Graphene / carbon nanotube composite material, preparation method and application thereof

Abstract

The application belongs to the field of carbon materials, and provides a graphene / carbon nanotube composite material, a preparation method and application thereof. The method for preparing the graphene / carbon nanotube composite material comprises the following steps: uniformly mixing graphene oxide, a soluble metal salt, an organic nitrogen source and water to obtain a suspension; performing hydrothermal reaction on the suspension to obtain a reduced graphene oxide hydrogel; immersing the reduced graphene oxide hydrogel in a phosphoric acid solution, and then performing first drying to obtain a reduced graphene oxide aerogel; and sending the reduced graphene oxide aerogel into a fluidized bed, performing gradient temperature rising treatment in the presence of mixed gas, and performing washing and second drying on the obtained product to obtain the graphene / carbon nanotube composite material. The method can improve the conductivity of the graphene / carbon nanotube composite material.

Description

Technical Field

[0001] This application belongs to the field of carbon materials, specifically, it provides a graphene / carbon nanotube composite material, its preparation method, and its application. Background Technology

[0002] Graphene has found some applications in lithium-ion batteries due to its high conductivity, large specific surface area, and good chemical stability. However, graphene is prone to agglomeration and stacking during use, which greatly reduces its effective specific surface area and makes it difficult to fully utilize its excellent properties. Furthermore, due to its two-dimensional sheet structure, high-current operation can cause Li-ion degradation. + Increased diffusion resistance leads to a decrease in the rate performance of the battery. Carbon nanotubes can be viewed as one-dimensional tubular nanomaterials formed by rolling up graphene sheets. Carbon nanotubes not only have excellent conductivity but also a fibrous structure, which is beneficial for forming an effective conductive network in the electrode.

[0003] Combining graphene with carbon nanotubes holds promise for leveraging the advantages of both to improve the electrochemical performance of lithium-ion batteries. Traditional methods involve mechanically mixing graphene and carbon nanotubes to prepare a slurry, followed by solid-liquid separation and drying to obtain a graphene-carbon nanotube composite. However, this simple mechanical mixing relies solely on physical bonding between the carbon nanotubes and graphene, resulting in excessively high contact resistance and failing to effectively improve conductivity. Related technologies also mention vapor-depositing carbon sources on the graphene surface, allowing graphene to adsorb the carbon source and form carbon nanotubes on its surface. However, graphene's ability to adsorb carbon sources is limited, potentially leading to uneven distribution and stacking of the deposited carbon nanotubes. Summary of the Invention

[0004] To address the aforementioned problems in the existing technology, the purpose of this application is to provide a graphene / carbon nanotube composite material, its preparation method, and its applications. The method of this application can improve the uniformity of carbon nanotube distribution on the graphene surface and enhance the electrical conductivity of the graphene / carbon nanotube composite material.

[0005] The first aspect of this application provides a method for preparing graphene / carbon nanotube composite materials, comprising:

[0006] S1: Graphene oxide, soluble metal salt, organic nitrogen source and water are mixed evenly to obtain a suspension, wherein the metal salt includes at least one of nickel salt, iron salt and cobalt salt;

[0007] S2: The suspension is subjected to a hydrothermal reaction to obtain a reduced graphene oxide hydrogel;

[0008] S3: The reduced graphene oxide hydrogel is impregnated with phosphoric acid solution and then dried to obtain reduced graphene oxide aerogel.

[0009] S4: The reduced graphene oxide aerogel is fed into a fluidized bed and subjected to gradient heating in the presence of a mixed gas. The resulting product is washed and then dried to obtain the graphene / carbon nanotube composite material. The mixed gas contains an inert gas and hydrogen, and the gradient heating process includes three stages: Stage I, Stage II, and Stage III, performed sequentially.

[0010] Stage I: The mixed gas is introduced into the fluidized bed, heated to a first temperature T1, and held at the first temperature T1 for 1 to 3 hours to form metal-nitrogen-phosphorus co-doped graphene;

[0011] Stage II: Heat to the second temperature T2, then introduce carbon source gas, and keep at the second temperature T2 for 0.5 to 2 hours to form carbon nanotubes on the surface of the metal-nitrogen-phosphorus co-doped graphene.

[0012] Phase III: Stop the flow of carbon source gas, then raise the temperature to the third temperature T3, and hold at the third temperature T3 for 1 to 3 hours;

[0013] T1, T2, and T3 satisfy the following relationship: 100℃≤T2-T1≤300℃, 150℃≤T3-T2≤300℃, and T1 is 400~600℃.

[0014] In the method provided in this application, metal ions are anchored by the electrostatic interaction of oxygen-containing functional groups on the surface of graphene oxide, which can fix the growth sites of carbon nanotubes. By adopting a three-stage gradient heating, sufficient pores can be formed in the graphene, promoting the effective doping of N and P, increasing defect sites, and promoting the fixation and uniform distribution of carbon nanotubes on the surface of graphene in the presence of metal particles (formed after the reduction of metal ions by hydrogen), thereby improving the conductivity of graphene / carbon nanotube composite materials.

[0015] In some embodiments of this application, the graphene oxide has a sheet diameter of 1–20 μm, a thickness of 1–5 nm, and an oxygen content of 30%–60%.

[0016] In some embodiments of this application, step S1 includes the following process:

[0017] Graphene oxide was dispersed in water to prepare a graphene oxide dispersion.

[0018] The soluble metal salt and the organic nitrogen source are added to the graphene oxide dispersion and mixed evenly to obtain the suspension.

[0019] Furthermore, the concentration of the graphene oxide dispersion is 1–20 mg / mL.

[0020] In some embodiments of this application, the organic nitrogen source is selected from at least one of urea, glycine, dicyandiamide, thiourea, and melamine.

[0021] In some embodiments of this application, the mass ratio of the organic nitrogen source to the graphene oxide is (1-5):1.

[0022] In some embodiments of this application, the iron salt is selected from one or more of ferric chloride, ferric nitrate, and ferric sulfate.

[0023] In some embodiments of this application, the nickel salt is selected from one or more of nickel chloride, nickel nitrate, and nickel sulfate.

[0024] In some embodiments of this application, the cobalt salt is selected from one or more of cobalt chloride, cobalt nitrate, and cobalt sulfate.

[0025] In some embodiments of this application, the mass ratio of the soluble metal salt to the graphene oxide is (2-7):1.

[0026] In some embodiments of this application, the temperature of the hydrothermal reaction is 150–250°C, and the reaction time is 6–24 hours.

[0027] In some embodiments of this application, the concentration of the phosphoric acid solution is 0.05 to 1 mol / L.

[0028] Furthermore, the impregnation time is 12–24 hours. This ensures a high phosphorus doping level while minimizing graphene structure collapse caused by excessively long impregnation times.

[0029] In some embodiments of this application, the first drying method is freeze drying, and the freeze drying time is 15 to 30 hours.

[0030] In some embodiments of this application, the volume ratio of inert gas to hydrogen in the mixed gas is (1-5):1.

[0031] In some embodiments of this application, the inert gas is argon.

[0032] In some embodiments of this application, in stages I, II and III, the flow rate of the mixed gas is 1 to 5 L / min relative to 10 g of the reduced graphene oxide aerogel.

[0033] In some embodiments of this application, the carbon source gas is selected from at least one of methane, ethane, ethylene, and acetylene.

[0034] In some embodiments of this application, in stage II, the flow rate of the carbon source gas is 0.5 to 2 L / min relative to 10 g of the reduced graphene oxide aerogel.

[0035] In some embodiments of this application, the second temperature T2 is 700–850°C.

[0036] In some embodiments of this application, the third temperature T3 is 900–1100°C.

[0037] The second aspect of this application provides a graphene / carbon nanotube composite material prepared by the method described in the first aspect of this application.

[0038] The third aspect of this application provides the application of the graphene / carbon nanotube composite material described in the second aspect of this application in lithium-ion batteries.

[0039] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0040] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, wherein:

[0041] Figure 1 This is a schematic diagram of the process for preparing graphene / carbon nanotube composite materials according to one embodiment.

[0042] Figure 2 This is a schematic diagram of the structure of a graphene / carbon nanotube composite material prepared according to one embodiment.

[0043] Figure 3 Impedance comparison diagrams of graphene / carbon nanotube composite materials used in Examples 1 and Comparative Examples 1-3 in lithium-ion battery applications.

[0044] Figure 4 The graph shows a comparison of the rate performance of the graphene / carbon nanotube composite materials used in Examples 1 and Comparative Examples 1-3 in lithium-ion battery applications. Detailed Implementation

[0045] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0046] The "scope" disclosed in this application is defined in the form of a lower limit and / or an upper limit, whereby a given scope is defined by selecting a lower limit and / or an upper limit. This type of scope may or may not include endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form an undefined scope, and any lower limit can be combined with other lower limits to form an undefined scope, similarly, any upper limit can be combined with any other upper limit to form an undefined scope. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and can be combined with any other point or single value, or with other lower or upper limits, to form an undefined scope.

[0047] Unless otherwise specified, all embodiments and optional embodiments of this application may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0048] In this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated.

[0049] In a first aspect, this application provides a method for preparing graphene / carbon nanotube composite materials, the method comprising the following steps S1 to S4.

[0050] S1: Mix graphene oxide, soluble metal salt, organic nitrogen source and water evenly to obtain a suspension;

[0051] S2: The suspension is subjected to a hydrothermal reaction to obtain a reduced graphene oxide hydrogel;

[0052] S3: The reduced graphene oxide hydrogel is impregnated with phosphoric acid solution and then dried to obtain reduced graphene oxide aerogel.

[0053] S4: The reduced graphene oxide aerogel is fed into a fluidized bed and subjected to gradient heating in the presence of a mixed gas (including inert gas and hydrogen). The resulting product is washed and then dried to obtain a graphene / carbon nanotube composite material.

[0054] According to this application, step S1 enables in-situ fixation of metal ions and organic nitrogen sources on graphene oxide.

[0055] In this application, the diameter of the graphene oxide sheet can be 1–20 μm, such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, etc.; the thickness of the graphene oxide can be 1–5 nm, such as 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, etc.; the oxygen content of the graphene oxide can be 30%–60%, such as 30%, 35%, 40%, 42%, 45%, 47%, 50%, 60%, etc.

[0056] In this application, the graphene oxide can be obtained commercially or prepared by methods well known in the art, such as the Hummers process using various graphite as raw materials.

[0057] In this application, the soluble metal salt refers to a metal salt that can dissolve in water, specifically including at least one of nickel salts, iron salts, and cobalt salts. During the gradient heating in step S4, the soluble metal salt is reduced to metal particles by hydrogen, thus acting as a catalyst.

[0058] As some preferred examples, the nickel salt is selected from one or more of nickel chloride, nickel nitrate, and nickel sulfate.

[0059] As some preferred examples, the iron salt is selected from one or more of ferric chloride, ferric nitrate, and ferric sulfate.

[0060] As some preferred examples, the cobalt salt is selected from one or more of cobalt chloride, cobalt nitrate, and cobalt sulfate.

[0061] In this application, the mass ratio of the soluble metal salt to the graphene oxide is typically (1–10):1, for example, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc. To promote carbon nanotube formation while minimizing metal particle agglomeration due to excessive metal salt content, the preferred mass ratio of the soluble metal salt to the graphene oxide is (2–7):1.

[0062] In this application, the organic nitrogen source can be selected from various organic compounds containing one or more amino groups. The amino groups crosslink with the oxygen-containing functional groups in graphene oxide, fixing the N element sites and hindering graphene aggregation.

[0063] In some embodiments, the organic nitrogen source is selected from at least one of urea, glycine, dicyandiamide, thiourea, and melamine.

[0064] In some embodiments, the mass ratio of the organic nitrogen source to the graphene oxide can be (1-5):1, for example, 1:1, 2:1, 2.5:1, 3:1, 4:1, 5:1, etc.

[0065] In some implementations, step S1 includes the following process:

[0066] Graphene oxide was dispersed in water to prepare a graphene oxide dispersion.

[0067] The soluble metal salt and the organic nitrogen source are added to the graphene oxide dispersion and mixed evenly to obtain the suspension.

[0068] Furthermore, the concentration of the graphene oxide dispersion can be 1 to 20 mg / mL, such as 1 mg / mL, 2 mg / mL, 3 mg / mL, 4 mg / mL, 5 mg / mL, 6 mg / mL, 8 mg / mL, 10 mg / mL, 12 mg / mL, 15 mg / mL, 20 mg / mL, etc.

[0069] This application does not have any special requirements for the mixing method, as long as the soluble metal salt and organic nitrogen source are evenly distributed in the graphene oxide dispersion. The mixing method is, for example, but not limited to, ultrasonic or magnetic stirring.

[0070] According to this application, during the hydrothermal reaction in step S2, graphene oxide (GO) in the suspension forms hydrogen bonds with water using its surface oxygen-containing functional groups (such as hydroxyl and carboxyl groups), causing GO to self-assemble into a gel structure in water. Furthermore, the amino groups provided by the organic nitrogen source have reducing properties, enabling partial reduction of GO to obtain a reduced graphene oxide hydrogel. The hydrothermal reaction can be carried out in a high-pressure reactor.

[0071] In some embodiments, the temperature of the hydrothermal reaction can be 150–250°C, for example, 150°C, 160°C, 165°C, 170°C, 172°C, 175°C, 180°C, 190°C, 193°C, 200°C, 210°C, 220°C, 235°C, 240°C, 250°C, etc.; the time of the hydrothermal reaction can be 6–24 hours, for example, 6 hours, 8 hours, 9.5 hours, 10 hours, 12 hours, 15 hours, 16 hours, 17 hours, 18 hours, 20 hours, 22 hours, 24 hours, etc. Preferably, the temperature of the hydrothermal reaction is 160–200°C, and the reaction time is 10–24 hours.

[0072] According to this application, in step S3, impregnating the reduced graphene oxide hydrogel with a phosphoric acid solution promotes the uniform distribution of the phosphorus source in the hydrogel. Furthermore, the relatively large diameter of phosphorus atoms, when co-doped with nitrogen, can further increase the interlayer spacing, prevent graphene stacking, and increase defect sites.

[0073] In some embodiments, the concentration of the phosphoric acid solution can be 0.05 to 1 mol / L, for example 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.5 mol / L, 0.7 mol / L, 1 mol / L, etc.

[0074] In some embodiments, the soaking time is 12 to 24 hours, such as 12 hours, 13 hours, 14 hours, 14.5 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 22 hours, 23 hours, 24 hours, etc.

[0075] In step S3, in order to remove impurities such as free ammonia and metal ions, step S3 preferably further includes: washing the reduced graphene oxide hydrogel, for example, soaking the reduced graphene oxide hydrogel in water for 12-24 hours, then washing it with water until neutral, and then impregnating it with phosphoric acid solution.

[0076] In step S3, the first drying aims to rapidly remove moisture from the hydrogel to form an aerogel. Preferably, the first drying method is freeze-drying, and the freeze-drying time can be 15 to 30 hours, such as 15 hours, 17 hours, 19 hours, 20 hours, 22 hours, 23 hours, 24 hours, 25 hours, 26 hours, 27 hours, 28 hours, 30 hours, etc.

[0077] In some embodiments, the particle size of the reduced graphene oxide aerogel prepared in step S3 is 2–10 μm. This particle size aerogel, when subjected to heating treatment in a fluidized bed, can further increase the contact area between the reduced graphene oxide (rGO) and the gaseous carbon source, thereby improving the adsorption capacity of rGO for the gaseous carbon source.

[0078] According to this application, in step S4, the gradient heating process is carried out in a mixed gas. The mixed gas contains an inert gas and hydrogen. The inert gas can be selected from various gases that do not participate in the reaction, such as argon, nitrogen, etc., preferably argon. In some embodiments, the volume ratio of the inert gas to hydrogen in the mixed gas is (1-5):1, for example, 1:1, 2:1, 3:1, 4:1, 5:1, etc.

[0079] In step S4, the gradient heating process includes three stages: stage I, stage II, and stage III, performed sequentially.

[0080] Stage I: The mixed gas is introduced into the fluidized bed and heated to a first temperature T1 (400-600℃), and held at the first temperature T1 for 1-3 hours to form metal-nitrogen-phosphorus co-doped graphene. In this stage, the reduced graphene oxide aerogel is calcined at low temperature in the mixed gas. On the one hand, the metal ions are reduced to metal nanoparticles under the action of hydrogen. On the other hand, small molecules (such as NH3, H2O, etc.) in the aerogel slowly escape to achieve pore formation and doping of heteroatoms (N and P).

[0081] As some specific examples, the first temperature T1 can be 400℃, 420℃, 450℃, 480℃, 500℃, 520℃, 530℃, 550℃, 560℃, 580℃, 600℃, etc., and the holding time at the first temperature T1 can be 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0082] In stage I, the fluidized bed is preferably heated to the first temperature T1 at a heating rate of 1 to 10 °C / min (e.g., 5 °C / min, 10 °C / min).

[0083] Stage II: The temperature is raised to a second temperature T2 (100℃≤T2-T1≤300℃), and then a carbon source gas is continuously introduced while maintaining the temperature at T2 for 0.5–2 hours to form carbon nanotubes on the metal-nitrogen-phosphorus co-doped graphene. In this stage, the carbon source gas introduced in the presence of the mixed gas enables the carbon nanotubes to grow in situ at the metal sites of the graphene.

[0084] In some embodiments, the second temperature T2 is 700 to 850°C, for example 700°C, 720°C, 750°C, 770°C, 800°C, 810°C, 815°C, 830°C, 850°C, etc.

[0085] In stage II, the fluidized bed is preferably heated to the second temperature T2 at a heating rate of 1 to 10 °C / min (e.g., 5 °C / min, 10 °C / min).

[0086] In Stage II, the carbon source gas can be selected from various organic carbon source gases. According to some embodiments, the carbon source gas is selected from at least one of methane, ethane, ethylene, and acetylene.

[0087] In some embodiments, during stage II, the flow rate of the carbon source gas can be 0.5 to 2 L / min relative to each 10 g of the reduced graphene oxide aerogel, for example, 0.5 L / min, 0.7 L / min, 1 L / min, 1.2 L / min, 1.5 L / min, 1.6 L / min, 2 L / min, etc.

[0088] As some examples, the holding time at the second temperature T2 can be 1 hour, 1.5 hours, 2 hours, etc.

[0089] Stage III: Stop the flow of the carbon source gas, then raise the temperature to a third temperature T3 (150℃≤T3-T2≤300℃), and hold at the third temperature T3 for 1–3 hours. This high-temperature treatment reduces oxygen-containing functional groups and defects in graphene, increases the graphitization degree of graphene and carbon nanotubes, enhances their order and crystallinity, and further improves the conductivity of the composite material.

[0090] In some embodiments, the third temperature T3 is 900 to 1100°C, such as 900°C, 920°C, 950°C, 960°C, 965°C, 970°C, 1000°C, 1050°C, 1100°C, etc.

[0091] In stage III, the fluidized bed is preferably heated to the third temperature T3 at a heating rate of 1 to 5 °C / min (e.g., 1 °C / min, 5 °C / min).

[0092] As examples, the holding time at the third temperature T2 can be 1h, 1.5h, 2h, 2.5h, 3h, etc.

[0093] In some embodiments, in stages I, II and III, the flow rate of the mixed gas relative to each 10g of the reduced graphene oxide aerogel can be 1 to 5 L / min, for example 1 L / min, 2 L / min, 3 L / min, 4 L / min, 5 L / min, etc.

[0094] Step S4 further includes washing and drying the product after gradient heating treatment. The washing method may include first acid washing the product, followed by water washing until neutral, for example, using a 0.5–1 mol / L hydrochloric acid or sulfuric acid solution for acid washing.

[0095] In some embodiments, the second drying method is vacuum drying. The temperature for vacuum drying can be, for example, 50–90°C, and the drying time can be 15–30 hours.

[0096] According to some specific implementation methods, such as Figure 1 As shown, the process for preparing the graphene / composite material includes:

[0097] Graphene oxide was dispersed in water to prepare a graphene oxide dispersion.

[0098] A reduced graphene oxide hydrogel was prepared by uniformly dispersing graphene oxide dispersion, soluble metal salt and organic nitrogen source and then undergoing hydrothermal reaction.

[0099] Reduced graphene oxide hydrogel was impregnated with phosphoric acid solution and then freeze-dried to obtain reduced graphene oxide aerogel.

[0100] Reduced graphene oxide aerogel was subjected to gradient heating treatment, and the resulting product was acid washed, water washed and vacuum dried to obtain a graphene / carbon nanotube composite material.

[0101] In the method of this application, during the gradient heating process of reduced graphene oxide aerogel, the organic nitrogen source and phosphoric acid on the graphene release gases upon heating. This allows for one-step heteroatom doping and pore formation, reducing graphene agglomeration and increasing its specific surface area. Furthermore, the use of a fluidized bed increases the contact area between the graphene and the gaseous carbon source, reducing the inhomogeneity of carbon nanotube deposition. Additionally, the metal ions in situ fixed on the graphene are reduced to metal nanoparticles during the gradient heating process. These nanoparticles act as catalysts, promoting the in-situ growth of carbon nanotubes on the graphene and resolving the issue of loose contact caused by physical mixing.

[0102] Therefore, in a second aspect, this application provides a graphene / carbon nanotube composite material prepared by the method described in the first aspect of this application. According to some embodiments, a schematic diagram of the structure of the graphene / carbon nanotube composite material is shown below. Figure 2 As shown, the graphene in the composite material has a two-dimensional planar structure (shown in black), and carbon nanotubes grow radially in one dimension within the pores of the graphene sheets (shown in longitudinal lines).

[0103] Thirdly, this application provides the application of the graphene / carbon nanotube composite material described in the second aspect of this application in lithium-ion batteries.

[0104] The graphene / carbon nanotube composite material of this application can be used as a conductive agent in lithium-ion batteries, for example, as a conductive agent in a negative electrode material. When the graphene / carbon nanotube composite material is combined with components such as a negative electrode material and binder to form a negative electrode film, it can achieve "point-line-surface" contact with the negative electrode material, thereby better promoting the electron transfer process.

[0105] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0106] In the following examples and comparative examples, the argon-hydrogen mixture refers to a mixture of argon and hydrogen in a volume ratio of 2:1. Furthermore, unless otherwise stated, the argon-hydrogen mixture is continuously introduced during the three-stage gradient heating process, and a carbon source gas is introduced in stage II.

[0107] Example 1

[0108] Graphene oxide (GO, 2 μm in diameter, 3 nm in thickness, and 40% oxygen content) was dispersed in water to prepare a GO dispersion A1 with a concentration of 5 mg / mL.

[0109] Nickel nitrate was added to dispersion A1, and the mixture was ultrasonically dispersed for 60 min. Then, it was stirred at 300 rpm for 30 min. Glycine was added, and the mixture was stirred for another 30 min to obtain a suspension. The mass ratio of nickel nitrate, glycine, and GO in dispersion A1 was 5:4:1.

[0110] After the suspension was transferred into a high-pressure reactor, it was reacted at 180°C for 12 hours to obtain reduced graphene oxide (rGO) hydrogel B1.

[0111] Hydrogel B1 was soaked and washed with water until neutral, then immersed in 0.1 mol / L phosphoric acid solution for 18 h, and then transferred to a freeze dryer for 24 h to obtain rGO aerogel C1;

[0112] 10g of aerogel C1 was transferred into a fluidized bed, and an argon-hydrogen mixture was introduced while the temperature was gradually increased. The first stage involved increasing the temperature to 500℃ at 5℃ / min and holding for 2 hours, with the argon-hydrogen mixture flowing at a rate of 3L / min. The second stage involved increasing the temperature to 800℃ at 5℃ / min, then introducing a carbon source (acetylene) and holding for 1 hour, with the argon-hydrogen mixture flowing at a rate of 3L / min and the carbon source flowing at a rate of 1L / min. The third stage involved increasing the temperature to 1000℃ at 2℃ / min and holding for 2 hours, with the argon-hydrogen mixture flowing at a rate of 3L / min. Finally, the temperature was lowered to room temperature to obtain dry powder D1.

[0113] The dry powder D1 was placed in a 1 mol / L sulfuric acid solution, stirred at 300 rpm for 60 min, washed with water until neutral, and then vacuum dried at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-1.

[0114] Example 2

[0115] Graphene oxide (GO, 5 μm in diameter, 3 nm in thickness, and 40% oxygen content) was dispersed in water to prepare a GO dispersion A2 with a concentration of 5 mg / mL.

[0116] Ferric chloride was added to GO dispersion A2, and the mixture was ultrasonically dispersed for 40 min, then stirred at 300 rpm for 30 min. Glycine was then added, and the mixture was stirred for another 30 min to obtain a suspension. The mass ratio of ferric chloride, glycine, and GO in dispersion A2 was 4:4:1.

[0117] After the suspension was transferred into a high-pressure reactor, it was reacted at 180°C for 12 hours to obtain reduced graphene oxide (rGO) hydrogel B2.

[0118] Hydrogel B2 was soaked and washed with water until neutral, then immersed in 0.2 mol / L phosphoric acid solution for 14 h, and then transferred to a freeze dryer for 24 h to obtain rGO aerogel C2.

[0119] 10g of aerogel C2 was transferred into a fluidized bed, and an argon-hydrogen mixture was introduced while the temperature was gradually increased. The first stage involved increasing the temperature to 600℃ at 10℃ / min and holding for 1.5h, with the argon-hydrogen mixture flowing at a rate of 4L / min. The second stage involved increasing the temperature to 850℃ at 5℃ / min, then introducing a carbon source (methane) and holding for 1h, with the argon-hydrogen mixture flowing at a rate of 4L / min and the carbon source flowing at a rate of 1L / min. The third stage involved increasing the temperature to 950℃ at 2℃ / min and holding for 2h, with the argon-hydrogen mixture flowing at a rate of 4L / min. Finally, the temperature was lowered to room temperature to obtain dry powder D2.

[0120] The dry powder D2 was placed in a 1 mol / L hydrochloric acid solution, stirred at 300 rpm for 60 min, washed with water until neutral, and then vacuum dried at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-2.

[0121] Example 3

[0122] Graphene oxide (GO, 2 μm in diameter, 3 nm in thickness, and 50% oxygen content) was dispersed in water to prepare a GO dispersion A3 with a concentration of 2 mg / mL.

[0123] Nickel nitrate was added to dispersion A3, and the mixture was ultrasonically dispersed for 60 min. Then, it was stirred at 300 rpm for 30 min. Glycine was added, and the mixture was stirred for another 30 min to obtain a suspension. The mass ratio of nickel nitrate, glycine, and GO in dispersion A3 was 5:5:1.

[0124] After the suspension was transferred into a high-pressure reactor, it was reacted at 160°C for 18 hours to obtain reduced graphene oxide (rGO) hydrogel B3.

[0125] Hydrogel B3 was soaked and washed with water until neutral, then immersed in 0.1 mol / L phosphoric acid solution for 18 h, and then transferred to a freeze dryer for 24 h to obtain rGO aerogel C3.

[0126] 10g of aerogel C3 was transferred into a fluidized bed, and an argon-hydrogen mixture was introduced while the temperature was gradually increased. The first stage involved increasing the temperature to 550℃ at 5℃ / min and holding for 2 hours, with the argon-hydrogen mixture flowing at a rate of 3L / min. The second stage involved increasing the temperature to 750℃ at 5℃ / min, then introducing a carbon source (acetylene) and holding for 1.5 hours, with the argon-hydrogen mixture flowing at a rate of 3L / min and the carbon source flowing at a rate of 0.5L / min. The third stage involved increasing the temperature to 1050℃ at 2℃ / min and holding for 1.5 hours, with the argon-hydrogen mixture flowing at a rate of 3L / min. Finally, the temperature was lowered to room temperature to obtain dry powder D3.

[0127] The dry powder D3 was placed in a 1.5 mol / L sulfuric acid solution, stirred at 350 rpm for 60 min, washed with water until neutral, and then vacuum dried at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-3.

[0128] Example 4

[0129] The graphene / carbon nanotube composite material was prepared according to the method of Example 1, except that glycine was replaced with melamine and the mass ratio of melamine to graphene oxide was 3:1. The prepared graphene / carbon nanotube composite material was denoted as G / C-4.

[0130] Example 5

[0131] The graphene / carbon nanotube composite material was prepared according to the method of Example 1, except that glycine was replaced with thiourea and the mass ratio of thiourea to graphene oxide was 5:1. The prepared graphene / carbon nanotube composite material is denoted as G / C-5.

[0132] Example 6

[0133] The graphene / carbon nanotube composite material was prepared according to the method of Example 1, except that nickel nitrate was replaced with cobalt nitrate and the mass ratio of cobalt nitrate to graphene oxide was 2:1. The prepared graphene / carbon nanotube composite material is denoted as G / C-6.

[0134] Comparative Example 1

[0135] Graphene oxide (GO, 2 μm in diameter, 3 nm in thickness, and 40% oxygen content) was dispersed in water to prepare a GO dispersion A1 with a concentration of 5 mg / mL.

[0136] Nickel nitrate was added to dispersion A1, and the mixture was ultrasonically dispersed for 60 min, then stirred at 300 rpm for 30 min to obtain a suspension; wherein the mass ratio of nickel nitrate to GO in dispersion A1 was 5:1.

[0137] After the suspension was transferred into a high-pressure reactor, it was reacted at 180°C for 18 hours to obtain reduced graphene oxide (rGO) hydrogel B-11.

[0138] Hydrogel B-11 was soaked and washed with water until neutral, and then transferred to a freeze dryer for 24 hours to obtain rGO aerogel C-11;

[0139] 10g of aerogel C-11 was transferred into a fluidized bed, and an argon-hydrogen mixture was introduced while the temperature was gradually increased. The first stage involved increasing the temperature to 500℃ at 5℃ / min and holding for 2 hours, with the argon-hydrogen mixture flowing at a rate of 3L / min. The second stage involved increasing the temperature to 800℃ at 5℃ / min, then introducing a carbon source (acetylene) and holding for 1 hour, with the argon-hydrogen mixture flowing at a rate of 3L / min and the carbon source flowing at a rate of 1L / min. The third stage involved increasing the temperature to 1000℃ at 2℃ / min and holding for 2 hours, with the argon-hydrogen mixture flowing at a rate of 3L / min. Finally, the temperature was lowered to room temperature to obtain dry powder D-11.

[0140] The dry powder D-11 was placed in a 1 mol / L sulfuric acid solution, stirred at 300 rpm for 60 min, washed with water until neutral, and then vacuum dried at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-d1.

[0141] Comparative Example 2

[0142] Graphene oxide (GO, 2 μm in diameter, 3 nm in thickness, and 40% oxygen content) was dispersed in water to prepare a GO dispersion A1 with a concentration of 5 mg / mL.

[0143] Nickel nitrate was added to dispersion A1, and the mixture was ultrasonically dispersed for 60 min. Then, it was stirred at 300 rpm for 30 min. Glycine was added, and the mixture was stirred for another 30 min to obtain a suspension. The mass ratio of nickel nitrate, glycine, and GO in dispersion A1 was 5:4:1.

[0144] After the suspension was transferred into a high-pressure reactor, it was reacted at 180°C for 12 hours to obtain reduced graphene oxide (rGO) hydrogel B1.

[0145] Hydrogel B1 was soaked and washed with water until neutral, then immersed in 0.1 mol / L phosphoric acid solution for 18 h, and then transferred to a freeze dryer for 24 h to obtain rGO aerogel C1;

[0146] 10g of aerogel C1 was transferred into a rotary kiln, and an argon-hydrogen mixture was introduced with a gradient temperature increase. The first temperature increase was at 5℃ / min to 500℃, held for 2 hours, with an argon-hydrogen mixture flow rate of 3L / min. The second temperature increase was at 5℃ / min to 800℃, followed by the introduction of a carbon source (acetylene) and holding for 1 hour, with an argon-hydrogen mixture flow rate of 3L / min and a carbon source flow rate of 1L / min. The third temperature increase was at 2℃ / min to 1000℃, held for 2 hours, with an argon-hydrogen mixture flow rate of 3L / min. Finally, the temperature was cooled to room temperature to obtain dry powder D-12.

[0147] The dry powder D-12 was placed in a 1 mol / L sulfuric acid solution, stirred at 300 rpm for 60 min, washed with water until neutral, and then vacuum dried at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-d2.

[0148] Comparative Example 3

[0149] Graphene oxide (GO, 2 μm in diameter, 3 nm in thickness, and 40% oxygen content) was dispersed in water to prepare a GO dispersion A1 with a concentration of 5 mg / mL.

[0150] Nickel nitrate was added to dispersion A1, and the mixture was ultrasonically dispersed for 60 min. Then, it was stirred at 300 rpm for 30 min. Glycine was added, and the mixture was stirred for another 30 min to obtain a suspension. The mass ratio of nickel nitrate, glycine, and GO in dispersion A1 was 5:4:1.

[0151] After the suspension was transferred into a high-pressure reactor, it was reacted at 180°C for 12 hours to obtain reduced graphene oxide (rGO) hydrogel B1.

[0152] Hydrogel B1 was soaked and washed with water until neutral, then immersed in 0.1 mol / L phosphoric acid solution for 18 h, and then transferred to a freeze dryer for 24 h to obtain rGO aerogel C1;

[0153] 10g of aerogel C1 was transferred into a fluidized bed, and an argon-hydrogen mixture was introduced and subjected to a temperature increase of 5℃ / min to 800℃. Then, a carbon source (acetylene) was introduced for 1h, and the temperature was maintained at 800℃ for a total of 5h. The argon-hydrogen mixture flow rate was 3L / min. Finally, the temperature was cooled to room temperature to obtain dry powder D-13.

[0154] The dry powder D-13 was placed in a 1 mol / L sulfuric acid solution, stirred at 300 rpm for 60 min, washed with water until neutral, and then vacuum dried at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-d3.

[0155] Test case

[0156] The test examples are used to illustrate the application performance of the graphene / carbon nanotube composite materials G / C-1 to G / C-6 and G / C-d1 to G / C-d3 prepared in the above examples and comparative examples.

[0157] 1. Resistivity test: The resistivity of the powder was tested using a GM-I type multifunctional automatic powder resistivity tester. The test results are shown in Table 1.

[0158] Table 1

[0159] Example 1 G / C-1 0.05 Example 2 G / C-2 0.03 Example 3 G / C-3 0.08 Example 4 G / C-4 0.06 Example 5 G / C-5 0.03 Example 6 G / C-6 0.04 Comparative Example 1 G / C-d1 0.12 Comparative Example 2 G / C-d2 0.23 Comparative Example 3 G / C-d3 0.18

[0160] As can be seen from Table 1, comparing Examples 1-6 with Comparative Examples 1-3, the graphene / carbon nanotube composite materials prepared in Examples 1-6 have lower resistivity and higher conductivity.

[0161] Comparing Example 1 with Comparative Examples 1-3, it can be seen that Example 1 uses N and P co-doping to change the electronic structure of graphene, improves electronic conductivity, increases defect sites, and improves electrochemical activity; the fluidized bed ensures full contact between the gaseous carbon source and the material to ensure uniform growth of carbon nanotubes; the three-stage heating ensures sufficient pore formation, doping, and uniform growth of carbon nanotubes, thereby improving the overall conductivity.

[0162] 2. Electrical performance testing

[0163] G / C-1 to G / C-6 and G / C-d1 to G / C-d3 were used as conductive agents to make button cells, and their performance was tested.

[0164] A conductive slurry with a conductive agent content of 3% and a dispersant concentration of 1wt% was prepared by mixing graphene / carbon nanotube composite material, dispersant (CMC) and water, and then ultrasonically dispersing it with a rod for 1 hour.

[0165] A silicon-carbon anode material (as the active material), a binder (model LA133), and a conductive paste were mixed in a mass ratio of active material: binder: conductive agent = 95.5:4:0.5, using water as a solvent to prepare the anode paste. The paste was then coated onto copper foil, with a lithium sheet as the counter electrode. Together with an electrolyte (1M LiPF6 electrolyte, solvent composition: ethylene carbonate (EC) and dimethyl carbonate (DMC), v / v = 50:50) and a PE porous separator, a 2032-type button battery was fabricated for evaluation.

[0166] (1) Impedance test

[0167] The EIS of the battery was obtained using an electrochemical workstation. The test frequency range was 0.01–1,000,000 Hz, and the scan voltage was 10 mV. The results are as follows: Figure 3 As shown.

[0168] (2) Ratio Performance Test

[0169] At 25°C, the battery was charged with low current in CC-CV charging mode, and 3 cycles of charge and discharge were performed, with a voltage range of 2.8 to 4.25V. The battery was then charged at a constant current of 0.3C and discharged at constant currents of 1C, 2C, and 3C to obtain the capacity retention rate.

[0170] The results are shown in Table 2 and Figure 4 As shown.

[0171] Table 2

[0172] G / C-1 100％ 94.60％ 85.45％ 70.76％ G / C-2 100％ 95.12％ 86.77％ 72.13％ G / C-3 100％ 93.76％ 84.56％ 70.02％ G / C-4 100％ 94.32％ 85.12％ 70.21％ G / C-5 100％ 95.21％ 86.44％ 71.92％ G / C-6 100％ 94.86％ 85.68％ 70.95％ G / C-d1 100％ 92.12％ 80.06％ 65.44％ G / C-d2 100％ 89.34％ 74.45％ 53.12％ G / C-d3 100％ 91.15％ 78.22％ 60.45％

[0173] Figure 3 The figure shows the battery impedance diagram, where the horizontal axis Z' represents the real part of the impedance and the vertical axis Z” represents the negative imaginary part. As can be seen from the figure, compared with Comparative Examples 1-3, the lithium battery prepared by the graphene / carbon nanotube composite material (conductive agent G / C-1) in Example 1 exhibits low resistance and high conductivity.

[0174] Table 2 and Figure 4 For the scaling effect, combine Table 2 and Figure 4 It can be seen that, compared with Comparative Examples 1-3, the graphene / carbon nanotube composite materials prepared in Examples 1-6 maintain a high capacity retention rate as conductive agents at different rates, which can effectively improve the rate performance of lithium batteries.

[0175] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for preparing graphene / carbon nanotube composite materials, characterized in that, include: S1: Graphene oxide, soluble metal salt, organic nitrogen source and water are mixed evenly to obtain a suspension, wherein the soluble metal salt includes at least one of nickel salt, iron salt and cobalt salt; S2: The suspension is subjected to a hydrothermal reaction to obtain a reduced graphene oxide hydrogel; S3: The reduced graphene oxide hydrogel is impregnated with phosphoric acid solution and then dried to obtain reduced graphene oxide aerogel; wherein the concentration of the phosphoric acid solution is 0.05~1 mol / L; S4: The reduced graphene oxide aerogel is fed into a fluidized bed and subjected to gradient heating in the presence of a mixed gas. The resulting product is washed and then dried to obtain a graphene / carbon nanotube composite material. The mixed gas contains an inert gas and hydrogen. The gradient heating process includes three stages: Stage I, Stage II, and Stage III, performed sequentially. Stage I: The mixed gas is introduced into the fluidized bed, heated to a first temperature T1, and held at the first temperature T1 for 1 to 3 hours to form metal-nitrogen-phosphorus co-doped graphene; Stage II: The temperature is raised to the second temperature T2, and then carbon source gas is introduced and kept at the second temperature T2 for 0.5~2h to form carbon nanotubes on the surface of the metal-nitrogen-phosphorus co-doped graphene; wherein, the flow rate of the carbon source gas is 0.5~2L / min relative to each 10g of the reduced graphene oxide aerogel. Phase III: Stop the flow of carbon source gas, then raise the temperature to the third temperature T3, and hold at the third temperature T3 for 1~3 hours; T1, T2, and T3 satisfy the following relationship: 100℃≤T2-T1≤300℃, 150℃≤T3-T2≤300℃, and T1 is 400~600℃.

2. The method according to claim 1, characterized in that, The graphene oxide has a sheet diameter of 1~20μm, a thickness of 1~5nm, and an oxygen content of 30%~60%.

3. The method according to claim 1, characterized in that, Step S1 includes the following process: Graphene oxide was dispersed in water to prepare a graphene oxide dispersion. The soluble metal salt and the organic nitrogen source are added to the graphene oxide dispersion and mixed evenly to obtain the suspension.

4. The method according to claim 3, characterized in that, The concentration of the graphene oxide dispersion is 1~20 mg / mL.

5. The method according to any one of claims 1-4, characterized in that, The organic nitrogen source is selected from at least one of urea, glycine, dicyandiamide, thiourea, and melamine.

6. The method according to any one of claims 1-4, characterized in that, The mass ratio of the organic nitrogen source to the graphene oxide is (1~5):

1.

7. The method according to any one of claims 1-4, characterized in that, The iron salt is selected from one or more of ferric chloride, ferric nitrate, and ferric sulfate.

8. The method according to any one of claims 1-4, characterized in that, The nickel salt is selected from one or more of nickel chloride, nickel nitrate, and nickel sulfate.

9. The method according to any one of claims 1-4, characterized in that, The cobalt salt is selected from one or more of cobalt chloride, cobalt nitrate, and cobalt sulfate.

10. The method according to any one of claims 1-4, characterized in that, The mass ratio of the soluble metal salt to the graphene oxide is (2~7):

1.

11. The method according to any one of claims 1-4, characterized in that, The hydrothermal reaction is carried out at a temperature of 150~250℃ for 6~24h.

12. The method according to any one of claims 1-4, characterized in that, The soaking time is 12-24 hours.

13. The method according to any one of claims 1-4, characterized in that, The first drying method is freeze drying, and the freeze drying time is 15-30 hours.

14. The method according to any one of claims 1-4, characterized in that, In the mixed gas, the volume ratio of inert gas to hydrogen is (1~5):

1.

15. The method according to any one of claims 1-4, characterized in that, The inert gas is argon.

16. The method according to any one of claims 1-4, characterized in that, In stages I, II, and III, the flow rate of the mixed gas is 1 to 5 L / min relative to each 10 g of the reduced graphene oxide aerogel.

17. The method according to any one of claims 1-4, characterized in that, The carbon source gas is selected from at least one of methane, ethane, ethylene, and acetylene.

18. The method according to any one of claims 1-4, characterized in that, The second temperature T2 is 700~850℃.

19. The method according to any one of claims 1-4, characterized in that, The third temperature T3 is 900~1100℃.

20. A graphene / carbon nanotube composite material prepared by the method according to any one of claims 1-19.

21. The application of the graphene / carbon nanotube composite material of claim 20 in lithium-ion batteries.

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