Carbon nanotube graphene composite conductive agent, preparation method thereof and lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a carbon nanotube graphene composite conductive agent and a preparation method thereof and a lithium ion battery, wherein the carbon nanotube graphene composite conductive agent comprises the following raw materials in parts by weight: modified graphene 1.5-3 parts, carbon nanotubes 7-9 parts, a surfactant 1.2-3.2 parts and N-methyl pyrrolidone 85-95 parts; the modified graphene is prepared by grafting hyperbranched polymers on the surface of graphene oxide and then reducing; the modified graphene has the advantages of improving the dispersibility of graphene and carbon nanotubes, thereby improving the conductive performance of the carbon nanotube graphene composite conductive agent.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries, and in particular to a carbon nanotube-graphene composite conductive agent, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are rechargeable batteries that primarily function by the movement of lithium ions between the positive and negative electrodes. Adding a certain amount of conductive agent during electrode fabrication ensures good charge-discharge performance. The conductive agent collects micro-currents between the active materials and between the active materials and the current collector, reducing electrode contact resistance and accelerating electron movement. It also effectively increases the migration rate of lithium ions within the electrode material, thereby improving the charge-discharge efficiency of the electrode.

[0003] Currently, graphene-carbon nanotube composite conductive agents are considered to be of relatively high quality. These agents are typically obtained by treating graphene with PVP and then mixing it with carbon nanotubes. However, in practical applications, this method cannot achieve sufficient dispersion of graphene and carbon nanotubes, resulting in insufficient conductivity of the graphene-carbon nanotube composite conductive agent. Summary of the Invention

[0004] To improve the dispersibility of graphene and carbon nanotubes, thereby enhancing the conductivity of the carbon nanotube-graphene composite conductive agent, this application provides a carbon nanotube-graphene composite conductive agent, its preparation method, and a lithium-ion battery.

[0005] In a first aspect, this application provides a carbon nanotube-graphene composite conductive agent, employing the following technical solution:

[0006] A carbon nanotube-graphene composite conductive agent comprises the following raw materials in parts by weight: 1.5-3 parts modified graphene, 7-9 parts carbon nanotubes, 1.2-3.2 parts surfactant, and 85-95 parts N-methylpyrrolidone.

[0007] The modified graphene is prepared by grafting hyperbranched polymers onto the surface of graphene oxide and then reducing it.

[0008] By adopting the above technical solution, carbon nanotube-graphene composite conductive agent is applied to the active material of lithium battery electrode. On the one hand, carbon nanotubes fill the spaces between graphene sheets, playing a supporting and isolating role, forming a one-dimensional and two-dimensional interwoven network structure, preventing graphene sheets from overlapping, and allowing graphene to be better dispersed in the electrode material and exert its excellent conductivity. On the other hand, this application modifies the graphene surface by grafting hyperbranched polymers, which can increase the steric hindrance between graphene sheets, thereby reducing its agglomeration and further improving the dispersibility of graphene in the electrode material system. Moreover, the use of hyperbranched polymers to modify graphene can greatly reduce its viscosity compared with conventional polymers. Under the combined effect of these factors, graphene can be uniformly dispersed, greatly improving its conductivity.

[0009] Preferably, the surfactant is a mixture of polyvinylpyrrolidone and small molecule surfactants.

[0010] By adopting the above technical solution, polyvinylpyrrolidone and small molecule surfactants have a synergistic effect. Polyvinylpyrrolidone molecules are relatively large and easily form large micelles in the dispersion system, thereby reducing the effect on graphene. By adding small molecule surfactants, which have smaller molecular weights, they move more flexibly in the system and can easily penetrate between graphene sheets, thus improving the dispersibility of both graphene and carbon nanotubes.

[0011] Preferably, the weight ratio of the polyvinylpyrrolidone to the small molecule surfactant is 1:(0.1-0.4).

[0012] By adopting the above technical solution, the amount of small molecule surfactant added needs to be within a certain range to achieve the best technical effect. As the amount added gradually increases, the conductivity of various properties of carbon nanotube-graphene composite conductive agents tends to decrease; carbon nanotube-graphene composite conductive agents prepared when the weight ratio of polyvinylpyrrolidone to small molecule surfactant is within the range of 1:(0.1-0.4) have more advantages in practical applications.

[0013] Preferably, the small molecule surfactant is one or both of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate.

[0014] By adopting the above technical solutions, the technical effects of this application can be achieved when the small molecule surfactant is one or both of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate.

[0015] Preferably, the modified graphene is prepared by the following method:

[0016] S1. Disperse 0.2-0.4 parts by weight of graphene oxide in 30-40 parts by weight of water to obtain a graphene oxide dispersion.

[0017] S2. Add 3-5 parts by weight of terminal amino hyperbranched polyamide to the graphene oxide dispersion, adjust the pH to 8-10, then heat to 40-50℃ and stir for 20-30 hours to obtain a graphene oxide grafted hyperbranched polymer dispersion.

[0018] S3. Continue to add reducing agent to reduce the hyperbranched polymer grafted with graphene oxide to obtain modified graphene.

[0019] By adopting the above technical solution, the hyperbranched polymer can be successfully grafted onto graphene oxide through the reaction between the carboxyl groups on the surface of graphene oxide and the amino groups in the terminal amino hyperbranched polyamide molecules. Then, through a reduction reaction, graphene oxide can be reduced back to graphene without affecting its conductivity.

[0020] Preferably, the reduction method for the graphene oxide-grafted hyperbranched polymer is as follows:

[0021] Add 0.01-0.03 parts by weight of hydrazine hydrate, heat to 75-85℃, react for 7-9 hours, and then filter to obtain modified graphene.

[0022] By adopting the above technical solution and controlling the amount of hydrazine hydrate, the graphene oxide groups of the hyperbranched polymer grafted onto graphene oxide can be reduced, and based on its steric hindrance, the grafted hyperbranched polymer is unaffected, thereby obtaining modified graphene.

[0023] Preferably, the average diameter of the carbon nanotubes is 30-60 nm.

[0024] By employing the above technical solutions, the average diameter of carbon nanotubes has a significant impact on the conductivity of carbon nanotube-graphene composite conductive agents. An average diameter in the range of 30-60 nm is more suitable for practical applications, while an average diameter greater than 60 nm significantly reduces the conductivity of the carbon nanotube-graphene composite conductive agent.

[0025] Secondly, this application provides a method for preparing a carbon nanotube-graphene composite conductive agent, employing the following technical solution:

[0026] A method for preparing a carbon nanotube-graphene composite conductive agent includes the following preparation steps:

[0027] The surfactant and N-methylpyrrolidone were stirred and mixed to ensure that the surfactant was fully dissolved in the N-methylpyrrolidone.

[0028] Then add carbon nanotubes and modified graphene and stir to disperse them to obtain a pre-dispersed slurry;

[0029] The pre-dispersed slurry was ground for 3-5 hours and then cooled to obtain a carbon nanotube-graphene composite conductive agent.

[0030] By adopting the above technical solution, the preparation method of this application does not have any technical difficulties, does not require modification of production equipment, and is relatively conventional and can be realized in general factories, making it suitable for large-scale production.

[0031] Thirdly, this application provides a lithium-ion battery, which adopts the following technical solution:

[0032] A lithium-ion battery, wherein the conductive agent in the lithium-ion battery electrode is the carbon nanotube-graphene composite conductive agent.

[0033] By adopting the above technical solution, the lithium-ion battery prepared using the carbon nanotube graphene composite conductive agent of this application has a high discharge capacity, with an initial efficiency of 91.3% or higher. At the same time, its capacity decay after 300 cycles is less than 7.3%.

[0034] In summary, this application includes at least one of the following beneficial technical effects:

[0035] 1. The carbon nanotube-graphene composite conductive agent is applied to the active material of lithium battery electrode. On the one hand, carbon nanotubes fill the spaces between graphene sheets, providing support and isolation, forming a one-dimensional and two-dimensional interwoven network structure. This prevents the graphene sheets from overlapping, allowing the graphene to be better dispersed in the electrode material and exert its excellent conductivity. On the other hand, this application modifies the graphene surface by grafting hyperbranched polymers, which increases the steric hindrance between graphene sheets, thereby reducing agglomeration and further improving the dispersibility of graphene in the electrode material system. Moreover, the use of hyperbranched polymers to modify graphene significantly reduces its viscosity compared to conventional polymers. Under these multiple effects, graphene can be uniformly dispersed, greatly improving its conductivity.

[0036] 2. Polyvinylpyrrolidone (PVP) and small-molecule surfactants exhibit a synergistic effect. PPVP molecules are relatively large, easily forming large micelles in the dispersion system, thus reducing its effect on graphene. Adding small-molecule surfactants, with their smaller molecular weight, allows for more flexible movement within the system and easier insertion between graphene sheets, simultaneously improving the dispersibility of both graphene and carbon nanotubes. The amount of small-molecule surfactant added needs to be within a certain range to achieve optimal technical results. As the amount added gradually increases, the conductivity of the carbon nanotube-graphene composite conductive agent tends to decrease. Carbon nanotube-graphene composite conductive agents prepared with a PPVP to small-molecule surfactant weight ratio of 1:(0.1-0.4) are more advantageous in practical applications.

[0037] 3. The lithium-ion batteries prepared using the carbon nanotube-graphene composite conductive agent of this application have high discharge capacity, with initial efficiency of 91.3% and above, and a maximum of 92.2%. At the same time, the capacity decay after 300 cycles is less than 7.3%, with a minimum of 5.4%. This indicates that the carbon nanotube-graphene composite conductive agent prepared in this application has excellent conductivity and the prepared lithium-ion batteries exhibit excellent durability. Detailed Implementation

[0038] The following provides a more detailed description of this application in conjunction with specific details.

[0039] raw material

[0040] In the embodiments of this application, the graphene oxide was prepared by the Hummers method and its thickness was less than 10 nm as determined by AFM; the terminal amino hyperbranched polyamide was of type HyPer N; the polyvinylpyrrolidone was of type K30; and the remaining raw materials were all commercially available products.

[0041] Preparation Example

[0042] Preparation Example 1

[0043] A modified graphene, the preparation method of which is as follows:

[0044] S1. Disperse 0.3g of graphene oxide in 35g of water to obtain a graphene oxide dispersion.

[0045] S2. Add 4g of terminal amino hyperbranched polyamide to the graphene oxide dispersion, adjust the pH to 9, then heat to 45℃ and stir for 25h to obtain a graphene oxide grafted hyperbranched polymer dispersion.

[0046] S3. Add 0.02 g of hydrazine hydrate, heat to 80 °C, react for 8 h, and after the reaction solution cools to room temperature, filter to obtain a precipitate. Wash the precipitate with distilled water, then with methanol, and then vacuum dry at 40 °C to obtain modified graphene. Example

[0047] Example 1

[0048] A carbon nanotube-graphene composite conductive agent, the raw materials and their amounts are shown in Table 1, and its preparation method is as follows:

[0049] The surfactant and N-methylpyrrolidone were stirred and mixed to ensure that the surfactant was fully dissolved in the N-methylpyrrolidone.

[0050] Then add carbon nanotubes and modified graphene and stir to disperse them to obtain a pre-dispersed slurry;

[0051] The pre-dispersed slurry was ground for 4 hours and then cooled to obtain a carbon nanotube-graphene composite conductive agent.

[0052] The surfactant is a mixture of polyvinylpyrrolidone and a small molecule surfactant, the small molecule surfactant being sodium dodecyl sulfate, and the weight ratio of polyvinylpyrrolidone to the small molecule surfactant being 1:0.1; the average diameter of the carbon nanotubes is 30 nm; and the modified graphene is derived from Preparation Example 1.

[0053] Table 1. Raw materials and their amounts (g) in Example 1

[0054]

[0055] Example 2

[0056] A carbon nanotube-graphene composite conductive agent differs from Example 1 in that the weight ratio of polyvinylpyrrolidone to small molecule surfactant is 1:0.2, while the remaining steps are the same as in Example 1.

[0057] Example 3

[0058] A carbon nanotube-graphene composite conductive agent differs from Example 1 in that the weight ratio of polyvinylpyrrolidone to small molecule surfactant is 1:0.3, while the remaining steps are the same as in Example 1.

[0059] Example 4

[0060] A carbon nanotube-graphene composite conductive agent differs from Example 1 in that the weight ratio of polyvinylpyrrolidone to small molecule surfactant is 1:0.4, while the remaining steps are the same as in Example 1.

[0061] Example 5

[0062] A carbon nanotube-graphene composite conductive agent differs from Example 3 in that the average diameter of its carbon nanotubes is 45 nm, while the remaining steps are the same as in Example 3.

[0063] Example 6

[0064] A carbon nanotube-graphene composite conductive agent differs from Example 3 in that the average diameter of its carbon nanotubes is 60 nm, while the remaining steps are the same as in Example 3.

[0065] Comparative Example

[0066] Comparative Example 1

[0067] A carbon nanotube-graphene composite conductive agent differs from Example 6 in that the modified graphene is replaced with an equal mass of graphene, and the preparation method of the graphene is as follows:

[0068] S1. Disperse 0.3g of graphene oxide in 35g of water to obtain a graphene oxide dispersion.

[0069] S2. Then add 0.02g of hydrazine hydrate, heat to 80℃, react for 8h, and after the reaction solution cools to room temperature, filter to obtain a precipitate. Wash the precipitate with distilled water, then with methanol, and then vacuum dry at 40℃ to obtain modified graphene.

[0070] Comparative Example 2

[0071] A carbon nanotube-graphene composite conductive agent differs from Example 6 in that its surfactant is replaced with an equal mass of polyvinylpyrrolidone, while the remaining steps are the same as in Example 6.

[0072] Comparative Example 3

[0073] A carbon nanotube-graphene composite conductive agent differs from Example 6 in that its surfactant is replaced with an equal mass of sodium dodecyl sulfate, while the remaining steps are the same as in Example 6.

[0074] Comparative Example 4

[0075] A carbon nanotube-graphene composite conductive agent differs from Example 6 in that the average diameter of its carbon nanotubes is 70 nm, while the remaining steps are the same as in Example 6.

[0076] Performance testing

[0077] Detection methods / test methods

[0078] Carbon nanotube-graphene composite conductive agents were prepared according to the preparation methods of Examples 1-6 and Comparative Examples 1-4, and then fabricated into 18650 lithium-ion batteries. The following tests were performed, and the test results are shown in Table 2.

[0079] Table 2. Detection results of Examples 1-6 and Comparative Examples 1-4

[0080]

[0081] As can be seen from Examples 1-6, Comparative Examples 1-4, and the test data in Table 2, the lithium-ion batteries prepared using the carbon nanotube-graphene composite conductive agent of this application have high discharge capacity, with initial efficiencies of 91.3% and above, reaching a maximum of 92.2%. Furthermore, the capacity decay after 300 cycles is less than 7.3%, with a minimum of 5.4%. This indicates that the carbon nanotube-graphene composite conductive agent prepared in this application has excellent conductivity, and the prepared lithium-ion batteries exhibit excellent durability.

[0082] The test data from Example 1 and Comparative Example 1 show that by modifying the graphene surface with hyperbranched polymers, this application can increase the steric hindrance between graphene sheets, thereby reducing their aggregation. On the other hand, the viscosity of graphene modified with hyperbranched polymers can be greatly reduced compared with conventional polymers. Under the combined effect of these two aspects, graphene can be evenly dispersed, greatly improving its conductivity.

[0083] Examples 1-4 and Comparative Examples 2-3 show that the surfactants in this application exhibit a synergistic effect between polyvinylpyrrolidone (PVP) and small molecule surfactants. PPVP molecules are relatively large, easily forming large micelles in the dispersion system, thus reducing its effect on graphene. The addition of small molecule surfactants, with their smaller molecular weight, allows for more flexible movement within the system, easily penetrating between graphene sheets, thereby improving dispersibility. Furthermore, the test data from Examples 1-4 show that the amount of small molecule surfactant added needs to be within a certain range to achieve optimal technical effects. As the amount added gradually increases, the conductivity of the carbon nanotube-graphene composite conductive agent tends to decrease. The best performance is achieved when the weight ratio of PPVP to small molecule surfactant is within the range of 1:(0.2-0.3).

[0084] The test data from Examples 3, 5-6, and Comparative Example 4 show that the average diameter of carbon nanotubes has a significant impact on the conductivity of the carbon nanotube-graphene composite conductive agent. An average diameter in the range of 30-60 nm is more suitable for practical applications, while an average diameter greater than 60 nm significantly reduces the conductivity of the carbon nanotube-graphene composite conductive agent.

[0085] The embodiments described above are preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A carbon nanotube graphene composite conductive agent, characterized by: It comprises the following raw materials in parts by weight: 1.5-3 parts modified graphene, 7-9 parts carbon nanotubes, 1.2-3.2 parts surfactant and 85-95 parts N-methylpyrrolidone; The modified graphene was prepared by grafting terminal amino hyperbranched polyamide onto the surface of graphene oxide and then reducing it.

2. The carbon nanotube-graphene composite conductive agent according to claim 1, characterized in that: The surfactant is a mixture of polyvinylpyrrolidone and small molecule surfactants.

3. The carbon nanotube-graphene composite conductive agent according to claim 2, characterized in that: The weight ratio of polyvinylpyrrolidone to small molecule surfactant is 1:(0.1-0.4).

4. The carbon nanotube-graphene composite conductive agent according to claim 3, characterized in that: The small molecule surfactant is one or both of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate.

5. The carbon nanotube-graphene composite conductive agent according to claim 1, characterized in that: The modified graphene is prepared as follows: S1. Disperse 0.2-0.4 parts by weight of graphene oxide in 30-40 parts by weight of water to obtain a graphene oxide dispersion. S2. Add 3-5 parts by weight of terminal amino hyperbranched polyamide to the graphene oxide dispersion, adjust the pH to 8-10, then heat to 40-50℃ and stir for 20-30 hours to obtain a graphene oxide grafted hyperbranched polymer dispersion. S3. Continue to add reducing agent to reduce the hyperbranched polymer grafted with graphene oxide to obtain modified graphene.

6. The carbon nanotube-graphene composite conductive agent according to claim 5, characterized in that: The reduction method of the graphene oxide-grafted hyperbranched polymer is as follows: Add 0.01-0.03 parts by weight of hydrazine hydrate, heat to 75-85℃, react for 7-9 hours, and then filter to obtain modified graphene.

7. The carbon nanotube-graphene composite conductive agent according to claim 1, characterized in that: The average diameter of the carbon nanotubes is 30-60 nm.

8. A method for preparing a carbon nanotube-graphene composite conductive agent according to any one of claims 1-7, characterized in that: It includes the following preparation steps: The surfactant and N-methylpyrrolidone were stirred and mixed to ensure that the surfactant was fully dissolved in the N-methylpyrrolidone. Then add carbon nanotubes and modified graphene and stir to disperse them to obtain a pre-dispersed slurry; The pre-dispersed slurry was ground for 3-5 hours and then cooled to obtain a carbon nanotube-graphene composite conductive agent.

9. A lithium-ion battery, characterized in that, The conductive agent in the lithium-ion battery electrode is any one of the carbon nanotube-graphene composite conductive agents described in claims 1-7.

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