Method for preparing modified carbon nanotube, modified carbon nanotube, negative electrode slurry, and battery

Abstract

A method for preparing a modified carbon nanotube, a modified carbon nanotube, a negative electrode slurry, and a battery. The method comprises: carboxylating a carbon nanotube; placing the carboxylated carbon nanotube in a first mixed acid solution to obtain a first inner-wall carboxylated carbon nanotube; placing the first inner-wall carboxylated carbon nanotube in a second mixed acid solution to obtain a second inner-wall carboxylated carbon nanotube; and mixing the second inner-wall carboxylated carbon nanotube and polyethylene glycol to obtain a modified carbon nanotube.

Description

Preparation method of modified carbon nanotubes, modified carbon nanotubes, negative electrode slurry and battery

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on September 27, 2023, with application number 202311272918.7. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the technical field of lithium batteries, and specifically relates to a preparation method of modified carbon nanotubes, modified carbon nanotubes, negative electrode slurry and batteries. Background Art

[0003] In lithium-ion batteries, the primary function of conductive agents is to enhance the conductive contact between active materials and improve electronic conductivity. Since positive electrode materials are mostly transition metal phosphates, they have poor conductivity. During repeated charge and discharge, negative electrode graphite expands and contracts, severely impairing its conductivity. Although conductive agents are used in relatively small quantities (accounting for approximately 2% of the cost of a lithium-ion battery), they play an irreplaceable role in the battery system.

[0004] In related technologies, the conductive agents used on the market can be divided into point, line, and surface contact conductive networks according to the contact form. Traditional conductive agents such as carbon black SP and conductive graphite are rigid nanoparticles that increase the contact between active substances on a point-to-point basis. New conductive agents mainly include carbon nanofibers, carbon nanotubes, and graphene. Carbon nanotubes are flexible long chains that increase the contact between active substances on a line-to-point basis; graphene is a flexible sheet that increases the contact between active substances on a surface-to-point basis. Due to their higher contact area, new conductive agents have better conductivity than traditional conductive agents. However, graphene-based conductive agents are more expensive, so their use is limited.

[0005] As a new type of conductive agent, carbon nanotubes are widely used in conductive agents in various industries due to their good conductivity and simple preparation method. Especially in the emerging technology field of lithium-ion batteries, the use of carbon nanotubes has greatly improved the conductive performance of lithium batteries.

[0006] In related technologies, in order to improve the conductivity of carbon nanotubes, researchers mainly use doping modification, coating modification, etc. to modify carbon nanotubes, but the mechanical properties of the obtained carbon nanotubes are relatively weak. Technical issues

[0007] The present application provides a preparation method of modified carbon nanotubes, modified carbon nanotubes, negative electrode slurry and battery to solve the above technical problems. Technical Solutions

[0008] In a first aspect, the present invention provides a method for preparing modified carbon nanotubes, comprising the following steps:

[0009] Placing the carbon nanotubes in a nitric acid solution, reacting at 65-75° C. for 10-14 hours, washing, and drying to obtain carboxylated carbon nanotubes; wherein the mass fraction of HNO 3 in the nitric acid solution is not less than 20%;

[0010] placing the carboxylated carbon nanotubes in a first mixed acid solution, reacting for 10 to 14 hours, washing, and drying to obtain first inner-wall carboxylated carbon nanotubes; the first mixed acid solution is a composite solution of concentrated nitric acid and concentrated hydrochloric acid, the volume ratio of the concentrated nitric acid to the concentrated hydrochloric acid being 1:2 to 5; the mass fraction of HNO3 in the concentrated nitric acid being 60 to 70%; and the mass fraction of HCl in the concentrated hydrochloric acid being 30 to 38%;

[0011] placing the first inner-wall carboxylated carbon nanotubes in a second mixed acid solution, allowing to stand for 10 to 14 hours, washing, and drying to obtain second inner-wall carboxylated carbon nanotubes; the second mixed acid solution is a composite solution of concentrated sulfuric acid and the concentrated hydrochloric acid, and the volume ratio of the concentrated sulfuric acid to the concentrated hydrochloric acid is 1:1 to 1.5;

[0012] The second inner wall carboxyl carbon nanotubes, polyethylene glycol, dispersant and water are mixed, and the pH of the mixed system is controlled to be 1-4. The mixture is refluxed at 78-85° C. for 3-5 hours under nitrogen, washed and dried to obtain modified carbon nanotubes.

[0013] In a second aspect, an embodiment of the present application provides a modified carbon nanotube, which is prepared by utilizing the preparation method of the modified carbon nanotube as described above.

[0014] In a third aspect, an embodiment of the present application provides a negative electrode slurry, comprising a conductive agent, wherein the conductive agent is the modified carbon nanotube as described above.

[0015] In a fourth aspect, an embodiment of the present application provides a battery, comprising a negative electrode sheet, wherein the negative electrode sheet is prepared using the negative electrode slurry as described above. Beneficial effects

[0016] Beneficial effects of this application:

[0017] The carbon nanotubes prepared by the preparation method provided in the present application have carboxyl groups on their inner walls, and the carboxyl groups on the inner walls react with the hydroxyl groups of polyethylene glycol, so that the carboxyl groups on the inner walls and the polyethylene glycol form a "bridge" structure in the carbon nanotubes. On the one hand, the diameter of the carbon nanotubes is reduced, and the mechanical properties of the carbon nanotubes, such as tensile strength, are improved, which is beneficial to improving the peeling phenomenon caused by volume change of the electrode during the charging and discharging process. On the other hand, the carbon nanotubes with smaller diameters form exclusive transport channels for lithium ions, greatly improving the conductivity of lithium ions in lithium batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG1 is a mechanical properties testing system for carbon nanotubes in this application;

[0019] FIG2 is a schematic flow diagram of a method for preparing modified carbon nanotubes according to an embodiment of the present application;

[0020] FIG3 is a schematic structural diagram of a battery provided in one embodiment of the present application;

[0021] FIG4 is a schematic structural diagram of the core package in FIG3 . Modes for Carrying Out the Invention

[0022] The present application provides a method for preparing modified carbon nanotubes, which effectively improves the conductivity of the carbon nanotubes. Under specific preparation methods, the pore size of the carbon nanotubes can be reduced, thereby increasing the tensile strength of the carbon nanotubes. The modified carbon nanotubes prepared by the method described in the present application can be applied to lithium batteries, effectively improving the conductivity of the lithium battery. Furthermore, the modified carbon nanotubes have higher tensile strength, which can effectively improve the exfoliation caused by volume changes during the charge and discharge process of the lithium battery, enhance the structural stability of the lithium battery, and optimize the cycle performance of the lithium battery.

[0023] According to a first aspect of the present application, a method for preparing modified carbon nanotubes is provided. Please refer to Figure 2. The preparation method comprises the following steps: S1. placing carbon nanotubes in a nitric acid solution, reacting at 65-75°C for 10-14 hours, washing, and drying to obtain carboxylated carbon nanotubes; the mass fraction of HNO3 in the nitric acid solution is not less than 20%; S2. placing the carboxylated carbon nanotubes in a first mixed acid solution, reacting for 10-14 hours, washing, and drying to obtain first carbon nanotubes with carboxylated inner walls; the first mixed acid solution is a composite solution of concentrated nitric acid and concentrated hydrochloric acid, and the volume ratio of concentrated nitric acid to concentrated hydrochloric acid is 1:2-5; in the concentrated nitric acid, H The mass fraction of NO3 is 60-70%; the mass fraction of HCl in concentrated hydrochloric acid is 30-38%; S3. The first inner-wall carboxylated carbon nanotubes are placed in a second mixed acid solution, allowed to stand for 10-14 hours, washed and dried to obtain second inner-wall carboxylated carbon nanotubes; the second mixed acid solution is a composite solution of concentrated sulfuric acid and concentrated hydrochloric acid, and the volume ratio of concentrated sulfuric acid to concentrated hydrochloric acid is 1:1-1.5; S4. The second inner-wall carboxylated carbon nanotubes, polyethylene glycol, a dispersant and water are mixed, and the pH of the mixed system is controlled to be 1-4, and the mixture is refluxed at 78-85°C under nitrogen conditions for 3-5 hours, washed and dried to obtain modified carbon nanotubes.

[0024] In the above S1, the temperature can be, for example, 65°C, 70°C, or 75°C, the reaction time can be, for example, 10h, 11h, 12h, 13h, or 14h, and the mass fraction of HNO3 can be, for example, 20%, 25%, 30%, or 40%; in the above S2, the reaction time can be, for example, 10h, 11h, 12h, 13h, or 14h, the volume ratio of concentrated nitric acid to concentrated hydrochloric acid can be, for example, 1:2, 1:3, 1:4, or 1:5, the mass fraction of HNO3 in concentrated nitric acid can be, for example, 60%, 65%, or 70%, and the mass fraction of HCl in concentrated hydrochloric acid can be, for example, 1: is 30%, 33%, 35%, 38%; in the above S3, the standing time can be, for example, 10h, 11h, 12h, 13h, 14h, and the volume ratio of concentrated sulfuric acid and concentrated hydrochloric acid can be, for example, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5; in the above S4, the pH can be, for example, 1, 2, 3, 4, the reaction temperature can be, for example, 78°C, 80°C, 85°C, and the reaction time can be, for example, 3h, 4h, 5h; the above reaction conditions are not limited to the listed values, and other values ​​not listed within the numerical range are equally applicable.

[0025] In the method for preparing modified carbon nanotubes provided herein, first, in step S1, nitric acid treatment is performed. The strong oxidizing properties of nitric acid are utilized to oxidize groups on the outer surface and inner wall of the carbon nanotubes, such as nitrile groups, hydroxyl groups, and ester groups, to carboxyl groups. Under specific reaction temperatures and reaction times, substantially all groups on the outer surface and inner wall of the carbon nanotubes, except for carboxyl groups, are oxidized to carboxyl groups. Next, in step S2, the carboxylated carbon nanotubes are reacted in a first mixed acid solution. The first mixed acid solution is a mixture of concentrated nitric acid and concentrated hydrochloric acid in a specific ratio and has strong corrosive properties. While the mixed acid solution can remove carboxyl groups from the surface of the carbon nanotubes, due to the relatively small inner wall space of the carbon nanotubes, the carboxyl groups on the inner wall of the carbon nanotubes are not removed, and essentially only the carboxyl groups on the inner wall of the carbon nanotubes are retained. Then, in S3, the first internal carboxylated carbon nanotubes with the surface carboxyl groups removed are subjected to acid treatment under strong acidic conditions to further remove the byproducts generated on the outer surface of the carbon nanotubes during the acid treatment in S2, so as to avoid these byproducts from adversely affecting the subsequent reaction between the carbon nanotubes and polyethylene glycol. Finally, the second internal carboxylated carbon nanotubes, which have undergone multiple acid treatments, are subjected to a reflux reaction with polyethylene glycol and a dispersant. At a specific pH and temperature, the carboxyl groups on the inner wall of the carbon nanotubes react with the hydroxyl groups of the polyethylene glycol, so that the carboxyl groups on the inner wall and the polyethylene glycol form a "bridge" structure in the carbon nanotubes. On the one hand, the diameter of the carbon nanotubes is reduced, and the mechanical properties of the carbon nanotubes, such as tensile strength, are improved, which is beneficial to improving the peeling phenomenon caused by volume change of the electrode during the charge and discharge process. On the other hand, the carbon nanotubes with smaller diameters form exclusive transport channels for lithium ions, greatly improving the conductivity of lithium ions in lithium batteries.

[0026] In S1, the carbon nanotubes are first treated with a nitric acid solution, which helps convert other groups on the outer surface and interior of the carbon nanotubes into carboxyl groups, thereby increasing the carboxyl content of the carbon nanotubes and facilitating the subsequent reaction of the carbon nanotubes with the hydroxyl groups in the polyethylene glycol. In S2, the carboxyl groups on the carbon nanotube surfaces are removed with the first mixed acid solution so that only the carboxyl groups on the inner wall react with the polyethylene glycol in the subsequent reaction of the polyethylene glycol with the carboxyl groups. Because the inner wall of the carbon nanotube has relatively little space, this prevents the polyethylene glycol from reacting first with the carboxyl groups on the outer surface of the carbon nanotubes. Only a small amount of polyethylene glycol reacts with the carboxyl groups on the inner wall of the carbon nanotubes, which cannot form a good "bridge" structure and cannot effectively reduce the diameter of the carbon nanotubes, resulting in an ineffective improvement in the tensile strength and lithium ion conductivity of the carbon nanotubes. Furthermore, pre-processing the carbon nanotubes in S2 by removing the carboxyl groups from their surfaces can reduce the amount of polyethylene glycol used, avoiding the need for excessive polyethylene glycol grafting onto the carbon nanotubes, and thus reducing raw material costs. Continuing to acid-treat the carbon nanotubes in S3 effectively removes byproducts generated by the acid-treatment in S2, preventing these byproducts from interfering with subsequent reactions between the carbon nanotubes and polyethylene glycol.

[0027] In some embodiments, in S2, the volume ratio of concentrated nitric acid to concentrated hydrochloric acid in the first mixed acid solution is 1:3. At this ratio, the carboxyl groups on the surface of the carbon nanotubes can be more fully removed without affecting the carboxyl groups on the inner wall of the carbon nanotubes.

[0028] In some embodiments, in S1, the mass fraction of HNO3 in the nitric acid solution is 25-40%. At this mass fraction, nitric acid can effectively oxidize other groups on the outer surface and inner wall of the carbon nanotubes into carboxyl groups while avoiding waste of resources caused by excessive HNO3.

[0029] In some embodiments, in S4, the mass ratio of the second inner-wall carboxylated carbon nanotubes, polyethylene glycol, and dispersant is 55-85:20-50:3-5. The mass ratio of the second inner-wall carboxylated carbon nanotubes, polyethylene glycol, and dispersant can be, for example, 55:50:5, 58:19:4, 60:30:5, 65:40:5, 70:38:5, 70:40:4, 75:21:5, or 85:20:3, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable. Controlling the feed ratio of each material facilitates good dispersion of the carbon nanotubes in the reaction system, allowing the carboxyl groups on the inner walls of the carbon nanotubes to fully react with the polyethylene glycol, forming a stable "bridge" structure within the carbon nanotubes, facilitating a reduction in the diameter of the carbon nanotubes and enhancing the stability of the carbon nanotubes after the diameter reduction.

[0030] In some embodiments, in S1, the diameter of the carbon nanotubes is 10-30 nm, for example, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, 22 nm, 25 nm, 28 nm, or 30 nm, but is not limited to the values ​​listed above, and other values ​​not listed within the numerical range are also applicable.

[0031] In some embodiments, in S1, the carbon nanotubes are multi-walled carbon nanotubes. Multi-walled carbon nanotubes contain a large number of active groups on their outer surfaces and inner walls, which facilitates the oxidation of more groups on the carbon nanotubes to carboxyl groups, thereby increasing the carboxyl content of the carbon nanotubes. This facilitates the reaction of more carboxyl groups on the inner walls of the carbon nanotubes with polyethylene glycol, significantly reducing the diameter of the carbon nanotubes and improving the lithium ion conductivity of the carbon nanotubes.

[0032] In some embodiments, in S4, the dispersant includes at least one of an ionic surfactant and a non-ionic surfactant.

[0033] In some embodiments, the ionic surfactant includes at least one of sodium dodecylbenzenesulfonate, sodium lauryl sulfate, and dodecyldimethylbenzyl ammonium chloride; and the nonionic surfactant includes TNWDIS. TNWDIS herein refers to an alkylphenol polyoxyethylene ether (APEO)-free, aromatic-group-containing nonionic surfactant.

[0034] In some embodiments, the dispersant is TNWDIS. Using TNWDIS as a dispersant can improve the dispersion of the reaction mixture, promote the full reaction of the carbon nanotubes and polyethylene glycol, effectively reduce the diameter of the carbon nanotubes, and enhance the lithium ion conductivity of the carbon nanotubes.

[0035] In some embodiments, in S1, the number average molecular weight of the polyethylene glycol is 600 to 4000 g / mol. For example, it can be 600 g / mol, 1000 g / mol, 1500 g / mol, 2000 g / mol, 3000 g / mol, or 4000 g / mol, but the values ​​listed are not limited thereto. Other values ​​not listed within the numerical range are also applicable. Controlling the molecular weight of the polyethylene glycol within a certain range is beneficial for regulating the diameter of the carbon nanotubes and helps the polyethylene glycol provide a certain support force within the diameter of the carbon nanotubes, making the diameter of the carbon nanotubes relatively stable and less susceptible to change, thereby effectively improving the mechanical properties of the carbon nanotubes.

[0036] In other embodiments, the number average molecular weight of polyethylene glycol is 1000-2000 g / mol.

[0037] According to a second aspect of the present application, a modified carbon nanotube is provided, which is prepared by utilizing the above-mentioned method for preparing the modified carbon nanotube.

[0038] According to a third aspect of the present application, a negative electrode slurry is provided, comprising a conductive agent, wherein the conductive agent is the modified carbon nanotubes described above. Using the modified carbon nanotubes described herein as the conductive agent in the negative electrode slurry effectively improves the conductivity of the negative electrode slurry. Furthermore, a dispersant with good adhesion to the surface of the carbon nanotubes improves the stability of the slurry.

[0039] According to a fourth aspect of the present application, a battery 200 is provided, comprising a negative electrode sheet 100, wherein the negative electrode sheet 100 is prepared using the negative electrode slurry described above. The battery 200 prepared in the present application not only has improved conductivity within the battery 200 and, due to the use of the modified carbon nanotubes described in the present application, has superior tensile strength, but also improves the peeling phenomenon of the negative electrode sheet caused by volume changes during charge and discharge, thereby improving the battery's cycling stability.

[0040] Referring to Figures 3 and 4 , in some embodiments, a battery 200 includes an outer casing 10 and a core pack 20. The outer casing 10 is composed of a shell 11 and a cover 12. The shell 11 defines a housing space within which the core pack 20 is mounted. The core pack 20 includes a positive electrode sheet 101, a separator 102, and the negative electrode sheet 100, which are stacked in sequence. The negative electrode sheet 100 is prepared using the aforementioned negative electrode slurry. Furthermore, in some embodiments, the core pack 20 also includes a positive electrode tab 51 connected to the positive electrode sheet 101 and a negative electrode tab 61 connected to the negative electrode sheet 100. The core pack 20 is filled with electrolyte. Furthermore, the battery 200 includes a positive electrode post 52 and a negative electrode post 62 disposed within the outer casing 10. The positive electrode post 52 is disposed corresponding to the positive electrode tab 51, passes through the outer casing 10, and is connected to the positive electrode tab 51 via a first connecting tab 53. The negative electrode post 62 is disposed corresponding to the negative electrode tab 61, passes through the outer casing 10, and is connected to the negative electrode tab 61 via a second connecting tab 63.

[0041] Example 1

[0042] (1) Preparation of modified carbon nanotubes

[0043] The method for preparing modified carbon nanotubes of this embodiment includes the following steps:

[0044] S1. Carbon nanotubes were placed in a nitric acid solution and ultrasonically dispersed at 70°C for 12 hours. After multiple washings with deionized water, they were vacuum-dried at 25°C for 24 hours to obtain carboxylated carbon nanotubes. The mass fraction of HNO3 in the nitric acid solution was 30%. The carbon nanotubes were multi-walled carbon nanotubes with a diameter of 20 nm.

[0045] S2. The carboxylated carbon nanotubes were placed in a first mixed acid solution, stirred for reaction for 12 hours, washed multiple times with deionized water, filtered through a 0.2 μm microporous filter membrane, and vacuum dried at 20-25°C for 24 hours to obtain first inner-wall carboxylated carbon nanotubes; the first mixed acid solution was a composite solution of concentrated nitric acid and concentrated hydrochloric acid, with a volume ratio of concentrated nitric acid to concentrated hydrochloric acid of 1:3; the mass fraction of HNO3 in the concentrated nitric acid was 65%; and the mass fraction of HCl in the concentrated hydrochloric acid was 36%;

[0046] S3. The first inner wall of the carboxylated carbon nanotubes was placed in a second mixed acid solution, allowed to stand for 12 hours, washed several times with deionized water, and then dried under vacuum at 25 ° C for 24 hours to obtain a second inner wall of the carboxylated carbon nanotubes; the second mixed acid solution was a composite solution of concentrated sulfuric acid and concentrated hydrochloric acid, and the volume ratio of concentrated sulfuric acid and concentrated hydrochloric acid was 1: 1.2;

[0047] S4. The second inner-wall carboxylated carbon nanotubes, polyethylene glycol, TNWDIS, and water were mixed, and the pH of the mixture was controlled to 2. The mixture was refluxed at 80°C under nitrogen for 4 hours. After multiple washings with deionized water, the mixture was vacuum-dried at 25°C for 24 hours to obtain modified carbon nanotubes. The mass ratio of the second inner-wall carboxylated carbon nanotubes, polyethylene glycol, and TNWDIS was 60:36:4. The number average molecular weight of the polyethylene glycol was 1500 g / mol.

[0048] (2) Preparation of negative electrode slurry

[0049] Graphite, the modified carbon nanotubes, a thickener (CMC), and a binder (SBR) were mixed in a mass ratio of 96.7:1.0:1.5:0.8 to obtain a negative electrode slurry.

[0050] Example 2

[0051] (1) Preparation of modified carbon nanotubes

[0052] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that in S4, the dispersant is too little, and the ratio of the second inner-wall carboxylated carbon nanotubes, polyethylene glycol, and dispersant is 63:30:1. The rest is the same as embodiment 1.

[0053] (2) Preparation of negative electrode slurry

[0054] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0055] Example 3

[0056] (1) Preparation of modified carbon nanotubes

[0057] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that, in S4, the dispersant is sodium dodecylbenzenesulfonate, and the rest is the same as embodiment 1.

[0058] (2) Preparation of negative electrode slurry

[0059] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0060] Example 4

[0061] (1) Preparation of modified carbon nanotubes

[0062] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that, in S4, the dispersant is sodium lauryl sulfate, and the rest is the same as embodiment 1.

[0063] (2) Preparation of negative electrode slurry

[0064] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0065] Example 5

[0066] (1) Preparation of modified carbon nanotubes

[0067] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that, in S4, the dispersant is dodecyldimethylbenzylammonium chloride, and the rest is the same as embodiment 1.

[0068] (2) Preparation of negative electrode slurry

[0069] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0070] Example 6

[0071] (1) Preparation of modified carbon nanotubes

[0072] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that in S4, the number average molecular weight of polyethylene glycol is 600 g / mol, and the rest is consistent with embodiment 1.

[0073] (2) Preparation of negative electrode slurry

[0074] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0075] Example 7

[0076] (1) Preparation of modified carbon nanotubes

[0077] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that in S4, the number average molecular weight of polyethylene glycol is 1000 g / mol, and the rest is consistent with embodiment 1.

[0078] (2) Preparation of negative electrode slurry

[0079] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0080] Example 8

[0081] (1) Preparation of modified carbon nanotubes

[0082] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that in S4, the number average molecular weight of polyethylene glycol is 2000 g / mol, and the rest is consistent with embodiment 1.

[0083] (2) Preparation of negative electrode slurry

[0084] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0085] Example 9

[0086] (1) Preparation of modified carbon nanotubes

[0087] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that in S4, the number average molecular weight of polyethylene glycol is 4000 g / mol, and the rest is consistent with embodiment 1.

[0088] (2) Preparation of negative electrode slurry

[0089] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0090] Example 10

[0091] (1) Preparation of modified carbon nanotubes

[0092] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that in S4, the number average molecular weight of polyethylene glycol is 5000 g / mol, and the rest is consistent with embodiment 1.

[0093] (2) Preparation of negative electrode slurry

[0094] The preparation of the negative electrode slurry in this embodiment is consistent with that in Example 1.

[0095] Comparative Example 1

[0096] (1) Preparation of modified carbon nanotubes

[0097] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that the carbon nanotubes are not subjected to the acid treatment in S2 , and the rest are the same as embodiment 1.

[0098] (2) Preparation of negative electrode slurry

[0099] The preparation of the negative electrode slurry in this comparative example is consistent with that in Example 1.

[0100] Comparative Example 2

[0101] (1) Preparation of modified carbon nanotubes

[0102] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that the carbon nanotubes are not subjected to the acid treatment in S3 , and the rest are the same as embodiment 1.

[0103] (2) Preparation of negative electrode slurry

[0104] The preparation of the negative electrode slurry in this comparative example is consistent with that in Example 1.

[0105] Comparative Example 3

[0106] (1) Preparation of modified carbon nanotubes

[0107] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that polyethylene glycol is not added in the reaction in S4, and the rest is the same as embodiment 1.

[0108] (2) Preparation of negative electrode slurry

[0109] The preparation of the negative electrode slurry in this comparative example is consistent with that in Example 1.

[0110] Comparative Example 4

[0111] (1) Preparation of modified carbon nanotubes

[0112] In the preparation of modified carbon nanotubes, this embodiment differs from embodiment 1 in that the dispersant TNWDIS is not added in the reaction in S4, and the rest is the same as embodiment 1.

[0113] (2) Preparation of negative electrode slurry

[0114] The preparation of the negative electrode slurry in this comparative example is consistent with that in Example 1.

[0115] Comparative Example 5

[0116] The carbon nanotubes used in this comparative example are unmodified carbon nanotubes, that is, in preparing the negative electrode slurry of this comparative example, the conductive agent used is unmodified carbon nanotubes, which is the carbon nanotubes that have been modified in any way in S1 of Example 1.

[0117] The preparation of the negative electrode slurry in this comparative example is consistent with that in Example 1.

[0118] Test Case

[0119] 1. Experimental Construction Method

[0120] (1) Test of tensile strength of carbon nanotubes

[0121] The modified carbon nanotubes or carbon nanotubes prepared in the above examples and comparative examples were tested for tensile strength, and the specific steps were as follows:

[0122] 1) Referring to the carbon nanotube fiber mechanical properties testing system at high temperature shown in FIG. 1 , three quartz tubes 2 are vertically fixed on an iron stand 1 . Two ends of a carbon nanotube fiber 3 to be tested are connected via two wires 4 , which pass through the upper and lower ends of the three quartz tubes 2 . Two clamps 7 of a universal material testing machine 6 are used to clamp the two wires 4 , respectively, maintaining the carbon nanotube fiber 3 in a vertical position.

[0123] 2) Connect the two wires 4 to the positive and negative terminals of a DC power supply 5, respectively. Set the current value and keep it constant. Before the test begins, introduce inert gas into the three quartz tubes 2 through the inert gas introduction mechanism 8 to remove air (for one minute). Continue to introduce inert gas during the test.

[0124] 3) Aim the eyepiece of infrared thermometer 9 at the middle of the carbon nanotube fiber 3 to be tested, open it in advance, and record the temperature change during the power-on process (the infrared thermometer measures the energy radiated by the carbon nanotube fiber during the power-on process to obtain its temperature. This application tests the tensile strength of the carbon nanotube fiber at 1500°C);

[0125] 4) Simultaneously start the DC power supply 5 and the universal material testing machine 6 to test the tensile strength of the carbon nanotube fiber under high temperature.

[0126] (2) Conductivity test of negative electrode slurry (expressed as resistivity)

[0127] The resistivity of the negative electrode slurry prepared in the above embodiment and comparative example is tested. The specific method is as follows: first, 10ML of negative electrode slurry is placed on a PP film, and the negative electrode slurry is evenly coated on the PP film using a small coating machine. Then, it is placed in an oven and baked at 100°C for 5 minutes. After the negative electrode slurry is cooled and solidified, it is taken out and the resistivity is tested using a four-probe method. The specific steps include the following: (1) Place the sample to be tested on the test bench and contact the sample surface with four probes respectively; (2) Start the current source and apply a constant current; (3) Measure the voltage difference between the two groups of electrodes using a voltmeter and record it; (4) Repeat the above steps multiple times for different positions and directions to obtain more accurate measurement data. After the measurement is completed, the obtained data can be processed, such as removing outliers, averaging, etc., and then the resistivity of the sample is calculated. At the same time, it is also necessary to analyze and compare the measurement results, compare them with the standard values, and judge the test error and reliability issues.

[0128] (3) Battery cycle performance test

[0129] A negative electrode sheet was prepared using the negative electrode slurry prepared in the above examples and comparative examples. A positive electrode sheet was prepared using the positive electrode slurry obtained by mixing the positive electrode active material, a conductive agent (CB, conductive carbon black), a dispersant (PVP, polyvinyl pyrrolidone), and a binder (PVDF) in a mass ratio of 97.2:0.5:0.6:1.7. The positive electrode sheet, separator, and negative electrode sheet were assembled, and an electrolyte (1 mol / L LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (EDC) (volume ratio 1:1)) was injected to produce a battery.

[0130] The obtained battery is then subjected to a cycle performance test, and the specific steps are as follows:

[0131] 1) Under 25℃ conditions, ① the battery is discharged at a constant current of 1C(A) to 2.5V and allowed to stand for 5min. ② It is charged at a constant current and constant voltage of 1C(A) with a cut-off voltage of 3.65V and a cut-off current of 0.05C(A) and allowed to stand for 5min. ③ The battery is discharged at a constant current of 1C(A) to 2.5V. Repeat the above operation 13 times. The last discharge capacity is taken as the initial capacity and recorded as Q0. 2) Let it stand for 3min. 3) Cyclic discharge: discharge at 1C to 2.5V. 4) Let it stand for 1h. 5) Constant current and constant voltage charge: charge at 1C to 3.65V with a cut-off current of 0.05C. 6) Let it stand for 1h. 7) Cycling: repeat operations 3 to 6. When the battery cycles 1500 times, record the discharge capacity as Q1. Capacity retention rate = Q1 / Q0×100%.

[0132] 2. Experimental Results

[0133] The relevant performance tests were performed on the test objects in the above embodiments and comparative examples, and the results are shown in Table 1.

[0134] Table 1 Test results of relevant properties of the test objects in the embodiments and comparative examples

[0135] As shown in Table 1, the modified carbon nanotubes prepared by the preparation method provided in this application exhibit stronger tensile strength, and the negative electrode slurry prepared using them has lower resistivity, that is, higher conductivity. For details, see Examples 1-10. The higher tensile strength of the modified carbon nanotubes can effectively improve the spalling phenomenon caused by volume change during the charge and discharge process of lithium batteries, thereby enhancing the structural stability of lithium batteries. The improved conductivity is beneficial to the conductivity of lithium ions in lithium batteries. These two factors together optimize the cycling performance of lithium batteries, allowing them to maintain a high capacity retention rate after multiple charge and discharge cycles. The improved tensile strength and conductivity of the modified carbon nanotubes are due to the formation of a "bridge" structure between the polyethylene glycol and the carboxyl groups on the inner wall of the carbon nanotubes. This reduces the diameter of the carbon nanotubes, improving their mechanical properties, such as tensile strength, and helping to mitigate spalling caused by volume change during charge and discharge. Furthermore, the smaller diameter carbon nanotubes form dedicated transport channels for lithium ions, significantly improving the conductivity of lithium ions in lithium batteries.

[0136] Further, comparing Example 1 and Comparative Examples 1 to 5, compared with Example 1, Comparative Example 1 does not undergo the acid treatment in S2, Comparative Example 2 does not undergo the acid treatment in S3, Comparative Example 3 does not add polyethylene glycol to S4, Comparative Example 4 does not add the dispersant TNWDIS to S4, and Comparative Example 5 does not undergo any modification of the carbon nanotubes. Ultimately, the tensile strength of the modified carbon nanotubes or unmodified carbon nanotubes in Comparative Examples 1 to 5 and the conductivity of the negative electrode slurries prepared using them are poor, resulting in poor cycle capacity retention of the lithium battery. This shows that each processing step in the preparation method of modified carbon nanotubes in this application may affect the tensile strength and conductivity of the final modified carbon nanotubes, especially the acid treatment in S2 and S3 and the addition of polyethylene glycol for modification, which directly have a significant impact on the tensile strength and conductivity of the final carbon nanotubes, thereby affecting the cycle performance of the lithium battery. Furthermore, comparing Examples 1 and 2, the insufficient amount of dispersant in Example 2 prevented the carbon nanotubes from fully dispersing and reacting with the polyethylene glycol. This, in turn, resulted in a low grafting rate of polyethylene glycol onto the inner walls of the carbon nanotubes, preventing a more effective reduction in the diameter of the carbon nanotubes. This reduced the tensile strength and conductivity of the modified carbon nanotubes, and also reduced the cycling performance of the lithium battery. Further comparing Examples 1 and 3-5, the tensile strength and conductivity of the modified carbon nanotubes in Examples 3-5 were lower than those in Example 1. This demonstrates that the use of TNWDIS dispersant facilitates the dispersion of the carbon nanotubes, further facilitating the grafting reaction between the carbon nanotubes and polyethylene glycol, thereby improving the tensile strength and conductivity of the carbon nanotubes and further optimizing the cycling performance of the lithium battery.

[0137] Further comparing Examples 1, 6, and 10, the molecular weight of the polyethylene glycol used in Examples 1, 6, and 9 was in the range of 600 to 4000 g / mol. Compared with Example 10 (polyethylene glycol number average molecular weight of 5000 g / mol), the modified carbon nanotubes had better tensile strength and conductivity. This shows that the molecular weight of the polyethylene glycol also has a certain impact on the performance of the modified carbon nanotubes. If the molecular weight of the polyethylene glycol is small and the polyethylene glycol chain is short, it may be detrimental to the grafting of the polyethylene glycol ends to the inner wall of the carbon nanotube, and the inner diameter of the carbon nanotube cannot be effectively reduced, resulting in a decrease in the tensile strength and conductivity of the carbon nanotube. If the molecular weight of the polyethylene glycol is too large and the molecular chain is long, the polyethylene glycol cannot provide a large tensile force to reduce the inner diameter of the carbon nanotube, and the inner diameter of the carbon nanotube cannot be effectively reduced, resulting in a decrease in the tensile strength and conductivity of the carbon nanotube.

Claims

1. A method for preparing modified carbon nanotubes, comprising the following steps: Placing the carbon nanotubes in a nitric acid solution, reacting at 65-75° C. for 10-14 hours, washing and drying to obtain carboxylated carbon nanotubes; wherein the mass fraction of HNO3 in the nitric acid solution is not less than 20%; The carboxylated carbon nanotubes are placed in a first mixed acid solution, reacted for 10 to 14 hours, washed and dried to obtain first inner wall carboxylated carbon nanotubes; the first mixed acid solution is a composite solution of concentrated nitric acid and concentrated hydrochloric acid, the volume ratio of the concentrated nitric acid to the concentrated hydrochloric acid is 1:2 to 5; the mass fraction of HNO3 in the concentrated nitric acid is 60 to 70%; the mass fraction of HCl in the concentrated hydrochloric acid is 30 to 38%; The first inner wall carboxylated carbon nanotubes are placed in a second mixed acid solution, left to stand for 10 to 14 hours, washed, and dried to obtain second inner wall carboxylated carbon nanotubes; the second mixed acid solution is a composite solution of concentrated sulfuric acid and the concentrated hydrochloric acid, and the volume ratio of the concentrated sulfuric acid to the concentrated hydrochloric acid is 1:1 to 1.5; The second inner wall carboxylation carbon nanotubes, polyethylene glycol, dispersant and water are mixed, and the pH of the mixed system is controlled to be 1-4. The mixture is refluxed at 78-85° C. for 3-5 hours under nitrogen conditions, and then washed and dried to obtain modified carbon nanotubes.

2. The method for preparing modified carbon nanotubes according to claim 1, wherein: In the step of mixing the second inner-wall carboxylated carbon nanotubes, polyethylene glycol, a dispersant and water, controlling the pH of the mixed system to be 1-4, refluxing the mixture at 78-85°C for 3-5 hours under nitrogen conditions, washing and drying to obtain modified carbon nanotubes, the mass ratio of the second inner-wall carboxylated carbon nanotubes, the polyethylene glycol and the dispersant is 55-85:20-50:3-5.

3. The method for preparing modified carbon nanotubes according to claim 1, wherein: In the step of placing carbon nanotubes in a nitric acid solution, reacting at 65-75° C. for 10-14 hours, washing and drying to obtain carboxylated carbon nanotubes, the carbon nanotubes have a diameter of 10-30 nm.

4. The method for preparing modified carbon nanotubes according to claim 3, wherein: In the step of placing carbon nanotubes in a nitric acid solution, reacting at 65-75° C. for 10-14 hours, washing and drying to obtain carboxylated carbon nanotubes, the carbon nanotubes are multi-walled carbon nanotubes.

5. The method for preparing modified carbon nanotubes according to claim 1, wherein: In the step of placing carbon nanotubes in a nitric acid solution, reacting at 65-75° C. for 10-14 hours, washing and drying to obtain carboxylated carbon nanotubes, the mass fraction of HNO 3 in the nitric acid solution is 25%-40%.

6. The method for preparing modified carbon nanotubes according to claim 1, wherein: The dispersant includes at least one of an ionic surfactant and a non-ionic surfactant.

7. The method for preparing modified carbon nanotubes according to claim 6, wherein: The ionic surfactant includes at least one of sodium dodecylbenzene sulfonate, sodium dodecyl sulfate, and dodecyldimethylbenzyl ammonium chloride.

8. The method for preparing modified carbon nanotubes according to claim 6, wherein: The nonionic surfactant includes TNWDIS.

9. The method for preparing modified carbon nanotubes according to any one of claims 1 to 8, wherein: The dispersant is TNWDIS.

10. The method for preparing modified carbon nanotubes according to claim 1, wherein: In the S4, the number average molecular weight of the polyethylene glycol is 600 to 4000 g / mol.

11. The method for preparing modified carbon nanotubes according to claim 10, wherein: The number average molecular weight of the polyethylene glycol is 1000-2000 g / mol.

12. A modified carbon nanotube prepared by the method for preparing modified carbon nanotubes according to any one of claims 1 to 11.

13. The modified carbon nanotube according to claim 12, wherein: The inner wall of the modified carbon nanotube is connected with a carboxyl group.

14. The modified carbon nanotube according to claim 13, wherein: The modified carbon nanotube further comprises polyethylene glycol, and the polyethylene glycol is connected to the carboxyl group. 15 . A negative electrode slurry comprising a conductive agent, wherein the conductive agent is the modified carbon nanotube according to claim 12 .

16. The negative electrode slurry according to claim 15, wherein: The negative electrode slurry further includes graphite, a thickener and a binder.

17. A battery comprising a negative electrode sheet, wherein the negative electrode sheet is prepared using the negative electrode slurry according to claim 15 or 16.

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