A graphene capsule molten salt phase change material and its preparation method and application

Abstract

The present application relates to the technical field of molten salt phase change materials, and discloses a graphene capsule molten salt phase change material, its preparation method and application. The preparation method of the graphene capsule molten salt phase change material comprises: dispersing a few-layer graphene oxide, polyvinyl alcohol and sodium ascorbate in water, stirring to obtain a uniform slurry; adding the slurry into a mold, filling the solar salt to the center of the slurry, applying pressure to the mold so that the slurry tightly coats the solar salt, and freeze-drying to obtain a capsule precursor; calcining the capsule precursor for the first time in an inert atmosphere, and then heating it for the second time to obtain a graphene capsule molten salt phase change material. The molten salt phase change material prepared in the present application is coated with graphene aerogel, has high thermal conductivity, and the molten salt is not easy to leak, and has high long-term stability; it combines the high thermal conductivity of graphene and the high latent heat storage capacity of molten salt, significantly improving the thermal management and storage performance of the phase change material, and has broad application prospects.

Description

Technical Field

[0001] The present application relates to the technical field of molten salt phase change materials, and in particular to a graphene capsule molten salt phase change material and a preparation method and application thereof. Background Art

[0002] With the continuous advancement of global science, technology, and industry, the demand for efficient and reliable new functional materials is growing. The application of high-performance energy storage materials is crucial for improving efficiency and achieving technological breakthroughs in key areas such as energy storage, environmental purification, electronic equipment, and metal smelting and processing. As a new energy storage material, graphene-enhanced molten salt phase change materials are becoming a hot topic in research and industrial applications due to their unique physical and chemical properties.

[0003] Graphene-based molten salt phase-change materials (PCMs) combine the high thermal conductivity of graphene with the high latent heat storage capacity of molten salts, offering a solution that maintains excellent thermal performance even in extreme environments. Graphene's high thermal conductivity significantly enhances the PCM's thermal response speed and thermal management capabilities, while the high latent heat of molten salts enhances its thermal energy storage efficiency. Zhang et al. found that incorporating graphene into PCMs significantly improves their thermal conductivity and stability, making them promising for broad applications in solar thermal energy storage and building heating and cooling systems. Furthermore, this composite material exhibits excellent thermal cycling stability, maintaining high thermal energy storage and release efficiencies over multiple thermal cycles. Li et al. studied graphene-enhanced PCMs, demonstrating their improved thermal properties and potential for advanced energy storage systems. Kim et al. developed a PCM based on graphene powder embedded in a capsule, demonstrating its potential for intelligent and flexible thermal response. Furthermore, Li et al. investigated the thermal properties of a PCM composite containing palmitic acid / bentonite / graphene, offering new options for thermal energy storage.

[0004] However, despite the theoretically superior performance of graphene-based molten salt phase-change energy storage materials, their practical application still faces some challenges, including poor long-term stability and poor integration compatibility with existing systems. There is an urgent need to develop new graphene-based molten salt phase-change materials. Summary of the Invention

[0005] The present application provides a graphene capsule molten salt phase change material and its preparation method and application, aiming to solve the problems of poor long-term stability and poor integration compatibility with existing systems of existing graphene-based molten salt phase change materials.

[0006] In order to achieve the above objectives, this application adopts the following technical solutions.

[0007] In a first aspect of the present application, a method for preparing a graphene capsule molten salt phase change material is provided, comprising:

[0008] S1, dispersing few-layer graphene oxide, polyvinyl alcohol and sodium ascorbate in water, heating and stirring to obtain a uniform slurry;

[0009] S2, adding the slurry into a mold, filling the solar salt to the center of the slurry, applying pressure to the mold so that the slurry tightly covers the solar salt, and freeze-drying to obtain a capsule precursor;

[0010] S3, calcining the capsule precursor in an inert atmosphere for the first time, and then heating it for the second time to obtain a graphene capsule molten salt phase change material.

[0011] In some embodiments, the few-layer graphene oxide is prepared by the following method:

[0012] Mix graphite and sulfuric acid evenly, add sodium nitrate and phosphorus pentoxide, stir at 80-100°C for 12-24 hours, separate and collect the solid phase, wash and air-dry to obtain the precursor;

[0013] The precursor is dispersed in excess concentrated sulfuric acid, and potassium permanganate is slowly added thereto, stirred at -10 to 0°C, and then diluted with water. After that, the temperature is raised to 90°C, and hydrogen peroxide is added until the solution turns bright yellow to obtain a dispersion.

[0014] The dispersion is diluted with water, centrifuged at a speed of 5000-7000 r / min to remove multilayer graphene oxide, and then centrifuged at a speed of 10000-12000 r / min to remove impurities, and the concentrated liquid obtained by centrifugation is freeze-dried to obtain few-layer graphene oxide.

[0015] In some embodiments, the mass ratio of the graphite, sulfuric acid, sodium nitrate and phosphorus pentoxide is (1-2):(18-20):(1.5-2):(1.5-2);

[0016] The mass ratio of the potassium permanganate to the graphite is 5:1.

[0017] In some embodiments, the heating temperature is 80° C.; the mass of the few-layer graphene oxide in the slurry is 10-20 mg / mL, the concentration of polyvinyl alcohol is 120-180 mg / mL, and the concentration of sodium ascorbate is 0.3-0.8 mg / mL.

[0018] In some embodiments, the solar salt is prepared by the following method: NaNO3 and KNO3 are mixed in a mass ratio of 6:4, ground and heated to 400°C to melt, and then cooled and ground again to obtain solar salt.

[0019] In some embodiments, the diameter of the mold is 1 to 3 cm, and the pressure applied to the mold is 10 to 15 MPa.

[0020] In some embodiments, the freeze-drying comprises:

[0021] The mold is frozen with liquid nitrogen for 10 to 12 hours; and then dried for 24 to 48 hours at a temperature of -70 to -80°C and a vacuum degree of <15 Pa.

[0022] In some embodiments, the temperature of the first calcination is 250-400° C.; the temperature of the second calcination is 500-800° C.;

[0023] The inert atmosphere is an argon atmosphere.

[0024] In a second aspect of the present application, a graphene capsule molten salt phase change material prepared by the above preparation method is provided.

[0025] The third aspect of the present application provides the use of the graphene capsule molten salt phase change material prepared by the above preparation method in an energy storage device or energy storage system.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] The present application forms a slurry by mixing a few-layer graphene oxide, polyvinyl alcohol and sodium ascorbate, pouring the slurry into a steel mold, filling it with molten salt and pressurizing it to form a tightly coated precursor. After freeze-drying, the precursor is calcined at a high temperature to complete the thermal reduction of the graphene and the crystallization of the molten salt, thereby finally obtaining a ceramicized graphene capsule molten salt phase change material with high mechanical strength and thermal stability. The molten salt phase change material is coated with graphene aerogel, has high thermal conductivity, is not easy to leak molten salt, and has high long-term stability.

[0028] The graphene capsule molten salt phase change material of the present application combines the high thermal conductivity of graphene and the high latent heat storage capacity of molten salt, significantly improves the thermal management and storage performance of the phase change material, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To more clearly illustrate the technical solutions of the embodiments of this application, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some of the embodiments described in this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

[0030] Figure 1 This is a scanning electron microscope image of graphene capsule molten salt phase change material;

[0031] Figure 2 This is the XRD pattern of graphene capsule molten salt phase change material;

[0032] Figure 3This is the DSC test result diagram of graphene capsule molten salt phase change material;

[0033] Figure 4 This is the thermal conductivity test result of graphene capsule molten salt phase change material. DETAILED DESCRIPTION

[0034] The following will be combined with the accompanying drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0035] In the following description of this embodiment, the terms "include", "comprising", "having" and "containing" are open-ended terms, meaning including but not limited to.

[0036] In the following description of this embodiment, the term "and / or" is used to describe the association relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, B exists alone, and both A and B exist. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0037] In the following description of this embodiment, the term "at least one" refers to one or more, and "plurality" refers to two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0038] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0039] Those skilled in the art should understand that in the following description of the embodiments of the present application, the order of serial numbers does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0040] It will be understood by those skilled in the art that the numerical ranges in the examples of the present application are to be understood as specifically disclosing each intermediate value between the upper and lower limits of the range. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also included in the present application. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.

[0041] Unless otherwise indicated, the technical / scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which this application belongs. Although this application describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail.

[0042] In the first aspect, the present application provides a method for preparing a graphene capsule molten salt phase change material, which comprises mixing a few-layer graphene oxide, polyvinyl alcohol and sodium ascorbate to form a slurry, pouring the slurry into a steel mold, filling the molten salt and pressurizing the slurry to form a tightly coated precursor, and after freeze-drying, calcining the slurry at a high temperature to complete the thermal reduction of the graphene and the crystallization of the molten salt, thereby finally obtaining a ceramic graphene capsule molten salt phase change material with high mechanical strength and thermal stability, which specifically comprises:

[0043] S1, dispersing few-layer graphene oxide, polyvinyl alcohol and sodium ascorbate in water, heating and stirring to obtain a uniform slurry;

[0044] In this application, few-layer graphene oxide, polyvinyl alcohol, and sodium ascorbate are dispersed in water and stirred at 80° C. for 24 to 48 hours to form a uniform slurry. The slurry should be in a uniform and stable suspension state with high viscosity and a consistent dark appearance.

[0045] Wherein, the mass of the few-layer graphene oxide in the slurry is 10-20 mg / mL, the concentration of polyvinyl alcohol is 120-180 mg / mL, and the concentration of sodium ascorbate is 0.3-0.8 mg / mL. Wherein, the polyvinyl alcohol is used to replace water as the main solvent, and it melts at 80°C. The molten polyvinyl alcohol can effectively prevent the stratification of salt and graphene oxide and significantly improve the viscosity of the resulting slurry. In addition, the solidification temperature of polyvinyl alcohol is high and the speed is fast, which can ensure that the capsules formed in the mold have good mechanical strength and provide support before the graphene oxide is reduced. The sodium ascorbate is used to reduce the few-layer graphene oxide to obtain conductive graphene nanosheets.

[0046] S2, adding the slurry into a mold, filling the solar salt to the center of the slurry, applying pressure to the mold so that the slurry tightly covers the solar salt, and freeze-drying to obtain a precursor capsule;

[0047] In the present application, the solar salt is prepared by the following method: NaNO3 and KNO3 are mixed in a mass ratio of 6:4, ground and heated to 400°C to melt, and then cooled and ground again to obtain solar salt.

[0048] In this application, the mold is preferably a cylinder with a diameter of 1-3 cm. After the slurry is added to the mold, the solar salt is pressed into the center of the slurry in the mold. A pressure of 10-15 MPa is applied to the mold to tightly coat the solar salt with the slurry. The mold is then frozen with liquid nitrogen for 10-12 hours and then dried at a temperature of -70-80°C and a vacuum of <15 Pa for 24-48 hours to obtain the encapsulated capsule precursor.

[0049] S3, calcining the capsule precursor in an inert atmosphere for the first time, and then heating it for the second time to obtain a graphene capsule molten salt phase change material.

[0050] In this application, the inert atmosphere is preferably an argon atmosphere, and both calcinations are performed in an argon atmosphere. The first calcination temperature is preferably 250-400°C to remove the polyethylene glycol in the capsule precursor; the second calcination temperature is 500-800°C to thermally reduce the graphene, melt the solar salt, and crystallize it, thereby enhancing the mechanical strength and stability of the material, thereby obtaining a graphene capsule molten salt phase change material with a molten salt phase change material as the inner core and graphene as the outer layer.

[0051] In this application, the few-layer graphene oxide is prepared by the following method:

[0052] Graphite and sulfuric acid are mixed evenly, sodium nitrate and phosphorus pentoxide are added, and the mixture is stirred at 80-100° C. for 12-24 hours. The solid phase is separated and collected, washed, and dried in the shade to obtain a precursor; wherein the mass ratio of the graphite, sulfuric acid, sodium nitrate, and phosphorus pentoxide is preferably (1-2):(18-20):(1.5-2):(1.5-2).

[0053] The precursor is dispersed in excess concentrated sulfuric acid, and potassium permanganate is slowly added thereto, stirred at -10 to 0°C, and then diluted with water. After that, the temperature is raised to 90°C, and hydrogen peroxide is added until the solution turns bright yellow to obtain a dispersion; the mass ratio of potassium permanganate to graphite is 5:1.

[0054] The dispersion is diluted with water, centrifuged at a speed of 5000-7000 r / min to remove multilayer graphene oxide, and then centrifuged at a speed of 10000-12000 r / min to remove impurities, and the concentrated liquid obtained by centrifugation is freeze-dried to obtain few-layer graphene oxide.

[0055] The graphene-encapsulated molten salt phase-change material prepared in this application is coated with graphene aerogel, exhibiting high thermal conductivity, low molten salt leakage, and high long-term stability. Combining the high thermal conductivity of graphene with the high latent heat storage capacity of molten salt, this material significantly improves the thermal management and storage performance of the phase-change material. It can be used in energy storage devices or systems, with applications in thermal energy storage and conversion systems, building energy conservation, electronic device thermal management, and ambient temperature control.

[0056] The present application is further described below through examples.

[0057] In the examples of this application, solar salt was prepared by mixing NaNO3 and KNO3 in a mass ratio of 6:4 and grinding the mixture in a ball mill at 500 rpm for 60 minutes to obtain a mixed salt powder. The mixed salt powder was heated to 400°C to melt, then cooled at a cooling rate of 5°C / min and ground again to obtain solar salt.

[0058] Example 1

[0059] This embodiment provides a method for preparing a graphene capsule molten salt phase change material, comprising:

[0060] S1. Preparation of few-layer graphene oxide: 2g of flake graphite was mixed with 10ml of sulfuric acid and stirred at 80°C. 1.7g of sodium nitrate and 1.7g of phosphorus pentoxide were then added sequentially and stirred for 12 hours. The mixture was filtered twice and air-dried at room temperature for 3 hours to obtain a precursor. 80ml of sulfuric acid was added to a beaker, along with the precursor. 10g of potassium permanganate was slowly added to the beaker at -10°C and stirred for 4.5 hours. The mixture was then treated in a 35°C water bath for 4 hours. The mixture was diluted in 1000ml of water and stirred for 15 minutes. The mixture was heated to 90°C and hydrogen peroxide was added until the solution turned bright yellow to obtain a dispersion. The dispersion was centrifuged at 11,000 rpm for 7 minutes, then at 6,000 rpm for 5 minutes to remove impurities, and then centrifuged again at 11,000 rpm for 15 minutes to obtain a concentrate. The concentrate was freeze-dried for 48 hours to obtain few-layer graphene oxide.

[0061] Dissolve few-layer graphene oxide, polyvinyl alcohol, and sodium ascorbate in water to prepare a mixture of 20 mg / mL few-layer graphene oxide, 180 mg / mL polyvinyl alcohol, and 0.8 mg / mL sodium ascorbate, and stir at 80°C for 48 hours to obtain a uniform slurry.

[0062] S2: Pour the slurry into a 3cm diameter stainless steel mold and press the solar salt into the center of the mold. Apply a pressure of 15MPa to the mold to tightly encapsulate the slurry. Freeze the mold with liquid nitrogen for 12 hours. After freezing, place the mold in a freeze dryer at -70°C and a vacuum of <15Pa for 48 hours to obtain the capsule precursor.

[0063] S3, heating the capsule precursor at 250°C for 2 hours in an argon atmosphere to remove the polyvinyl alcohol; then heating to 500°C and heating for 2 hours and then cooling to thermally reduce the graphene, and the solar salt melts and crystallizes to obtain a graphene capsule molten salt phase change material.

[0064] Example 2

[0065] This embodiment provides a method for preparing a graphene capsule molten salt phase change material, comprising:

[0066] S1. Preparation of few-layer graphene oxide: 2g of flake graphite was mixed with 10ml of sulfuric acid and stirred at 80°C. 1.7g of sodium nitrate and 1.7g of phosphorus pentoxide were then added sequentially and stirred for 12 hours. The mixture was filtered twice and air-dried at room temperature for 3 hours to obtain a precursor. 80ml of sulfuric acid was added to a beaker, along with the precursor. 10g of potassium permanganate was slowly added to the beaker at -10°C and stirred for 4.5 hours. The mixture was then treated in a 35°C water bath for 4 hours. The mixture was diluted in 1000ml of water and stirred for 15 minutes. The mixture was heated to 90°C and hydrogen peroxide was added until the solution turned bright yellow to obtain a dispersion. The dispersion was centrifuged at 10,000 rpm for 7 minutes, then at 6,000 rpm for 5 minutes to remove impurities, and then centrifuged again at 10,000 rpm for 15 minutes to obtain a concentrate. The concentrate was freeze-dried for 48 hours to obtain few-layer graphene oxide.

[0067] Dissolve few-layer graphene oxide, polyvinyl alcohol, and sodium ascorbate in water to prepare a mixture of 15 mg / mL few-layer graphene oxide, 150 mg / mL polyvinyl alcohol, and 0.5 mg / mL sodium ascorbate, and stir at 80°C for 48 hours to obtain a uniform slurry.

[0068] S2: Pour the slurry into a 2-cm-diameter stainless steel mold. Press the solar salt into the center of the mold and apply a pressure of 10 MPa to tightly encapsulate the slurry. Freeze the mold using liquid nitrogen for 12 hours. After freezing, place the mold in a freeze dryer at -70°C and a vacuum of <15 Pa. Dry for 24 hours to obtain the capsule precursor.

[0069] S3, heating the capsule precursor at 400°C for 2 hours in an argon atmosphere to remove the polyvinyl alcohol; then heating to 800°C and heating for 2 hours and then cooling to thermally reduce the graphene, and the solar salt melts and crystallizes to obtain a graphene capsule molten salt phase change material.

[0070] Example 3

[0071] This embodiment provides a method for preparing a graphene capsule molten salt phase change material, comprising:

[0072] S1. Preparation of few-layer graphene oxide: 2g of flake graphite was mixed with 10ml of sulfuric acid and stirred at 80°C. 1.7g of sodium nitrate and 1.7g of phosphorus pentoxide were then added sequentially and stirred for 12 hours. The mixture was filtered twice and air-dried at room temperature for 3 hours to obtain a precursor. 80ml of sulfuric acid was added to a beaker, along with the precursor. 10g of potassium permanganate was slowly added to the beaker at -10°C and stirred for 4.5 hours. The mixture was then treated in a 35°C water bath for 4 hours. The mixture was diluted in 1000ml of water and stirred for 15 minutes. The mixture was heated to 90°C and hydrogen peroxide was added until the solution turned bright yellow to obtain a dispersion. The dispersion was centrifuged at 10,000 rpm for 7 minutes, then at 6,000 rpm for 5 minutes to remove impurities, and then centrifuged again at 10,000 rpm for 15 minutes to obtain a concentrate. The concentrate was freeze-dried for 48 hours to obtain few-layer graphene oxide.

[0073] Dissolve few-layer graphene oxide, polyvinyl alcohol, and sodium ascorbate in water to prepare a mixture of 10 mg / mL few-layer graphene oxide, 120 mg / mL polyvinyl alcohol, and 0.3 mg / mL sodium ascorbate, and stir at 80°C for 24 hours to obtain a uniform slurry.

[0074] S2: Pour the slurry into a 1cm diameter stainless steel mold and press the solar salt into the center of the mold. Apply a pressure of 15MPa to the mold to tightly encapsulate the slurry. Freeze the mold using liquid nitrogen for 12 hours. After freezing, place the mold in a freeze dryer at -70°C and a vacuum of <15Pa for 24 hours to obtain the capsule precursor.

[0075] S3, heating the capsule precursor at 300°C for 2 hours in an argon atmosphere to remove the polyvinyl alcohol; then heating to 600°C and heating for 2 hours and then cooling to thermally reduce the graphene, and the solar salt melts and crystallizes to obtain a graphene capsule molten salt phase change material.

[0076] The performance of the graphene capsule molten salt phase change material prepared in Example 1 was evaluated. Figure 1 This is a scanning electron microscope image of graphene capsule molten salt phase change material. Figure 1 It can be seen that the crystal structure is complete and evenly distributed, with high crystallinity, showing excellent uniformity, proving that the eutectic salt and graphene oxide have achieved good bonding.

[0077] Figure 2 This is the XRD pattern of graphene capsule molten salt phase change material. Figure 2 It can be seen that it has obvious crystallization peaks, indicating that a good crystalline structure has been formed between the graphene capsules and the molten salt. The appearance of these characteristic peaks indicates the effective loading and interaction between the eutectic salt and graphene oxide, indicating that the prepared composite material has high crystallinity and stable structure.

[0078] Figure 3 This is the DSC (differential scanning calorimetry) test result of graphene capsule molten salt phase change material. Figure 3 It can be seen that the graphene capsule molten salt phase change material has excellent energy absorption and release capabilities during the thermal process, has good energy storage performance, and can stably store and release energy during temperature changes.

[0079] The thermal conductivity of the graphene aerogel, eutectic salt, and molten salt phase change energy storage material prepared in Example 1 was tested using a laser flash method, specifically including:

[0080] 1. Prepare samples: Cut and prepare samples, which are square slices.

[0081] 2. Install the sample: Fix the sample in the test equipment, ensuring that the sample surface is flat and in good contact with the equipment.

[0082] 3. Laser irradiation: Use short pulse laser to irradiate one side of the sample.

[0083] 4. Temperature measurement: Record the temperature response on the other side of the sample, usually using an infrared sensor.

[0084] 5. Data analysis: Calculate the thermal diffusivity from the sample's temperature response curve, and then calculate the thermal conductivity based on the sample's density and specific heat capacity.

[0085] The test results are as follows Figure 4 As shown. Figure 4It can be seen that the thermal conductivity of the graphene capsule molten salt phase change material is 0.68W / m·K, which is significantly higher than that of the graphene aerogel alone (0.12W / m·K) and close to the thermal conductivity of solar salt (0.81W / m·K). This shows that the graphene capsule molten salt phase change material, although the molten salt phase change material is coated with graphene aerogel, still has high thermal conductivity; and under the protection of the graphene capsule, the molten salt phase change material is not easy to leak and has high long-term stability. The graphene capsule molten salt phase change material of this application takes into account both high thermal conductivity and long-term stability, significantly improves the thermal management and storage performance of the phase change material in the energy storage scenario, and has broad application prospects.

[0086] The same test was performed on the graphene capsule molten salt phase change materials of Example 2 and Example 3, and their structures and thermal conductivities were similar to those of the graphene capsule molten salt phase change material of Example 1.

[0087] Although this specification has been used to fully describe the present application using general descriptions and specific embodiments, it is obvious to those skilled in the art that modifications or improvements may be made based on the present application. Therefore, such modifications or improvements made without departing from the spirit of the present application are within the scope of protection claimed in this application.

Claims

1. A method for preparing a graphene capsule molten salt phase change material, characterized in that: include: S1, dispersing few-layer graphene oxide, polyvinyl alcohol and sodium ascorbate in water, heating and stirring to obtain a uniform slurry; Wherein, the heating temperature is 80°C; In the slurry, the mass of the few-layer graphene oxide is 10-20 mg / mL, the concentration of polyvinyl alcohol is 120-180 mg / mL, and the concentration of sodium ascorbate is 0.3-0.8 mg / mL; S2, adding the slurry into a mold, filling the solar salt to the center of the slurry, applying pressure to the mold so that the slurry tightly covers the solar salt, and freeze-drying to obtain a capsule precursor; The solar salt is prepared by the following method: NaNO3 and KNO3 are mixed in a mass ratio of 6:4, ground, heated to 400°C for melting, cooled, and ground again to obtain solar salt; S3, calcining the capsule precursor in an inert atmosphere for the first time, and then heating it for the second time to obtain a graphene capsule molten salt phase change material; The temperature of the first calcination is 250-400°C; the temperature of the second calcination is 500-800°C; The inert atmosphere is an argon atmosphere.

2. The preparation method according to claim 1, characterized in that The few-layer graphene oxide is prepared by the following method: Mix graphite and sulfuric acid evenly, add sodium nitrate and phosphorus pentoxide, stir at 80-100°C for 12-24 hours, separate and collect the solid phase, wash and air-dry to obtain the precursor; The precursor is dispersed in excess concentrated sulfuric acid, and potassium permanganate is slowly added thereto. The mixture is stirred at -10 to 0°C, and then diluted with water. The mixture is then heated to 90°C and hydrogen peroxide is added until the solution turns bright yellow to obtain a dispersion. The dispersion is diluted with water, centrifuged at a speed of 5000-7000 r / min to remove multilayer graphene oxide, and then centrifuged at a speed of 10000-12000 r / min to remove impurities. The concentrated solution obtained by centrifugation is freeze-dried to obtain few-layer graphene oxide.

3. The preparation method according to claim 2, wherein: The mass ratio of the graphite, sulfuric acid, sodium nitrate and phosphorus pentoxide is (1-2):(18-20):(1.5-2):(1.5-2); The mass ratio of the potassium permanganate to the graphite is 5:

1.

4. The preparation method according to claim 1, characterized in that The diameter of the mold is 1-3 cm, and the pressure applied to the mold is 10-15 MPa.

5. The preparation method according to claim 1, characterized in that The freeze-drying comprises: The mold is frozen with liquid nitrogen for 10 to 12 hours, and then dried at -70 to -80°C and a vacuum degree of <15 Pa for 24 to 48 hours.

6. The graphene capsule molten salt phase change material prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the graphene capsule molten salt phase change material prepared by the preparation method according to any one of claims 1 to 5 in an energy storage device or energy storage system.

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