Graphene composite membrane as well as preparation method and application thereof

Abstract

The invention discloses a graphene composite film and a preparation method and application thereof. The graphene composite film comprises a composite layer composed of graphene and hexagonal boron nitride and a graphene layer arranged on at least one surface of the composite layer, and the defect degree of graphene in the graphene layer is higher than that of graphene in the composite layer. The graphene composite film contains the hexagonal boron nitride powder, is high in heat conductivity coefficient and is an insulating material, and the resistivity can be improved while the heat conductivity is improved; the graphene layer with high defect degree can further improve the resistivity, so that the composite film has heating and heat dissipation functions, and can be used as a heating and heat dissipation integrated element to be applied to the field of thermal management of chemical power supplies such as lithium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of thermal management, specifically relating to a graphene composite film with both good heat dissipation and heating performance, as well as its preparation method and application. Background Technology

[0002] In recent years, with the development of the new energy era and the continuous improvement of the industrialization of various automobiles, lithium batteries, as a commonly used energy storage device in new energy electric vehicles, have attracted widespread attention. During battery operation, the battery pack is in a high-rate discharge state, generating a large amount of heat. Excessive temperature directly affects the discharge capacity of the battery pack, reduces the stable operation of the entire system, and can cause thermal runaway, seriously increasing the probability of fire and explosion. Therefore, heat dissipation management of lithium batteries is necessary. Furthermore, at low temperatures, the internal chemical reaction rate of lithium batteries slows down, electronic conductivity deteriorates, polarization increases, and usable capacity decreases. Simultaneously, low temperatures slow down the diffusion rate of lithium ions, potentially leading to the accumulation of irregularly shaped lithium metal, forming lithium dendrites that penetrate the battery's thin film, causing short circuits and reducing battery safety. Typically, a temperature control system is required in low-temperature environments to maintain a suitable operating temperature. Therefore, research on high-performance thermally conductive and heat-dissipating materials has been put forward new requirements, simultaneously achieving heat / cooling of lithium batteries.

[0003] Graphene, with its two-dimensional carbon atom structure and excellent thermal conductivity, possesses unique advantages in integrated thermal management systems. Ideally, pure graphene exhibits a thermal conductivity of 5300 W·m. -1 ·K -1 With an electrical conductivity of 10 6 S / m, carrier mobility reaches 15000 cm⁻¹ 2 ·V -1 ·s -1 Graphene is currently the material with the best thermal and electrical conductivity and the highest strength in the world. At present, graphene films are widely used for heat conduction and heat dissipation. However, its production mainly adopts the oxidation-reduction method. The product usually undergoes carbon atom rearrangement due to the graphitization process, which greatly reduces material defects. Therefore, graphene films can achieve a thermal conductivity of 1500W / mK. However, due to the low resistivity of the material with low defect density, it cannot form electric heating performance as an electric heating product. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a graphene composite film with both high thermal conductivity and high heating performance, a method for preparing the same, a heat dissipation and heating element utilizing the composite film, and an electronic device employing the element.

[0005] The first aspect of the present invention provides a graphene composite film comprising a composite layer composed of graphene and hexagonal boron nitride and a graphene layer disposed on at least one surface of the composite layer, wherein the graphene in the graphene layer has a higher defect rate than the graphene in the composite layer.

[0006] According to one embodiment of the present invention, the mass percentage of hexagonal boron nitride in the composite layer is less than 50%.

[0007] According to another embodiment of the present invention, the thickness of the composite layer is 20 to 100 micrometers, and the thickness of the graphene layer is 10 to 50 micrometers.

[0008] According to another embodiment of the present invention, the thermal conductivity of the graphene composite film is 1300–2000 W·m. -1 ·K -1 The surface resistance is 5 to 50 ohms / □.

[0009] A second aspect of the present invention provides a method for preparing the above-mentioned graphene composite film, comprising: forming a slurry containing graphene oxide and hexagonal boron nitride into a film and then performing a carbonization treatment to obtain a graphene-boron nitride layer; and coating a graphene oxide slurry onto the surface of the graphene-boron nitride layer and then performing a graphitization treatment.

[0010] According to one embodiment of the present invention, the method further includes a pressing process for the graphitized film layer, wherein the pressing pressure is 40-120 MPa and the holding time is 1-6 h.

[0011] According to another embodiment of the present invention, the solid content in the slurry containing graphene oxide and hexagonal boron nitride is 4% to 5%.

[0012] According to another embodiment of the present invention, the carbonization treatment is carried out under an inert atmosphere, with a heating rate of 1-5°C / min, heated to 1000-1200°C, and held for 0.5-2 hours; the graphitization treatment is first rapidly heated to 1000-1200°C and held for 2-4 hours, then rapidly heated to 2800-3100°C and held for 2-4 hours; the rapid heating rate is 20-30°C / min, and the slow heating rate is 5-8°C / min.

[0013] According to another embodiment of the present invention, a pretreatment step is further included before the carbonization treatment, wherein the heating rate of the pretreatment is 1 to 5 °C / min, the temperature is 200 to 300 °C, and the holding time is 0.5 to 2 h.

[0014] A third aspect of the present invention provides an integrated heating and heat dissipation element, comprising the above-mentioned graphene composite film and an electrode, wherein the electrode is disposed on the surface of the graphene layer.

[0015] According to one embodiment of the present invention, the element further includes an encapsulation layer.

[0016] A fourth aspect of the present invention provides an electrochemical device comprising the above-described integrated heating and heat dissipation element and a lithium-ion battery unit.

[0017] The graphene composite film of the present invention contains hexagonal boron nitride powder, which has a high thermal conductivity and is an insulating material, and can improve resistivity while improving thermal conductivity; the inclusion of a graphene layer with high defect degree can further improve resistivity, so that the composite film has both heating and heat dissipation functions, and can be used as an integrated heating and heat dissipation element in the field of thermal management of chemical power sources such as lithium-ion batteries. Attached Figure Description

[0018] Figure 1 This is an overall morphological diagram of the graphene composite film prepared in Example 1.

[0019] Figure 2 This is a Raman image of the graphene composite film prepared in Example 1. Detailed Implementation

[0020] The present invention will now be described in detail with reference to specific embodiments.

[0021] The graphene composite film of the present invention comprises a composite layer composed of graphene and hexagonal boron nitride, and a graphene layer disposed on at least one surface of the composite layer, wherein the graphene in the graphene layer has a higher defect rate than the graphene in the composite layer. The graphene composite film of the present invention utilizes the high thermal conductivity and insulation properties of hexagonal boron nitride to improve resistivity while maintaining high thermal conductivity. Furthermore, the inclusion of a graphene layer with a high defect rate further enhances resistivity, thereby enabling the composite film to possess both heat dissipation and heating properties, making it suitable for use in battery thermal management. When the battery temperature is too high, it functions as a heat dissipation element, rapidly dissipating heat and lowering the battery temperature. At low temperatures, it can act as a heating element to heat the battery, maintaining it at an appropriate temperature to improve the efficiency of internal chemical reactions and prevent side reactions. Using the graphene composite film of the present invention as a heat dissipation and heating element allows the battery to operate at a suitable temperature, thereby improving the battery's energy conversion efficiency, lifespan, and safety.

[0022] Graphene composite films can include two graphene layers, meaning that graphene layers are formed on both surfaces of the composite layer. Alternatively, they can contain only one graphene layer, meaning that graphene layers are formed on only one surface of the composite layer.

[0023] The mass percentage of hexagonal boron nitride in the composite layer is less than 50%. When the content of hexagonal boron nitride is greater than 50%, film formation is difficult. The content of hexagonal boron nitride in the composite layer can determine the resistivity of the graphene composite film, and an appropriate content can be selected according to actual needs, such as, but not limited to, 49%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5%, etc.

[0024] The thickness of the composite layer is 20-100 micrometers, and the thickness of the graphene layer is 10-50 micrometers. The thickness and defect level of the graphene layer directly affect the sheet resistance of the composite film. The sheet resistance decreases as the graphene thickness increases. An appropriate thickness can be selected according to actual needs, such as, but not limited to, 10 micrometers, 20 micrometers, 30 micrometers, 40 micrometers, 50 micrometers, etc. Compared with the graphene in the composite layer, the graphene in the graphene layer has only undergone graphitization treatment and lacks the carbonization process. Therefore, its defect level is high and its sheet resistance is large.

[0025] The thermal conductivity of graphene composite films is 1300–2000 W·m. -1 ·K -1 The surface resistance is 5 to 50 ohms / □.

[0026] The preparation method of the aforementioned graphene composite film may include: forming a film from a slurry containing graphene oxide and hexagonal boron nitride, followed by carbonization to obtain a graphene-boron nitride layer; and coating a graphene oxide slurry onto the surface of the graphene-boron nitride layer followed by graphitization. After carbonization of the slurry containing graphene oxide and hexagonal boron nitride, the graphene oxide is reduced to defective graphene, meaning the graphene in the graphene-boron nitride layer has a high degree of graphene defect. After graphene oxide is coated onto the surface of the graphene-boron nitride layer and then graphitized, the graphene within the graphene-boron nitride layer rearranges to form a composite layer comprising graphene with low defect levels and hexagonal boron nitride. During this process, the graphene oxide is reduced to defective graphene. Therefore, a composite film comprising a composite layer and a graphene layer is formed after graphitization, wherein the graphene in the graphene layer has a higher degree of graphene defect than the graphene in the composite layer. Because the graphene in the graphene layer has defects, the resistivity of the layer is high, which can increase the resistance of the composite film and thus give the composite film electrothermal properties.

[0027] A slurry containing graphene oxide and hexagonal boron nitride can be formed by conventional methods, preferably with a solid content of 4%–5%, meaning the total mass content of graphene oxide and hexagonal boron nitride in the slurry is 4%–5%. The slurry formation process can be, but is not limited to, the following: Add 128–268 parts of deionized water to a high-speed disperser and adjust the speed to 300–400 rpm. Add 3–8 parts of graphene oxide to a container while stirring until the graphene oxide filter cake is completely dispersed. Add 1%–3% hexagonal boron nitride powder to the container, add 5–10 parts of ammonia water to the container, adjust the pH to 6–8, and continue increasing the stirring speed to 2000–2300 rpm for 0.5–1.5 hours to prepare a graphene oxide-boron nitride slurry with a solid content of approximately 4%–5%. The dispersed graphene oxide-boron nitride slurry was added to a homogenizer, and the homogenization pressure was set to 70-80 MPa. The homogenization was repeated 3-4 times. The homogenized graphene oxide-boron nitride slurry was then stirred under vacuum, with the vacuum pressure set to 0.01-0.08 MPa and the stirring speed set to 800-1500 rpm. The mixture was degassed for 5-10 minutes.

[0028] The film-forming method of the slurry can be any suitable method, such as, but not limited to, blade coating, spraying, etc. The specific process can be, but is not limited to, the following: The degassed slurry is coated onto a polypropylene substrate to a thickness of 2–4 mm. It is then transferred to a forced-air drying oven and dried at 40–60°C for 5–10 hours to form a film. The film is then separated from the substrate to obtain a graphene oxide film. After drying, the film thickness is 110–140 micrometers.

[0029] A pretreatment process can be included before carbonizing the membrane layer. This pretreatment process can release gases and prevent hazards such as explosions caused by violent gas release. Of course, this step is not mandatory. If the subsequent carbonization process can avoid the danger caused by violent gas release, pretreatment can be omitted. The pretreatment process can be as follows: two membrane layers are separated by graphite paper, stacked in 8 to 20 layers, with a counterweight placed on top of the stack, and placed in a forced-air drying oven for pretreatment. The heating rate of the pretreatment is 1 to 5℃ / min, the temperature is 200 to 300℃, and the holding time is 0.5 to 2 hours.

[0030] The carbonization process is carried out under an inert atmosphere (e.g., nitrogen) with a heating rate of 1–5 °C / min, reaching 1000–1200 °C, and holding for 0.5–2 h. The resulting carbonization process yields a graphene-boron nitride layer.

[0031] Subsequently, graphene oxide slurry is coated onto the surface of the graphene-boron nitride layer. The graphene oxide slurry can be coated on one side or both sides. The preparation process of the graphene oxide slurry can be similar to that of the graphene oxide-boron nitride slurry, i.e., without the step of adding boron nitride powder. The coating process of the graphene oxide slurry can be any suitable method. The coating thickness on one side can be 1–2 mm. It can then be transferred to a forced-air drying oven and dried at 40–60°C for 5–10 hours to form a graphene oxide film. After drying, the thickness of the composite film is 170–190 micrometers.

[0032] The graphitization process can be as follows: first, the temperature is raised to 1000-1200℃ at high speed and held for 2-4 hours, then the temperature is raised to 2800-3100℃ at low speed and held for 2-4 hours. The heating rate for high-speed heating is 20-30℃ / min, and the heating rate for low-speed heating is 5-8℃ / min.

[0033] For graphitized films, the need for pressing can be determined based on their thermal conductivity. Pressing removes air from the film, reducing interfacial resistance and improving thermal conductivity. If the graphitized film already meets the requirements, pressing is unnecessary. If further improvement in thermal conductivity is needed, pressing can be performed on the graphitized film. The pressing pressure can be 40–120 MPa, and the holding time can be 1–6 hours.

[0034] The aforementioned graphene composite film, used as an integrated heating and heat dissipation element, also includes electrodes disposed on the surface of the graphene layer. The electrodes can be made of any suitable material, preferably materials with high conductivity, such as silver or copper. Silver paste strips can be screen-printed onto both sides of the graphene composite film's surface, and then dried in an oven at 100-130°C for 15-30 minutes. Copper electrodes are then laid on top of the silver paste strips. The silver paste strips and copper strips together form a strip-shaped electrode.

[0035] Finally, the above components can be encapsulated. A coating device can be used to encapsulate the top surface of the composite film with electrodes using a transparent polymer film with an adhesive layer.

[0036] The integrated heating and heat dissipation element of the present invention can be configured to provide heat dissipation and heating for the lithium-ion battery cell surface.

[0037] The present invention is further described below through specific examples. However, these examples are merely exemplary and do not constitute any limitation on the scope of protection of the present invention. Unless otherwise specified, the reagents, materials, and instruments used in the following embodiments and comparative examples are commercially available.

[0038] Example 1

[0039] Add 268 parts of deionized water to a high-speed disperser and adjust the speed to 400 rpm. Crush 8 parts of graphene oxide filter cake and add it to the container while stirring until the graphene oxide filter cake is completely dispersed. Add 3% boron nitride powder to the container, add ammonia water to adjust the pH to 7, and continue to increase the stirring speed to 2300 rpm for 1.5 hours to prepare a graphene oxide-boron nitride slurry with a solid content of approximately 4.5%. Add the dispersed graphene oxide-boron nitride slurry to a homogenizer and homogenize it 4 times at a pressure of 80 MPa. Stir the homogenized graphene oxide-boron nitride slurry under vacuum at a pressure of 0.08 MPa and a stirring speed of 1500 rpm for 10 minutes to remove bubbles.

[0040] Using a manual coating machine, the degassed slurry was coated onto a polypropylene substrate to a thickness of 4 mm. It was then transferred to a forced-air drying oven and dried at 60°C for 10 hours to form a film. The film was then separated from the substrate and dried again to obtain a film layer with a thickness of 130 micrometers.

[0041] The dried film was placed in a forced-air drying oven for pretreatment. The heating rate was 5℃ / min, the temperature was 300℃, and the holding time was 2h. Then, the pretreated film was carbonized in a vacuum / atmosphere box furnace with nitrogen circulating atmosphere, a heating rate of 5℃ / min, heated to 1200℃, and held for 2h to obtain a graphene-boron nitride film.

[0042] Graphene oxide slurry was coated on both sides of the graphene-boron nitride film. The preparation process of the graphene oxide slurry was similar to that of the graphene oxide-boron nitride slurry, except that the step of adding boron nitride powder was omitted. The coating thickness was 2 mm. The film was then transferred to a forced-air drying oven and dried at 60°C for 10 hours to form a film. The thickness of the dried film was 180 micrometers.

[0043] The coated film was then graphitized in a graphite furnace. The temperature was first rapidly increased to 1200℃ and held for 4 hours, then rapidly increased to 2800℃ and held for another 4 hours. The rapid heating rate was 30℃ / min, and the slow heating rate was 8℃ / min. Afterwards, a pressing process was performed at a pressure of 120 MPa for 6 hours, yielding the graphitized graphene film. A photograph of the actual product is shown below. Figure 1 As shown.

[0044] Figure 2 The Raman spectrum of the graphene composite film is shown. The figure shows that the defect level of graphene in the graphene layer is higher than that in the composite layer.

[0045] The obtained graphene composite film has a thermal conductivity of 1300 W / mK and a sheet resistance of 16 Ω / □.

[0046] Example 2

[0047] The rest is the same as in Example 1, except that the amount of boron nitride powder added is reduced to 1%.

[0048] The obtained graphene composite film has a thermal conductivity of 2000 W / mK and a sheet resistance of 7 Ω / □.

[0049] Comparative Example 1

[0050] Except for not recoating the graphene oxide slurry layer, the other steps are the same as in Example 1. Finally, a composite layer consisting of graphene and hexagonal boron nitride is obtained.

[0051] The obtained thin film has a thermal conductivity of 1200 W / mK and a sheet resistance of 8.2 Ω / □.

[0052] Comparative Example 2

[0053] Except for the absence of boron nitride, the other steps are the same as in Example 1, and a composite layer consisting of a low-defect graphene layer and a high-defect graphene layer is finally obtained.

[0054] The obtained thin film has a thermal conductivity of 800 W / mK and a sheet resistance of 5 Ω / □.

[0055] Comparative Example 3

[0056] The graphitization process first involves rapidly heating to 1200°C and holding at that temperature for 4 hours, then rapidly heating to 2500°C and holding at that temperature for another 4 hours. The other steps are the same as in Example 1.

[0057] The obtained thin film has a thermal conductivity of 1000 W / mK and a sheet resistance of 20 Ω / □.

[0058] The data from Examples 1 and 2 and Comparative Examples 1 and 2 show that the sheet resistance can be effectively improved by adding boron nitride and a high-defect graphene layer. As can be seen from Examples 1 and 3, the graphitization temperature will affect both thermal conductivity and sheet resistance. In order to obtain matching heating and heat dissipation performance, a suitable graphitization temperature can be selected.

[0059] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A graphene composite film, characterized in that, It includes a composite layer composed of graphene and hexagonal boron nitride and a graphene layer disposed on at least one surface of the composite layer, wherein the graphene in the graphene layer has a higher defect rate than the graphene in the composite layer.

2. The graphene composite film according to claim 1, characterized in that, The mass percentage of hexagonal boron nitride in the composite layer is less than 50%.

3. The graphene composite film according to claim 1, characterized in that, The thickness of the composite layer is 20–100 micrometers, and the thickness of the graphene layer is 10–50 micrometers.

4. The graphene composite film according to claim 1, characterized in that, The thermal conductivity of the graphene composite film is 1300–2000 W·m. -1 ·K -1 The surface resistance is 5 to 50 ohms / □.

5. A method for preparing a graphene composite film according to any one of claims 1-4, characterized in that, include: A graphene-boron nitride layer is obtained by carbonizing a slurry containing graphene oxide and hexagonal boron nitride. as well as After coating the graphene oxide slurry onto the surface of the graphene-boron nitride layer, graphitization treatment is performed.

6. The method for preparing the graphene composite film according to claim 5, characterized in that, It also includes a pressing process for the graphitized film, wherein the pressing pressure is 40-120 MPa and the holding time is 1-6 h.

7. The method for preparing the graphene composite film according to claim 5, characterized in that, The slurry containing graphene oxide and hexagonal boron nitride has a solid content of 4% to 5%.

8. The method for preparing the graphene composite film according to claim 5, characterized in that, The carbonization process is carried out under an inert atmosphere, with a heating rate of 1–5 °C / min, heating to 1000–1200 °C, and holding time of 0.5–2 h. The graphitization process involves first rapidly heating the temperature to 1000–1200°C and holding it at that temperature for 2–4 hours, then rapidly heating it to 2800–3100°C and holding it at that temperature for 2–4 hours. The rapid heating rate is 20–30°C / min, and the slow heating rate is 5–8°C / min.

9. The method for preparing the graphene composite film according to claim 8, characterized in that, The carbonization process includes a pretreatment step, wherein the heating rate of the pretreatment is 1-5℃ / min, the temperature is 200-300℃, and the holding time is 0.5-2h.

10. A heating and heat dissipation integrated element, characterized in that, The invention comprises a graphene composite film and an electrode as described in any one of claims 1-4, wherein the electrode is disposed on the surface of the graphene layer.

11. The integrated heating and heat dissipation element according to claim 10, characterized in that, The component also includes an encapsulation layer.

12. An electrochemical device, characterized in that, It includes the integrated heating and heat dissipation element and the lithium-ion battery cell as described in claim 10 or 11.

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