Binder composition and preparation method thereof, secondary battery and electronic device

Abstract

The invention provides a binder composition and a preparation method thereof, a secondary battery and an electronic device. The binder composition comprises a binder and deionized water, the binder comprises a carbon nanotube and polyacrylate located on at least part of the surface of the carbon nanotube, the defect degree of the binder is ID / IG, the ID / IG is larger than or equal to 0.1 and smaller than or equal to 0.6, and the emulsion particle size Dv50 of the binder is 0.2-0.5 [mu] m; based on the mass of the binder composition, the mass percentage X of the binder is 3.3-5.3%, the optical density OD of the binder composition is greater than or equal to 0.6 and less than or equal to 1. The binder composition disclosed by the invention is used in the secondary battery, and the ID / IG value, the Dv50 value, the X value and the OD value are regulated and controlled in the range, so that a stable and uniformly dispersed binding conductive network can be formed in the negative electrode plate, the porosity of the negative electrode plate after circulation is reduced, and the cycle performance of the secondary battery is improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technology, and in particular to a binder composition and a preparation method thereof, a secondary battery, and an electronic device. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, have the advantages of high energy density, high power, and long cycle life. They are widely used in consumer electronics, electric bicycles, and electric vehicles. As their application scope continues to expand, the requirements for the energy density and cycle performance of lithium-ion batteries continue to increase.

[0003] Currently, common graphite anode materials for lithium-ion batteries no longer meet energy density requirements. While silicon-containing materials, as anode active materials, offer high theoretical specific capacity and are ideal for replacing graphite anodes and improving the energy density of lithium-ion batteries, their large cyclic expansion during lithium intercalation and deintercalation can disrupt the bonded conductive network within the anode material layer, leading to excessive porosity in the anode sheet and poor cycle life for lithium-ion batteries. This has limited the widespread application of silicon-containing materials. Summary of the Invention

[0004] The purpose of this application is to provide a binder composition and its preparation method, a secondary battery, and an electronic device to reduce the porosity of the negative electrode sheet after cycling and improve the cycling performance of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of the present application provides a binder composition comprising a binder and deionized water, wherein the binder comprises carbon nanotubes and polyacrylate located on at least a portion of the surface of the carbon nanotubes. The binder has a defectivity of ID / IG, 0.1≤ID / IG≤0.6, and an emulsion particle size Dv50 of the binder is 0.2μm to 0.5μm. The binder has a mass percentage X of 3.3% to 5.3% based on the mass of the binder composition, and an optical density OD of 0.6≤OD≤1. In the binder composition of the present application, the binder comprises carbon nanotubes and polyacrylate located on at least a portion of the surface of the carbon nanotubes. The polyacrylate has good elasticity. Modifying the polyacrylate within the carbon nanotube network structure can increase the interfacial force between the binder and the silicon-containing material, thereby alleviating damage to the bonding network caused by external forces generated by silicon expansion during the charge and discharge of the secondary battery. By regulating the values ​​of ID / IG, Dv50, X and OD within the above ranges, the binder composition is used to prepare the negative electrode sheet, which can increase the interfacial force between the binder and the silicon-containing material, form a stable and uniformly dispersed bonding conductive network in the negative electrode material layer, and play a role in stress buffering and stress release during the charge and discharge process of the secondary battery. At the same time, it is beneficial to resist the damage to the bonding network caused by the external force generated by silicon expansion during the charge and discharge process of the secondary battery, improve the stability of the bonding conductive network during the charge and discharge cycle, and reduce the porosity of the negative electrode sheet after the cycle, thereby improving the space utilization of the negative electrode sheet and improving the cycle performance of the secondary battery.

[0006] In some embodiments of the present application, the weight percentage of carbon nanotubes is 7% to 11% and the weight percentage of polyacrylate is 89% to 93% based on the weight of the binder. Regulating the weight percentages of carbon nanotubes and polyacrylate within these ranges helps improve the stability of the bonded conductive network in the negative electrode material layer during charge and discharge cycles, further reduces the porosity of the negative electrode sheet after cycling, and improves the cycle performance of the secondary battery.

[0007] In some embodiments of the present application, the binder further comprises lithium carboxymethyl cellulose, and based on the mass of the binder, the mass percentage of the carbon nanotubes is 5.4% to 10.5%, the mass percentage of the polyacrylate is 69.2% to 90.6%, and the mass percentage of the lithium carboxymethyl cellulose is 2% to 21.4%. The binder further comprises lithium carboxymethyl cellulose, and the mass percentages of the carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose are regulated within the above-mentioned ranges. This can form a stable and more evenly dispersed bonding conductive network in the negative electrode material layer, further facilitating the stability of the bonding conductive network during charge and discharge cycles, further reducing the porosity of the negative electrode sheet after cycling, and improving the cycle performance of the secondary battery.

[0008] In some embodiments of the present application, the monomers forming the polyacrylate include n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate, and acrylic acid. The n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate, and acrylic acid respectively constitute the first, second, third, and fourth structural units of the polyacrylate. Based on the sum of the moles of the first, second, third, and fourth structural units, the mole percentage of the first structural unit is 64.5% to 72.5%, the mole percentage of the second structural unit is 17.5% to 21.5%, the mole percentage of the third structural unit is 0% to 4%, and the mole percentage of the fourth structural unit is 6% to 10%. Selecting the above monomers to polymerize the polyacrylate and regulating the mole percentages of the first, second, third, and fourth structural units within the above ranges can improve the reactivity between the carbon nanotubes and the monomers forming the polyacrylate, ensuring uniform distribution of the polyacrylate within the carbon nanotube network, thereby better performing the binder function. Furthermore, the binder can have excellent flexibility, bonding properties, and stability. Therefore, applying the binder to the negative electrode sheet is beneficial to reducing the porosity of the negative electrode sheet after cycling and improving the cycle performance of the secondary battery.

[0009] In some embodiments of the present application, the viscosity of the binder composition is η mPa·s, 800 ≤ η ≤ 3500, and 233 ≤ η / X ≤ 660. The values ​​of η and η / X being within the above ranges indicate that the binder in the binder composition is uniformly dispersed, which is beneficial for improving the stability of the bonded conductive network in the negative electrode material layer during charge and discharge cycles, thereby further reducing the porosity of the negative electrode sheet after cycling and improving the cycle performance of the secondary battery.

[0010] In some embodiments of the present application, the binder composition is allowed to stand for 48 hours, and the optical density of the binder composition after standing is OD1, 0 < (OD1-OD) / OD ≤ 17.0%. The value of (OD1-OD) / OD is within the above range, indicating that the carbon nanotubes in the binder are uniformly distributed in the binder composition and have good stability. In this way, when the binder composition is used to prepare a negative electrode sheet, it is beneficial for the binder composition to be uniformly dispersed in the negative electrode slurry, which is beneficial to improving the stability of the bonding conductive network in the negative electrode material layer during the charge and discharge cycle, thereby further reducing the porosity of the negative electrode sheet after cycling and improving the cycle performance of the secondary battery.

[0011] In some embodiments of the present application, the binder satisfies at least one of the following characteristics: (1) the glass transition temperature Tg of the binder is -20°C to -10°C; (2) the tensile strength of the binder film is 10 MPa to 50 MPa; (3) the binder film is immersed in electrolyte at 60±5°C for 12 hours, the elongation at break of the binder film before immersion in electrolyte is L0, and the elongation at break of the binder film after immersion in electrolyte is L1, 100%≤L1≤130%, 1.42≤L1 / L0≤1.74; (4) 0.2≤ID / IG≤0.5. Satisfying at least one of the above characteristics is beneficial to reducing the porosity of the negative electrode sheet after cycling and improving the cycling performance of the secondary battery.

[0012] In some embodiments of the present application, in the infrared spectrum of the binder, there is a -1 Stretching vibration absorption peak, 1450±5cm -1 and 1720±2cm -1 The absorption peak of 3437±25cm -1 Stretching vibration absorption peak, 1730±2cm -1 The sharp stretching vibration absorption peak, 1061±5cm -1 、1161±2cm -1 、1158±2cm -1 、1242±5cm -1 The infrared spectrum of the binder has the above-mentioned characteristic peaks, indicating that the binder includes carbon nanotubes and polyacrylate. The binder with the above-mentioned infrared characteristic peaks has good dispersibility and can form a stable and uniformly dispersed bonding conductive network in the negative electrode material layer, thereby further reducing the porosity of the negative electrode sheet after cycling and improving the cycling performance of the secondary battery.

[0013] A second aspect of the present application provides a method for preparing the binder composition provided in any of the above embodiments, comprising the following steps:

[0014] (1) adding carbon nanotubes to a mixture of concentrated sulfuric acid and concentrated nitric acid for acidification, and then washing and drying to obtain acidified carbon nanotubes, wherein the acidification temperature is 50° C. to 70° C. and the acidification time is 0.5 h to 1.5 h;

[0015] (2) under a nitrogen atmosphere, n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid are mixed to obtain a mixed monomer, wherein, based on the sum of the moles of n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid, the mole percentage of n-butyl acrylate is 64.5% to 72.5%, the mole percentage of n-butyl methacrylate is 17.5% to 21.5%, the mole percentage of isooctyl acrylate is 0% to 4%, and the mole percentage of acrylic acid is 6% to 10%; the mixed monomer is divided into two groups, namely a first mixed monomer and a second mixed monomer, and the mass ratio of the first mixed monomer to the second mixed monomer is 1:2;

[0016] (3) under a nitrogen atmosphere, adding an emulsifier and a first mixed monomer to deionized water at 60° C. to 70° C. for pre-emulsification, wherein the pre-emulsification time is 15 min to 25 min; then adding an initiator and a cross-linking agent to carry out a first reaction to obtain a first mixed solution, wherein the temperature of the first reaction is 70° C. to 80° C., the time of the first reaction is 1.5 h to 3 h, and the cross-linking agent includes tetraethylene glycol dimethacrylate;

[0017] (4) slowly adding the second mixed monomer and the acidified carbon nanotubes to the first mixed solution for 2 to 4 hours; after the addition is completed, performing a second reaction to obtain a second mixed solution, wherein the temperature of the second reaction is 65° C. to 85° C. and the time is 1 to 3 hours;

[0018] (5) Adding the initiator to the second mixed solution again, and continuing the third reaction, the temperature of the third reaction is 70° C. to 80° C., and the time is 0.5 h to 2 h; after the third reaction is completed, adjusting the pH to neutral to obtain a binder composition.

[0019] In the above-mentioned preparation method, acidification of the carbon nanotubes facilitates the uniform distribution of the polyacrylate in the carbon nanotube network structure. In addition, the acidification treatment can also increase the interaction between the polar functional groups on the surface of the carbon nanotubes and the negative electrode active material, thereby improving the bonding force between the carbon nanotubes and the negative electrode active material. Pre-emulsification can reduce the occurrence of problems such as emulsion aggregation and stratification. The use of tetraethylene glycol dimethacrylate as a cross-linking agent can increase the cross-linking density of the polyacrylate and reduce the swelling of the polyacrylate in the electrolyte. In addition, the mixed monomers are divided into two groups and distributed according to the above-mentioned mass ratio. The first mixed monomer is added first to facilitate the formation of more nucleation points on the surface of the carbon nanotubes, increasing the number of polyacrylate particles. The second mixed monomer is then added to continue the polymerization, which can cause the polyacrylate particles to grow. The initiator is then added again to carry out the third reaction, thereby obtaining the adhesive composition of the present application. The binder composition prepared by the above preparation method is uniformly dispersed in deionized water in the form of emulsion particles, so that the negative electrode material layer prepared by using the above composition has a stable and uniformly dispersed bonding conductive network, which is beneficial to reducing the porosity of the negative electrode plate after cycling and improving the cycle performance of the secondary battery.

[0020] In some embodiments of the present application, the mass percentage of the carbon nanotubes is 7% to 11%, and the mass percentage of the mixed monomers is 89% to 93%, based on the combined mass of the carbon nanotubes and the mixed monomers. Regulating the mass percentages of the carbon nanotubes and the mixed monomers within these ranges helps improve the stability of the bonded conductive network in the negative electrode material layer during charge and discharge cycles, further reduces the porosity of the negative electrode sheet after cycling, and improves the cycle performance of the secondary battery.

[0021] In some embodiments of the present application, the preparation method further includes: after adjusting the pH to neutral, adding lithium carboxymethyl cellulose, based on the sum of the mass of carbon nanotubes, mixed monomers and lithium carboxymethyl cellulose, the mass percentage of carbon nanotubes is 5.4% to 10.5%, the mass percentage of mixed monomers is 69.2% to 90.6%, and the mass percentage of lithium carboxymethyl cellulose is 2% to 21.4%. By adding lithium carboxymethyl cellulose, the suspension stability of the binder emulsion particles and the ion conductivity of the binder can be improved. In this way, when the binder composition is used to prepare the negative electrode sheet, it is conducive to the uniform dispersion of the binder composition in the negative electrode slurry, and a stable and uniformly dispersed bonding conductive network can be formed in the negative electrode material layer, thereby improving the stability of the above-mentioned bonding conductive network during the charge and discharge cycle, thereby further reducing the porosity of the negative electrode sheet after the cycle and improving the cycle performance of the secondary battery.

[0022] In a third aspect, the present application provides a secondary battery comprising a positive electrode plate, an electrolyte, and a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a silicon-containing material and a binder, the binder comprising carbon nanotubes and a polyacrylate disposed on at least a portion of the surface of the carbon nanotubes, the binder having a defectivity of ID / IG, 0.1≤ID / IG≤0.6, and an emulsion particle size Dv50 of the binder of 0.2μm to 0.5μm. The negative electrode material layer comprising the silicon-containing material and the above-mentioned binder can increase the interfacial force between the binder and the silicon-containing material, thereby alleviating damage to the bonding network caused by external forces generated by silicon expansion during the charge and discharge process of the secondary battery. By regulating the values ​​of ID / IG and Dv50 within the above range, the interfacial force between the binder and the silicon-containing material can be increased, forming a stable and uniformly dispersed bonding conductive network in the negative electrode sheet, which can play a role in stress buffering and stress release during the charge and discharge process of the secondary battery. At the same time, it is beneficial to resist the damage to the bonding network caused by the external force generated by silicon expansion during the charge and discharge process of the secondary battery, improve the stability of the above-mentioned bonding conductive network during the charge and discharge cycle, reduce the porosity of the negative electrode sheet after the cycle, and improve the cycle performance of the secondary battery.

[0023] In some embodiments of the present application, based on the mass of the negative electrode material layer, the mass percentage of silicon is 7.4% to 54%, and the mass percentage of the binder is 3% to 9%. The inclusion of the binder in the negative electrode material layer can reduce the proportion of the binder in the negative electrode material layer, thereby increasing the proportion of the negative electrode active material, which is beneficial for increasing the gram capacity of the negative electrode material layer, thereby enabling the secondary battery to have a higher energy density. Furthermore, it is beneficial for improving the stability of the aforementioned bonding conductive network during charge and discharge cycles, reducing the porosity of the negative electrode sheet after cycling, and improving the cycling performance of the secondary battery.

[0024] In some embodiments of the present application, the negative electrode material layer further comprises lithium carboxymethyl cellulose, and the mass percentage of the lithium carboxymethyl cellulose is 0.25% to 1.58% based on the mass of the negative electrode material layer. The negative electrode material layer further comprises lithium carboxymethyl cellulose, and the mass percentage of the lithium carboxymethyl cellulose is regulated within the above range. This can form a more uniformly dispersed bonded conductive network in the negative electrode material layer, further facilitating the stability of the bonded conductive network during charge and discharge cycles, further reducing the porosity of the negative electrode sheet after cycling, and improving the cycle performance of the secondary battery.

[0025] In some embodiments of the present application, the weight percentage of carbon nanotubes is 7% to 11% and the weight percentage of polyacrylate is 89% to 93% based on the weight of the binder. Regulating the weight percentages of carbon nanotubes and polyacrylate within these ranges helps improve the stability of the bonded conductive network in the negative electrode material layer during charge and discharge cycles, further reduces the porosity of the negative electrode sheet after cycling, and improves the cycle performance of the secondary battery.

[0026] In some embodiments of the present application, the binder further comprises lithium carboxymethyl cellulose, and based on the mass of the binder, the mass percentage of the carbon nanotubes is 5.4% to 10.5%, the mass percentage of the polyacrylate is 69.2% to 90.6%, and the mass percentage of the lithium carboxymethyl cellulose is 2% to 21.4%. The binder further comprises lithium carboxymethyl cellulose, and the mass percentages of the carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose are regulated within the above-mentioned ranges. This can form a stable and more evenly dispersed bonding conductive network in the negative electrode material layer, further facilitating the stability of the bonding conductive network during charge and discharge cycles, further reducing the porosity of the negative electrode sheet after cycling, and improving the cycle performance of the secondary battery.

[0027] In a fourth aspect, the present application provides an electronic device comprising the secondary battery provided in any one of the above embodiments. The secondary battery of the present application has good cycle performance, so that the electronic device of the present application has a long service life.

[0028] Beneficial effects of this application:

[0029] The present application provides an adhesive composition, a preparation method thereof, a secondary battery, and an electronic device. The adhesive composition includes an adhesive and deionized water, wherein the adhesive includes carbon nanotubes and a polyacrylate located on at least a portion of the surface of the carbon nanotubes. The adhesive has a defectivity of ID / IG, 0.1 ≤ ID / IG ≤ 0.6, and an emulsion particle size Dv50 of the adhesive is 0.2 μm to 0.5 μm. The mass percentage X of the adhesive, based on the mass of the adhesive composition, is 3.3% to 5.3%, and the optical density of the adhesive composition is OD, 0.6 ≤ OD ≤ 1. In the binder composition of the present application, the binder includes carbon nanotubes and polyacrylate located on at least a portion of the surface of the carbon nanotubes, and the values ​​of ID / IG, Dv50, X and OD are regulated within the above-mentioned ranges, so that a stable and uniformly dispersed bonding conductive network can be formed in the negative electrode material layer, which can play a role in stress buffering and stress release during the charge and discharge process, while resisting the damage to the bonding network caused by the external force generated by silicon expansion during the charge and discharge process, thereby improving the stability of the above-mentioned bonding conductive network during the charge and discharge cycle, reducing the porosity of the negative electrode sheet after the cycle, and improving the cycle performance of the secondary battery.

[0030] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0032] Figure 1 This is a scanning electron microscope photograph of the negative electrode material layer of Example 1-1;

[0033] Figure 2 This is the infrared spectrum of the adhesive of Example 1-1. DETAILED DESCRIPTION

[0034] The following will be combined with the embodiments of the present application and the accompanying drawings to clearly and completely describe the technical solutions in this application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0035] It should be noted that, in the specific embodiments of the present application, a lithium-ion battery is used as an example of an electrochemical device to explain the present application, but the electrochemical device of the present application is not limited to a lithium-ion battery.

[0036] The first aspect of the present application provides an adhesive composition, which includes an adhesive and deionized water, the adhesive including carbon nanotubes and polyacrylate located on at least a portion of the surface of the carbon nanotubes, the defectivity of the adhesive is ID / IG, 0.1≤ID / IG≤0.6, preferably 0.2≤ID / IG≤0.5; the emulsion particle size Dv50 of the adhesive is 0.2μm to 0.5μm; based on the mass of the adhesive composition, the mass percentage X of the adhesive is 3.3% to 5.3%, and the optical density of the adhesive composition is OD, 0.6≤OD≤1. For example, the value of ID / IG can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or a range consisting of any two of these values; Dv50 can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or a range consisting of any two of these values; the value of X can be 3.3%, 3.5%, 3.8%, 4%, 4.5%, 4.8%, 5%, 5.3%, or a range consisting of any two of these values; and the value of OD can be 0.6, 0.7, 0.8, 0.9, 1, or a range consisting of any two of these values.

[0037] In the binder composition of the present application, the binder includes carbon nanotubes and polyacrylate modified in the carbon nanotube network. The polyacrylate has good elasticity, which can increase the interfacial force between the binder and the silicon-containing material, and alleviate the damage to the bonding network caused by the external force generated by the expansion of silicon during the charging and discharging process of the secondary battery; at the same time, a uniformly dispersed three-dimensional conductive bonding network can be formed on the surface of the negative active material, which plays a role in stress buffering and stress release during the charging and discharging process of the secondary battery. It also has good bonding and conductive properties, which can reduce the number and probability of pores generated by the destruction of the bonding network due to the expansion of the negative active material after the cycle, reduce the porosity of the negative electrode sheet after the cycle, thereby improving the space utilization of the negative electrode sheet and improving the cycle performance of the secondary battery.

[0038] The binder's defectivity is used to indicate the degree of modification of polar groups (e.g., hydroxyl and carboxyl groups) on the carbon nanotube surface. When the ID / IG value is relatively low, such as less than 0.1, it indicates that the carbon nanotube surface defects in the binder are too low, and the content of hydroxyl and carboxyl groups on the carbon nanotube surface is too low. This is not conducive to uniform modification of the polyacrylate on the carbon nanotube surface and its uniform dispersion in the carbon nanotube network structure, resulting in the binder agglomerating and the particle size of the emulsion being too large, which is not conducive to the binder function. When the ID / IG value is relatively high, such as greater than 0.6, it indicates that the carbon nanotube surface defects in the binder are too high, and the carbon nanotube conductivity is poor. This leads to poor conductivity of the conductive bonding network formed by the binder in the negative electrode material layer, affecting the cycle performance and kinetic performance of the secondary battery. When the ID / IG value is within the range of this application, it indicates that the carbon nanotube surface in the binder is modified with an appropriate amount of polar groups, which can increase the interfacial interaction between the carbon nanotubes and the silicon-containing material, resulting in high mechanical strength, thereby reducing the porosity of the negative electrode plate after cycling and improving the cycle performance of the secondary battery. When Dv50 is too small, for example, less than 0.2 μm, the particle size of the binder emulsion particles is too small, resulting in poor bonding performance of the binder, too little cohesion when applied to the negative electrode material layer, and the conductive network is easily damaged by stress during the secondary battery cycle, thereby failing to improve the cycle performance of the secondary battery; when Dv50 is too large, for example, greater than 0.5 μm, the particle size of the binder emulsion particles is too large, resulting in uneven dispersion of the binder emulsion particles in the binder composition, which is not conducive to uniform dispersion of the binder in the negative electrode material layer, and is unable to form a three-dimensional network of efficient bonding conductive networks, which is not conducive to resisting the damage to the bonding network caused by external forces generated by silicon expansion during the charge and discharge process of the secondary battery, thereby failing to reduce the porosity of the negative electrode sheet after cycling, and failing to improve the cycle performance of the secondary battery. The binder emulsion particle size Dv50 is within the range of this application, indicating that the binder emulsion particles in the binder composition have good dispersibility and bonding performance, which is conducive to reducing the porosity of the negative electrode sheet after cycling and improving the cycle performance of the secondary battery.

[0039] When the value of X is too small, for example, less than 3.3%, the viscosity of the binder composition is too low, the binder particles in the binder composition are affected by gravity and are easy to settle, and the emulsion particles of the binder in the binder composition are unevenly dispersed, resulting in uneven dispersion of the binder in the negative electrode material layer. When the value of X is too large, for example, greater than 5.3%, the viscosity of the binder composition is too large, which is not conducive to uniform dispersion of the binder in the negative electrode material layer. In this way, it is not conducive to the formation of a three-dimensional network of efficient bonding conductive network in the negative electrode material layer, and is not conducive to resisting the damage to the bonding network caused by external forces generated by silicon expansion during the charge and discharge process of the secondary battery, thereby failing to reduce the porosity of the negative electrode sheet after cycling and failing to improve the cycle performance of the secondary battery.

[0040] The optical density (OD) of the binder composition is used to assess the dispersion and stability of the carbon nanotubes within the binder composition. If light is uniformly absorbed or scattered when passing through the binder composition, the binder composition exhibits a low optical density (OD) value. The aforementioned "Based on the mass of the binder composition, the binder mass percentage (X) is between 3.3% and 5.3%, and the optical density (OD) of the binder composition is 0.6 ≤ OD ≤ 1" indicates that when the solids content of the binder composition is between 3.3% and 5.3%, the optical density (OD) of the binder composition is low, indicating that the binder absorbs less light, indicating that the carbon nanotubes are uniformly dispersed and stable within the binder composition. If the optical density (OD) of the binder composition is too high, for example, greater than 1, it indicates that the carbon nanotubes are not uniformly dispersed within the binder composition, and the binder cannot form a uniformly distributed three-dimensional bonding network when used in the negative electrode material layer. Therefore, an optical density (OD) value of the binder composition within the range of this application indicates that the carbon nanotubes are uniformly dispersed and stable within the binder composition.

[0041] Therefore, by regulating the values ​​of ID / IG, Dv50, X and OD within the above ranges, a stable and uniformly dispersed bonding conductive network can be formed in the negative electrode material layer, thereby improving the stability of the above bonding conductive network during the charge and discharge cycle, reducing the porosity of the negative electrode sheet after the cycle, and improving the cycle performance of the secondary battery.

[0042] In some embodiments of the present application, based on the mass of the binder, the mass percentage of the carbon nanotubes is 7% to 11%, and the mass percentage of the polyacrylate is 89% to 93%. For example, the mass percentage of the carbon nanotubes can be 7%, 7.4%, 8%, 8.5%, 9%, 9.4%, 10%, 10.5%, 11%, or a range consisting of any two of these values, and the mass percentage of the polyacrylate can be 89%, 89.5%, 90%, 90.6%, 91%, 91.3%, 92%, 92.6%, 93%, or a range consisting of any two of these values. Regulating the mass percentages of the carbon nanotubes and polyacrylate within the above ranges is conducive to better exerting the role of the binder, forming a stable and uniformly dispersed bonding conductive network in the negative electrode material layer, and is more conducive to improving the stability of the bonding conductive network during the charge and discharge cycle, further reducing the porosity of the negative electrode sheet after cycling, and improving the cycle performance of the secondary battery.

[0043] In some embodiments of the present application, the binder further comprises lithium carboxymethyl cellulose. Based on the mass of the binder, the mass percentage of the carbon nanotubes is 5.4% to 10.5%, the mass percentage of the polyacrylate is 69.2% to 90.6%, and the mass percentage of the lithium carboxymethyl cellulose is 2% to 21.4%. For example, the mass percentage of the carbon nanotubes can be 5.4%, 6%, 7%, 7.4%, 8%, 8.5%, 9%, 9.4%, 10%, 10.5% or any of the above. The mass percentage of polyacrylate can be 69.2%, 70%, 72%, 75%, 78%, 80%, 82%, 83%, 85%, 86.6%, 88%, 90.6%, or a range consisting of any two of these values. The mass percentage of lithium carboxymethyl cellulose can be 2%, 2.2%, 5%, 8%, 10%, 12.2%, 14.2%, 17%, 20%, 21.4%, or a range consisting of any two of these values. The binder includes carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose, and the mass percentages of the carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose are adjusted within the above ranges to form a stable and more uniformly dispersed bonding conductive network in the negative electrode material layer, which is more conducive to improving the stability of the bonding conductive network during charge and discharge cycles, further reducing the porosity of the negative electrode sheet after cycling, and improving the cycle performance of the secondary battery.

[0044] In some embodiments of the present application, the monomers forming the polyacrylate include n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate, and acrylic acid. The n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate, and acrylic acid respectively constitute the first structural unit, the second structural unit, the third structural unit, and the fourth structural unit of the polyacrylate. Based on the sum of the molar numbers of the first structural unit, the second structural unit, the third structural unit, and the fourth structural unit, the molar percentage of the first structural unit is 64.5% to 72.5%, the molar percentage of the second structural unit is 17.5% to 21.5%, the molar percentage of the third structural unit is 0% to 4%, and the molar percentage of the fourth structural unit is 6% to 10%. For example, the molar percentage of the first structural unit may be 64.5%. , 66.2%, 68.3%, 69.5%, 70%, 71%, 71.5%, 72%, 72.5% or a range consisting of any two of the numerical values ​​therein, the molar percentage of the second structural unit may be 17.5%, 18%, 18.6%, 19%, 19.3%, 20%, 21%, 21.5% or a range consisting of any two of the numerical values ​​therein, the molar percentage of the third structural unit may be 0%, 0.01%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.6%, 4% or a range consisting of any two of the numerical values ​​therein, and the molar percentage of the fourth structural unit may be 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10% or a range consisting of any two of the numerical values ​​therein. Among the monomers forming the polyacrylate, n-butyl acrylate is a soft monomer, providing flexibility and elasticity to the polyacrylate and also resulting in a lower glass transition temperature for the polymer. n-butyl methacrylate is a hard monomer, providing hardness and strength to the polyacrylate, resulting in a higher bond strength in the binder. Isooctyl acrylate is even more flexible than n-butyl acrylate, further enhancing the flexibility and elasticity of the polyacrylate. Acrylic acid provides crosslinking capability, and the carboxyl groups in acrylic acid have strong polarity, providing electrostatic repulsion and improving the stability of the binder composition. The carboxyl groups also enhance polar hydrogen bonding forces between the polyacrylate and carbon nanotubes, and between the binder and the negative electrode active material, thereby improving the binder's bond strength. In the present application, the above-mentioned monomers are used to form an in-situ polymer on the surface of carbon nanotubes to form a binder. The binder includes carbon nanotubes and polyacrylate located on at least a portion of the surface of the carbon nanotubes. The molar percentages of the first structural unit, the second structural unit, the third structural unit, and the fourth structural unit are regulated within the above-mentioned ranges. This helps to improve the reactivity between the carbon nanotubes and the monomers that form the polyacrylate, so that the polyacrylate located in the carbon nanotube network structure is evenly distributed, thereby better exerting the function of the binder; and can also make the binder have excellent flexibility, bonding properties, and stability.Therefore, applying the binder to the negative electrode plate can form a stable and evenly dispersed bonding conductive network in the negative electrode plate, improve the stability of the above bonding conductive network during the charge and discharge cycle, and further reduce the porosity of the negative electrode plate after the cycle and improve the cycle performance of the secondary battery.

[0045] In some embodiments of the present application, the viscosity of the adhesive composition is ηmPa·s, 800≤η≤3500, 233≤η / X≤660, for example, the value of η can be 800, 1000, 1200, 1500, 1800, 2000, 2300, 2500, 2750, 3000, 3300, 3500 or a range consisting of any two values ​​therein, and the value of η / X can be 233, 280, 300, 350, 400, 430, 480, 500, 530, 580, 600, 620, 660 or a range consisting of any two values ​​therein. The values ​​of η and η / X being within the above ranges indicate that the binder in the binder composition is uniformly dispersed. When the binder composition is used to prepare a negative electrode sheet, it is beneficial for the binder composition to be uniformly dispersed in the negative electrode slurry, thereby forming a stable and uniformly dispersed bonding conductive network in the negative electrode sheet, thereby improving the stability of the bonding conductive network during the charge and discharge cycle, thereby further reducing the porosity of the negative electrode sheet after cycling and improving the cycle performance of the secondary battery.

[0046] In some embodiments of the present application, the adhesive composition is allowed to stand for 48 hours, and the optical density of the adhesive composition after standing is OD1, 0<(OD1-OD) / OD≤17.0%, for example, (OD1-OD) / OD can be 0.1%, 1.0%, 1.5%, 2.5%, 4.0%, 6.0%, 8.0%, 10.0%, 11.2%, 12.5%, 13.4%, 14.2%, 15.5%, 16.0%, 17.0% or a range consisting of any two values ​​therein. OD is the optical density of the adhesive composition before standing. In the adhesive composition of the present application, the polyacrylate-modified carbon nanotubes are uniformly dispersed in deionized water in the form of emulsion particles. When light passes through the adhesive composition, it will be uniformly absorbed or scattered, and the optical density value is relatively stable. In some embodiments, the optical density OD1 of the adhesive composition after standing is 0.61 to 1.17. The (OD1-OD) / OD value of the binder composition is within the above range, indicating that the carbon nanotubes in the binder are uniformly distributed and stable in the binder composition. Thus, when the binder composition is used to prepare a negative electrode sheet, it is beneficial for the binder composition to be evenly dispersed in the negative electrode slurry, forming a stable and uniformly dispersed bonding conductive network in the negative electrode sheet, thereby improving the stability of the bonding conductive network during charge and discharge cycles, thereby further reducing the porosity of the negative electrode sheet after cycling and improving the cycling performance of the secondary battery.

[0047] In some embodiments of the present application, the binder has a glass transition temperature (Tg) of -20°C to -10°C. For example, Tg can be -20°C, -18°C, -16°C, -15°C, -14°C, -13°C, -12°C, -11°C, -10°C, or a range consisting of any two of these values. Regulating the binder's glass transition temperature within this range helps release stress on the bonding network caused by silicon expansion during the charge and discharge process of the secondary battery, reducing damage to the bonding network. This serves as a stress buffer and stress release during the charge and discharge process of the secondary battery, reducing the number and probability of pores generated by bond network damage due to expansion of the negative electrode active material after cycling, and reducing the porosity of the negative electrode sheet after cycling, thereby increasing the space utilization of the negative electrode sheet and improving the cycling performance of the secondary battery.

[0048] In some embodiments of the present application, the tensile strength of the adhesive film is 10 MPa to 50 MPa. For example, the tensile strength of the adhesive film can be 10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa, or a range consisting of any two of these values. Regulating the tensile strength of the binder within the above range is beneficial for improving the stress buffering and stress release effects of the adhesive network during the charge and discharge process of the secondary battery, reducing the number and probability of pores generated by the adhesive network being destroyed due to expansion of the negative electrode active material after cycling, and reducing the porosity of the negative electrode sheet after cycling, thereby improving the space utilization of the negative electrode sheet and improving the cycling performance of the secondary battery.

[0049] In some embodiments of the present application, the adhesive film is immersed in an electrolyte at 60±5°C for 12 hours, the elongation at break of the adhesive film before being immersed in the electrolyte is L0, and the elongation at break of the adhesive film after being immersed in the electrolyte is L1, 100%≤L1≤130%, 1.42≤L1 / L0≤1.74, for example, L1 can be 100%, 115%, 120%, 125%, 128%, 130% or a range consisting of any two values ​​therein, and L1 / L0 can be 1.42, 1.46, 1.50, 1.54, 1.58, 1.62, 1.66, 1.70, 1.74 or a range consisting of any two values ​​therein. In some embodiments, 65%≤L1≤85%, L0 can be 65%, 70%, 72%, 75%, 78%, 80%, 85% or a range consisting of any two values ​​therein. Regulating the values ​​of L1 and L1 / L0 within the above ranges indicates that the adhesive film has good tensile properties before and after immersion in the electrolyte, which is beneficial to improving the stress buffering and stress release effects of the bonding network during the charge and discharge process of the secondary battery, reducing the number and probability of pores generated by the destruction of the bonding network due to the expansion of the negative electrode active material after cycling, and reducing the porosity of the negative electrode sheet after cycling, thereby improving the space utilization of the negative electrode sheet and improving the cycle performance of the secondary battery.

[0050] In some embodiments of the present application, in the infrared spectrum of the binder, there is a -1 Stretching vibration absorption peak, 1450±5cm -1 and 1720±2cm -1 The absorption peak of 3437±25cm -1 Stretching vibration absorption peak, 1730±2cm -1 The sharp stretching vibration absorption peak, 1061±5cm -1 、1161±2cm -1 、1158±2cm -1 、1242±5cm -1 The stretching vibration absorption peak is located at 1580±5cm -1 The stretching vibration absorption peak corresponds to the absorption peak of the carbon nanotube skeleton, which is located at 1450±5cm -1 and 1720±2cm -1 The absorption peak is more obvious, corresponding to the hydroxyl and carboxyl groups on the surface of acidified carbon nanotubes, 3437±25cm -1 The stretching vibration absorption peak is the stretching vibration peak of hydroxyl, located at 1730±2cm -1 The sharp stretching vibration absorption peak is caused by the stretching vibration of the carbonyl group in polyacrylate, 1061±5cm -1 、1161±2cm -1、1158±2cm -1 、1242±5cm -1 The stretching vibration absorption peak is caused by the stretching vibration of the ester group in the polyacrylate. The infrared spectrum of the binder has the above characteristic peaks, indicating that the binder includes carbon nanotubes and polyacrylate. Therefore, the binder with the above infrared characteristic peaks has good dispersibility and can form a stable and uniformly dispersed bonding conductive network in the negative electrode sheet, improving the stability of the bonding conductive network during charge and discharge cycles, thereby further reducing the porosity of the negative electrode sheet after cycling and improving the cycling performance of the secondary battery.

[0051] A second aspect of the present application provides a method for preparing the binder composition provided in any of the above embodiments, comprising the following steps:

[0052] (1) adding carbon nanotubes to a mixture of concentrated sulfuric acid and concentrated nitric acid for acidification, and then washing and drying to obtain acidified carbon nanotubes, wherein the acidification temperature is 50° C. to 70° C. and the acidification time is 0.5 h to 1.5 h;

[0053] (2) under a nitrogen atmosphere, n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid are mixed to obtain a mixed monomer, wherein, based on the sum of the moles of n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid, the mole percentage of n-butyl acrylate is 64.5% to 72.5%, the mole percentage of n-butyl methacrylate is 17.5% to 21.5%, the mole percentage of isooctyl acrylate is 0% to 4%, and the mole percentage of acrylic acid is 6% to 10%; the mixed monomer is divided into two groups, namely a first mixed monomer and a second mixed monomer, and the mass ratio of the first mixed monomer to the second mixed monomer is 1:2;

[0054] (3) under a nitrogen atmosphere, adding an emulsifier and a first mixed monomer to deionized water at 60° C. to 70° C. for pre-emulsification, wherein the pre-emulsification time is 15 min to 25 min; then adding an initiator and a cross-linking agent to carry out a first reaction to obtain a first mixed solution, wherein the temperature of the first reaction is 70° C. to 80° C., the time of the first reaction is 1.5 h to 3 h, and the cross-linking agent includes tetraethylene glycol dimethacrylate;

[0055] (4) slowly adding the second mixed monomer and the acidified carbon nanotubes to the first mixed solution for 2 to 4 hours; after the addition is completed, performing a second reaction to obtain a second mixed solution, wherein the temperature of the second reaction is 65° C. to 85° C. and the time is 1 to 3 hours;

[0056] (5) Adding the initiator to the second mixed solution again, and continuing the third reaction, the temperature of the third reaction is 70° C. to 80° C., and the time is 0.5 h to 2 h; after the third reaction is completed, adjusting the pH to neutral to obtain a binder composition.

[0057] In the above-mentioned preparation method, by acidifying the carbon nanotubes, functional groups such as carboxyl and hydroxyl groups can be added to the surface of the carbon nanotubes, which facilitates bonding with the mixed monomers to achieve better in-situ polymerization, resulting in a uniform distribution of the polyacrylate obtained by in-situ polymerization on the surface of the carbon nanotubes. In addition, the acidification treatment can also increase the interaction between the polar functional groups on the carbon nanotubes and the surface of the negative electrode active material, thereby improving the bonding force between the binder and the negative electrode active material. Pre-emulsification can ensure that the mixed monomers form a uniform monomer distribution on the surface of the carbon nanotubes, making nucleation more uniform during the emulsion polymerization process, improving the stability of the emulsion, and reducing the occurrence of emulsion aggregation and stratification problems. Selecting tetraethylene glycol dimethacrylate as a cross-linking agent can increase the cross-linking density of the polyacrylate while improving the mechanical strength and chemical resistance of the polymer, improving the thermal stability and solvent resistance of the polyacrylate, and reducing the swelling of the polyacrylate in the electrolyte. In addition, the mixed monomers are divided into two groups and distributed according to the above-mentioned mass ratio. Adding the first mixed monomer first is beneficial to forming more nucleation points on the surface of the carbon nanotubes, increasing the number of polyacrylate particles, and then adding the second mixed monomer to carry out the second reaction to continue polymerization, which can make the polyacrylate particles grow. The initiator is added again to carry out the third reaction and further polymerization, consuming the monomers in the reaction system, improving the utilization rate of the monomers, and finally obtaining the adhesive composition of the present application.

[0058] The binder composition prepared using the above-mentioned preparation method comprises a binder and deionized water. The binder is uniformly dispersed in the deionized water as emulsion particles, resulting in a stable and uniformly dispersed bonding conductive network in the negative electrode material layer prepared using the above-mentioned composition. The binder comprises carbon nanotubes and a polyacrylate uniformly distributed within the carbon nanotube network structure through in-situ polymerization. The polyacrylate exhibits excellent elasticity, which increases the interfacial force between the binder and the silicon-containing material while mitigating damage to the bonding network caused by external forces generated by silicon expansion during charge and discharge. This helps reduce the porosity of the negative electrode sheet after cycling and improves the cycling performance of the secondary battery.

[0059] In the above step (1), the temperature of the acidification treatment can be 50°C, 55°C, 60°C, 65°C, 67°C, 70°C or a range consisting of any two of the values, and the time of the acidification treatment can be 0.5h, 0.8h, 1.0h, 1.2h, 1.5h or a range consisting of any two of the values. The volume ratio of concentrated sulfuric acid and concentrated nitric acid in the mixture of concentrated sulfuric acid and concentrated nitric acid can be (2 to 4):1. The ratio of the volume of the above-mentioned mixture of concentrated sulfuric acid and concentrated nitric acid to the mass of the carbon nanotubes can be (15 to 25)mL:1g. The present application has no particular limitation on the cleaning and drying processes in step (1), as long as the purpose of the present application can be achieved.

[0060] In the above step (2), based on the sum of the moles of n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid, the mole percentage of n-butyl acrylate can be 64.5%, 66.2%, 68.3%, 69.5%, 70%, 71%, 71.5%, 72%, 72.5% or a range consisting of any two of these values, and the mole percentage of n-butyl methacrylate can be 17.5%, 18%, 18.6%, 19%, 19.3% or more. , 20%, 21%, 21.5% or a range consisting of any two of the values ​​therein, the molar percentage of isooctyl acrylate may be 0%, 0.01%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.6%, 4% or a range consisting of any two of the values ​​therein, and the molar percentage of acrylic acid may be 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10% or a range consisting of any two of the values ​​therein.

[0061] In the above step (3), the temperature of the deionized water can be 60°C, 62°C, 65°C, 67°C, 70°C or a range consisting of any two of the numerical values ​​therein, and the time for pre-emulsification can be 15 min, 18 min, 20 min, 21 min, 22 min, 23 min, 25 min or a range consisting of any two of the numerical values ​​therein. The temperature of the first reaction can be 70°C, 71°C, 72°C, 74°C, 75°C, 77°C, 78°C, 80°C or a range consisting of any two of the numerical values ​​therein, and the time for the first reaction can be 1.5 h, 2 h, 2.2 h, 2.5 h, 2.7 h, 3 h or a range consisting of any two of the numerical values ​​therein. The present application has no particular limitation on the type of emulsifier, as long as the purpose of the present application can be achieved, for example, it can include but is not limited to at least one of STD-1 emulsifier and A-501 emulsifier, preferably, including STD-1 emulsifier and A-501 emulsifier. The present application does not particularly limit the type of initiator, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to at least one of potassium persulfate, sodium persulfate, ammonium persulfate, dibenzoyl peroxide, or azobisisobutyronitrile. Based on 100 parts by weight of the binder, the amount of emulsifier added in step (3) may be 0.4 to 0.6 parts, the amount of initiator may be 0.3 to 0.5 parts, the amount of cross-linking agent may be 0.1 to 0.2 parts, and the amount of deionized water may be 600 to 1200 parts.

[0062] In the above step (4), the time for slow addition can be 2 h, 2.2 h, 2.5 h, 2.7 h, 3 h or a range consisting of any two values ​​therein; the temperature of the second reaction can be 65°C, 70°C, 72°C, 74°C, 75°C, 78°C, 80°C, 85°C or a range consisting of any two values ​​therein; and the time can be 1 h, 1.5 h, 2 h, 2.2 h, 2.5 h, 2.7 h, 3 h or a range consisting of any two values ​​therein.

[0063] In the above-mentioned step (5), the temperature of the third reaction can be 70°C, 72°C, 74°C, 75°C, 77°C, 78°C, 80°C or a range consisting of any two of the numerical values, and the time can be 0.5h, 0.6h, 0.7h, 0.8h, 0.9, 1h or a range consisting of any two of the numerical values. The type of initiator added in step (5) is the same as that in step (3). Based on the mass of the binder as 100 parts, the number of initiators added in step (5) can be 0.03 to 0.05 parts. After the third reaction is completed, lithium hydroxide can be used to adjust the pH to neutral. This application is not particularly limited to this, as long as the purpose of this application can be achieved. After the pH is adjusted to neutral, the solid content of the binder composition can be adjusted to a suitable value by adding deionized water or by rotary evaporation to increase or decrease the content of deionized water in the reaction product. This application is not particularly limited to this, as long as the purpose of this application can be achieved.

[0064] In some embodiments of the present application, based on the sum of the mass of the carbon nanotubes and the mixed monomers, the mass percentage of the carbon nanotubes is 7% to 11%, and the mass percentage of the mixed monomers is 89% to 93%. For example, the mass percentage of the carbon nanotubes can be 7%, 7.4%, 8%, 8.5%, 9%, 9.4%, 10%, 10.5%, 11%, or a range consisting of any two of these values, and the mass percentage of the polyacrylate can be 89%, 89.5%, 90%, 90.6%, 91%, 91.3%, 92%, 92.6%, 93%, or a range consisting of any two of these values. Regulating the mass percentage of the carbon nanotubes and the mixed monomers within the above range is conducive to better exerting the role of the binder, forming a stable and uniformly dispersed bonding conductive network in the negative electrode sheet, and is more conducive to improving the stability of the bonding conductive network during the charge and discharge cycle, further reducing the porosity of the negative electrode sheet after cycling, and improving the cycle performance of the secondary battery.

[0065] In some embodiments of the present application, the preparation method further comprises: after adjusting the pH to neutral, adding lithium carboxymethyl cellulose, and finally obtaining a binder composition. Based on the sum of the mass of the carbon nanotubes, the mixed monomers and the lithium carboxymethyl cellulose, the mass percentage of the carbon nanotubes is 5.4% to 10.5%, the mass percentage of the mixed monomers is 69.2% to 90.6%, and the mass percentage of the lithium carboxymethyl cellulose is 2% to 21.4%. For example, the mass percentage of the carbon nanotubes can be 5.4%, 6%, 7%, 7.4%, 8%, 8.5%, 9%, 9.4%, 10%, 10.5% or any two of these values. The mass percentage of polyacrylate can be 69.2%, 70%, 72%, 75%, 78%, 80%, 82%, 83%, 85%, 86.6%, 88%, 90.6%, or a range consisting of any two of these values; the mass percentage of lithium carboxymethyl cellulose can be 2%, 2.2%, 5%, 8%, 10%, 12.2%, 14.2%, 17%, 20%, 21.4%, or a range consisting of any two of these values. By adding lithium carboxymethyl cellulose, the suspension stability of the emulsion particles of the binder and the ion conductivity of the binder can be improved. In this way, when the binder composition is used to prepare a negative electrode sheet, it is beneficial for the binder composition to be uniformly dispersed in the negative electrode slurry, and a stable and uniformly dispersed bonding conductive network can be formed in the negative electrode sheet, thereby improving the stability of the bonding conductive network during the charge and discharge cycle, thereby further reducing the porosity of the negative electrode sheet after the cycle and improving the cycle performance of the secondary battery.

[0066] In some embodiments, the binder comprises carbon nanotubes and polyacrylate, and the mass of the binder can be calculated based on the mass of the acidified carbon nanotubes and the mass of the mixed monomer, based on 100 parts by mass of the binder. In other embodiments, the binder comprises carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose, and the mass of the binder can be calculated based on the mass of the acidified carbon nanotubes, the mass of the mixed monomer, and the mass of the lithium carboxymethyl cellulose, based on 100 parts by mass of the binder.

[0067] Typically, the binder's defectivity ID / IG can be altered by varying the acidification temperature and duration, as well as the amount of the concentrated sulfuric acid / nitric acid mixture used. With other conditions remaining constant, increasing the acidification temperature increases the binder's defectivity, while decreasing it decreases. Prolonging the acidification time increases the binder's defectivity, while decreasing it decreases. Increasing the ratio of the volume of the concentrated sulfuric acid / nitric acid mixture to the mass of the carbon nanotubes increases the binder's defectivity, while decreasing it decreases.

[0068] The binder's emulsion particle size Dv50 can be changed by changing the emulsifier concentration, initiator concentration, monomer concentration, the temperature and time of the second reaction. When other conditions remain unchanged, increasing the emulsifier concentration increases the binder's emulsion particle size Dv50, and vice versa. Increasing the initiator concentration increases the binder's emulsion particle size Dv50, and vice versa. Increasing the monomer concentration increases the binder's emulsion particle size Dv50, and vice versa. Increasing the temperature of the second reaction increases the binder's emulsion particle size Dv50, and vice versa. Prolonging the second reaction time increases the binder's emulsion particle size Dv50, and vice versa. In this application, "binder's emulsion particle size Dv50" refers to the particle size of the binder's emulsion particles in the volume-based particle size distribution, starting from the small particle size side, reaching 50% of the volume accumulation.

[0069] In a third aspect, the present application provides a secondary battery, which includes a positive electrode plate, an electrolyte and a negative electrode plate, the negative electrode plate includes a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the negative electrode material layer includes a silicon-containing material and a binder, the binder includes carbon nanotubes and polyacrylate located on at least part of the surface of the carbon nanotubes, the defectivity of the binder is ID / IG, 0.1≤ID / IG≤0.6, and the emulsion particle size Dv50 of the binder is 0.2μm to 0.5μm. For example, the value of ID / IG can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 or a range consisting of any two of these values, and Dv50 can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5 or a range consisting of any two of these values. The negative electrode material layer includes a silicon-containing material and the aforementioned binder, which can increase the interfacial forces between the binder and the silicon-containing material, mitigating damage to the bonding network caused by external forces generated by silicon expansion during the charge and discharge process of the secondary battery. Regulating the ID / IG and Dv50 values ​​within the aforementioned ranges can increase the interfacial forces between the binder and the silicon-containing material, forming a stable and evenly dispersed bonding conductive network within the negative electrode sheet. This provides stress buffering and stress relief during the charge and discharge process, while also helping to resist damage to the bonding network caused by external forces generated by silicon expansion during the charge and discharge process. This improves the stability of the bonding conductive network during charge and discharge cycles, reduces the porosity of the negative electrode sheet after cycling, and improves the cycling performance of the secondary battery.

[0070] In some embodiments of the present application, based on the mass of the negative electrode material layer, the mass percentage of silicon element is 7.4% to 54%, and the mass percentage of the binder is 3% to 9%. For example, the mass percentage of silicon element can be 7.4%, 8%, 10%, 12.2%, 14.2%, 17%, 20%, 23%, 26%, 30%, 33%, 35%, 37%, 40%, 43%, 45%, 47%, 50%, 54%, or a range consisting of any two of them, and the mass percentage of the binder can be 3%, 3.5%, 4%, 4.6%, 5%, 5.7%, 6%, 6.4%, 7%, 7.5%, 8%, 9%, or a range consisting of any two of them. The negative electrode material layer includes the binder of the present application, which can reduce the proportion of the binder in the negative electrode material layer, thereby increasing the proportion of the negative electrode active material. The mass percentage of the binder in the negative electrode material layer is lower than the total mass of the conductive agent and the binder added to the existing silicon-containing negative electrode, which is beneficial to increasing the gram capacity of the negative electrode material layer, thereby making the secondary battery have a higher energy density; and it is also beneficial to improve the stability of the above-mentioned bonding conductive network during the charge and discharge cycle, reduce the porosity of the negative electrode plate after the cycle, and improve the cycle performance of the secondary battery.

[0071] In some embodiments of the present application, based on the mass of the binder, the mass percentage of carbon nanotubes is 7% to 11%, and the mass percentage of polyacrylate is 89% to 93%. Regulating the mass percentages of carbon nanotubes and polyacrylate within these ranges helps to better utilize the binder, forming a stable and uniformly dispersed bonding conductive network in the negative electrode sheet. This further helps to improve the stability of the bonding conductive network during charge and discharge cycles, further reducing the porosity of the negative electrode sheet after cycling, and improving the cyclic performance of the secondary battery.

[0072] In some embodiments of the present application, the binder further comprises lithium carboxymethyl cellulose, and based on the mass of the binder, the mass percentage of carbon nanotubes is 5.4% to 10.5%, the mass percentage of polyacrylate is 69.2% to 90.6%, and the mass percentage of lithium carboxymethyl cellulose is 2% to 21.4%. The binder comprises carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose, and the mass percentages of the carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose are regulated within the above-mentioned ranges. This can form a stable and more evenly dispersed bonding conductive network in the negative electrode material layer, further facilitating the stability of the bonding conductive network during charge and discharge cycles, further reducing the porosity of the negative electrode sheet after cycling, and improving the cycle performance of the secondary battery.

[0073] In some embodiments of the present application, the negative electrode material layer further includes lithium carboxymethyl cellulose, and the mass percentage of the lithium carboxymethyl cellulose is 0.25% to 1.58% based on the mass of the negative electrode material layer. For example, the mass percentage of the lithium carboxymethyl cellulose can be 0.25%, 0.4%, 0.5%, 0.7%, 0.9%, 1.0%, 1.2%, 1.35%, 1.48%, 1.58%, or a range consisting of any two values ​​therein. The negative electrode material layer further includes lithium carboxymethyl cellulose, and the mass percentage of the lithium carboxymethyl cellulose is regulated within the above range. A more uniformly dispersed bonding conductive network can be formed in the negative electrode plate, which is more conducive to improving the stability of the above bonding conductive network during the charge and discharge cycle, further reducing the porosity of the negative electrode plate after the cycle, and improving the cycle performance of the secondary battery.

[0074] In some embodiments, the binder includes lithium carboxymethyl cellulose, and the lithium carboxymethyl cellulose in the negative electrode material layer is entirely derived from the binder. Based on the mass of the binder, the mass percentage of lithium carboxymethyl cellulose is 8.3% to 21.4%. In other embodiments, the binder includes lithium carboxymethyl cellulose, and lithium carboxymethyl cellulose is additionally added when preparing the negative electrode slurry. The lithium carboxymethyl cellulose in the negative electrode material layer is partially derived from the binder, and the remainder is derived from the additional lithium carboxymethyl cellulose added to the negative electrode slurry. Based on the mass of the binder, the mass percentage of lithium carboxymethyl cellulose is 2% to 8.3%. In still other embodiments, the binder does not include lithium carboxymethyl cellulose, and additional lithium carboxymethyl cellulose is required when preparing the negative electrode slurry. The lithium carboxymethyl cellulose in the negative electrode material layer is entirely derived from the additional lithium carboxymethyl cellulose added to the negative electrode slurry. Selecting any of the above embodiments is conducive to forming a relatively uniformly dispersed bonding conductive network in the negative electrode plate, which can improve the stability of the bonding conductive network during the charge and discharge cycle, reduce the porosity of the negative electrode plate after cycling, and improve the cycle performance of the secondary battery. Preferably, the binder includes lithium carboxymethyl cellulose, and the lithium carboxymethyl cellulose in the negative electrode material layer is entirely derived from the binder, which can reduce the mass proportion of lithium carboxymethyl cellulose in the negative electrode material layer, increase the mass proportion of the negative electrode active material, increase the gram capacity of the negative electrode material layer, and make the secondary battery have a higher energy density; at the same time, it is also beneficial to form a more evenly dispersed bonding conductive network in the negative electrode plate, reduce the porosity of the negative electrode plate after cycling, and improve the cycle performance of the secondary battery.

[0075] In the present application, silicon-containing materials may include but are not limited to at least one of silicon, silicon carbon, silicon nitrogen or silicon oxygen. The negative electrode material layer of the present application also includes a carbon material, and the above-mentioned carbon material may include but is not limited to at least one of natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon or soft carbon. The negative electrode material layer of the present application may also include a conductive agent. The present application has no particular restrictions on the conductive agent, as long as the purpose of the present application can be achieved. For example, the conductive agent may include but is not limited to at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers. Conductive carbon black may include but is not limited to at least one of acetylene black or Ketjen black. The above-mentioned carbon nanotubes may include but are not limited to at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes or oligo-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nano-carbon fibers. The above-mentioned metal materials may include but are not limited to metal powder and / or metal fibers. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The conductive polymer may include, but is not limited to, at least one of a polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The present application does not particularly limit the weight percentage of the conductive agent, as long as it achieves the objectives of the present application. For example, based on the weight of the negative electrode material layer, the weight percentage of the conductive agent may be 0.5% to 2%.

[0076] The phrase "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or on both surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can refer to the entire surface of the negative electrode current collector or a portion of the surface of the negative electrode current collector. This is not particularly limited in this application, as long as the objectives of this application can be achieved.

[0077] The present application has no particular restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. For example, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc. The present application has no particular restrictions on the thickness of the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4μm to 15μm. The present application has no particular restrictions on the thickness of the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode material layer is 30μm to 120μm.

[0078] In the present application, there is no particular limitation on the method for preparing the negative electrode sheet, as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: mixing the negative electrode active material and the conductive agent, optionally adding the dispersant lithium carboxymethyl cellulose, then adding the prepared binder composition, and then adding it to the solvent and stirring evenly to obtain a negative electrode slurry with a solid content of 15wt% to 45wt%. The negative electrode slurry is evenly coated on one surface of the negative electrode collector, and after drying, a negative electrode sheet coated with a negative electrode material layer on one side is obtained. Then, the above coating steps are repeated on the other surface of the negative electrode collector, and after drying, a negative electrode sheet coated with a negative electrode material layer on both sides is obtained. After coating is completed, the negative electrode sheet is obtained by cold pressing and cutting.

[0079] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector. The above-mentioned "positive electrode material layer arranged on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be arranged on one surface of the positive electrode current collector along the thickness direction of itself, or on two surfaces of the positive electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the surface of the positive electrode current collector, or it can be a partial area of ​​the surface of the positive electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the positive electrode current collector, as long as the purpose of this application can be achieved. For example, it can include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector). This application has no special restrictions on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm, and the thickness of the positive electrode material layer is 30μm to 120μm.

[0080] The positive electrode material layer includes a positive electrode active material. The present application has no particular limitation on the positive electrode active material as long as the purpose of the present application can be achieved. For example, the positive electrode active material may include but is not limited to at least one of lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium iron manganese phosphate or lithium titanate.

[0081] The positive electrode material layer may also include a positive electrode conductor and a positive electrode binder. The present application has no particular restrictions on the positive electrode conductor, as long as the purpose of the present application can be achieved. For example, the positive electrode conductor may include but is not limited to at least one of conductive carbon black (SuperP), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers, and conductive carbon black may include but is not limited to at least one of acetylene black or Ketjen black. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nano-carbon fibers. The above-mentioned metal materials may include but are not limited to metal powder and / or metal fibers. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymers may include but are not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. The present application has no particular limitation on the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the positive electrode binder may include but is not limited to at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, polyamide-imide, styrene-butadiene rubber or polyvinylidene fluoride.

[0082] This application does not particularly limit the preparation method of the positive electrode sheet. Preparation methods known in the art can be used as long as the purpose of this application can be achieved. This application does not particularly limit the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application can be achieved.

[0083] The present application has no particular restrictions on the isolation membrane, as long as the purpose of the present application can be achieved. For example, the material of the isolation membrane may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of isolation membrane may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane. The present application has no particular restrictions on the thickness of the isolation membrane, as long as the purpose of the present application can be achieved.

[0084] In one embodiment of the present application, the secondary battery includes an electrolyte. The present application does not particularly limit the type of the electrolyte, and those skilled in the art can select an electrolyte known in the art according to actual needs, as long as the purpose of the present application can be achieved.

[0085] The secondary battery also includes a shell for accommodating the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the above-mentioned other components. This application does not particularly limit the shell, and it can be a shell known in the art, as long as it can achieve the purpose of this application. For example, the shell can be a hard shell or a flexible shell. The material of the hard shell can be metal. This application does not limit the type of metal. A metal hard shell known in the art can be used, as long as it can achieve the purpose of this application. The flexible shell can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0086] The preparation process of the secondary battery of the present application is well known to those skilled in the art and is not particularly limited in the present application. For example, the preparation process of the secondary battery may include but is not limited to the following steps: stacking the positive electrode sheets, the separator and the negative electrode sheets in order, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a shell, injecting the electrolyte into the shell and sealing it to obtain a secondary battery. Alternatively, stacking the positive electrode sheets, the separator and the negative electrode sheets in order, and then fixing the four corners of the entire laminated structure with tape to obtain an electrode assembly with a laminated structure, placing the electrode assembly in a shell, injecting the electrolyte into the shell and sealing it to obtain a secondary battery. In addition, as needed, overcurrent protection elements, guide plates, etc. may be placed in the shell to prevent pressure rise and overcharge and discharge inside the secondary battery.

[0087] The secondary battery of the present application is not particularly limited and may include any device that undergoes an electrochemical reaction. For example, the secondary battery may include but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0088] In a fourth aspect, the present application provides an electronic device comprising the secondary battery provided in any one of the above embodiments. The secondary battery of the present application has good cycle performance, so that the electronic device of the present application has a long service life.

[0089] The present application does not particularly limit the type of electronic device, and it can be any electronic device known in the prior art. In some embodiments of the present application, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0090] Example

[0091] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0092] Test methods and equipment:

[0093] Defect ID / IG test:

[0094] The binder composition was dried in a vacuum oven at 110°C for 6 h to obtain a dry binder powder. The spectrum was collected using a microscopic laser confocal Raman spectrometer (instrument model HORIBA LabRAM HR Evolution, L73600). The laser wavelength was 532 nm, the intensities of the D peak and the G peak were recorded, and the ID / IG ratio was calculated.

[0095] Binder emulsion particle size Dv50 test:

[0096] The particle size of the binder in the binder composition was measured using a Malvern MasterSizer 2000 particle size analyzer. First, rinse the sample inlet system with water. Select the refractive index as 1.8, the absorbance as 1, the dispersant as water, the dispersant refractive index as 1.333, and the analysis model as a universal spherical. When the background light intensity is <200 or the laser intensity is >70%, add sample until the obscuration falls within the range of 8% to 12%. Stop adding sample, let it stand for 30 seconds, and observe if the obscuration remains stable before measuring and obtaining a particle size distribution. Dv50 is the particle size at which 50% of the volume accumulation is achieved, starting from the smaller particle size side, in the volume-based particle size distribution.

[0097] Optical density (OD) test:

[0098] The adhesive composition was diluted to 0.043 mg / mL to obtain a sample. The optical density (OD) of the adhesive composition was measured using a spectrophotometer (ThermoScientific NanoDrop 2000c) at a wavelength of 600 nm using a 1 cm cuvette. The sample was allowed to stand for 48 hours and then tested again according to the above method to obtain the optical density OD1 of the adhesive composition after standing for 48 hours. (OD1-OD) / OD was calculated.

[0099] Test of mass percentage of structural units in polyacrylate:

[0100] The monomer composition of the polyacrylate after pyrolysis was determined by pyrolysis gas chromatography-mass spectrometry (py GC-MS), and the mass percentage of each structural unit in the polyacrylate was analyzed, where the molar percentage of the first structural unit was recorded as D1, the molar percentage of the second structural unit was recorded as D2, the molar percentage of the third structural unit was recorded as D3, and the molar percentage of the fourth structural unit was recorded as D4. The adhesive composition was placed in a vacuum oven at 110°C and dried for 6 hours. The dried adhesive was then tested in a pyrolysis gas chromatography-mass spectrometry instrument (Agilent 7890B / 5977B), using a mass selective detector (MSD) and a pyrolysis gas injector (EGA / PY-3030D). The column temperature was 40°C to 320°C, and the pyrolysis temperature was 40°C to 800°C.

[0101] Viscosity test:

[0102] The viscosity of the adhesive composition was measured using a digital rotational viscometer (LVDV-1, Shanghai Jingtian Electronic Instrument Co., Ltd.) at 25°C, a stirring speed of 12 rpm, and a stirring time of 2 min. The appropriate rotor and speed were selected based on the adhesive composition to be measured. For viscosities ranging from 501 mPa·s to 1000 mPa·s, rotor 62#L2# was used at a speed of 12 rpm. For viscosities ranging from 1001 mPa·s to 10,000 mPa·s, rotor 63#L3# was used at a speed of 12 rpm. The rotor was slowly tilted and placed into the adhesive composition to soak it, preventing bubbles from forming at the bottom of the rotor. The adhesive composition liquid level was located in the middle of the rotor groove. The rotational viscometer was started. After three minutes, the reading was taken when the display remained stable. This was recorded as the viscosity η of the adhesive composition in mPa·s.

[0103] Glass transition temperature (Tg) test:

[0104] The adhesive composition was dried in a vacuum oven at 110°C for 6 hours to obtain a dried adhesive. The adhesive's Tg was determined using differential scanning calorimetry (DSC) using a DSC214 instrument, nitrogen atmosphere, a purge gas flow rate of 40 mL / min, a shielding gas flow rate of 60 mL / min, and a heating rate of 10°C / min. The DSC curve was used to determine the Tg.

[0105] Infrared spectrum test:

[0106] The binder composition was dried in a vacuum oven at 110° C. for 6 h to obtain a dry binder powder. The infrared spectrum of the binder was measured using a Fourier transform infrared spectrometer in accordance with the national standard GB / T 21186-2007 “Fourier transform infrared spectrometer”.

[0107] Composition test of adhesive:

[0108] The composition of the binder was tested by thermogravimetric method using a synchronous thermal analyzer (NETZSCH STA449F3, Germany) and reference standard JY-T 0589.1-2020 "General Rules for Thermal Analysis Methods". Use N2 atmosphere, purge gas flow rate of 60 mL / min, protective gas flow rate of 20 mL / min, heating rate of 10 ° C / min, and temperature range of 25 ° C to 600 ° C. The thermogravimetric curve of the binder has weight loss peaks at 300 ± 2 ° C, 388 ± 2 ° C, and 403 ± 2 ° C, and the mass weight loss rate change rate, the weight loss rate of 300 ± 2 ° C corresponds to the mass proportion of lithium carboxymethyl cellulose, the weight loss rate of 388 ± 2 ° C and 403 ± 2 ° C corresponds to the mass proportion of polyacrylate, and the remainder is the mass proportion of carbon nanotubes.

[0109] Tensile strength and elongation at break tests:

[0110] The adhesive composition was poured into a mold and dried at 80°C for 4 hours to produce an adhesive film. The resulting films were then cut to produce specimens for the following tests. The specimen dimensions were 2.5 mm thick, 10 mm wide, and 100 mm long. These specimens were standard dumbbell-shaped specimens, and the central, uniform 30 mm section was used for testing.

[0111] The tensile strength and elongation at break of the adhesive film were tested using a tensile testing machine according to the national standard GB / T 30776-2014, "Test Method for Tensile Strength and Elongation at Break of Adhesive Tapes." The sample was clamped between the upper and lower clamps of the tensile testing machine, with the initial spacing between the upper and lower clamps set to H0 (i.e., the initial length of the sample was H0). The upper and lower clamps were stretched at a constant rate of 50 mm / min until the sample broke. The stress-strain curve of the sample was obtained, and the spacing between the upper and lower clamps at the time of sample fracture was recorded as H1. The elongation at break, L0, is calculated as (H1 - H0) / H0 × 100%. The tensile force at sample fracture is F, and the tensile strength of the adhesive film is calculated as F / S, where S = width × thickness = 10 mm × 2.5 mm.

[0112] The same method was used to prepare a sample, which was immersed in a 60°C electrolyte for 12 hours. The elongation at break of the film after immersion in the electrolyte was measured according to the above steps as L1, and L1 / L0 was calculated. The composition of the above electrolyte was the same as that of Example 1-1.

[0113] Silicon mass percentage test:

[0114] Disassemble the lithium-ion battery, remove the negative electrode sheet, soak the negative electrode sheet in dimethyl carbonate (DMC) for 12 hours, completely immerse the negative electrode sheet in the liquid, pour out the solvent after soaking, and repeat the above cleaning steps twice. After cleaning, place the negative electrode sheet in a 60°C oven and dry it for 8 hours. Use conductive carbon glue to fix the negative electrode sheet on the sample stage, and place the sample stage on the bracket. Place the bracket and sample stage in a cross-section polisher (CP) and vacuum to 10 -4 After the CP test, the negative electrode sheet was cut along its thickness using argon gas. The sample cut after CP was placed on the SEM sample stage with the cross section facing up and observed using a Philips XL-30 field emission scanning electron microscope (SEM). The test was performed at an accelerating voltage of 10kV, an emission current of 10mA, and a magnification of 3000x. The mass percentage of silicon in the negative electrode material layer was measured using an energy dispersive spectrometer (EDS) equipped with the SEM.

[0115] Binder mass percentage test:

[0116] The lithium-ion battery was discharged at a current of 0.2C to 3.0V, and this step was repeated three times to prevent voltage rebound. The lithium-ion battery was disassembled in a dry room (water content <2%), and the negative electrode was removed. The negative electrode was then cleaned with dimethyl carbonate (DMC) solvent. The cleaning steps were as follows: the negative electrode was soaked in DMC for 12 hours, completely submerging the negative electrode. After soaking, the solvent was discarded and the cleaning step was repeated twice. After cleaning, the negative electrode was dried in a 60°C oven for 8 hours. The negative electrode material layer on the surface of the negative electrode was then scraped off and sampled for analysis using a pyrolysis gas chromatography-mass spectrometry instrument to determine the binder mass percentage. The instrument model was Agilent 7890B / 5977B, the detector was a mass selective detector (MSD), the injector was a pyrolysis gas EGA / PY-3030D, the column oven temperature was 40°C to 320°C, and the pyrolysis temperature was 40°C to 800°C.

[0117] Porosity test of negative electrode after cycling:

[0118] The porosity of the negative electrode sheet after cycling is used to evaluate the space utilization and stability of the bonded conductive network. The fewer the number of pores and the lower the probability of pores resulting from the expansion of the negative electrode active material leading to the destruction of the bonded conductive network after cycling, the lower the porosity of the negative electrode sheet after cycling, indicating higher space utilization and better stability of the bonded conductive network after cycling. Conversely, lower porosity indicates lower space utilization and worse stability of the bonded conductive network after cycling. At 25°C, the lithium-ion battery was charged at a constant current of 2C to 4.53V, then charged at a constant voltage of 4.53V to a current of 0.05C, and then discharged at a constant current of 0.5C to 3.0V. The lithium-ion battery was cycled 600 times under the above conditions. The cycled lithium-ion battery was disassembled in a drying room (water content <2%), the negative electrode sheet was removed, and the negative electrode sheet was cleaned with a solvent of dimethyl carbonate (DMC). The cleaning steps are as follows: soak the negative electrode sheet in DMC for 12 hours, and the liquid completely immerses the negative electrode sheet. After soaking, the solvent is poured out and the above cleaning steps are repeated twice. After cleaning, the negative electrode sheet is placed in a 60°C oven for 8 hours. Then refer to the national standard GB / T 24586-2009 "Determination of apparent density, true density and porosity of iron ore" test method, and the equipment model used is: The II 1340 fully automatic true density meter uses the gas replacement method to test the porosity of the negative electrode sheet, and the replacement gas is N2.

[0119] Cyclic performance test:

[0120] At 25°C, charge the lithium-ion battery at a constant current of 1C to a full charge voltage of 4.53V. Continue charging at a constant voltage of 4.53V to a cutoff current of 0.05C. Then discharge the lithium-ion battery at a constant current of 0.7C to 3.0V. This is one cycle of charge and discharge, and the first discharge capacity is recorded as C1. Repeat the above operation for 600 cycles, and record the discharge capacity at the 600th cycle as C2. The capacity retention rate of the lithium-ion battery (%) = C2 / C1×100%.

[0121] The lithium-ion battery used as an example in this application has a charge cut-off voltage of 4.53 V and a discharge cut-off voltage of 3.0 V. The charge and discharge cut-off voltages of lithium-ion batteries can be based on the outer packaging printed on the battery. The charge and discharge rates of lithium-ion batteries can also be calculated based on the rated capacity printed on the outer packaging.

[0122] Example 1-1

[0123] <Preparation of Adhesive Composition>

[0124] (1) Add carbon nanotubes to a mixture of concentrated sulfuric acid (molar concentration of 18.4 mol / L) and concentrated nitric acid (molar concentration of 14.5 mol / L), and mechanically stir the mixture in an oil bath at 60°C for 1 hour for acidification, i.e., the temperature of the acidification treatment is 60°C, and the acidification time is 1 hour. Then, wash with deionized water, filter until neutral, and dry to obtain acidified carbon nanotubes, the surface of which contains functional groups such as hydroxyl and carboxyl groups. The ratio of the volume of the mixture of concentrated sulfuric acid and concentrated nitric acid to the mass of the carbon nanotubes is 20 mL:1 g, and the volume ratio of concentrated sulfuric acid to concentrated nitric acid is 3:1.

[0125] (2) Under a nitrogen atmosphere, n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate, and acrylic acid were mixed in a molar ratio of 70.5:19.5:2:8 to obtain 77.2 parts of a mixed monomer; the mixed monomer was divided into two groups, namely a first mixed monomer and a second mixed monomer, and the mass ratio of the first mixed monomer to the second mixed monomer was 1:2.

[0126] (3) Under a nitrogen atmosphere, 0.46 parts of STD-1 emulsifier, 0.02 parts of A-501 emulsifier, and the first mixed monomer were mixed and added to 900 parts of deionized water at 65°C for pre-emulsification, and the pre-emulsification time was 20 minutes; then 0.27 parts of initiator potassium persulfate and 0.12 parts of cross-linking agent tetraethylene glycol dimethacrylate were added to carry out a first reaction to obtain a first mixed solution. The temperature of the first reaction was 75°C, and the first reaction time was 2 hours.

[0127] (4) Slowly adding the second mixed monomer and 7.4 parts of acidified carbon nanotubes to the first mixed solution for 3 hours; after the addition is completed, a second reaction is carried out to obtain a second mixed solution. The temperature of the second reaction is 75° C. and the time is 2 hours.

[0128] (5) 0.03 parts of initiator potassium persulfate was added to the second mixed solution again, and the third reaction was continued. The temperature of the third reaction was 75° C. and the time was 1 hour. After the third reaction was completed, lithium hydroxide was added to adjust the pH to neutral, and then 15.4 parts of lithium carboxymethyl cellulose was added. Finally, deionized water was added as a solvent to prepare a binder composition. The mass percentage X of the binder in the binder composition was 4.3%.

[0129] The mass percentage of carbon nanotubes A, the mass percentage of polyacrylate B, and the mass percentage of lithium carboxymethyl cellulose C in the binder are shown in Table 2.

[0130] <Preparation of negative electrode sheet>

[0131] The silicon-containing material silicon carbon (silicon carbon mass ratio of 50:50), graphite and conductive agent carbon nanotubes were mixed, and the binder composition prepared above was added and stirred evenly. Deionized water was then added as a solvent to prepare a slurry with a solid content of 45wt%. After stirring evenly with a vacuum mixer, a negative electrode slurry was obtained. The negative electrode slurry was evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm and dried at 120℃ to obtain a negative electrode sheet with a single-side coating of the negative electrode material layer. The coating weight of the negative electrode material layer was 100mg / 1540mm 2 Repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet coated on both sides with a negative electrode material layer. After drying at 120°C and cold pressing, the sheet is cut and the tabs are welded to obtain a negative electrode sheet measuring 78mm x 875mm for future use. The thickness of the negative electrode material layer on one side is 35μm.

[0132] Based on the mass of the negative electrode material layer, the mass percentage of the silicon-containing material is 60%, the mass percentage of silicon element is 30%, the mass percentage of the binder is 6%, the mass percentage of the conductive agent is 0.5%, and the balance is graphite.

[0133] <Preparation of positive electrode sheet>

[0134] The positive electrode active material LiCoO2, the positive electrode conductive agent Super P, and the positive electrode binder polyvinylidene fluoride were mixed in a mass ratio of 97.9:0.9:1.2, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75wt%. After vacuum stirring, the positive electrode slurry was obtained. The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm and dried at 120℃ to obtain a positive electrode sheet with a single-sided positive electrode material layer. The coating weight of the positive electrode material layer was 267.8mg / 1540mm 2 Repeat the above steps on the other side of the aluminum foil to obtain a double-sided positive electrode sheet coated with a positive electrode material layer. After drying at 120°C and cold pressing, the sheet is cut and the tabs are welded to obtain a 74mm x 867mm positive electrode sheet ready for use. The thickness of the positive electrode material layer on one side is 42μm.

[0135] <Preparation of Electrolyte>

[0136] In an environment with a water content of less than 10 ppm, ethylene carbonate (EC) and propylene carbonate (PC) are mixed in a 1:1 mass ratio to form a non-aqueous solvent. Lithium salt LiPF6 is then added to the non-aqueous solvent and mixed thoroughly to form an electrolyte. The lithium salt accounts for 12.5% ​​by mass of the electrolyte, with the remainder being the non-aqueous solvent.

[0137] <Diaphragm>

[0138] A porous polyethylene film with a thickness of 7 μm (supplied by Celgard) was used as the separator.

[0139] <Preparation of lithium-ion batteries>

[0140] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order, with the separator positioned between the positive and negative electrode sheets to act as a separator, and then wound to form an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dehydrated at 80°C, and then injected with the electrolyte prepared above. The lithium-ion battery is produced through vacuum packaging, standing, forming, degassing, and trimming. The upper limit of the formation voltage is 4.15V, the formation temperature is 70°C, and the formation standing time is 2 hours.

[0141] Example 1-2 to Example 1-5

[0142] Except for adjusting the temperature and time of the acidification treatment so that the defectivity ID / IG value of the adhesive and related parameters are as shown in Table 1, the rest is the same as Example 1-1.

[0143] Example 1-6 to Example 1-7

[0144] Except for adjusting the concentration of the initiator so that the value of the binder emulsion particle size Dv50 and related parameters are as shown in Table 1, the rest is the same as Example 1-1.

[0145] Example 1-8 to Example 1-9

[0146] Except for adjusting the proportion of deionized water in the adhesive composition so that the mass percentage X of the adhesive in the adhesive composition is as shown in Table 1, the rest is the same as Example 1-1.

[0147] Example 1-10 to Example 1-11

[0148] The process was the same as Example 1-1 except that the temperature of the second reaction was adjusted to make the relevant parameters as shown in Table 1. The temperature of the second reaction of Example 1-10 was adjusted to 85°C, and the temperature of the second reaction of Example 1-11 was adjusted to 65°C.

[0149] Example 2-1 to Example 2-14

[0150] The same procedures as in Example 1-1 were used, except that the mass percentages A of the carbon nanotubes, B of the polyacrylate, and C of the lithium carboxymethyl cellulose in the binder were adjusted according to Table 2, and lithium carboxymethyl cellulose was added to the negative electrode slurry so that the mass percentage Cz of the lithium carboxymethyl cellulose in the negative electrode material layer was as shown in Table 2. The mass percentage of the lithium carboxymethyl cellulose added to the negative electrode slurry based on the mass of the negative electrode material layer was Cf.

[0151] Example 2-15 to Example 2-17

[0152] The same procedures as in Example 1-1 were used except that the mass percentages A of the carbon nanotubes and B of the polyacrylate in the binder were adjusted according to Table 2, lithium carboxymethyl cellulose was not added in step (5) of <Preparation of the Binder Composition>, and lithium carboxymethyl cellulose was additionally added to the negative electrode slurry so that the mass percentage Cz of lithium carboxymethyl cellulose in the negative electrode material layer was as shown in Table 2. In Examples 2-15 to 2-17, Cz = Cf.

[0153] Example 3-1 to Example 3-9

[0154] Except for adjusting the molar ratio of n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid in the mixed monomer so that the molar percentage content of the first structural unit D1, the molar percentage content of the second structural unit D2, the molar percentage content of the third structural unit D3, the molar percentage content of the fourth structural unit D4 and related parameters are as shown in Table 3, the rest is the same as Example 1-1.

[0155] Example 4-1 to Example 4-4

[0156] The same procedures as in Example 1-1 were used except that the silicon-carbon mass ratio and / or its mass percentage in the silicon-containing material was adjusted so that the mass percentage of silicon was as shown in Table 4. When the mass percentage of the silicon-containing material was changed, the mass percentage of graphite was also changed, while the mass percentages of the binder and the conductive agent remained unchanged.

[0157] Example 4-5 to Example 4-8

[0158] The process was the same as Example 1-1 except that the mass percentage of the binder was adjusted as shown in Table 4. When the mass percentage of the binder was changed, the mass percentage of the graphite was changed accordingly, while the mass percentages of the silicon-containing material and the conductive agent remained unchanged.

[0159] Comparative Example 1

[0160] The preparation process was the same as that of Example 1-1, except that the mixed monomer was replaced by acrylic acid monomer in the preparation of the adhesive composition, so that polyacrylate was replaced by polyacrylic acid in the adhesive.

[0161] Comparative Example 2

[0162] The process was the same as Example 1-1 except that the carbon nanotubes were not subjected to acidification treatment and the acidified carbon nanotubes were replaced with non-acidified carbon nanotubes in the process of <Preparation of Binder Composition>.

[0163] Comparative Example 3

[0164] Except for adjusting the temperature and time of the acidification treatment so that the value of ID / IG is as shown in Table 1, the rest is the same as Example 1-1.

[0165] Comparative Example 4 to Comparative Example 5

[0166] Except for adjusting the concentration of the initiator so that the particle size Dv50 of the binder emulsion particles is as shown in Table 1, the rest is the same as Example 1-1.

[0167] Comparative Example 6 to Comparative Example 7

[0168] Except for adjusting the proportion of deionized water in the adhesive composition so that the mass percentage X of the adhesive in the adhesive composition is as shown in Table 1, the rest is the same as Example 1-1.

[0169] Comparative Example 8

[0170] Except for adjusting the time of the first reaction to 0.5 h so that the relevant parameters are as shown in Table 1, the rest is the same as Example 1-1.

[0171] The relevant parameters and performance tests of each embodiment and each comparative example are shown in Tables 1 to 4.

[0172]

[0173]

[0174]

[0175] Referring to Tables 1 to 3, it can be seen from Examples 1-1 to 1-11 and Comparative Examples 1 to 8 that the binder includes carbon nanotubes and polyacrylate, and the ID / IG, Dv50, X, and OD values ​​are adjusted within the range of this application, that the porosity of the negative electrode sheet after cycling is low and the capacity retention rate of the lithium-ion battery is high, indicating that the negative electrode sheet has high space utilization and the lithium-ion battery has good cycling performance. In Comparative Example 1, the binder includes carbon nanotubes and polyacrylic acid, but does not include polyacrylate; and in Comparative Examples 2 to 8, at least one of the ID / IG, Dv50, X, and OD values ​​is outside the range of this application. Consequently, the porosity of the negative electrode sheet after cycling is high and the capacity retention rate of the lithium-ion battery is low, indicating that the negative electrode sheet has low space utilization and the cycling performance of the lithium-ion battery is poor.

[0176] ID / IG usually affects the porosity of the negative electrode sheet after cycling and the cycling performance of the lithium-ion battery. From Examples 1-1 to 1-5 and Comparative Examples 2 to 3, it can be seen that when the ID / IG value is too small, such as in Comparative Example 2, the carbon nanotubes in Comparative Example 2 have not been acidified, the defectivity is too small, the emulsion particles of the binder are easily agglomerated, and the conductive bonding network formed in the negative electrode material layer has poor dispersion uniformity. After cycling, the porosity of the negative electrode sheet is high and the capacity retention rate of the lithium-ion battery is low. When the ID / IG value is too large, such as in Comparative Example 3, the ID / IG value of the binder in Comparative Example 3 is too large and the defectivity is too large, the conductive bonding network in the negative electrode material layer has poor conductivity, the porosity of the negative electrode sheet is high after cycling, and the capacity retention rate of the lithium-ion battery is low. Therefore, by adjusting the ID / IG value within the scope of this application, the porosity of the negative electrode sheet can be low and the capacity retention rate of the lithium-ion battery can be high after cycling, indicating that the negative electrode sheet has high space utilization and the lithium-ion battery has good cycling performance.

[0177] The value of Dv50 usually affects the porosity of the negative electrode sheet after cycling and the cycling performance of the lithium-ion battery. From Examples 1-1, 1-6 to 1-7, and Comparative Examples 4 to 5, it can be seen that when the value of Dv50 is too small, such as in Comparative Example 4, the adhesive performance of the binder is poor, the cohesive force is too small when applied to the negative electrode material layer, and the conductive adhesive network in the negative electrode material layer is easily damaged by stress during cycling. The porosity of the negative electrode sheet after cycling is high and the capacity retention rate of the lithium-ion battery is low. When the value of Dv50 is too large, such as in Comparative Example 5, the particle size of the binder emulsion is too large, resulting in uneven dispersion of the binder emulsion particles in the binder composition, which is not conducive to uniform dispersion of the binder in the negative electrode material layer. The porosity of the negative electrode sheet after cycling is high and the capacity retention rate of the lithium-ion battery is low. By regulating the value of Dv50 within the scope of this application, the porosity of the negative electrode plate can be lower and the capacity retention rate of the lithium-ion battery can be higher after cycling, indicating that the negative electrode plate has a higher space utilization rate and the lithium-ion battery has good cycle performance.

[0178] The value of X usually affects the porosity of the negative electrode sheet after cycling and the cycle performance of the lithium-ion battery. It can be seen from Example 1-1, Example 1-8 to Example 1-9, and Comparative Example 6 to Comparative Example 7 that when the value of X is too small, such as Comparative Example 6, the viscosity of the binder composition is too small, the binder particles in the binder composition are affected by gravity and are easy to settle, and the binder is unevenly distributed in the negative electrode material layer; when the value of X is too large, such as Comparative Example 7, the viscosity of the binder composition is too large, which is not conducive to the uniform distribution of the binder in the negative electrode material layer, resulting in a higher porosity of the negative electrode sheet after cycling and a lower capacity retention rate of the lithium-ion battery. By regulating the value of X within the scope of this application, the porosity of the negative electrode sheet after cycling can be lowered and the capacity retention rate of the lithium-ion battery can be higher, indicating that the negative electrode sheet has a higher space utilization rate and the lithium-ion battery has good cycle performance.

[0179] The OD value typically affects the porosity of the negative electrode sheet after cycling and the cycling performance of the lithium-ion battery. As can be seen from Examples 1-1 to 1-11, Comparative Examples 2, 4, and 6 to 8, regulating the OD value within the scope of this application can result in lower porosity in the negative electrode sheet after cycling and higher capacity retention in the lithium-ion battery, indicating higher space utilization in the negative electrode sheet and good cycling performance in the lithium-ion battery.

[0180] The composition of the binder generally affects the porosity of the negative electrode sheet after cycling and the cycling performance of the lithium-ion battery. As can be seen from Examples 1-1, 2-1, and 2-9, the binder includes carbon nanotubes, polyacrylate, and lithium carboxymethyl cellulose, and the mass percentage of the carbon nanotubes (A), the mass percentage of the polyacrylate (B), and the mass percentage of the lithium carboxymethyl cellulose (C) are adjusted within the scope of this application. This allows the binder composition to have a high viscosity, glass transition temperature, tensile strength, and elongation at break, as well as suitable η / X and L1 / L0. This can result in a low porosity of the negative electrode sheet after cycling and a high capacity retention rate of the lithium-ion battery, indicating that the negative electrode sheet has high space utilization and the lithium-ion battery has good cycling performance. It can be seen from Examples 2-15 to 2-17 that the binder includes carbon nanotubes and polyacrylate, and the mass percentage A of the carbon nanotubes and the mass percentage B of the polyacrylate are regulated within the scope of this application. This can result in a lower porosity of the negative electrode sheet after cycling and a higher capacity retention rate of the lithium-ion battery, indicating that the negative electrode sheet has a higher space utilization rate and the lithium-ion battery has good cycle performance.

[0181] The mass percentage of lithium carboxymethyl cellulose (Cz) in the negative electrode material layer typically affects the porosity of the negative electrode sheet after cycling and the cycling performance of the lithium-ion battery. As can be seen from Examples 1-1, 2-8, and 2-14, regulating the Cz value within the range of this application can result in lower porosity in the negative electrode sheet and higher capacity retention in the lithium-ion battery after cycling, indicating high space utilization in the negative electrode sheet and good cycling performance in the lithium-ion battery.

[0182] The composition of polyacrylates typically affects the porosity of the negative electrode sheet after cycling and the cycling performance of lithium-ion batteries. As can be seen from Examples 1-1 and 3-1 to 3-9, the monomers forming the polyacrylate include n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate, and acrylic acid, and regulating the molar percentages of the corresponding structural units D1, D2, D3, and D4 within the scope of this application can result in a binder composition having high viscosity, glass transition temperature, tensile strength, and elongation at break, as well as suitable η / X and L1 / L0. This results in a low porosity of the negative electrode sheet after cycling and a high capacity retention rate of the lithium-ion battery, indicating high space utilization of the negative electrode sheet and good cycling performance of the lithium-ion battery.

[0183] The viscosity η, η / X, (OD1-OD) / OD, glass transition temperature Tg of the binder composition, the tensile strength of the binder film, and the elongation at break L1 and L1 / L0 after soaking in electrolyte usually affect the porosity of the negative electrode sheet after cycling and the cycling performance of the lithium-ion battery. From Examples 1-1 to 1-11, 2-1 to 2-17, and 3-1 to 3-9, it can be seen that by regulating the above parameters within the scope of this application, the porosity of the negative electrode sheet after cycling can be lowered and the capacity retention rate of the lithium-ion battery can be higher, indicating that the negative electrode sheet has a higher space utilization rate and the lithium-ion battery has good cycling performance.

[0184] Figure 1 A scanning electron microscope photograph of the negative electrode material layer of Example 1-1 is shown. Figure 1 The linear carbon nanotubes and the spherical polyacrylates are shown. The carbon nanotubes are relatively long, forming a long-range conductive network within the negative electrode material layer. The polyacrylic acid is located on at least a portion of the surface of the carbon nanotubes, modifying them and distributing them within the carbon nanotube network. The binder, comprising the carbon nanotubes and polyacrylate, is evenly attached to the surface of the negative electrode active material particles, forming a uniformly dispersed three-dimensional conductive bonding network on the surface of the negative electrode active material particles.

[0185] Figure 2 The infrared spectrum of the adhesive of Example 1-1 is shown. Figure 2 Middle, located at 1579.96cm -1 The stretching vibration absorption peak corresponds to the absorption peak of the carbon nanotube skeleton, which is located at 1450.06 cm -1 and 1720.28cm -1 The absorption peak of 3437.81cm is more obvious, corresponding to the hydroxyl and carboxyl groups on the surface of acidified carbon nanotubes. -1 The stretching vibration absorption peak is the stretching vibration peak of hydroxyl group, 2958.24cm -1 、2933.22cm -1 、2873.38cm -1 The stretching vibration and deformation vibration absorption peaks corresponding to methylene and methyl groups are located at 1729.10 cm -1 The sharp stretching vibration absorption peak at 1061.59 cm is caused by the stretching vibration of the carbonyl group in polyacrylate. -1 、1161.61cm -1 、1158.62cm -1 、1242.62cm -1 The stretching vibration absorption peak is caused by the stretching vibration of the ester group in polyacrylate.

[0186] Table 4

[0187]

[0188] The mass percentage of silicon element usually affects the porosity of the negative electrode plate after cycling and the cycling performance of the lithium-ion battery. From Example 1-1, Example 4-1 to Example 4-4, it can be seen that regulating the mass percentage of silicon element within the scope of this application can make the porosity of the negative electrode plate lower after cycling and the capacity retention rate of the lithium-ion battery higher, indicating that the negative electrode plate has a higher space utilization rate and the lithium-ion battery has good cycling performance. In Example 4-3, the mass percentage of silicon element in the negative electrode material layer is relatively low, and the gram capacity of the negative electrode material layer is relatively low. Although the porosity of the negative electrode plate of the lithium-ion battery is relatively low and the capacity retention rate of the lithium-ion battery is relatively high after cycling, the energy density of the lithium-ion battery is relatively low.

[0189] The weight percentage of the binder typically affects the porosity of the negative electrode sheet after cycling and the cycling performance of the lithium-ion battery. As can be seen from Examples 1-1, 4-5, and 4-8, adjusting the weight percentage of the binder within the range of this application can result in lower porosity in the negative electrode sheet after cycling and higher capacity retention in the lithium-ion battery, indicating higher space utilization in the negative electrode sheet and good cycling performance in the lithium-ion battery.

[0190] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article comprising a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, or article.

[0191] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0192] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A binder composition comprising a binder and deionized water, wherein the binder comprises carbon nanotubes and a polyacrylate located on at least a portion of the surface of the carbon nanotubes. The defectivity of the binder is ID / IG, 0.1≤ID / IG≤0.6, and the particle size Dv50 of the emulsion particles of the binder is 0.2 μm to 0.5 μm; Based on the mass of the adhesive composition, the mass percentage X of the adhesive is 3.3% to 5.3%, and the optical density of the adhesive composition is OD, 0.6≤OD≤1.

2. The adhesive composition according to claim 1, wherein Based on the mass of the binder, the mass percentage of the carbon nanotubes is 7% to 11%, and the mass percentage of the polyacrylate is 89% to 93%.

3. The adhesive composition according to claim 1, wherein The binder also includes lithium carboxymethyl cellulose. Based on the mass of the binder, the mass percentage of the carbon nanotubes is 5.4% to 10.5%, the mass percentage of the polyacrylate is 69.2% to 90.6%, and the mass percentage of the lithium carboxymethyl cellulose is 2% to 21.4%.

4. The adhesive composition according to claim 1, wherein The monomers forming polyacrylates include n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid. n-Butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid respectively constitute the first structural unit, the second structural unit, the third structural unit and the fourth structural unit of the polyacrylate. Based on the sum of the molar numbers of the first structural unit, the second structural unit, the third structural unit and the fourth structural unit, the molar percentage of the first structural unit is 64.5% to 72.5%, the molar percentage of the second structural unit is 17.5% to 21.5%, the molar percentage of the third structural unit is 0% to 4%, and the molar percentage of the fourth structural unit is 6% to 10%.

5. The adhesive composition according to any one of claims 1 to 4, wherein The viscosity of the adhesive composition is ηmPa·s, 800≤η≤3500, and 233≤η / X≤660.

6. The adhesive composition according to any one of claims 1 to 4, wherein The adhesive composition is allowed to stand for 48 hours. After the standing, the optical density of the adhesive composition is OD1, 0<(OD1-OD) / OD≤17.0%.

7. The adhesive composition according to any one of claims 1 to 4, wherein The adhesive satisfies at least one of the following characteristics: (1) The glass transition temperature Tg of the binder is -20°C to -10°C; (2) The tensile strength of the adhesive film is 10 MPa to 50 MPa; (3) The adhesive film of the adhesive is immersed in an electrolyte at 60°C ± 5°C for 12 hours. The elongation at break of the adhesive film before immersion in the electrolyte is L0, and the elongation at break of the adhesive film after immersion in the electrolyte is L1, 100% ≤ L1 ≤ 130%, 1.42 ≤ L1 / L0 ≤ 1.74; (4)0.2≤ID / IG≤0.

5.

8. The adhesive composition according to any one of claims 1 to 4, wherein In the infrared spectrum of the binder, there is a -1 Stretching vibration absorption peak, 1450±5cm -1 and 1720±2cm -1 The absorption peak of 3437±25cm -1 Stretching vibration absorption peak, 1730±2cm -1 The sharp stretching vibration absorption peak, 1061±5cm -1 、1161±2cm -1 、1158±2cm -1 、1242±5cm -1 stretching vibration absorption peak.

9. A method for preparing the binder composition according to any one of claims 1 to 8, comprising the following steps: (1) adding the carbon nanotubes to a mixture of concentrated sulfuric acid and concentrated nitric acid for acidification, and then washing and drying to obtain acidified carbon nanotubes, wherein the acidification temperature is 50° C. to 70° C. and the acidification time is 0.5 h to 1.5 h; (2) under a nitrogen atmosphere, n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid are mixed to obtain a mixed monomer, wherein, based on the sum of the moles of n-butyl acrylate, n-butyl methacrylate, isooctyl acrylate and acrylic acid, the mole percentage of n-butyl acrylate is 64.5% to 72.5%, the mole percentage of n-butyl methacrylate is 17.5% to 21.5%, the mole percentage of isooctyl acrylate is 0% to 4%, and the mole percentage of acrylic acid is 6% to 10%; the mixed monomer is divided into two groups, namely a first mixed monomer and a second mixed monomer, and the mass ratio of the first mixed monomer to the second mixed monomer is 1:2; (3) under a nitrogen atmosphere, adding the emulsifier and the first mixed monomer to deionized water at 60° C. to 70° C. for pre-emulsification, wherein the pre-emulsification time is 15 min to 25 min; then adding an initiator and a cross-linking agent to carry out a first reaction to obtain a first mixed solution, wherein the temperature of the first reaction is 70° C. to 80° C., and the time of the first reaction is 1.5 h to 3 h, wherein the cross-linking agent includes tetraethylene glycol dimethacrylate; (4) slowly adding the second mixed monomer and the acidified carbon nanotubes to the first mixed solution, wherein the slow addition time is 2 hours to 4 hours; after the addition is completed, performing a second reaction to obtain a second mixed solution, wherein the temperature of the second reaction is 65° C. to 85° C., and the time is 1 hour to 3 hours; (5) Adding an initiator to the second mixed solution again, and continuing the third reaction. The temperature of the third reaction is 70° C. to 80° C., and the time is 0.5 h to 2 h. After the third reaction is completed, the pH is adjusted to neutral to obtain a binder composition.

10. The preparation method according to claim 9, wherein Based on the sum of the mass of the carbon nanotubes and the mixed monomers, the mass percentage of the carbon nanotubes is 7% to 11%, and the mass percentage of the mixed monomers is 89% to 93%.

11. The preparation method according to claim 9, wherein The preparation method comprises: adjusting the pH to neutral, adding lithium carboxymethyl cellulose, Based on the sum of the masses of the carbon nanotubes, the mixed monomers and lithium carboxymethyl cellulose, the mass percentage of the carbon nanotubes is 5.4% to 10.5%, the mass percentage of the mixed monomers is 69.2% to 90.6%, and the mass percentage of lithium carboxymethyl cellulose is 2% to 21.4%.

12. A secondary battery comprising a positive electrode sheet, an electrolyte, and a negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises a silicon-containing material and a binder. The binder includes carbon nanotubes and polyacrylate located on at least a portion of the surface of the carbon nanotubes. The defectivity of the binder is ID / IG, 0.1≤ID / IG≤0.6, and the particle size Dv50 of the emulsion particles of the binder is 0.2 μm to 0.5 μm.

13. The secondary battery according to claim 12, wherein Based on the mass of the negative electrode material layer, the mass percentage of silicon element is 7.4% to 54%, and the mass percentage of the binder is 3% to 9%. The negative electrode material layer further includes lithium carboxymethyl cellulose. Based on the mass of the negative electrode material layer, the mass percentage of lithium carboxymethyl cellulose is 0.25% to 1.58%.

14. The secondary battery according to claim 12, wherein Based on the mass of the binder, the mass percentage of the carbon nanotubes is 7% to 11%, and the mass percentage of the polyacrylate is 89% to 93%.

15. The secondary battery according to claim 12, wherein The binder also includes lithium carboxymethyl cellulose. Based on the mass of the binder, the mass percentage of the carbon nanotubes is 5.4% to 10.5%, the mass percentage of the polyacrylate is 69.2% to 90.6%, and the mass percentage of the lithium carboxymethyl cellulose is 2% to 21.4%. 16 . An electronic device comprising the secondary battery according to claim 12 .

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