethylenetriamine between layers, which can better combine with the chemical intercalating agent, reduce the heat consumption of the kiln when the graphite expands, improve production efficiency, and reduce costs.

[0018] In some embodiments, the preparation steps of the tackifier are as follows:

[0019] Stir the sodium ethoxide solution and imidazole evenly. Under nitrogen protection, add hexadecyl bromide, and stir at 90-100 °C for 40-50 h. Extract, collect the upper organic layer, wash with distilled water, dry with anhydrous sodium sulfate, filter to obtain a filtrate. Add anhydrous ethanol and 1,2-dibromoethane to the filtrate, stir at 90-100 °C for 40-50 h, rotary evaporate, recrystallize, and dry to obtain the tackifier.

[0020] In some embodiments, the molar ratio of the imidazole to the hexadecyl bromide is 1:(1.2-1.8).

[0021] In some embodiments, the molar ratio of the imidazole to the 1,2-dibromoethane is 1:(0.5-0.7).

[0022] Due to the phenomenon that resin precipitates and accumulates on the surface of the electrode plate during hot pressing, which will lead to technical problems such as a decrease in the conductivity of the electrode plate, an increase in the contact resistance with the diffusion layer, and local foaming or swelling caused by different thermal effects during the hot pressing process. The tackifier provided by the present invention exhibits highly viscoelastic worm-like micelles at high temperatures. These worm-like micelles are held together by physical interactions. As the pressure and temperature change, the physical interactions continuously break and recombine, entangling with the thermosetting resin and the carbon-based mixture in an instantaneous network, endowing the mixture with certain viscoelasticity, alleviating the problems caused by resin precipitation, and effectively solving the problem of inconvenient demolding of the composite electrode plate.

[0023] On the other hand, the present invention provides a thermosetting high-conductive graphite bipolar plate obtained by the above preparation method, and the conductivity of the composite electrode plate is ≥300 S / cm.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention realizes the uniform mixing of carbon materials, thermosetting resins, and tackifiers through air convection, and solves the problem of resin overflowing to the surface during hot pressing through a special tackifier, improving the conductivity and making the bipolar plate easy to demold.

[0026] 2. The present invention uses an intercalation method and high-temperature expansion to prepare graphite worms. By aminating natural flake graphite, it can better combine with chemical intercalating agents, reducing the heat consumption of the kiln during expansion, improving production efficiency, and reducing costs.

[0027] 3. The tackifier provided in the present application exhibits highly viscoelastic worm-like micelles at high temperatures. These worm-like micelles are entangled with the thermosetting resin and the carbon-based mixture in an instantaneous network, and the graphite worms in the carbon-based mixture can strengthen the entanglement effect. The two cooperate to endow the mixture with certain viscoelasticity, alleviating the problems caused by resin precipitation, and effectively solving the problem of inconvenient demolding of the composite electrode plate. Specific Embodiments

[0028] The following will illustrate the present invention in conjunction with specific implementation schemes. It should be noted that the following examples are examples of the present invention, only used to illustrate the present invention, and not used to limit the present invention. Other combinations and various improvements within the concept of the present invention can be made without departing from the main idea or scope of the present invention.

[0029] To facilitate those skilled in the art to implement the present invention, some raw materials and manufacturers of the examples and comparative examples are described as follows: Polyimide is purchased from Shenzhen Zhibang Technology Co., Ltd.

[0030] Preparation Example 1

[0031] The preparation steps of tackifier - A are as follows:

[0032] Stir 30 mL of 0.1 mol / L sodium ethoxide solution and 1 mol of imidazole evenly. Under nitrogen protection, add 1.5 mol of hexadecyl bromide, stir at 95 °C for 48 h, extract, collect the upper organic layer, wash twice with distilled water, dry with anhydrous sodium sulfate, filter to obtain a filtrate. Add 800 mL of absolute ethanol and 0.6 mol of 1,2-dibromoethane to the filtrate, stir at 95 °C for 48 h, rotary evaporate, recrystallize, and dry to obtain the tackifier-A.

[0033] Preparation Example 2

[0034] The preparation steps of the tackifier-B are as follows:

[0035] The difference between this preparation example and Preparation Example 1 is that 1.1 mol of hexadecyl bromide is used.

[0036] Preparation Example 3

[0037] The preparation steps of the tackifier-C are as follows:

[0038] The difference between this preparation example and Preparation Example 1 is that 1.9 mol of hexadecyl bromide is used.

[0039] Preparation Example 4

[0040] The preparation steps of the tackifier-D are as follows:

[0041] The difference between this preparation example and Preparation Example 1 is that 0.45 mol of 1,2-dibromoethane is used.

[0042] Preparation Example 5

[0043] The preparation steps of the tackifier-E are as follows:

[0044] The difference between this preparation example and Preparation Example 1 is that 0.75 mol of 1,2-dibromoethane is used.

[0045] Preparation Example 6

[0046] The preparation steps of the modified graphite worm-A are as follows:

[0047] S1. Disperse 4 g of natural flake graphite in 500 mL of dimethylformamide, add 0.1 g of 1-ethyl-(3-dimethylaminopropyl) carbodiimide and 0.1 g of N-hydroxysuccinimide, stir at 5 °C for 2 h, then add 25 ml of diethylenetriamine; continue to stir at 180 °C for 18 h. After stirring, wash with distilled water, filter by suction, and vacuum freeze-dry at -50 °C to constant weight to obtain aminated flake graphite;

[0048] S2. The aminated flake graphite obtained in step S1, KMnO 4Mix with 75 wt% sulfuric acid at a mass ratio of 1:0.2:5.5, stir and react for 60 min, then hydrolyze for 30 min, wash until the pH value is 5 - 7, and then dry until the moisture content ≤ 1.0% to obtain intercalated graphite. Heat-treat at 900 °C for 12 s to obtain modified graphite worm - A.

[0049] Preparation Example 7

[0050] The preparation steps of modified graphite worm - B are as follows:

[0051] The difference between this preparation example and Preparation Example 6 is that: the aminated flake graphite obtained in step S1 and KMnO 4 are mixed at a mass ratio of 1:0.2.

[0052] Preparation Example 8

[0053] The preparation steps of modified graphite worm - C are as follows:

[0054] The difference between this preparation example and Preparation Example 6 is that: the aminated flake graphite obtained in step S1 and 75 wt% sulfuric acid are mixed at a mass ratio of 1:5.5.

[0055] Preparation Example 9

[0056] The preparation steps of graphite worm are as follows:

[0057] Mix natural flake graphite, KMnO 4 and 75 wt% sulfuric acid at a mass ratio of 1:0.2:5.5, stir and react for 60 min, then hydrolyze for 30 min, wash until the pH value is 5 - 7, and then dry until the moisture content ≤ 1.0% to obtain intercalated graphite. Heat-treat at 900 °C for 12 s to obtain graphite worm.

[0058] Example 1

[0059] A preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate comprises the following steps:

[0060] Use an air convection device to mix a carbon-based mixture, polyimide and adhesive - A with a mass ratio of 1:0.25:0.1 to obtain a mixture; spread the powder and pre-press to obtain a blank plate; then mold the blank plate, first hot-press at a hot plate temperature of 260 °C and a pressure of 10 MPa for 4 min; then hot-press at a hot plate temperature of 200 °C and a pressure of 18 MPa for 3 min; then mold the flow channel at this temperature; finally cool to room temperature and demold to obtain the graphite composite bipolar plate;

[0061] Among them, the carbon-based mixture contains modified graphite worm - A, carbon fiber and multi-walled carbon nanotube with a mass ratio of 1:0.15:0.1.

[0062] Example 2

[0063] A preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate, comprising the following steps:

[0064] Using air convection to mix a carbon-based mixture, polyimide and adhesive-A with a mass ratio of 1:0.2:0.07 to obtain a mixture; spreading the powder and pre-pressing to obtain a blank plate; then molding the blank plate by pressing, first hot-pressing at a hot plate temperature of 240 °C and a pressure of 14 MPa for 5 min; then hot-pressing at a hot plate temperature of 180 °C and a pressure of 22 MPa for 4 min; then molding the flow channels at this temperature; finally cooling to room temperature and demolding to obtain the graphite composite bipolar plate;

[0065] Among them, the carbon-based mixture contains modified graphite worm-A, carbon fiber and multi-walled carbon nanotube with a mass ratio of 1:0.1:0.05.

[0066] Example 3

[0067] A preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate, comprising the following steps:

[0068] Using air convection to mix a carbon-based mixture, polyimide and adhesive-A with a mass ratio of 1:0.3:0.12 to obtain a mixture; spreading the powder and pre-pressing to obtain a blank plate; then molding the blank plate by pressing, first hot-pressing at a hot plate temperature of 280 °C and a pressure of 8 MPa for 3 min; then hot-pressing at a hot plate temperature of 220 °C and a pressure of 15 MPa for 2 min; then molding the flow channels at this temperature; finally cooling to room temperature and demolding to obtain the graphite composite bipolar plate;

[0069] Among them, the carbon-based mixture contains modified graphite worm-A, carbon fiber and multi-walled carbon nanotube with a mass ratio of 1:0.2:0.15.

[0070] Example 4

[0071] This example provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation method is the same as that of Example 1, except that adhesive-A is replaced by an equal amount of adhesive-B.

[0072] Example 5

[0073] This example provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation method is the same as that of Example 1, except that adhesive-A is replaced by an equal amount of adhesive-C.

[0074] Example 6

[0075] This embodiment provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation manner is the same as that of Embodiment 1, except that the adhesive - A is replaced by an equal amount of adhesive - D.

[0076] Embodiment 7

[0077] This embodiment provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation manner is the same as that of Embodiment 1, except that the adhesive - A is replaced by an equal amount of adhesive - E.

[0078] Embodiment 8

[0079] This embodiment provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation manner is the same as that of Embodiment 1, except that the modified graphite worm - A is replaced by an equal amount of modified graphite worm - B.

[0080] Embodiment 9

[0081] This embodiment provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation manner is the same as that of Embodiment 1, except that the modified graphite worm - A is replaced by an equal amount of modified graphite worm - C.

[0082] Embodiment 10

[0083] This embodiment provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation manner is the same as that of Embodiment 1, except that the carbon - based mixture contains modified graphite worm - A and carbon fiber with a mass ratio of 1:0.15.

[0084] Embodiment 11

[0085] This embodiment provides a preparation method of a thermosetting highly conductive flexible graphite composite bipolar plate. The specific implementation manner is the same as that of Embodiment 1, except that the carbon - based mixture contains modified graphite worm - A and multi - walled carbon nanotubes with a mass ratio of 1:0.1.

[0086] Comparative Example 1

[0087] This comparative example provides a preparation method of a low - resistance carbon - plastic composite bipolar plate. The specific implementation manner is the same as that of Embodiment 1, except that the modified graphite worm - A is replaced by an equal amount of graphite worm.

[0088] Performance Test:

[0089] 1. Conductivity Test

[0090] The conductivity of the composite bipolar plate is tested by a four - probe low - resistance tester. During the measurement, specimen deformation and dust on the specimen surface should be avoided, and the average value is taken after repeating 5 times.

[0091] 2. Flexural Strength

[0092] Using a universal mechanical testing machine, the three-point bending test was carried out on the composite plates provided in each example according to the standard NB / T42007-2013 to measure the flexural strength of the samples. The loading speed was 0.5 mm / min. The size of the composite plate was 100 mm in length × 10 mm in width × 0.8 mm in thickness, and the span was 64 mm.

[0093] The results are shown in Table 1.

[0094] Table 1 Performance test results

[0095] Conductivity S / cm Flexural strength (MPa) Example 1 457 76.4 Example 2 423 75.9 Example 3 431 76.1 Example 4 395 67.6 Example 5 409 68.0 Example 6 387 67.2 Example 7 401 67.8 Example 8 382 68.4 Example 9 398 68.7 Example 10 412 63.7 Example 11 417 64.3 Comparative Example 1 368 65.8

[0096] It can be seen from the data in Table 1 that the graphite composite bipolar plates of Examples 1-3 have high conductivity and flexural strength; compared with Example 1, the amount of cetyl bromide in the adhesive promoter was changed in Examples 4-5, and the amount of cetyl bromide in the adhesive promoter was changed in Examples 6-7. These two changes may affect the composition of the adhesive promoter molecules, which is not conducive to the formation of worm-like micelles at high temperatures, and cannot effectively inhibit the precipitation of resin during hot pressing, resulting in a decrease in the conductivity of the graphite composite bipolar plate.

[0097] Compared with Example 1, the formula of the chemical intercalating agent in the modified graphite worms was changed in Examples 8-9, and the graphite worms in Comparative Example 1 were not aminated before expansion. Both of these changes will affect the transformation of flake graphite into expanded graphite worms, resulting in a decrease in conductivity. It can be seen from Example 1 and Examples 10-11 that the compounding of the types of carbon materials in the carbon-based mixture can improve the conductivity and flexural strength of the graphite composite bipolar plate.

[0098] The above-mentioned examples and comparative examples do not impose any formal restrictions on the present invention. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the above-disclosed technical content without departing from the technical solution of the present invention. However, as long as it does not depart from the technical solution content of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a thermosetting high-conductivity flexible graphite composite bipolar plate, characterized in that: The method comprises the following steps: mixing a carbon-based mixture, a nano-scale thermosetting resin and an adhesion promoter by air convection to obtain a mixture; spreading powder and pre-pressing to obtain a blank plate; and then compression molding the blank plate to obtain the thermosetting high-conductivity flexible graphite composite bipolar plate. Wherein, the carbon-based mixture comprises modified graphite worms, carbon fibers and multi-walled carbon nanotubes; The specific operation of the compression molding is: first, hot pressing for 3-5 minutes at a hot plate temperature of 240-280°C and a pressure of 8-14MPa; then, hot pressing for 2-4 minutes at a hot plate temperature of 180-220°C and a pressure of 15-22MPa; then, at this temperature, molding out the flow channel; finally, cooling to room temperature and demolding; The mass ratio of the carbon-based mixture, the nanoscale thermosetting resin, and the adhesion promoter is 1:(0.2-0.3):(0.07-0.12); The preparation steps of the modified graphite worms are as follows: S1. Disperse natural flake graphite in dimethylformamide, add 1-ethyl-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide, stir at 0-8°C for 1-2h, then add diethylenetriamine, continue stirring at 160-200°C for 16-20h, wash, filter and vacuum freeze-dry after stirring to obtain aminated flake graphite; S2, mixing the aminated flake graphite obtained in step S1 and the chemical intercalation agent, stirring for reaction, then hydrolyzing, washing, and drying to obtain intercalated graphite, and heat treating at 850-950° C. for 10-15 seconds to obtain the modified graphite worms; The preparation steps of the adhesion promoter are as follows: The sodium ethoxide solution and imidazole are stirred evenly, and under the protection of nitrogen, hexadecane bromide is added, and the mixture is stirred at 90-100° C. for 40-50 hours, extracted, and the upper organic layer is collected, washed with distilled water, dried over anhydrous sodium sulfate, filtered to obtain a filtrate, anhydrous ethanol and 1,2-dibromoethane are added to the filtrate, and the mixture is stirred at 90-100° C. for 40-50 hours, rotary evaporated, recrystallized, and dried to obtain the adhesion promoter.

2. The method for preparing a thermosetting high-conductivity flexible graphite composite bipolar plate according to claim 1, characterized in that: The mass ratio of the modified graphite worms, carbon fibers and multi-walled carbon nanotubes is 1:(0.1-0.2):(0.05-0.15).

3. The method for preparing a thermosetting high-conductivity flexible graphite composite bipolar plate according to claim 1, characterized in that: In step S2, the chemical intercalant comprises KMnO4 and 75 wt% sulfuric acid.

4. The method for preparing a thermosetting high-conductivity flexible graphite composite bipolar plate according to claim 1, characterized in that: The molar ratio of the imidazole to the hexadecane bromide is 1:(1.2-1.8).

5. The method for preparing a thermosetting high-conductivity flexible graphite composite bipolar plate according to claim 1, characterized in that: The molar ratio of the imidazole to 1,2-dibromoethane is 1:(0.5-0.7).

6. A thermosetting high-conductivity flexible graphite composite bipolar plate obtained by the preparation method according to any one of claims 1 to 5, characterized in that: The electrical conductivity of the composite bipolar plate is ≥300 S / cm.