Integrally-formed bipolar plate preparation process

Abstract

The invention belongs to the technical field of battery assembly manufacturing, and discloses a preparation process of an integrally formed bipolar plate. The preparation process comprises the following steps: mixing graphite powder, graphene sheets and carbon nanotubes; modifying the composite powder with a coupling agent; the pretreated composite powder and a thermoplastic polymer are mixed, granulated and kneaded; injecting the blank into a mold, and performing hot-press molding; carrying out gradient roasting on the plate; and performing post-treatment on the semi-finished product to obtain the bipolar plate. According to the invention, the quality stability and batch consistency from the raw materials to the final product are ensured from the screw parameter, temperature and vacuum degree of granulation to the rotating speed time of mixing and kneading, then to the pressure temperature of hot pressing and finally to the gradient temperature rise of roasting. Therefore, the method is not only an effective process for preparing the high-performance bipolar plate, but also guarantees high-efficiency, long-life and stable operation of the flow battery, further drives industrial chains of electrolyte, membrane materials and the like to develop, and promotes the flow battery to realize wider application.

Description

Technical Field

[0001] This invention relates to the field of battery module manufacturing technology, and in particular to a process for preparing an integrated bipolar plate. Background Technology

[0002] As a core component of flow batteries, bipolar plates play a crucial role in current collection and conduction, separating the positive and negative electrolytes, and supporting the battery stack structure. Their performance directly determines the energy conversion efficiency, cycle life, and overall manufacturing cost of the battery system. Currently, commercially available bipolar plates mainly fall into three categories: graphite plates, metal plates, and composite material plates. However, all of these types face significant technical bottlenecks in practical engineering applications.

[0003] The traditional graphite plate manufacturing process is extremely complex, with a production cycle typically lasting 6 to 7 months. It involves multiple processes such as raw material pretreatment, molding, multiple resin impregnation, high-temperature carbonization, repeated sealing treatment, and precision machining. Not only is it time-consuming and energy-intensive, but also, due to the high brittleness and poor impact resistance of graphite itself, it is prone to breakage and reduced yield when processing ultra-thick or ultra-thin structures. In particular, ultra-thin designs (such as those with a thickness ≤1.5mm) face serious problems of insufficient mechanical strength and high processing difficulty.

[0004] Although metal bipolar plates (such as stainless steel and titanium) have excellent mechanical strength and electrical conductivity, and are easy to achieve thin-layer production and mass production, they are prone to electrochemical corrosion in long-term working environments with strong acid or strong alkaline electrolytes. This not only leads to an increase in plate surface resistance, but also causes the dissolution of metal ions to contaminate the electrolyte, which in turn causes catalyst poisoning and battery performance degradation, seriously affecting the long-term operational stability and lifespan of flow batteries.

[0005] The challenge in manufacturing composite bipolar plates (such as graphite / polymer composites) lies in balancing the properties of the conductive filler and the polymer matrix. Increasing conductivity requires increasing the graphite content, leading to increased brittleness and molding difficulties. Conversely, improving toughness and processability necessitates increasing the polymer ratio, sacrificing conductivity and impermeability. Furthermore, in the manufacturing of large-size, ultra-thin composite plates, warping and deformation are highly likely due to factors such as material anisotropy, curing shrinkage, and uneven internal stress distribution. Flatness is often difficult to control within 0.1 mm / m, severely impacting the assembly sealing and current distribution uniformity of the fuel cell stack.

[0006] The aforementioned bipolar plates generally face problems such as complex manufacturing processes, limited production capacity, and difficulty in ensuring product consistency and reliability, which greatly restricts the promotion and application of flow batteries in large-scale grid-scale energy storage. Therefore, developing an integrated molding and manufacturing process that can achieve short-process, high-performance, and low-cost manufacturing has become an urgent need to promote the further development of the flow battery industry and is also an important research direction in the field of energy storage materials and manufacturing technology. Summary of the Invention

[0007] The object of the present invention is to provide an integrated molding process for preparing bipolar plates, so as to solve the problems of complex manufacturing processes, limited production capacity, and difficulty in ensuring product consistency and reliability in the existing bipolar plate preparation processes.

[0008] In order to achieve the above object of the invention, the present invention provides the following technical solutions:

[0009] The present invention provides an integrated molding process for preparing bipolar plates, comprising the following steps:

[0010] (1) Mix graphite powder, graphene sheets, and carbon nanotubes to obtain a composite powder; modify the composite powder with a coupling agent to obtain a pretreated composite powder;

[0011] (2) Mix and granulate the pretreated composite powder with a thermoplastic polymer to obtain granulated materials; knead the granulated materials to obtain a blank;

[0012] (3) Inject the blank into a mold and perform hot pressing to form a plate;

[0013] (4) Under a protective atmosphere, perform gradient roasting on the plate to obtain a semi-finished product;

[0014] (5) Perform post-treatment on the semi-finished product to obtain a bipolar plate;

[0015] Among them, in step (1), the graphite powder comprises a mixture of coarse particles, medium particles, and fine particles; the particle size d1 of the coarse particles is 50μm < d1 ≤ 100μm; the particle size d2 of the medium particles is 10μm ≤ d2 ≤ 50μm; the particle size d3 of the fine particles is 0.1μm ≤ d3 < 10μm; the mass ratio of the coarse particles, the medium particles, and the fine particles is 3 - 4:4 - 5:1 - 2; the particle size distribution span of the coarse particles, the medium particles, and the fine particles is independently D 90 / D 10 <2;

[0016] In step (1), the mass ratio of the graphite powder, the graphene sheets, and the carbon nanotubes is 80 - 85:3 - 7:1 - 2.

[0017] Further, in the integrated molding process for preparing bipolar plates, in step (1), the coupling agent is a silane coupling agent or a titanate coupling agent; the silane coupling agent is KH-550 or KH-792; the titanate coupling agent is HY-109 or HY-201;

[0018] In step (1), the mass of the coupling agent is 0.5 - 2% of the mass of the composite powder.

[0019] Furthermore, in the integrated bipolar plate fabrication process, the composite powder in step (1) also includes silicon carbide nanowires;

[0020] The mass of the silicon carbide nanowires is 1 to 2% of the mass of the composite powder.

[0021] Furthermore, in the integrated bipolar plate manufacturing process, the thermoplastic polymer in step (2) is polyphenylene sulfide or polyether ketone ketone;

[0022] In step (2), the mass ratio of the pretreated composite powder to the thermoplastic polymer is 85:15 to 90:10.

[0023] Furthermore, in the integrated bipolar plate preparation process, the mixing and granulation conditions in step (2) include: screw length-to-diameter ratio of 30 to 36:1; zone temperature: feeding section 280 to 300°C, compression section 300 to 310°C, homogenization section 310 to 320°C; screw speed 300 to 500 r / min; vacuum degree ≤ -0.08 MPa;

[0024] The mixing conditions in step (2) include: temperature of 300-350℃, rotation speed of 50-80r / min, and time of 30-60min.

[0025] Furthermore, in the integrated bipolar plate manufacturing process, the hot pressing conditions in step (3) include: pressure of 5-20 MPa, temperature of 320-380 °C, and time of 10-15 min.

[0026] Furthermore, in the integrated bipolar plate fabrication process, the gradient calcination conditions in step (4) include:

[0027] First stage: The room temperature is raised to 200℃ at a rate of 5-10℃ / min, and the holding time is 0.5-1h;

[0028] Second stage: Heating from 200℃ to 600℃ at a rate of 2-6℃ / min, with a holding time of 0.2-0.5h;

[0029] The third stage: heating from 600℃ to 1200℃ at a rate of 6-9℃ / min, and holding for 2-4 hours.

[0030] Furthermore, in the integrated bipolar plate manufacturing process, the post-processing in step (5) includes: polishing the semi-finished product to correct its flatness; the single correction amount of the polishing is ≤0.02mm / m.

[0031] Furthermore, in the integrated bipolar plate manufacturing process, the modification method in step (1) includes the following steps: mixing the coupling agent with a low-boiling-point organic solvent to obtain a coupling agent solution; spraying the coupling agent solution onto the composite powder and removing the solvent.

[0032] Furthermore, in the integrated bipolar plate manufacturing process, the conditions of the bipolar plate in step (5) include: a thickness of 1.2 to 1.5 mm and a flatness of ≤ ±0.05 mm / m.

[0033] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0034] The integrated molding bipolar plate fabrication process provided by this invention produces bipolar plates that exhibit significant advantages, specifically in the following aspects:

[0035] (1) This invention uses coarse, medium, and fine graphite powders of different particle sizes for graded filling. The fine graphite powder fills the gaps between the large particles, while the two-dimensional graphene sheets can encapsulate and connect the graphite particles, and the one-dimensional carbon nanotubes can penetrate into even finer pores, entangle and bridge adjacent particles and graphene sheets. This invention constructs a multi-dimensional synergistic structure of "zero-dimensional particles + one-dimensional tubes + two-dimensional sheets", forming a continuous and efficient three-dimensional interconnected conductive / thermal network inside the composite material. Even if the polymer binder carbonizes after calcination, this network can maintain extremely high integrity, thereby making the in-plane conductivity and thermal conductivity of the bipolar plate close to the level of pure graphite material, significantly reducing the contact resistance and local hot spot risk of the battery stack.

[0036] (2) This invention uses silane coupling agents or titanate coupling agents to pretreat the composite powder. The inorganic-loving groups of the coupling agent molecules bind to the surface of the carbon material, while the organic-loving groups become entangled or chemically react with the subsequently added thermoplastic polymer molecular chains. This greatly improves the problem of poor compatibility between inorganic carbon materials and organic polymers in traditional processes, effectively reducing phase separation and microscopic defects caused by weak interfacial bonding, making the material more uniform during mixing and molding. After calcination, this strong interfacial bonding is transformed into a denser and tougher carbon skeleton structure, which greatly improves the bending strength and impact resistance of the final product, solving the problem of high brittleness and easy breakage of graphite materials.

[0037] (3) In this invention, silicon carbide nanowires are further added to the composite powder as a high-performance ceramic reinforcing phase. They are uniformly dispersed in the matrix and can effectively hinder the propagation of microcracks. They consume fracture energy through pull-out, bridging and other effects, thereby giving the bipolar plate higher toughness and strength.

[0038] (4) This invention employs precise gradation of three-stage graphite powder (and requires that the particle size distribution of each component be concentrated, D)90 / D 10 <2>The initial porosity of the mixture is minimized from the source. Subsequent hot pressing and gradient calcination processes allow the polymer melt to fully fill these pores. During gradient calcination, the slow heating rate and segmented heat preservation allow the polymer binder to decompose and carbonize slowly and steadily, avoiding cracking, bubbling, and the formation of macroscopic pores caused by the rapid overflow of volatiles. The resulting semi-finished product has a dense structure and extremely low porosity, thus possessing extremely high gas barrier properties, meeting the requirements for long-term safe operation of the battery.

[0039] (5) This invention adopts a "granulation-kneading-hot pressing" route, which fully utilizes the characteristics of thermoplastic polymers to melt and flow after heating and to solidify after cooling. Compared with traditional molded expanded graphite plates or graphite sintered plates, its advantage lies in the one-time pressing and molding of bipolar plates with complex flow channel structures through precision molds, eliminating the need for secondary machining of the flow channels, avoiding processing losses and breakage risks, and resulting in extremely high production efficiency and product consistency. Compared with pure graphite plates, raw material costs and manufacturing costs are significantly reduced.

[0040] (6) This invention ensures the quality stability and batch consistency of the raw materials and final product by controlling the screw parameters, temperature, and vacuum degree during granulation, the rotation speed and time during kneading, the pressure, temperature, and heat preservation during hot pressing, and finally the gradient temperature rise during calcination. The micro-polishing in the post-processing ensures that the product achieves extremely high flatness (≤±0.05mm / m), thereby obtaining extremely low contact resistance and uniform pressure distribution in the stack assembly, improving the performance and life of the battery. Detailed Implementation

[0041] This invention provides a process for fabricating an integrally molded bipolar plate, comprising the following steps:

[0042] (1) Graphite powder, graphene sheets and carbon nanotubes are mixed to obtain composite powder; the composite powder is modified with a coupling agent to obtain pretreated composite powder;

[0043] (2) The pretreated composite powder is mixed with a thermoplastic polymer and granulated to obtain granules; the granules are kneaded to obtain a blank;

[0044] (3) The blank is injected into a mold and hot-pressed to obtain a sheet material;

[0045] (4) Under a protective atmosphere, the plate is subjected to gradient calcination to obtain a semi-finished product;

[0046] (5) The semi-finished product is post-processed to obtain a bipolar plate;

[0047] Among them, the graphite powder in step (1) comprises a mixture of coarse particles, medium particles and fine particles; the particle size d1 of the coarse particles is 50 μm < d1 ≤ 100 μm; the particle size d2 of the medium particles is 10 μm ≤ d2 ≤ 50 μm; the particle size d3 of the fine particles is 0.1 μm ≤ d3 < 10 μm; the mass ratio of the coarse particles, the medium particles and the fine particles is 3-4:4-5:1-2; the particle size distribution span of the coarse particles, the medium particles and the fine particles is independently D 90 / D 10 <2;

[0048] The mass ratio of the graphite powder, the graphene sheets and the carbon nanotubes in step (1) is 80-85:3-7:1-2.

[0049] In the present invention, the mass ratio of the coarse particles, the medium particles and the fine particles is preferably 3.2-3.8:4.2-4.8:1-1.8, more preferably 3.4-3.6:4.4-4.6:1-1.4, and still more preferably 3.5:4.5:1.

[0050] In the present invention, the particle size distribution span of the coarse particles, the medium particles and the fine particles is independently preferably D 90 / D 10 <1.8, more preferably <1.6, and still more preferably <1.5.

[0051] In the present invention, the physical and chemical properties of the graphene sheets in step (1) include: the sheet thickness is preferably ≤5 nm, more preferably ≤4 nm, and still more preferably ≤3 nm; the diameter is preferably 5-10 μm, more preferably 6-9 μm, and still more preferably 8 μm.

[0052] In the present invention, the physical and chemical properties of the carbon nanotubes in step (1) include: the diameter is preferably 20-50 nm, more preferably 30-40 nm, and still more preferably 30 nm; the length is preferably 5-10 μm, more preferably 6-9 μm, and still more preferably 8 μm.

[0053] In the present invention, the mass ratio of the graphene sheets and the carbon nanotubes in step (1) is preferably 3:1-5:1, more preferably 3:1-4:1, and still more preferably 3:1.

[0054] In the present invention, the mass ratio of the graphite powder, the graphene sheets and the carbon nanotubes in step (1) is preferably 8​​In this invention, the composite powder in step (1) further includes silicon carbide nanowires; the physicochemical properties of the silicon carbide nanowires include: a diameter preferably of 100-600 nm, more preferably of 150-300 nm, and more preferably of 200 nm; and a length preferably of 10-30 μm, more preferably of 15-25 μm, and more preferably of 20 μm.

[0056] In this invention, the mass of the silicon carbide nanowires is preferably 1 to 2% of the mass of the composite powder, more preferably 1.2 to 1.8%, and even more preferably 1.5%.

[0057] In this invention, the coupling agent in step (1) is preferably a silane coupling agent or a titanate coupling agent.

[0058] In this invention, the silane coupling agent is preferably KH-550 or KH-792, and more preferably KH-550.

[0059] In this invention, the titanate coupling agent is preferably HY-109 or HY-201, and more preferably HY-201.

[0060] In this invention, the mass of the coupling agent in step (1) is preferably 0.5 to 2% of the mass of the composite powder, more preferably 0.8 to 1.5%, and even more preferably 1%.

[0061] In this invention, the modification method described in step (1) includes the following steps: mixing the coupling agent with a low-boiling-point organic solvent to obtain a coupling agent solution; spraying the coupling agent solution onto the composite powder to remove the solvent.

[0062] In this invention, the low-boiling-point organic solvent is preferably ethanol.

[0063] In this invention, the composite powder is stirred during spraying; the stirring speed is preferably 200-500 r / min, more preferably 300-450 r / min, and even more preferably 400 r / min.

[0064] In this invention, heating and stirring are performed during solvent removal; the conditions for heating and stirring include: the rotation speed is preferably 400-800 r / min, more preferably 500-700 r / min, and even more preferably 600 r / min; the temperature is preferably 40-60°C, more preferably 45-55°C, and even more preferably 50°C.

[0065] In this invention, the thermoplastic polymer in step (2) is preferably polyphenylene sulfide or polyether ketone ketone, and more preferably polyphenylene sulfide.

[0066] In this invention, the weight-average molecular weight of the polyphenylene sulfide is preferably 12,000 to 55,000, more preferably 30,000 to 50,000, and even more preferably 40,000.

[0067] In this invention, the weight-average molecular weight of the polyether ketone is preferably 40,000 to 100,000, more preferably 60,000 to 90,000, and even more preferably 80,000.

[0068] In this invention, the particle size of the thermoplastic polymer in step (2) is preferably ≤50μm. Thermoplastic polymers that do not meet the particle size requirement can be crushed.

[0069] In this invention, the mass ratio of the pretreated composite powder to the thermoplastic polymer in step (2) is preferably 85:15 to 90:10, more preferably 85:15 to 88:12, and even more preferably 85:15.

[0070] In this invention, the mixing conditions for step (2) of mixing the pretreated composite powder with the thermoplastic polymer for granulation include: the rotation speed is preferably 30-50 r / min, more preferably 35-45 r / min, and even more preferably 40 r / min; the time is preferably 30-60 min, more preferably 40-50 min, and even more preferably 40 min.

[0071] In this invention, the conditions for mixing and granulation in step (2) include: the screw length-to-diameter ratio is preferably 30-36:1, more preferably 31-34:1, and even more preferably 32:1; the zone temperature is: the feeding zone is preferably 280-300℃, more preferably 285-295℃, and even more preferably 290℃; the compression zone is preferably 300-310℃, more preferably 302-308℃, and even more preferably 305℃; the homogenization zone is preferably 310-320℃, more preferably 312-318℃, and even more preferably 315℃; the screw speed is preferably 300-500 r / min, more preferably 350-450 r / min, and even more preferably 400 r / min; and the vacuum degree is ≤-0.08 MPa.

[0072] In this invention, the particle size of the granules in step (2) is preferably 0.5 to 2 mm.

[0073] In this invention, the mixing conditions in step (2) include: the temperature is preferably 300-350°C, more preferably 320-340°C, and even more preferably 330°C; the rotation speed is preferably 50-80 r / min, more preferably 55-70 r / min, and even more preferably 60 r / min; and the time is preferably 30-60 min, more preferably 40-50 min, and even more preferably 45 min.

[0074] In this invention, the mold is preheated before hot pressing in step (3); the preheating conditions include: the temperature is preferably 200-250°C, more preferably 210-240°C, and even more preferably 220°C; the time is preferably 3-5 min, more preferably 4-5 min, and even more preferably 5 min.

[0075] In this invention, the hot pressing conditions in step (3) include: pressure preferably 5-20 MPa, more preferably 10-18 MPa, and more preferably 15 MPa; temperature preferably 320-380°C, more preferably 340-360°C, and more preferably 350°C; and time preferably 10-15 min, more preferably 11-14 min, and more preferably 12 min.

[0076] In this invention, the protective atmosphere in step (4) is preferably nitrogen or argon; the purity of the protective atmosphere is preferably ≥99.999%.

[0077] In this invention, the conditions for gradient calcination in step (4) include:

[0078] First stage: The room temperature is raised to 200℃, the heating rate is preferably 5-10℃ / min, more preferably 8-10℃ / min, and even more preferably 10℃ / min; the holding time is preferably 0.5-1h, more preferably 0.8-1h, and even more preferably 1h;

[0079] Second stage: heating from 200℃ to 600℃, preferably at a rate of 2-6℃ / min, more preferably at 3-5℃ / min, and even more preferably at 3℃ / min; holding time preferably at 0.2-0.5h, more preferably at 0.4-0.5h, and even more preferably at 0.5h;

[0080] The third stage involves heating from 600℃ to 1200℃, with a heating rate preferably of 6-9℃ / min, more preferably 7-9℃ / min, and even more preferably 8℃ / min; the holding time is preferably 2-4h, more preferably 2.5-3.5h, and even more preferably 3h.

[0081] In this invention, step (4) preferably includes, after gradient calcination, continuing to introduce a protective atmosphere and naturally cooling to room temperature.

[0082] In this invention, the post-processing in step (5) includes polishing the semi-finished product and correcting its flatness.

[0083] In this invention, the single-pass correction amount of the polishing is preferably ≤0.02mm / m, more preferably ≤0.018mm / m, and even more preferably ≤0.015mm / m.

[0084] In the present invention, the conditions of the bipolar plate in step (5) include: the thickness is 1.2 - 1.5 mm, and the flatness ≤ ±0.05 mm / m.

[0085] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0086] Embodiment 1

[0087] This embodiment provides an integrated molding process for preparing a bipolar plate, including the following steps:

[0088] (1) Mix coarse graphite powder (50 μm < d1 ≤ 100 μm, average particle size 80 μm, D 90 / D 10 = 1.8), medium graphite powder (10 μm ≤ d2 ≤ 50 μm, average particle size 30 μm, D 90 / D 10 = 1.6), and fine graphite powder (0.1 μm ≤ d3 < 10 μm, average particle size 5 μm, D 90 / D 10 = 1.5) according to a mass ratio of 3.5:4.5:1; take 82.5 parts of the aforementioned mixed graphite powder, and mix it with 6.0 parts of graphene sheets (thickness 2 - 3 nm, diameter 8 μm) and 2.0 parts of carbon nanotubes (diameter 30 nm, length 8 μm) to obtain a composite powder; dissolve 1 wt% KH-550 silane coupling agent in absolute ethanol, spray it onto 100 parts of the composite powder under stirring at 400 r / min, and then stir and dry at 50 °C and 600 r / min to remove the solvent, obtaining a pretreated composite powder;

[0089] (2) Mix 85 parts of the pretreated composite powder with 15 parts of polyphenylene sulfide (PPS, weight average molecular weight Mw ≈ 40,000, particle size D 50 = 45 μm) in a high-speed mixer at 40 r / min for 40 min; then use a twin-screw granulator (aspect ratio 32:1, feeding section 290 °C, compression section 305 °C, homogenization section 315 °C, screw rotation speed 400 r / min, vacuum degree -0.09 MPa) for melt blending and granulation, and through cooling, crushing, and screening, obtain pellets with a particle size of 1 - 2 mm; put the pellets into a kneader, and knead at 330 °C and 60 r / min for 45 min to obtain a uniform blank;

[0090] (3) Preheat the mold to 220 °C and keep it warm for 5 min; inject the blank into the mold, and hot press and keep it warm at 350 °C and 15 MPa for 12 min to obtain a dense plate;

[0091] (4) Place the plate in a furnace protected by high-purity nitrogen (≥99.999%) and bake it according to the following procedure: ① Heat up to 200℃ at 10℃ / min and hold for 1h; ② Heat up to 600℃ at 3℃ / min and hold for 0.5h; ③ Heat up to 1200℃ at 8℃ / min and hold for 3h; After the procedure is completed, cool it to room temperature with the furnace to obtain a semi-finished product;

[0092] (5) The semi-finished product is precision polished, and the single correction amount is controlled at 0.015mm / m, so that a bipolar plate with a thickness of 1.35mm and a flatness of ±0.04mm / m is finally obtained.

[0093] Example 2

[0094] The preparation process is basically the same as in Example 1, except for the following parameters:

[0095] Carbon materials: The mass ratio of graphite powder, graphene sheets, and carbon nanotubes is 84:4:1;

[0096] Add 1.5 wt% silicon carbide nanowires (200 nm in diameter and 20 μm in length) to the composite powder (mixed with carbon materials);

[0097] Polymer: Polyetherketoneketone (PEKK) with a weight-average molecular weight Mw≈80,000 was used, and the mass ratio of pretreated composite powder to PEKK was 88:12;

[0098] Hot pressing: The hot pressing temperature is increased to 370℃ and the pressure is 18MPa.

[0099] Example 3

[0100] The preparation process is basically the same as in Example 1, except for the following parameters:

[0101] Graphite powder gradation: The mass ratio of coarse, medium, and fine particles is adjusted to 4:4:2;

[0102] Coupling agent: 1.2 wt% titanate coupling agent HY-201 was used;

[0103] Granulation: Screw speed is 450 r / min, homogenization section temperature is 320℃;

[0104] Calcination: The heating rate in the second stage is reduced to 2℃ / min.

[0105] Comparative Example 1

[0106] The main difference from Example 1 is that graphite powder gradation is not used; only a single particle size (D) is used. 50 The graphite powder (50 μm) was used, and other parameters and conditions were the same as in Example 1.

[0107] Comparative Example 2

[0108] The main difference from Example 1 is that the gradient calcination procedure was changed, the second stage of heat preservation was omitted, the temperature was directly increased from 200℃ to 600℃ at 8℃ / min, and the third stage adopted rapid heating at 15℃ / min to 1200℃. Other parameters and conditions were the same as in Example 1.

[0109] Comparative Example 3

[0110] The main difference from Example 1 is that thermosetting phenolic resin 2130 is used instead of polyphenylene sulfide, a molding curing process (180℃ / 20MPa / 30min curing) is adopted, and gradient calcination treatment is not performed. Other parameters and conditions are the same as in Example 1.

[0111] Test Example 1

[0112] The bipolar plates obtained in Examples 1-3 and Comparative Examples 1-3 were subjected to the following performance tests, and the test results are shown in Table 1.

[0113] (1) Volume resistivity test: The surface resistance of the bipolar plate was measured at room temperature (25℃) using the four-probe method, and the volume resistivity (unit: μΩ·m) was calculated from the thickness. The test area consisted of 3 points at the center and 3 points at the edge of the plate, and the average value was taken.

[0114] (2) Bending strength test: The sample size is 80mm×10mm×1.35mm (length×width×thickness), with a span of 50mm. A three-point bending test is carried out according to GB / T 9341-2008, with a loading rate of 2mm / min. The maximum load at fracture is recorded, and the bending strength (unit: MPa) is calculated.

[0115] (3) Corrosion resistance test: Simulates the electrolyte environment commonly used in flow batteries (vanadium redox flow battery, using 1.5 mol / L V). 3+ / V 4+ A bipolar plate sample (20 mm × 20 mm × 1.35 mm) was immersed in a 3 mol / L H₂SO₄ electrolyte solution and left to stand at 60 °C for 1000 h. The mass was measured before and after the test (accuracy 0.1 mg), and the weight loss rate (unit: %) was calculated. At the same time, the change rate of volume resistivity after immersion was measured (unit: %).

[0116] (4) Flatness verification: A laser flatness measuring instrument is used to take points every 50mm along the length of the electrode plate to detect the actual flatness (unit: mm / m) and compare it with the target value of ±0.05mm / m.

[0117] Table 1. Bipolar plate performance of Examples 1-3 and Comparative Examples 1-3

[0118]

[0119] As shown in Table 1, the bipolar plates prepared in Examples 1-3 of this invention possess excellent comprehensive performance. The core of this success lies in achieving ultra-high density through multi-scale graphite gradation, optimizing interfacial bonding through coupling agent modification, and obtaining a highly graphitized and defect-free structure through thermoplastic processes and gradient calcination. This results in bipolar plates with extremely low volume resistivity (7.5–8.5 μΩ·m), excellent flexural strength (45.2–52.6 MPa), and excellent corrosion resistance (corrosion weight loss rate of only 0.06–0.09%, resistivity change rate <6%), while ensuring extremely high flatness (within ±0.040 mm / m). In contrast, comparative examples show significant performance degradation in Comparative Example 1 due to the lack of graphite gradation and porous structure; Comparative Example 2 suffers from deteriorated performance due to microscopic defects introduced by the rapid calcination process; and Comparative Example 3, using thermosetting resin, exhibits a sharp deterioration in resistivity after corrosion (change rate 28.6%), leading to a significant decrease in durability. Therefore, this invention not only leads in static performance with high conductivity and high strength, but also has long-term operational stability, proving that the preparation process of this invention can produce high-performance, long-life bipolar plates.

[0120] Test Example 2

[0121] A 0.2 mm thick layer of graphite-based conductive sealant was applied to the sealing grooves of the bipolar plates in Examples 1-3 and Comparative Examples 1-3, respectively. Graphite felt electrodes (3 mm thick) and Nafion 117 proton exchange membranes were then matched and assembled into 50 cm² plates. 2 The following battery performance tests were conducted on a single cell (all-vanadium redox flow battery system), and the test results are shown in Table 2.

[0122] (1) Contact resistance test: Under a pressure of 15 MPa, the interfacial contact resistance between the bipolar plate and the graphite felt was measured using the four-probe method (unit: mΩ·cm). 2 ).

[0123] (2) Charge / discharge efficiency test: Charge / discharge cycles were performed at 0.1C, 0.2C, and 0.5C rates (charging cutoff voltage 1.65V, discharging cutoff voltage 0.8V). The voltage efficiency (VE) and energy efficiency (EE) were recorded after the 50th cycle. The calculation formula is as follows:

[0124] Voltage efficiency VE = Average discharge voltage / Average charging voltage × 100%;

[0125] Energy efficiency EE = Discharge energy / Charge energy × 100%.

[0126] (3) Cyclic stability test: Cycle 300 times at 0.2C, record the energy efficiency retention rate of each cycle (EE of the nth cycle / EE of the first cycle × 100%), and evaluate the long-term operational stability.

[0127] Table 2. Results obtained from Test Example 2

[0128]

[0129] As shown in Table 2, the bipolar plates prepared in Examples 1-3 of this invention exhibit extremely low interfacial contact resistance (16.2-19.3 mΩ·cm) in single-cell tests of flow batteries. 2 Thanks to its excellent electrical conductivity and surface smoothness, the battery exhibits higher voltage efficiency (88.8–91.2%) and energy efficiency (85.6–88.5%), indicating less ohmic polarization and lower energy loss during operation. More importantly, after 300 cycles, the example still maintains an energy efficiency of over 95.2%, demonstrating its high corrosion resistance and structural stability, which translates into an ultra-long lifespan. In contrast, Comparative Example 1 suffers from high contact resistance and lowest efficiency due to its porous structure; Comparative Example 2, due to defects in its fast-burning process, still performs significantly worse than the example; and Comparative Example 3, due to the inherent chemical instability of the thermosetting resin route, experiences a sharp drop in efficiency retention to 78.3% after cycling, revealing its durability deficiencies.

[0130] Therefore, this invention is not only an effective process for preparing high-performance bipolar plates, but also a guarantee for achieving high efficiency, long life and stable operation of flow batteries, thereby driving the development of the electrolyte, membrane materials and other industrial chains, and promoting the wider application of flow batteries.

[0131] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A process for fabricating an integrally molded bipolar plate, characterized in that, Includes the following steps: (1) Graphite powder, graphene sheets and carbon nanotubes are mixed to obtain composite powder; the composite powder is modified with a coupling agent to obtain pretreated composite powder; (2) The pretreated composite powder is mixed with a thermoplastic polymer and granulated to obtain granules; the granules are kneaded to obtain a blank; (3) The blank is injected into a mold and hot-pressed to obtain a sheet material; (4) Under a protective atmosphere, the plate is subjected to gradient calcination to obtain a semi-finished product; (5) The semi-finished product is post-processed to obtain a bipolar plate; Among them, the graphite powder described in step (1) includes a mixture of coarse particles, medium particles, and fine particles; the particle size d1 of the coarse particles is 50 μm < d1 ≤ 100 μm; the particle size d2 of the medium particles is 10 μm ≤ d2 ≤ 50 μm; the particle size d3 of the fine particles is 0.1 μm ≤ d3 < 10 μm; the mass ratio of the coarse particles, the medium particles, and the fine particles is 3 to 4: 4 to 5: 1 to 2; the particle size distribution span of the coarse particles, the medium particles, and the fine particles is independently D 90 / D 10 < 2; The mass ratio of the graphite powder, the graphene sheet, and the carbon nanotube in step (1) is 80-85:3-7:1-2.

2. The integrated bipolar plate fabrication process according to claim 1, characterized in that, The coupling agent in step (1) is a silane coupling agent or a titanate coupling agent; the silane coupling agent is KH-550 or KH-792; the titanate coupling agent is HY-109 or HY-201; The mass of the coupling agent in step (1) is 0.5 to 2% of the mass of the composite powder.

3. The integrated bipolar plate fabrication process according to claim 1, characterized in that, The composite powder in step (1) also includes silicon carbide nanowires; The mass of the silicon carbide nanowires is 1 to 2% of the mass of the composite powder.

4. The integrated bipolar plate fabrication process according to any one of claims 1 to 3, characterized in that, The thermoplastic polymer in step (2) is polyphenylene sulfide or polyether ketone ketone; In step (2), the mass ratio of the pretreated composite powder to the thermoplastic polymer is 85:15 to 90:

10.

5. The integrated bipolar plate fabrication process according to claim 1, characterized in that, The conditions for mixing and granulating in step (2) include: screw length-to-diameter ratio of 30 to 36:1; zone temperature: 280 to 300°C for feeding section, 300 to 310°C for compression section, and 310 to 320°C for homogenization section; screw speed of 300 to 500 r / min; and vacuum degree ≤ -0.08 MPa. The mixing conditions in step (2) include: temperature of 300-350℃, rotation speed of 50-80r / min, and time of 30-60min.

6. The integrated bipolar plate fabrication process according to claim 1, characterized in that, The conditions for hot pressing in step (3) include: pressure of 5-20 MPa, temperature of 320-380℃, and time of 10-15 min.

7. The integrated bipolar plate fabrication process according to claim 1, characterized in that, The conditions for gradient calcination in step (4) include: First stage: The room temperature is raised to 200℃ at a rate of 5-10℃ / min, and the holding time is 0.5-1h; Second stage: Heating from 200℃ to 600℃ at a rate of 2-6℃ / min, with a holding time of 0.2-0.5h; The third stage: heating from 600℃ to 1200℃ at a rate of 6-9℃ / min, and holding for 2-4 hours.

8. The integrated bipolar plate fabrication process according to claim 1, characterized in that, The post-processing in step (5) includes polishing the semi-finished product to correct its flatness; the single correction amount of the polishing is ≤0.02mm / m.

9. The integrated bipolar plate fabrication process according to any one of claims 1 to 3, characterized in that, The modification method described in step (1) includes the following steps: mixing the coupling agent with a low-boiling-point organic solvent to obtain a coupling agent solution; spraying the coupling agent solution onto the composite powder and removing the solvent.

10. The integrated bipolar plate fabrication process according to claim 1, characterized in that, The conditions for the bipolar plate in step (5) include: a thickness of 1.2 to 1.5 mm and a flatness of ≤ ±0.05 mm / m.

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