A shear-thickening graphene composite conductive paste and its preparation method

Abstract

The present invention discloses a graphene composite conductive paste with shear thickening properties. The paste comprises the following components in mass percentage: 85 - 91% of N-methylpyrrolidone, 7 - 9% of graphene, 1 - 3% of carbon nanotubes, and 1 - 3% of a dispersant; the pH value of the graphene composite conductive paste is 5 - 7. By adjusting and controlling the component content ratio and the pH value range of the graphene composite conductive paste, the present invention enables the graphene composite conductive paste to have shear thickening rheology without adding a rheology aid, thereby improving the liquid-phase exfoliation efficiency in a sand mill, shortening the sanding time, and obtaining graphene with a thinner thickness and improved conductivity. Meanwhile, the present invention also discloses a preparation method of the graphene composite conductive paste and its application in a lithium-ion secondary battery.

Description

Technical Field

[0001] The present invention relates to a conductive paste, in particular to a graphene composite conductive paste with shear thickening rheology, a preparation method thereof, and an application thereof in a lithium ion secondary battery. Background Art

[0002] Graphene is a thin-layer two-dimensional nanomaterial. According to the crystal structure of graphite, there is a dual combination of covalent bonds (σ bonds) and metallic bonds (large π bonds) between carbon atoms. Its binding force is very strong, the melting point is high, and the chemical properties are stable. The adjacent lamellar sheets are connected by weak van der Waals forces, which makes the force between adjacent lamellar sheets very small. Under the action of external shear force, it is easy to have relative sliding between layers, which is the process of changing the graphite crystal with a multi-layer stacked structure into thin-layer graphene. The core of graphene preparation is to make graphite thinner.

[0003] The ball milling method (also called sand milling method) is a method for mechanically exfoliating and manufacturing graphene that has been widely implemented in the industry. In the ball milling method, there are two types of graphite particle structure transformations during the process of graphite becoming graphene: one is that graphite is subjected to the extrusion, collision, and vertical impact of grinding balls, which breaks the covalent bonds within the plane layer and causes the plane structure to break into fragments. This type of structural change can be characterized by particle size distribution; the other is that the graphite sheet layer structure is subjected to shear force, which counteracts the van der Waals force and causes interlayer peeling of the graphite sheet layer, and the thickness of the graphite crystal becomes thinner. This type of structural change can be characterized by the thickness of graphene. When preparing graphene by the ball milling method, the first type of structural transformation (decrease in particle size) is more likely to occur at low viscosity; while the second type of structural transformation (decrease in thickness) is more likely to occur at slightly higher viscosity. Wang Caiwei clearly stated in the article titled "Discussion on Improving the Grinding Efficiency of Sand Mills": One of the reasons for the low grinding efficiency of sand mills is too low viscosity. The grinding efficiency increases with the increase in the viscosity of the color paste. Generally, the viscosity of the material is 300 - 5000 centipoise (Reference: "Paint Industry", 1988, Issue 5, Page 18). The composition and structure of the graphene composite conductive paste are similar to those of the paint color paste, and the same applies. Therefore, it is very necessary to maintain the sanding material at an appropriate viscosity. If the viscosity is too low, the material is likely to slip away from the grinding chamber of the sand mill; if the viscosity is too high, it will cause difficulty in feeding and blockage of the sand mill.

[0004] Conventional graphene composite conductive slurries are generally oil-based or water-based dispersions composed of graphene, carbon nanotubes or carbon black, plus polymer dispersants, stabilizers, etc. For conventional graphene composite conductive slurries, the viscosity is very high at the initial stage of sand grinding preparation. As the grinding time increases, the particle size becomes finer and finer, and the viscosity also becomes lower and lower. In addition, for this kind of dispersion like graphene composite conductive slurry, under the action of high shear, the instantaneous association between polymers (dispersants) undergoes "depolymerization" or the particle clusters are destroyed, resulting in shear thinning. The superposition of the two makes the viscosity of the graphene composite slurry very low (tens or one or two hundred centipoises) during the middle and late stages of sand grinding. In the high-speed collision and shear in the sand mill cavity, the vertical impact of the graphite flakes and the grinding balls becomes stronger while the shear friction is insufficient, which will cause the lateral size of the graphene to be ground smaller but the thickness not to be ground thinner, and finally the conductivity becomes worse during use, which is not desirable in graphene preparation.

[0005] Chinese Patent CN201711024702.3 discloses a method for preparing graphene by exfoliating graphite using a shear thickening system. This method mixes the shear thickening system with graphite, and through mechanical shear, the mixed liquid is in a high-viscosity state of shear thickening, so as to improve the efficiency of exfoliating graphite to prepare graphene. After removing the mechanical shear or reducing the shear rate, the mixed liquid returns to a low-viscosity state of non-shear thickening. That is, the mixed liquid after exfoliating graphite into graphene in this patent is a non-shear thickening low-viscosity mixed liquid. However, this method requires special preparation of the shear thickening system. Such shear thickening systems have complex compositions, use ethylene glycol or water as solvents, and add microspheres, inorganic salts, dispersants, polymer polymers, etc., which need to be removed by centrifugation in the later stage, and the process is complex and costly. In addition, when preparing, the material is in a high-viscosity state, which is not conducive to the transfer and transportation of the material, and the energy consumption during preparation also increases.

[0006] Chinese Patent CN201410466761.6 discloses a carbon nanotube and graphene composite conductive slurry for lithium ion secondary batteries. This slurry consists of 2 - 4.5% carbon nanotubes, 0.5 - 1.5% graphene, 0.1 - 1.5% dispersion stabilizer, and the balance is NMP. The viscosity of the composite conductive slurry obtained in this invention is ≤8000 mPa·s. The solid content of the composite slurry obtained by this method is generally 4 - 6%. In recent years, due to the increasing price of the solvent NMP (almost tripled), it is urgent to increase the solid content, reduce the solvent usage, and reduce the proportion of NMP in the conductive slurry. Summary of the Invention

[0007] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a graphene composite conductive slurry that reduces the solvent usage and has shear thickening characteristics without adding rheological aids; at the same time, the present invention also provides a preparation method of the graphene composite conductive slurry.

[0008] To achieve the above object, the technical solution adopted by the present invention is as follows: a shear-thickening graphene composite conductive paste, and the graphene composite conductive paste comprises the following components in mass percentage: 85-91% of N-methylpyrrolidone, 7-9% of graphene, 1-3% of carbon nanotubes, and 1-3% of a dispersant; the pH value of the graphene composite conductive paste is 5-7.

[0009] The inventors of the present application have found that conventional conductive pastes (including carbon nanotube conductive pastes, graphene conductive pastes, etc.) are basically shear-thinning pastes. Conventional carbon nanotube conductive pastes are shear-thinning, and the mechanism is that carbon nanotube conductive pastes usually consist of carbon nanotubes with a very large aspect ratio and a polymer dispersant. Under the action of shear force, the polymer chains in the paste depolymerize and the carbon nanotube clusters are destroyed, resulting in a decrease in viscosity. Conventional graphene conductive pastes are also shear-thinning, and the main mechanism is that the sheet-like graphene is oriented under shear force, resulting in a decrease in viscosity. However, the degree of shear thinning of graphene paste is not as large as that of carbon nanotube paste. In order to reduce the viscosity, conventional graphene or carbon nanotube conductive pastes generally make the paste alkaline, aiming to react the base with the carboxyl groups on the carbon tubes or graphene to produce acetate ions, thereby becoming negatively charged, and the carbon tubes or graphene generate electrostatic repulsion to reduce the viscosity and avoid agglomeration. The pH value is generally between 8 and 12. The inventors of the present application have found that by adjusting the ratio of graphene and carbon nanotubes and controlling the particle size of graphene (higher proportion of graphene and smaller particle size of graphene), and at the same time controlling the pH of the paste to be weakly acidic or neutral, a low-viscosity paste can also be obtained.

[0010] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the raw materials for preparing the graphene composite conductive paste comprise the following components in parts by weight: 85-91 parts of N-methylpyrrolidone, 7-9 parts of expanded graphite, 1-3 parts of carbon nanotubes, 1-3 parts of a dispersant, and 0.01-0.1 part of a pH value regulator.

[0011] The shear-thickening graphene composite conductive paste of the present invention is prepared from the raw materials in the specific quantitative ratio. The addition amount of the solvent N-methylpyrrolidone (NMP) is reduced, effectively reducing the cost. Moreover, due to the addition of the pH value regulator and the specific quantitative ratio of expanded graphite, carbon nanotubes, etc., the pH value of the shear-thickening graphene composite conductive paste is within a specific range, so that the viscosity of the graphene composite conductive paste reaches 100-2000 centipoises in the shear rate range of 1-50S -1 and shows a gradually increasing characteristic, achieving the technical effects of improving the liquid-phase exfoliation efficiency of the graphene composite conductive paste in a sand mill, shortening the sanding time, obtaining a thinner graphene thickness, and effectively improving the conductivity.

[0012] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the raw materials for preparing the graphene composite conductive paste comprise the following components in parts by weight: 87-90 parts of N-methylpyrrolidone, 7-8 parts of expanded graphite, 1.5-2.5 parts of carbon nanotubes, 1.5-2.5 parts of dispersant, and 0.02-0.06 part of pH regulator.

[0013] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the specific surface area of the expanded graphite is 20-50m 2 / g.

[0014] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the expansion ratio of the expanded graphite is greater than or equal to 200 times. Considering that the larger the expansion ratio of the expanded graphite, the thinner the graphite sheet layer and the easier it is to peel off subsequently, the expansion ratio of the expanded graphite is preferably greater than or equal to 250 times.

[0015] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the pH regulator is a small molecule compound that can be volatilized by heating.

[0016] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the pH regulator is at least one of ammonia water, organic amines, alkanolamines, piperazines, and alkanoic acids. The pH regulator of the present invention is preferably but not limited to at least one of ammonia water, organic amines, alkanolamines, piperazines, and alkanoic acids, and the pH regulator can also be other small molecule compounds that are well-known to those skilled in the art and can be volatilized by heating.

[0017] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the viscosity of the graphene composite conductive paste gradually increases in the range of shear rate of 1-50S -1 range, and the viscosity of the graphene composite conductive paste is 100-2000 centipoise. The viscosity of the graphene composite conductive paste of the present invention satisfies the following conditions: 100 centipoise (CP) < viscosity at a shear rate of 1S -1 < viscosity at a shear rate of 10S -1 < viscosity at a shear rate of 50S -1 < viscosity at a shear rate of 50S < 2000 centipoise, that is, the viscosity of the graphene composite conductive paste gradually increases in the range of shear rate of 1-50S -1 range, and the viscosity of the graphene composite conductive paste is 100-2000 centipoise. When the viscosity of the graphene composite conductive paste of the present invention is within this range and has the shear-thickening property, the liquid-phase exfoliation efficiency of the graphene composite conductive paste in a stone mill is significantly improved, the sanding time is effectively shortened, the obtained graphene has a thinner thickness, and the conductivity is significantly improved.

[0018] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the dispersant is at least one of polyethylene glycol, polyacrylamide, polyvinylpyrrolidone, polyvinylidene fluoride, sodium carboxymethyl cellulose, and polyester polyoxyethylene ether. As a more preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the dispersant is polyvinylpyrrolidone (PVP K30).

[0019] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the particle size D50 of the graphene is 4-8 μm; the tube diameter of the carbon nanotube is 20-80 nm.

[0020] As a preferred embodiment of the shear-thickening graphene composite conductive paste of the present invention, the tube diameter of the carbon nanotube is 30 nm, and the specific surface area is 190 m 2 / g.

[0021] In addition, the present invention also provides a preparation method of the shear-thickening graphene composite conductive paste as described above. The preparation method includes the following steps:

[0022] (1) N-methylpyrrolidone, expanded graphite, carbon nanotubes, and a dispersant are subjected to high-speed dispersion and mixing, and a pH regulator is added simultaneously to make the pH value of the mixture 5-7, obtaining a premix;

[0023] (2) The premix is fed into a sand mill for sanding. After liquid-phase exfoliation, a sanded material is obtained;

[0024] (3) The sanded material is fed into a high-pressure homogenizer for further high-pressure homogenization and exfoliation, and finally the graphene composite conductive paste is obtained.

[0025] It should be noted here that the order of sanding and homogenization can be changed. It can be homogenized first and then sanded; or only sanded without homogenization. By extending the sanding time, the graphene composite conductive paste of the present invention can also be obtained, but the efficiency is a little lower.

[0026] Finally, the present invention also provides the application of the shear-thickening graphene composite conductive paste as described above in lithium-ion secondary batteries. The shear-thickening graphene composite conductive paste of the present invention can be used as a conductive additive in lithium-ion secondary batteries. When it is used as a conductive agent, when it is mixed with the positive electrode main material, adhesive, and solvent, the rheology of the positive electrode paste is shear thinning and is not affected by the rheology of the conductive paste.

[0027] At present, in the conventional graphene composite slurry in the prior art, in the middle and late stages of sand milling, the viscosity of the material is very low (when the shear rate is 50 inverse seconds, the viscosity < 300 centipoises). The low viscosity will lead to insufficient internal friction. The lateral size of graphene is ground smaller but the thickness is not ground thinner, and finally the conductivity becomes worse when in use. The graphene composite conductive slurry provided by this application, by adding a specific ratio of pH regulator and combining with the selection of other components and their ratios, can obtain a shear-thickening low-viscosity and high-solid-content graphene composite slurry without adding rheological aids, which improves the liquid-phase exfoliation efficiency in the sand mill, shortens the sand milling time, and the obtained graphene has a thinner thickness and higher conductivity. Moreover, because the viscosity of the graphene composite slurry described in this application is low itself, more active ingredients (such as graphene, carbon nanotubes, etc.) can be added, and more than half of the solvent NMP can be saved. As NMP has become more and more expensive in recent years, this invention can effectively reduce costs. Description of the Drawings

[0028] Figure 1 Shows the viscosity curve of the graphene composite conductive slurry at different shear rates. Detailed Description of the Invention

[0029] To better illustrate the purpose, technical solution and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments and drawings.

[0030] Description of Raw Materials:

[0031] Among the raw materials used in the following examples and comparative examples, the expansion ratio of the expanded graphite is 250 times, the particle size D50 is 20 microns, and the metal impurities meet the lithium battery requirements. The diameter of the carbon nanotubes is 30 nm, the specific surface area is 190 m 2 / g, and the metal impurities meet the lithium battery requirements.

[0032] Among the above raw materials, the expansion ratio of the expanded graphite and the diameter of the carbon nanotubes can also be selected as other values within the ranges described in this application. The N-methylpyrrolidone (NMP) can also be purchased from other manufacturers or prepared by conventional methods known in the art, etc., and the results are similar to those of the following examples and comparative examples.

[0033] Description of Experimental Equipment:

[0034] Sand Mill: The volume of the rod pin sand mill cavity is 1 L, and the linear velocity of the sand mill is greater than 8 m / s, preferably greater than or equal to 12 m / s. The linear velocity of the sand mill in the following examples and comparative examples is around 13 m / s.

[0035] Homogenizer: The homogenization pressure of the high-pressure homogenizer is greater than 50 MPa, preferably 60 - 100 MPa.

[0036] Test Description:

[0037] pH value test: The graphene composite slurry is first diluted 10 times with pure water, stirred evenly, filtered with slow filter paper, and the filtrate is tested with a pH meter.

[0038] Viscosity test: Use a DV2T-RV viscometer (with rheological function) produced by Brookfield for testing. The rotor is No. 21, and the shear rate during testing is set to 0.1S -1 ,1S -1 ,10S -1 ,50S -1 ,100S -1 . The temperature during testing is controlled at 25 ± 2 °C.

[0039] Particle size test: The test equipment is a Dandong Baite laser particle size analyzer S2600, with NMP as the mobile phase. When testing, the refractive index of the substance is selected as graphite. After scanning the mobile phase background, take 1 small drop of the slurry sample with a small dropper and drop it into the sample cell. Before testing, ultrasonically treat it with the built-in ultrasonic machine of the laser particle size analyzer for 30 seconds, and take the average of 3 consecutive tests to read D50.

[0040] Example 1

[0041] An example of the shear thickening graphene composite conductive slurry of the present invention. The shear thickening graphene composite conductive slurry in this example contains the following components by mass percentage: N-methylpyrrolidone 88%, graphene 8%, carbon nanotubes 2%, dispersant 2%; the pH value of the graphene composite conductive slurry in this example is 6.1.

[0042] The preparation raw materials of the shear thickening graphene composite conductive slurry in this example contain the following components by weight: N-methylpyrrolidone (NMP) 5280 g, expanded graphite 480 g, carbon nanotubes 120 g, dispersant polyvinylidene fluoride (PVDF) 60 g, dispersant polyvinylpyrrolidone (PVP-K30) 60 g, pH value regulator anhydrous piperazine 2.4 g.

[0043] The shear thickening graphene composite conductive slurry in this example is prepared by the following method:

[0044] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant to a 10 L sealed dispersion tank, and use a dispersion disk to disperse and mix at a high speed of 3000 rpm for 1 h. During this period, add the pH value regulator anhydrous piperazine and test the pH value of the premixed slurry to be 6.1 to obtain a premixed material;

[0045] (2) Pass the premixed material into a sand mill with a chamber volume of 1 L, and the linear velocity of the sand mill is 13 m / s for sand grinding and liquid phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sand grinding and record the sand grinding time to obtain a sand ground material;

[0046] (3) Feed the abrasive into a high-pressure homogenizer with a homogenization pressure of 60 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0047] Example 2

[0048] An example of the shear-thickening graphene composite conductive paste of the present invention. The shear-thickening graphene composite conductive paste in this example contains the following components by mass percentage: N-methylpyrrolidone 88%, graphene 8%, carbon nanotubes 2%, and dispersant 2%; the pH value of the graphene composite conductive paste in this example is 6.0.

[0049] The raw materials for preparing the shear-thickening graphene composite conductive paste in this example contain the following components by weight: N-methylpyrrolidone (NMP) 5280 g, expanded graphite 480 g, carbon nanotubes 120 g, dispersant polyvinylidene fluoride (PVDF) 60 g, dispersant polyvinylpyrrolidone (PVP-K30) 60 g, and pH value regulator concentrated ammonia water 3.6 g.

[0050] The shear-thickening graphene composite conductive paste in this example is prepared by the following method:

[0051] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10 L sealed dispersion tank, and use a dispersion disk to disperse and mix at a high speed of 3000 rpm for 1 h. During this period, add the pH value regulator concentrated ammonia water and test that the pH value of the premixed paste is 6.0 to obtain a premix.

[0052] (2) Feed the premix into a sand mill with a chamber volume of 1 L and a linear velocity of 13 m / s for sanding and liquid-phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sanding and record the sanding time to obtain the sanded material.

[0053] (3) Feed the sanded material into a high-pressure homogenizer with a homogenization pressure of 60 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0054] Example 3

[0055] An example of the shear-thickening graphene composite conductive paste of the present invention. The shear-thickening graphene composite conductive paste in this example contains the following components by mass percentage: N-methylpyrrolidone 88%, graphene 7%, carbon nanotubes 3%, and dispersant 2%; the pH value of the graphene composite conductive paste in this example is 6.6.

[0056] The raw materials for preparing the shear thickening graphene composite conductive paste in this embodiment include the following components by weight: 5280 g of N-methylpyrrolidone (NMP), 420 g of expanded graphite, 180 g of carbon nanotubes, 120 g of dispersant polyvinylpyrrolidone (PVP-K30), and 6 g of pH regulator 2-amino-2-methyl-1-propanol.

[0057] The shear thickening graphene composite conductive paste in this embodiment is prepared by the following method:

[0058] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10 L sealed dispersion tank, and disperse and mix them at a high speed of 3000 rpm with a dispersion disc for 1 h. During this period, add the pH regulator 2-amino-2-methyl-1-propanol and test the pH value of the premixed paste to be 6.6 to obtain a premixed material;

[0059] (2) Feed the premixed material into a sand mill with a chamber volume of 1 L, and the linear velocity of the sand mill is 12 m / s for sanding and liquid-phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sanding and record the sanding time to obtain a sanded material;

[0060] (3) Feed the sanded material into a high-pressure homogenizer with a homogenization pressure of 60 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0061] Example 4

[0062] An embodiment of the shear thickening graphene composite conductive paste of the present invention. The shear thickening graphene composite conductive paste in this embodiment includes the following components by mass percentage: 88% of N-methylpyrrolidone, 9% of graphene, 1% of carbon nanotubes, and 2% of dispersant; the pH value of the graphene composite conductive paste in this embodiment is 5.8.

[0063] The raw materials for preparing the shear thickening graphene composite conductive paste in this embodiment include the following components by weight: 5280 g of N-methylpyrrolidone (NMP), 540 g of expanded graphite, 60 g of carbon nanotubes, 120 g of dispersant polyvinylpyrrolidone (PVP-K30), and 1.8 g of pH regulator 2-amino-2-methyl-1-propanol.

[0064] The shear thickening graphene composite conductive paste in this embodiment is prepared by the following method:

[0065] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10 L sealed dispersion tank, and disperse and mix them at a high speed of 3000 rpm with a dispersion disc for 1 h. During this period, add the pH regulator 2-amino-2-methyl-1-propanol and test the pH value of the premixed paste to be 5.8 to obtain a premixed material;

[0066] (2) Feed the premix into a sand mill with a chamber volume of 1 L. The linear velocity of the sand mill is 13 m / s for liquid-phase exfoliation by sanding. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sanding and record the sanding time to obtain the sanded material.

[0067] (3) Feed the sanded material into a high-pressure homogenizer with a homogenization pressure of 80 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0068] Example 5

[0069] An example of the shear-thickening graphene composite conductive paste of the present invention. The shear-thickening graphene composite conductive paste in this example contains the following components by mass percentage: 85% of N-methylpyrrolidone, 9% of graphene, 3% of carbon nanotubes, and 3% of dispersant; the pH value of the graphene composite conductive paste in this example is 6.2.

[0070] The raw materials for preparing the shear-thickening graphene composite conductive paste in this example contain the following components by weight: 5100 g of N-methylpyrrolidone (NMP), 540 g of expanded graphite, 180 g of carbon nanotubes, 180 g of dispersant polyethylene glycol, and 3.6 g of pH regulator concentrated ammonia water.

[0071] The shear-thickening graphene composite conductive paste in this example is prepared by the following method:

[0072] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10 L sealed dispersion tank, and disperse and mix them at a high speed at 3000 rpm for 1 h. During this period, add the pH regulator concentrated ammonia water and test the pH value of the premixed slurry to be 6.2 to obtain the premix.

[0073] (2) Feed the premix into a sand mill with a chamber volume of 1 L. The linear velocity of the sand mill is 13 m / s for liquid-phase exfoliation by sanding. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sanding and record the sanding time to obtain the sanded material.

[0074] (3) Feed the sanded material into a high-pressure homogenizer with a homogenization pressure of 55 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0075] Example 6

[0076] An example of the shear-thickening graphene composite conductive paste of the present invention. The shear-thickening graphene composite conductive paste in this example contains the following components by mass percentage: 91% of N-methylpyrrolidone, 7% of graphene, 1% of carbon nanotubes, and 1% of dispersant; the pH value of the graphene composite conductive paste in this example is 5.5.

[0077] The raw materials for preparing the shear thickening graphene composite conductive paste in this embodiment include the following components by weight: 5460 g of N-methylpyrrolidone (NMP), 420 g of expanded graphite, 60 g of carbon nanotubes, 60 g of dispersant sodium carboxymethyl cellulose, and 0.6 g of pH regulator anhydrous piperazine.

[0078] The shear thickening graphene composite conductive paste in this embodiment is prepared by the following method:

[0079] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10 L sealed dispersion tank, and use a dispersion disk to disperse and mix at a high speed of 3000 rpm for 1 h. During this period, add anhydrous piperazine as the pH regulator and test the pH value of the premixed paste to be 5.5 to obtain a premixed material;

[0080] (2) Feed the premixed material into a sand mill with a chamber volume of 1 L. The linear velocity of the sand mill is 13 m / s, and perform sand milling for liquid phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sand milling and record the sand milling time to obtain a sand milled material;

[0081] (3) Feed the sand milled material into a high-pressure homogenizer with a homogenization pressure of 80 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0082] Comparative Example 1

[0083] The graphene composite conductive paste in this comparative example contains the following components by mass percentage: 88% of N-methylpyrrolidone, 8% of graphene, 2% of carbon nanotubes, and 2% of dispersant; the pH value of the graphene composite conductive paste in this comparative example is 4.9.

[0084] The raw materials for preparing the graphene composite conductive paste in this comparative example include the following components by weight: 5280 g of N-methylpyrrolidone (NMP), 480 g of expanded graphite, 120 g of carbon nanotubes, 60 g of dispersant polyvinylidene fluoride (PVDF), and 60 g of dispersant polyvinylpyrrolidone (PVP-K30).

[0085] The graphene composite conductive paste in this comparative example is prepared by the following method:

[0086] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10 L sealed dispersion tank, and use a dispersion disk to disperse and mix at a high speed of 3000 rpm for 1 h. During this period, do not add a pH regulator, and test the pH value of the premixed paste to be 4.9 to obtain a premixed material;

[0087] (2) Feed the premix into a sand mill with a chamber volume of 1 L, with the linear velocity of the sand mill being 13 m / s, and conduct sand grinding for liquid-phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sand grinding and record the sand grinding time to obtain the sanded material;

[0088] (3) Feed the sanded material into a high-pressure homogenizer with a homogenization pressure of 60 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0089] Comparative Example 2

[0090] The graphene composite conductive paste of this comparative example contains the following components by mass percentage: 88% of N-methylpyrrolidone, 8% of graphene, 2% of carbon nanotubes, and 2% of dispersant; the pH value of the graphene composite conductive paste of this comparative example is 8.4.

[0091] The preparation raw materials of the graphene composite conductive paste of this comparative example contain the following components by weight: 5280 g of N-methylpyrrolidone (NMP), 480 g of expanded graphite, 120 g of carbon nanotubes, 60 g of dispersant polyvinylidene fluoride (PVDF), 60 g of dispersant polyvinylpyrrolidone (PVP-K30), and 12 g of pH value regulator anhydrous piperazine.

[0092] The graphene composite conductive paste of this comparative example is prepared by the following method:

[0093] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10-L sealed dispersion tank, and use a dispersion disc to disperse and mix at a high speed at 3000 rpm for 1 h. During this period, add the pH value regulator anhydrous piperazine and test the pH value of the premixed slurry to be 8.4 to obtain the premix;

[0094] (2) Feed the premix into a sand mill with a chamber volume of 1 L, with the linear velocity of the sand mill being 13 m / s, and conduct sand grinding for liquid-phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sand grinding and record the sand grinding time to obtain the sanded material;

[0095] (3) Feed the sanded material into a high-pressure homogenizer with a homogenization pressure of 80 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0096] Comparative Example 3

[0097] The graphene composite conductive paste of this comparative example contains the following components by mass percentage: 88% of N-methylpyrrolidone, 4% of graphene, 6% of carbon nanotubes, and 2% of dispersant; the pH value of the graphene composite conductive paste of this comparative example is 6.2.

[0098] The raw materials for preparing the graphene composite conductive paste of this comparative example include the following components by weight: 5280 g of N-methylpyrrolidone (NMP), 240 g of expanded graphite, 360 g of carbon nanotubes, 120 g of dispersant polyvinylpyrrolidone (PVP-K30), and 2.4 g of pH regulator anhydrous piperazine.

[0099] The graphene composite conductive paste of this comparative example is prepared by the following method:

[0100] (1) Add N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant into a 10 L sealed dispersion tank, and use a dispersion disk to disperse and mix at a high speed of 3000 rpm for 1 h. During this period, add anhydrous piperazine as the pH regulator and test the pH value of the premixed paste to be 6.2 to obtain a premix.

[0101] (2) Pass the premix into a sand mill with a chamber volume of 1 L, and the linear velocity of the sand mill is 13 m / s for sanding and liquid-phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sanding and record the sanding time to obtain a sanded material.

[0102] (3) Pass the sanded material into a high-pressure homogenizer with a homogenization pressure of 60 MPa until D50 < 7 μm to finally obtain the graphene composite conductive paste.

[0103] Comparative Example 4

[0104] Refer to Chinese Patent CN201410466761.6. The graphene composite conductive paste of this comparative example is prepared by the following method:

[0105] (1) Add 60 g of expanded graphite with a specific surface area of about 150 m 2 / g, 240 g of multi-walled carbon nanotubes, 60 g of dispersant PVPK30, and 5640 g of NMP into a 10 L sealed dispersion tank, and use a dispersion disk to disperse and mix at a high speed of 3000 rpm for 1 h. The pH value of the premixed paste is 5.2 to obtain a premix.

[0106] (2) Pass the premix into a sand mill with a chamber volume of 1 L, and the linear velocity of the sand mill is 13 m / s for sanding and liquid-phase exfoliation. When the particle size reaches D50 = 8.5 ± 0.2 μm, stop sanding and record the sanding time to obtain a sanded material, and the sanded material is the graphene composite conductive paste.

[0107] Example 7

[0108] Performance test of the graphene composite conductive paste of the present invention

[0109] 1. Physical property test of the graphene composite conductive paste of the present invention

[0110] The viscosity, particle size, etc. of the graphene composite conductive pastes of the above Examples 1-6 and Comparative Examples 1-4 were tested respectively, and the sanding time, paste pH value, etc. during the preparation process of each group of graphene composite conductive pastes were counted. The results are as Figure 1 shown in Table 1.

[0111] Table 1 Physical property test results of Examples and Comparative Examples

[0112]

[0113] It can be seen from the results in Table 1 that the viscosity of the graphene composite conductive pastes of Examples 1-6 gradually increases in the shear rate range of 1-50 S -1 while the viscosity of the graphene composite conductive pastes of Comparative Examples 1-4 gradually decreases in the shear rate range of 1-50 S -1 range. It can be seen that the graphene composite conductive paste of the present invention contains specific ratios of components such as graphene and carbon nanotubes, and has a specific pH value range, so that the viscosity of the graphene composite conductive paste has the property of shear thickening, and the liquid-phase exfoliation efficiency of the graphene composite conductive paste in the stone mill is significantly improved, and the sanding time is effectively shortened.

[0114] 2. Test on the sheet resistance and coin cell performance of the graphene composite conductive paste of the present invention

[0115] The graphene composite conductive pastes of Examples 1-6 and Comparative Examples 1-4 were respectively used as conductive additives in lithium-ion secondary batteries for sheet resistance and coin cell performance tests. The specific test methods are as follows:

[0116] Sheet resistance test: The graphene composite paste was used as a conductive additive and added to the main materials of the LFP battery for slurry mixing, coating, and drying, and then the sheet resistance of the electrode was tested. The sheet resistance can truly evaluate the conductivity of graphene in the application scenario. The following test equipment is required: four-probe resistivity meter (manufacturer: Suzhou Jingge Electronics Co., Ltd., model: ST2258C), coater (manufacturer: Shenzhen Kejing Zhida Technology Co., Ltd., model: MSK-AFA-SC200), forced air drying oven, tablet press, standard doctor blade coater, high-temperature polyester film (PET film) and other auxiliary tools. The specific test method is as follows: The materials were fed according to the mass ratio of lithium iron phosphate active material: polyvinylidene fluoride: graphene composite paste conductive agent (conductive agent solid content part) of 100:3:1. The solid content of the positive electrode slurry was adjusted to 55.5% by adding NMP. After vacuum stirring and slurry mixing for 2.5 hours, a positive electrode slurry was prepared, coated on the PET film, and dried at 90 degrees for 0.5 h to obtain a positive electrode sheet. The PET film electrode sheet was cut into a 4x4 cm square, the thickness of the electrode sheet was measured by a thickness gauge, and the sheet resistance of the electrode sheet was measured by a four-probe resistivity tester.

[0117] Button cell test: The LFP cathode slurry prepared with the resistivity of the above-mentioned test electrode sheet was coated on the carbon-coated aluminum foil. After drying in an oven, the electrode sheet was cut, and a lithium sheet was used as the anode to prepare a CR2025-type button cell in a glove box and tested with a Neware high-performance battery testing system.

[0118] The test results of the battery slurry mixing performance and the resistivity of the electrode sheets of Examples 1-6 and Comparative Examples 1-4 are shown in Table 2.

[0119] Table 2 Battery slurry mixing performance and electrode sheet performance of Examples and Comparative Examples

[0120]

[0121] The test results of the button cell performance of Examples 1-6 and Comparative Examples 1-4 are shown in Table 3.

[0122] Table 3 Button cell performance of Examples and Comparative Examples

[0123]

[0124] It can be seen from the results in Tables 1-3 that the component content of the graphene composite conductive slurry in Comparative Example 1 is the same as that in Example 1. However, the pH regulator was not added during the preparation of the graphene composite conductive slurry in Comparative Example 1, resulting in the excessive acidity (pH value of 4.9) of the graphene composite conductive slurry in Comparative Example 1. The viscosity of the slurry is high, which is not conducive to transportation and feeding, and is not conducive to sanding and peeling. The sanding time increased from 2.3 h in Example 1 to 3.4 h in Comparative Example 1, and the sanding and peeling efficiency decreased significantly. At the same time, due to the too high viscosity of the slurry in Comparative Example 1, the viscosity of the battery slurry mixing increased from 2678 cp in Example 1 to 5760 cp in Comparative Example 1; the resistivity of the electrode sheet increased from 12.3 Ω·cm to 17.2 Ω·cm, indicating that the conductivity of the graphene composite slurry prepared in Comparative Example 1 is worse than that in Example 1. After the conductivity becomes worse, the rate performance (5C and 10C) also decreases accordingly.

[0125] The component content of the graphene composite conductive slurry in Comparative Example 2 is also the same as that in Example 1. However, an excessive amount of pH regulator was added during the preparation of the graphene composite conductive slurry in Comparative Example 2, resulting in the excessive alkalinity (pH value of 8.4) of the graphene composite conductive slurry in Comparative Example 2. Therefore, shear thinning (at a shear rate of 1-50 reciprocal seconds) rheology is shown during sanding and after final homogenization. Coupled with the low viscosity of the slurry itself, it is only 106 cp at 50 reciprocal seconds after shear thinning, resulting in low sanding efficiency and insufficient graphene peeling. The resistivity of its electrode sheet is the highest, which also indirectly indicates that the graphene is thick and the conductivity is insufficient, resulting in a low capacity retention rate at 5C and 10C rates during button cell testing.

[0126] Although the pH value of the graphene composite conductive paste of Comparative Example 3 is within the range of 5-7, among the components of the graphene composite conductive paste of Comparative Example 3, the proportion of carbon nanotubes is higher than that of graphene. Therefore, it shows shear thinning (at a shear rate of 1-50 reciprocal seconds) during sanding and after final homogenization. The compaction density of the electrode sheet is relatively low, not as good as that of the examples where the proportion of graphene is higher than that of carbon nanotubes. The low compaction density will ultimately result in poor volume energy density of the battery.

[0127] In Comparative Example 4, high specific surface area graphene was used, and the proportion of carbon nanotubes in the raw material ratio was higher than that of graphene, ultimately resulting in high viscosity. The sanding took 3.1 h and the peeling efficiency was not high. With high specific surface area graphene, the conductivity of the paste was still good (low resistivity of the electrode sheet). However, the viscosity of the mixed paste was too high, reaching 12309 cp, which would cause insufficient mixing of the paste and uneven distribution of the electrode sheet materials, ultimately resulting in poor rate performance of the button cell. Due to the relatively large specific surface area of graphene, more irreversible lithium ions are consumed during the first charge and discharge process, and the first efficiency is relatively low.

[0128] Generally, the graphene composite conductive paste is a uniformly dispersed suspension composed of nano-powders + polymer dispersants + solvents. Under the action of shear force, the instantaneous association between the polymer dispersants "depolymerizes" and the nanoparticle clusters are destroyed, resulting in a decrease in viscosity and showing shear thinning. In the present invention, however, the graphene composite conductive paste shows shear thickening, and this shear thickening rheology is beneficial to the peeling of graphene in the sand mill, resulting in better conductivity of the obtained graphene.

[0129] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A shear thickening graphene composite conductive paste, characterized in that, The graphene composite conductive paste comprises the following components in mass percentage: 85-91% of N-methylpyrrolidone, 7-9% of graphene, 1-3% of carbon nanotubes, and 1-3% of dispersant; the pH value of the graphene composite conductive paste is 5-7; the viscosity of the graphene composite conductive paste gradually increases in the shear rate range of 1-50S -1 range.

2. The shear thickening graphene composite conductive paste according to claim 1, characterized in that, The preparation raw materials of the graphene composite conductive paste comprise the following components in parts by weight: 85-91 parts of N-methylpyrrolidone, 7-9 parts of expanded graphite, 1-3 parts of carbon nanotubes, 1-3 parts of dispersant, and 0.01-0.1 part of pH regulator.

3. The shear thickening graphene composite conductive paste according to claim 2, characterized in that, The preparation raw materials of the graphene composite conductive paste comprise the following components in parts by weight: 87-90 parts of N-methylpyrrolidone, 7-8 parts of expanded graphite, 1.5-2.5 parts of carbon nanotubes, 1.5-2.5 parts of dispersant, and 0.02-0.06 part of pH regulator.

4. The shear thickening graphene composite conductive paste according to claim 2 or 3, characterized in that, The pH regulator is a small molecule compound that can volatilize upon heating; the specific surface area of the expanded graphite is 20-50 m 2 / g.

5. The shear thickening graphene composite conductive paste according to claim 4, characterized in that, The pH regulator is at least one of ammonia water, organic amines, alkanolamines, piperazines, and alkyds.

6. The shear thickening graphene composite conductive paste according to claim 1, characterized in that, The viscosity of the graphene composite conductive paste is 100-2000 centipoises.

7. The shear thickening graphene composite conductive paste according to claim 1, characterized in that, The dispersant is at least one of polyethylene glycol, polyacrylamide, polyvinylpyrrolidone, polyvinylidene fluoride, sodium carboxymethyl cellulose, and polyester polyoxyethylene ether.

8. The shear thickening graphene composite conductive paste according to claim 1, characterized in that, The particle size D50 of the graphene is 4-8 μm; the tube diameter of the carbon nanotubes is 20-80 nm.

9. A method for preparing the shear thickening graphene composite conductive paste according to any one of claims 1-8, characterized in that, The preparation method comprises the following steps: (1) High-speed dispersing and mixing N-methylpyrrolidone, expanded graphite, carbon nanotubes, and dispersant, and simultaneously adding a pH regulator to make the pH value of the mixture 5-7 to obtain a premix; (2) Passing the premix into a sand mill for sanding, and after liquid-phase exfoliation, obtaining a sanded material; (3) Passing the sanded material into a high-pressure homogenizer for further high-pressure homogenization exfoliation to finally obtain the graphene composite conductive paste.

10. An application of the shear thickening graphene composite conductive paste according to any one of claims 1-8 in a lithium-ion secondary battery.

Images