Binder for negative pole piece, secondary battery and electronic device

Abstract

The invention provides a binder for a negative pole piece, a secondary battery and an electronic device. The adhesive comprises a copolymer, monomers forming the copolymer comprise a first monomer, a second monomer, a crosslinking monomer and a functional monomer, the first monomer comprises at least one of acrylic acid or methacrylic acid, the second monomer is selected from C7 to C19 acrylate compounds, and the functional monomer is selected from at least one of acrylic acid and methacrylic acid. The functional monomer comprises at least one of allyl triethoxy silane, N-propyl triethoxy silane or 3-aminopropyl triethoxy silane, and the functional monomer comprises at least one of allyl triethoxy silane, N-propyl triethoxy silane and 3-aminopropyl triethoxy silane; the phase angle lifting rate of the binder is alpha%, and alpha is larger than or equal to 25 and smaller than or equal to 75. The binding agent has good affinity with negative electrode active material particles, and when the binding agent is used for the negative electrode plate, the cycle performance and the dynamic performance of the secondary battery can be improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technology, and in particular to a binder for a negative electrode sheet, a secondary battery, and an electronic device. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, offer advantages such as high energy density, high operating voltage, and minimal environmental impact. They are widely used in various small portable devices, including mobile phones, digital cameras, laptops, and drones. However, demands for improved cycling and kinetic performance of lithium-ion batteries are increasing.

[0003] During the charge and discharge process of a secondary battery, the surface of the negative electrode active material will inevitably react with the electrolyte to form a solid electrolyte interface film (SEI film). During this process, the electrolyte is consumed, affecting the transmission of lithium ions in the later stage of the cycle, thereby causing the cycle capacity of the secondary battery to decay and affecting the cycle performance of the secondary battery. In order to improve the above problem, by increasing the amount of binder so that the binder is coated on the surface of the negative electrode active material particles, the contact area between the surface of the negative electrode active material and the electrolyte can be reduced. However, in the prior art, it is usually necessary to add too much binder to achieve the above effect. Too much binder will form an overly thick coating layer on the surface of the negative electrode active material particles, which is not conducive to lithium ion conduction and will deteriorate the dynamic performance of the secondary battery. Summary of the Invention

[0004] The purpose of this application is to provide a binder for negative electrode sheets, a secondary battery, and an electronic device to improve the cycle performance and dynamic performance of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of the present application provides a binder for a negative electrode sheet, wherein the binder comprises a copolymer, the monomers forming the copolymer comprise a first monomer, a second monomer, a cross-linking monomer, and a functional monomer, the first monomer comprises at least one of acrylic acid or methacrylic acid, the second monomer is selected from C7 to C 19 The acrylate compound comprises a functional monomer comprising at least one of allyltriethoxysilane, N-propyltriethoxysilane, or 3-aminopropyltriethoxysilane; the phase angle enhancement rate of the binder is α%, 25≤α≤75; under the conditions of an ambient temperature of 25±5°C, a shear stress of 0.1 Pa, and an applied shear stress frequency of 1 Hz, the phase angle of the binder is θ0°, the phase angle of the mixture of the binder and the negative electrode active material is θ1°, and the mass ratio of the binder to the negative electrode active material in the mixture is 7:3. In the binder of the present application, the first monomer, the second monomer, the crosslinking monomer, and the functional monomer respectively constitute the first monomer unit, the second monomer unit, the crosslinking monomer unit, and the functional monomer unit of the copolymer. The first monomer unit constitutes the main structure of the copolymer, which can make the binder have good hydrophilicity, which is conducive to the uniform dispersion of the binder in the negative electrode slurry and the uniform distribution of the binder in the negative electrode material layer. The second monomer unit provides hydrophobicity to the copolymer, which is conducive to improving the affinity between the binder and the negative electrode active material. The crosslinking monomer unit provides a crosslinked structure, which is conducive to improving the ion pass rate. The functional monomer unit can further provide the binder with affinity between the negative electrode active material and the negative electrode active material, so that the binder can be uniformly attached to the surface of the negative electrode active material particles. The phase angle enhancement rate α% of the binder is within the above range, indicating that the binder of the present application can uniformly adhere to the surface of the negative electrode active material particles. The binder of the present application has a high surface modification ability for the negative electrode active material particles, and the porosity between the copolymer chain segments is high, which is conducive to improving the lithium ion pass rate. Using the binder of the present application for the negative electrode plate can reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, reduce the side reactions of the electrolyte, and at the same time is beneficial to the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0006] In some embodiments of the present application, 40≤α≤60. Regulating the value of α within the above range indicates that the binder can be more evenly attached to the surface of the negative electrode active material particles, thereby using the binder of the present application in the negative electrode sheet is more conducive to improving the cycle performance and kinetic performance of the secondary battery.

[0007] In some embodiments of the present application, 12≤θ0≤18. The value of θ0 being within the above range indicates that the binder of the present application has a relatively high phase angle, which is beneficial for reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing the side reactions of the electrolyte, and also facilitating the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0008] In some embodiments of the present application, the crosslinking monomer includes at least one of pentaerythritol allyl ether, trimethylolpropane triacrylate, divinylbenzene, ethoxylated trimethylolpropane triacrylate, or pentaerythritol triallyl ether. The selection of these crosslinking monomers can adjust the uniformity of the binder film formation on the surface of the negative electrode active material particles, adjust the microstructure within the binder film, and increase the porosity between the copolymer segments, thereby increasing the lithium ion transmission rate, thereby reducing side reactions between the negative electrode active material particles and the electrolyte. It also facilitates lithium ion conduction, further improving the cycling performance and kinetic performance of the secondary battery.

[0009] In some embodiments of the present application, the first monomer, the second monomer, the crosslinking monomer, and the functional monomer constitute the first monomer unit, the second monomer unit, the crosslinking monomer unit, and the functional monomer unit, respectively, of the copolymer. Based on the mass of the copolymer, the weight percentage of the first monomer unit is 67.6% to 87.6%, the weight percentage of the second monomer unit is 10% to 30%, the weight percentage of the crosslinking monomer unit is 0.2% to 1%, and the weight percentage of the functional monomer unit is 1% to 3%. The first monomer unit constitutes the main structure of the copolymer, the second monomer provides hydrophobicity to the copolymer, the crosslinking monomer unit provides a crosslinking structure to the copolymer, and the functional monomer provides affinity for the negative electrode active material. Adjusting the weight percentage of the above monomer units within the above range can better synergize the functions of each monomer unit in the copolymer, improve the binder's ability to modify the surface of the negative electrode active material particles, and uniformly adhere to the surface of the negative electrode active material particles, thereby further reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing side reactions of the electrolyte, and further improving lithium ion conduction, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0010] In some embodiments of the present application, the mass percentage of the second monomer unit is 15% to 20%, and the mass percentage of the cross-linking monomer unit is 0.3% to 0.5%, based on the mass of the copolymer; and / or the mass ratio X of the functional monomer unit to the second monomer unit is 0.06 to 0.2. Meeting at least one of the above characteristics is more conducive to reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing side reactions of the electrolyte, and further improving the conduction of lithium ions, thereby enhancing the cycle performance and kinetic performance of the secondary battery.

[0011] In some embodiments of the present application, the second monomer is selected from C 10 to C 12 The use of the above-mentioned compound as the second monomer can impart suitable hydrophobicity to the copolymer, enhance the affinity between the binder and the negative electrode active material, facilitate the function of the second monomer unit, and enhance the binder's ability to modify the surface of the negative electrode active material particles, allowing the binder to evenly adhere to the surface of the negative electrode active material particles, thereby improving the cycling performance and kinetic performance of the secondary battery.

[0012] In some embodiments of the present application, the second monomer includes at least one of butyl acrylate, isooctyl acrylate, butyl methacrylate, lauryl methacrylate, hexadecyl acrylate, or lauryl acrylate. The use of these compounds as the second monomer enhances the function of the second monomer unit and further improves the cycling and kinetic performance of the secondary battery.

[0013] In some embodiments of the present application, the weight-average molecular weight (Mw) of the copolymer is between 50,000 g / mol and 100,000 g / mol. Regulating the weight-average molecular weight of the copolymer within this range facilitates the adsorption of the binder onto the surface of the negative electrode active material particles, reduces the contact area between the negative electrode active material particle surface and the electrolyte, reduces side reactions of the electrolyte, and also facilitates the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0014] In some embodiments of the present application, the binder has a swelling degree in the electrolyte of 10% to 30%, and / or a dissolution rate in the electrolyte of 2% to 3%. Meeting at least one of the above characteristics is more conducive to improving the cycle performance and kinetic performance of the secondary battery.

[0015] A second aspect of the present application provides a secondary battery comprising a positive electrode sheet 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 binder as provided in any of the aforementioned embodiments. The negative electrode material layer comprising the binder of the present application can reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, thereby reducing side reactions of the electrolyte, while also facilitating the conduction of lithium ions and improving the cycle performance and kinetic performance of the secondary battery.

[0016] In some embodiments of the present application, the negative electrode material layer includes a negative electrode active material, which includes graphite and a silicon-containing material. The weight percentage of silicon is 1.5% to 30% based on the weight of the negative electrode material layer. Selecting the above-mentioned negative electrode active material and regulating the weight percentage of silicon within the above-mentioned range can make the negative electrode material layer have a higher specific capacity. In combination with the binder of the present application, this further facilitates the utilization of the capacity of the negative electrode active material, thereby enabling the secondary battery to have a higher energy density while also having good cycling performance and kinetic performance.

[0017] In some embodiments of the present application, the binder content is 1% to 10% by weight based on the mass of the negative electrode material layer. Adjusting the binder content within this range facilitates the binder's effectiveness, reduces the contact area between the surface of the negative electrode active material particles and the electrolyte, reduces electrolyte side reactions, and facilitates lithium ion conduction, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0018] In some embodiments of the present application, the weight percentage of the binder is 2% to 5% based on the weight of the negative electrode material layer. Regulating the weight percentage of the binder within the above range is conducive to better exerting the role of the binder, thereby further improving the cycle performance and kinetic performance of the secondary battery.

[0019] The third aspect of the present application provides an electronic device, which includes the secondary battery provided in any of the aforementioned embodiments. Thus, the electronic device of the present application has a long service life and good performance.

[0020] Beneficial effects of this application:

[0021] The present application provides a binder for a negative electrode sheet, a secondary battery, and an electronic device. The binder includes a copolymer, wherein the monomers forming the copolymer include a first monomer, a second monomer, a cross-linking monomer, and a functional monomer, wherein the first monomer includes at least one of acrylic acid or methacrylic acid, and the second monomer is selected from C7 to C 19 The acrylate compound comprises a functional monomer comprising at least one of allyltriethoxysilane, N-propyltriethoxysilane, or 3-aminopropyltriethoxysilane; the binder has a phase angle enhancement rate of α%, 25≤α≤75. In the binder of the present application, the structural unit of the first monomer constitutes the main structure of the binder, which can make the binder have good hydrophilicity, which is conducive to the uniform dispersion of the binder in the negative electrode slurry; the second monomer provides hydrophobicity to the copolymer, which is conducive to improving the affinity between the binder and the negative electrode active material; the cross-linking monomer can provide a cross-linking structure, which is conducive to improving the ion pass rate; the functional monomer can further provide the binder with affinity between the negative electrode active material, so that the binder is uniformly attached to the surface of the negative electrode active material particles. The phase angle enhancement rate α% of the binder is within the above range, indicating that the binder of the present application can be uniformly attached to the surface of the negative electrode active material particles, the binder of the present application has a high surface modification ability for the negative electrode active material particles, and the porosity between the copolymer chain segments is high, which is conducive to improving the lithium ion pass rate. Using the binder of the present application for the negative electrode plate can reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, reduce the side reactions of the electrolyte, and at the same time is beneficial to the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0022] 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

[0023] 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.

[0024] Figure 1 is a graph showing the phase angle of the binder of Example 1-1 and the phase angle of the mixture of the binder and the negative electrode active material as a function of shear stress;

[0025] Figure 2 Graph showing the phase angle of the binder of Comparative Example 5 and the phase angle of the mixture of the binder and the negative electrode active material as a function of shear stress. DETAILED DESCRIPTION

[0026] 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.

[0027] It should be noted that in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0028] The first aspect of the present application provides a binder for a negative electrode sheet, wherein the binder comprises a copolymer, the monomers forming the copolymer comprise a first monomer, a second monomer, a cross-linking monomer, and a functional monomer, the first monomer comprises at least one of acrylic acid or methacrylic acid, the second monomer is selected from C7 to C 19 The acrylate compound, the functional monomer includes at least one of allyltriethoxysilane, N-propyltriethoxysilane or 3-aminopropyltriethoxysilane; the phase angle enhancement rate of the binder is α%, 25≤α≤75, preferably 40≤α≤60. For example, the value of α can be 25, 28, 30, 35, 40, 44, 45, 50, 53, 55, 59, 60, 65, 70, 75 or a range consisting of any two values therein. Under the conditions of an ambient temperature of 25±5°C, a shear stress of 0.1Pa, and an applied shear stress frequency of 1Hz, the phase angle of the binder is θ0°, the phase angle of the mixture of the binder and the negative electrode active material is θ1°, and the mass ratio of the binder to the negative electrode active material in the mixture is 7:3.

[0029] In the binder of the present application, the first monomer, the second monomer, the cross-linking monomer and the functional monomer constitute the first monomer unit, the second monomer unit, the cross-linking monomer unit and the functional monomer unit of the copolymer respectively. Among them, the first monomer unit constructs the main structure of the copolymer, which can make the binder have good hydrophilicity, which is conducive to the uniform dispersion of the binder in the negative electrode slurry and can be evenly distributed in the negative electrode material layer, so as to better play the role of the binder. The second monomer unit provides hydrophobicity to the copolymer, which can make the binder have suitable affinity with the negative electrode active material, which is conducive to the stretching of the copolymer chain segments and reduces the possibility of the copolymer chain segments curling up, so that the binder can be uniformly adhered to the surface of the negative electrode active material particles. Preferably, it is conducive to improving the binder's ability to modify the surface of graphite, thereby helping to reduce the contact area between the surface of the negative electrode active material particles and the electrolyte and reduce the side reactions of the electrolyte. The cross-linked monomer unit provides a cross-linked structure for the copolymer, which can adjust the film formation of the binder on the surface of the negative electrode active material particles and the microstructure of the binder film. The copolymer has suitable segment mobility, which enables the binder to have a high surface modification ability for the negative electrode active material particles, reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing the side reactions of the electrolyte, and the high porosity between the copolymer segments is conducive to improving the lithium ion pass rate. The functional monomer can further provide the binder with affinity for the negative electrode active material, so that the binder is more evenly attached to the surface of the negative electrode active material particles. Preferably, it is conducive to improving the surface modification ability of the binder for silicon-containing materials, thereby helping to reduce the contact area between the surface of the negative electrode active material particles and the electrolyte and reducing the side reactions of the electrolyte. In addition, the above-mentioned functional monomer and the second monomer have an interactive force. Through the hydrophobic compatibility between the alkyl groups, the functional monomer can be connected to the copolymer segment through the above-mentioned action, which can further improve the utilization rate of the functional monomer, thereby better playing the role of the functional monomer. Therefore, the selection of monomers for the above-mentioned copolymer not only allows the binder to be evenly distributed in the negative electrode material layer, but also improves the binder's ability to modify the surface of the negative electrode active material particles, allowing the binder to evenly adhere to the surface of the negative electrode active material particles. In addition, the high porosity between the copolymer chain segments is beneficial for improving the lithium ion pass rate. Therefore, using the binder of the present application in the negative electrode plate can reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing side reactions of the electrolyte, while also facilitating the conduction of lithium ions and improving the cycle performance and kinetic performance of the secondary battery.

[0030] In addition, the inventors discovered that for the binder, due to the strong interaction between its copolymer chain segments, the segments are entangled together, and the copolymer exhibits a partial rigidity. When it is subjected to stress, the elastic effect dominates, and the phase angle value is low. When the binder and the negative electrode active material are mixed, more of it will be adsorbed on the surface of the negative electrode active material particles, the free binder is reduced, the entanglement of the copolymer chain segments is reduced, and the interaction between the segments is reduced. When it is subjected to stress, the rigidity is reduced, and when it is subjected to stress, the proportion of the elastic effect is weakened, and the phase angle is increased. Therefore, the phase angle increase rate is used to characterize the surface modification ability of the binder on the particles. The higher the phase angle increase rate, the higher the surface modification ability of the binder on the negative electrode active material particles, the more uniform the distribution on the surface of the negative electrode active material particles, and the more binder adheres to the surface of the negative electrode active material particles. When the value of α is too small, for example, less than 25, the phase angle enhancement rate is too small, indicating that the binder has a low ability to modify the surface of the negative electrode active material particles and cannot evenly adhere to the surface of the negative electrode active material particles, which is not conducive to the role of the binder; when the value of α is too large, for example, greater than 75, the phase angle enhancement rate is too large, and too much binder adheres to the surface of the negative electrode active material, causing the ion conduction in the negative electrode material layer to be blocked, and deteriorating the dynamic performance of the lithium-ion battery. The phase angle enhancement rate α% of the binder is within the above range, indicating that the binder of the present application can evenly adhere to the surface of the negative electrode active material particles, and the binder has a high surface modification ability for the negative electrode active material particles. Therefore, using the binder of the present application for the negative electrode plate can reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, reduce the side reactions of the electrolyte, and is beneficial to improving the cycle performance and dynamic performance of the secondary battery.

[0031] In some embodiments of the present application, 12≤θ0≤18, for example, θ0 can be 12, 13, 14, 15, 16, 17, 18 or a range consisting of any two of these values. Commonly used binders, such as polyacrylic acid binders, have a phase angle of 4° to 6°. The value of θ0 of the binder of the present application is within the above range, indicating that the binder of the present application has a higher phase angle. Binders with a higher phase angle have a higher surface modification ability for negative electrode active material particles, thereby further reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing the side reactions of the electrolyte, and at the same time being beneficial to the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0032] In some embodiments of the present application, the crosslinking monomer includes at least one of pentaerythritol allyl ether, trimethylolpropane triacrylate, divinylbenzene, ethoxylated trimethylolpropane triacrylate, or pentaerythritol triallyl ether. The selection of these crosslinking monomers can adjust the uniformity of the binder film formation on the surface of the negative electrode active material particles, adjust the microstructure within the binder film, and increase the porosity between the copolymer segments, thereby increasing the lithium ion transmission rate, thereby reducing side reactions between the negative electrode active material particles and the electrolyte. It also facilitates lithium ion conduction, further improving the cycling performance and kinetic performance of the secondary battery.

[0033] In some embodiments of the present application, the first monomer, the second monomer, the cross-linking monomer, and the functional monomer constitute the first monomer unit, the second monomer unit, the cross-linking monomer unit, and the functional monomer unit of the copolymer, respectively. Based on the mass of the copolymer, the mass percentage A of the first monomer unit is 67.6% to 87.6%, the mass percentage B of the second monomer unit is 10% to 30%, the mass percentage C of the cross-linking monomer unit is 0.2% to 1%, and the mass percentage D of the functional monomer unit is 1% to 3%. For example, the mass percentage A of the first monomer unit can be 67.6%, 70%, 71%, 73%, 75%, 77%, 80%, 83%, 85%, 86%, 87%, 87.6%, or a range consisting of any two of these values, and the mass percentage B of the second monomer unit can be 10%, 13%, 15%, 18%, 20%, 22%, 24%, 25%, 28%, 30%, or a range consisting of any two of these values. The mass percentage content C of the cross-linking monomer unit can be 0.2%, 0.3%, 0.35%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or a range consisting of any two values therein, and the mass percentage content D of the functional monomer unit can be 1%, 1.1%, 1.5%, 1.6%, 1.8%, 2%, 2.4%, 2.8%, 3% or a range consisting of any two values therein. By regulating the mass percentage content of the first monomer unit within the above range, the binder can have a suitable molecular weight and water solubility, which is conducive to uniform dispersion of the binder in the negative electrode slurry, thereby better playing the role of the binder. Regulating the mass percentage of the second monomer unit within the above range can give the binder appropriate hydrophobicity and affinity with the negative electrode active material, which is beneficial to the stretching of the copolymer chain segments and reduces the possibility of their curling, thereby allowing the binder to adhere evenly to the surface of the negative electrode active material particles, thereby facilitating the reduction of the contact area between the surface of the negative electrode active material particles and the electrolyte and reducing the side reactions of the electrolyte. Regulating the mass percentage of the cross-linking monomer unit within the above range can give the copolymer an appropriate degree of cross-linking and porosity, so that the copolymer has appropriate segment mobility, allowing the binder to have a high surface modification ability for the negative electrode active material particles, reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing the side reactions of the electrolyte, and also allowing the porosity between the copolymer chain segments to be high, which is beneficial to improving the lithium ion passage rate. Regulating the mass percentage of the functional monomer unit within the above range is more conducive to the role of the functional monomer, allowing the binder to adhere more evenly to the surface of the negative electrode active material particles, further reducing the contact area between the surface of the negative electrode active material particles and the electrolyte and reducing the side reactions of the electrolyte.Therefore, adjusting the mass percentage of the monomer units to be within the above range is more conducive to playing the role of the binder and further improving the cycle performance and dynamic performance of the secondary battery.

[0034] In some embodiments of the present application, the weight percentage of the second monomer unit is 15% to 20%, and the weight percentage of the cross-linking monomer unit is 0.3% to 0.5%, based on the weight of the copolymer. Regulating the weight percentage of the second monomer unit and the weight percentage of the cross-linking monomer unit within the above ranges can better utilize the functions of the second monomer unit and the cross-linking monomer unit, further helping to reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing side reactions of the electrolyte, and further improving the conduction of lithium ions, thereby enhancing the cycle performance and kinetic performance of the secondary battery.

[0035] In some embodiments of the present application, the mass ratio X of the functional monomer unit to the second monomer unit is 0.06 to 0.2, for example, the value of X can be 0.06, 0.07, 0.08, 0.09, 0.1, 0.14, 0.15, 0.18, 0.2, or a range consisting of any two of these values. Regulating the mass ratio of the functional monomer unit to the second monomer unit within the above range can further improve the utilization rate of the functional monomer, thereby better exerting the role of the functional monomer, improving the surface modification ability of the binder on the negative electrode active material, and being more conducive to reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing side reactions of the electrolyte, and further improving the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0036] In some embodiments of the present application, the second monomer is selected from C 10 to C 12 For example, the second monomer may be C 10 Acrylate compounds, C 11 Acrylate compounds, C 12 The second monomer may be an acrylate compound, preferably isooctyl acrylate. Using the above-mentioned compound as the second monomer can impart suitable hydrophobicity to the copolymer and facilitate chain stretching. This enhances the affinity between the binder and the negative electrode active material, facilitates the function of the second monomer unit, and improves the binder's ability to modify the surface of the negative electrode active material particles, allowing the binder to evenly adhere to the surface of the negative electrode active material particles, thereby improving the cycling and kinetic performance of the secondary battery.

[0037] In some embodiments of the present application, the second monomer includes at least one of butyl acrylate, isooctyl acrylate, butyl methacrylate, lauryl methacrylate, hexadecyl acrylate, or lauryl acrylate. Using these compounds as the second monomer can impart suitable hydrophobicity to the copolymer and enhance chain stretchability. This improves the binder's affinity for the negative electrode active material, further facilitating the function of the second monomer unit, and further enhancing the binder's ability to modify the surface of the negative electrode active material particles, allowing the binder to evenly adhere to the surface of the negative electrode active material particles, further improving the cycling and kinetic performance of the secondary battery.

[0038] In the present application, the number of carbon atoms of the acrylate compound refers to the sum of all carbon atoms in the main chain and side chains of the acrylate compound, for example, the carbon number of butyl acrylate is C7, the carbon number of isooctyl acrylate is C11, the carbon number of butyl methacrylate is C8, the carbon number of lauryl methacrylate is C16, the carbon number of hexadecyl acrylate is C19, and the carbon number of lauryl acrylate is C15.

[0039] In some embodiments of the present application, the weight average molecular weight Mw of the copolymer is 50,000 g / mol to 100,000 g / mol, for example, the weight average molecular weight Mw of the copolymer can be 50,000 g / mol, 60,000 g / mol, 70,000 g / mol, 80,000 g / mol, 90,000 g / mol, 100,000 g / mol or a range consisting of any two of these values. By regulating the weight average molecular weight of the copolymer within the above range, the hydrophilicity or hydrophobicity of the copolymer can be closer to that of the negative electrode active material, and the affinity between the copolymer and the negative electrode active material is higher, which is more conducive to adsorption on the surface of the negative electrode active material, so that the binder is uniformly modified on the surface of the negative electrode active material to achieve a surface modification effect, which can reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, reduce the side reactions of the electrolyte, and also facilitate the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0040] In some embodiments of the present application, the swelling degree of the binder in the electrolyte is 10% to 30%. For example, the swelling degree of the binder in the electrolyte can be 10%, 14%, 15%, 18%, 20%, 24%, 25%, 28%, 30%, or a range consisting of any two of these values. The swelling degree of the binder in the electrolyte being within the above range indicates that the binder of the present application has a low swelling degree and does not swell easily after being immersed in the electrolyte, indicating low affinity with the electrolyte. This can reduce the probability of contact between the electrolyte and the particles of the negative electrode active material, reduce side reactions of the electrolyte, and thus improve the cycle performance and kinetic performance of the secondary battery.

[0041] In some embodiments of the present application, the dissolution rate of the binder in the electrolyte is 2% to 3%. For example, the dissolution rate of the binder in the electrolyte can be 2%, 2.1%, 2.2%, 2.4%, 2.5%, 2.7%, 2.9%, 3%, or a range consisting of any two of these values. The dissolution rate of the binder in the electrolyte being within the above range indicates that the binder has good stability in the electrolyte and a low dissolution rate, which can better play its role in the use of the secondary battery, reduce the exposed area of the active sites on the surface of the negative electrode active material particles, reduce the side reactions of the electrolyte, and is more conducive to improving the cycle performance and kinetic performance of the secondary battery.

[0042] The present application has no particular limitation on the preparation method of the binder, as long as the purpose of the present application can be achieved. For example, the method may include but is not limited to the following steps:

[0043] (1) First, mix water, pH regulator and initiator evenly to obtain mixed solution E.

[0044] (2) The first monomer and the second monomer are mixed evenly to obtain a monomer mixed solution 1, 30% to 60% by mass of the above monomer mixed solution 1 is taken and added to the mixed solution E, and stirred evenly at room temperature, and then a first reaction is carried out. The temperature T1 of the first reaction is 70°C to 90°C, and the reaction time t2 is 3 hours (h) to 5 hours to obtain a mixed solution H.

[0045] (3) Add the crosslinking monomer and functional monomer to the remaining monomer mixture 1 and stir evenly to obtain monomer mixture 2. Add the monomer mixture 2 to the above mixture H by dropwise addition for a time of t a The temperature of the mixture H is kept at T during the addition process. a After the addition is completed, the second reaction is carried out. The temperature T2 of the second reaction is 70°C to 90°C, and the time t2 is 0.5h to 2h. After the reaction is completed, a mixed solution K containing a binder is obtained.

[0046] (4) Adding a pH adjuster to the mixed solution K to adjust the pH to neutral, and finally filtering and vacuum treating to remove residual monomers to separate and obtain the binder.

[0047] The present application does not particularly limit the pH adjuster, as long as it can achieve the objectives of the present application. For example, it may include, but is not limited to, at least one of lithium carbonate, lithium hydroxide, or sodium hydroxide. The present application does not particularly limit the initiator, as long as it can achieve the objectives of the present application. For example, the initiator may include, but is not limited to, at least one of the following: inorganic peroxides, such as sodium persulfate, potassium persulfate, ammonium persulfate, a composite initiator of ammonium persulfate and sodium bisulfite, potassium perphosphate, and hydrogen peroxide; organic peroxides, such as tert-butyl peroxide, cumene hydroperoxide, terpane hydroperoxide, di-tert-butyl peroxide, tert-butyl cumyl peroxide, acetyl peroxide, isobutyl peroxide, octanoyl peroxide, dibenzoyl peroxide, 3,5,5-trimethylhexanol peroxide, tert-butyl peroxyisobutyrate; and azo compounds, such as at least one of azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, or dimethyl azobisisobutyrate.

[0048] In the present application, based on the total mass of the monomers, the mass percentage of the first monomer is 71% to 87%, the mass percentage of the second monomer is 10% to 30%, the mass percentage of the cross-linking monomer is 0.2% to 1%, and the mass percentage of the functional monomer is 1% to 3%. The above-mentioned "total mass of monomers" is the sum of the masses of the first monomer, the second monomer, the cross-linking monomer and the functional monomer. The mass ratio of the total mass of the monomers to water is 1: (3 to 19), the mass ratio of the total mass of the monomers to the initiator is 1: (0.025 to 0.1), and the mass ratio of the total mass of the monomers to the pH regulator is 1: (0.025 to 0.1). The present application has no particular restrictions on the above-mentioned stirring, as long as the purpose of the present application can be achieved. For example, the stirring rate can be 200rpm to 300rpm, and the stirring method can be magnetic stirring or single-rod stirring. The present application has no particular restrictions on the rate of dropwise addition, as long as the purpose of the present application can be achieved, for example, the rate of dropwise addition is 0.1mL / s to 1mL / s.

[0049] The present application does not particularly limit the process for vacuum treatment to remove residual monomers, as long as the purpose of the present application can be achieved. For example, it can be achieved by vacuuming at room temperature, under vacuum conditions of -60 kPa to -80 kPa, and at a stirring speed of 200 rpm to 300 rpm for 1.5 to 3 hours.

[0050] In the above step (4), after the residual monomers are removed by vacuum treatment, the obtained binder is dispersed in the solvent remaining in the reaction and exists in an emulsion state. It can be formulated to a suitable solid content and directly used in the preparation of the negative electrode slurry. Those skilled in the art can also dry it according to actual needs to obtain solid particles of the binder, and add water as a solvent to prepare the binder slurry in the preparation of the negative electrode sheet. This application has no special restrictions on this, as long as the purpose of this application can be achieved. The above drying temperature can be 80°C to 120°C, and the drying time can be 2h to 10h.

[0051] The present application also provides a binder composition comprising the binder provided in the first aspect of the present application, a pH adjuster, and water. Based on the mass of the binder composition, the binder comprises 10% to 20% by weight, the pH adjuster comprises 0.5% to 1% by weight, and the remainder is water. Using the binder composition with the above characteristics to prepare a negative electrode sheet can make the binder more evenly dispersed in the negative electrode material layer, and also improve the binder's ability to modify the surface of the negative electrode active material, thereby reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing side reactions of the electrolyte, and further improving the conduction of lithium ions, thereby enhancing the cycle performance and kinetic performance of the secondary battery.

[0052] The second aspect of the present application provides a secondary battery, which includes a positive electrode plate and a negative electrode plate, the negative electrode plate includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector, and the negative electrode material layer includes the binder provided in any of the aforementioned embodiments. The binder of the present application has a high surface modification ability for the negative electrode active material particles, and the porosity between the copolymer chain segments is high, which is beneficial to improving the pass rate of lithium ions. The negative electrode material layer includes the binder of the present application, which can reduce the contact area between the surface of the negative electrode active material particles and the electrolyte, reduce the side reactions of the electrolyte, and is also beneficial to the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0053] In some embodiments of the present application, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes graphite and a silicon-containing material. Based on the mass of the negative electrode material layer, the mass percentage of the silicon element is 1.5% to 30%, for example, the mass percentage of the silicon element can be 1.5%, 3%, 5%, 10%, 15%, 20%, 23%, 25%, 30%, or a range consisting of any two of these values. Selecting the above-mentioned negative electrode active material and regulating the mass percentage of the silicon element within the above-mentioned range can make the negative electrode material layer have a higher gram capacity. Combined with the binder of the present application, it is more conducive to the capacity of the negative electrode active material, thereby making the secondary battery have a higher energy density while having good cycle performance and kinetic performance.

[0054] In the present application, the silicon-containing material includes at least one of silicon, silicon carbon, or silicon oxide, and the graphite may include natural graphite and artificial graphite. There is no particular limitation on this in the present application, as long as the purpose of the present application can be achieved. Optionally, the negative electrode active material may also include a carbon material such as hard carbon.

[0055] In some embodiments of the present application, the weight percentage of the binder is 1% to 10%, preferably 2% to 5%, based on the weight of the negative electrode material layer. For example, the weight percentage of the binder can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two of these values. Regulating the weight percentage of the binder within the above range is beneficial to exerting the role of the binder, reducing the contact area between the surface of the negative electrode active material particles and the electrolyte, reducing side reactions of the electrolyte, and also facilitating the conduction of lithium ions, thereby improving the cycle performance and kinetic performance of the secondary battery.

[0056] 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.

[0057] 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 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. 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.

[0058] In some embodiments, the negative electrode material layer may further include a conductive agent and a dispersant. The present application does not particularly limit the types of the conductive agent and the dispersant, 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, and the 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. For example, the dispersant may include but is not limited to at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. The present application does not particularly limit the mass percentage of the conductive agent and dispersant in the negative electrode material layer. Those skilled in the art may select according to actual needs as long as the purpose of the present application can be achieved. For example, the mass percentage of the conductive agent may be 0% to 5%, and the mass percentage of the dispersant may be 0.2% to 2.

[0059] In the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector along its thickness direction, or on both surfaces of the positive electrode current collector along its thickness direction. It should be noted that the "surface" here can refer to the entire surface of the positive electrode current collector or a portion of the surface of the positive electrode current collector. This is not particularly limited in the present application, as long as the purpose of this application can be achieved.

[0060] This application does not particularly limit the positive electrode current collector, as long as it can achieve the purpose of this application. 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 does not particularly limit the thickness of the positive electrode current collector and the positive electrode material layer, as long as it can achieve the purpose of this application. 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.

[0061] 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.

[0062] 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.

[0063] The present application does not particularly limit the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer. Those skilled in the art can select the ratio according to actual needs as long as the purpose of the present application can be achieved.

[0064] In the present application, the secondary battery also includes a diaphragm. The present application has no particular restrictions on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or at least one of aramid. The type of diaphragm 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. In some embodiments, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film or a composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. In the present application, the thickness of the separator is not particularly limited as long as the purpose of the present application can be achieved. For example, the thickness of the separator may be 3 μm to 30 μm.

[0065] In the present application, the secondary battery also includes an electrolyte, and the electrolyte includes a lithium salt and a non-aqueous solvent. The present application has no particular restrictions on the lithium salt, as long as the purpose of the present application can be achieved. For example, the lithium salt may include but is not limited to at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalatoborate) (LiBOB) or lithium difluoroborate. The present application has no particular restrictions on the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, the non-aqueous solvent may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvents. The above-mentioned carbonate compound may include but is not limited to at least one of a chain carbonate compound, a cyclic carbonate compound or a fluorinated carbonate compound. The above-mentioned linear carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC) or methyl ethyl carbonate (MEC). The above-mentioned cyclic carbonate may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound may include but is not limited to at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate or trifluoromethylethylene carbonate. The above-mentioned carboxylate compounds may include but are not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid, valerolactone or caprolactone. The above-mentioned ether compounds may include but are not limited to at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate. The present application does not particularly limit the content of the non-aqueous solvent in the electrolyte, as long as the purpose of the present application can be achieved.

[0066] 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.

[0067] 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.

[0068] The third aspect of the present application provides an electronic device, which includes the secondary battery provided in any of the aforementioned embodiments. Thus, the electronic device of the present application has a long service life and good performance.

[0069] 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.

[0070] Example

[0071] 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.

[0072] Test methods and equipment:

[0073] Phase angle test:

[0074] The phase angle is tested by a rotational rheometer (TA Company, USA, Model: DHR-2), and the phase angle improvement rate α% of the binder is calculated. The binder is dispersed in deionized water to obtain an aqueous dispersion of the binder with a solid content of 15%. The aqueous dispersion of the binder is loaded onto the bottom plate of the testing instrument, and the excess sample is scraped off. The test parameters are set to temperature: 25±5°C, frequency: 1Hz, and shear stress range: 0.01Pa to 8Pa. Then start the test and start the amplitude scan, then the phase angle curve of the binder can be obtained, and finally analyze the curve to obtain the phase angle θ0 of the binder. First, the binder is dispersed in deionized water, and then the negative electrode active material (the same composition as in Example 1-1) is added. The mass ratio of the binder to the negative electrode active material is 7:3, and an aqueous dispersion of a mixture of the binder and the negative electrode active material with a solid content of 21.4wt% is obtained. Then, the mixture was stirred and dispersed at 1000 rpm for 1 hour to obtain an aqueous dispersion of the mixture of the binder and the negative electrode active material. The aqueous dispersion of the mixture of the binder and the negative electrode active material was then loaded onto the bottom plate of the testing instrument for testing. The testing steps were the same as the test operation process of the phase angle of the binder mentioned above, and the phase angle θ1 of the mixture of the binder and the negative electrode active material was obtained.

[0075] Composition test of copolymer:

[0076] The monomer composition of the copolymer after pyrolysis was determined by pyrolysis gas chromatography-mass spectrometry (py GC-MS), and the weight percentage of the monomer units in the copolymer was analyzed. The binder was dried in an 80°C oven for 8 hours and 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 ranged from 40°C to 320°C, and the pyrolysis temperature ranged from 40°C to 800°C.

[0077] Weight average molecular weight test:

[0078] Gel permeation chromatography (GPC) was performed using an Agilent 1260 HPLC-based gel permeation chromatography instrument equipped with a differential refractive index detector (RID) and a 79911GP-502 column to characterize the weight-average molecular weight (Mw) of the binder. The specific steps included dispersing 5 mg of the binder sample in 2 mL of deionized water (HPLC grade), maintaining a column temperature of 30°C and a mobile phase of HPLC-grade water at a flow rate of 1.0 mL / min. -1 .

[0079] Swelling and dissolution rate test:

[0080] Take a sample of the adhesive and weigh the mass (W0). Then, soak the sample in the electrolyte for 48 hours (h). Wipe off the solvent on the sample surface and weigh the mass of the swollen sample (W1). Each sample is weighed three times during the test. The degree of swelling of the adhesive (%) = (W1-W0) / W0×100%. Dry the swollen sample at 80°C for 8 hours. The mass of the dried sample is weighed as W2. The dissolution rate of the adhesive (%) = (W2-W0) / W0×100%.

[0081] The electrolyte used above is the same as that in Example 1-1.

[0082] Silicon mass percentage test:

[0083] Disassemble the lithium-ion battery and remove the negative electrode. Use conductive carbon glue to fix the negative electrode on the sample stage and place the sample stage on the bracket. Place the bracket and sample stage in the 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.

[0084] Binder mass percentage test:

[0085] Thermogravimetric analysis (TGA) is used to characterize the mass percentage of the binder in the negative electrode. The lithium-ion battery is disassembled, the negative electrode is removed, and after cleaning, a 5mm x 5mm sample is taken and placed on the sample pan of the TGA instrument, and the initial mass is recorded. The TGA heating program is set at a heating rate of 10°C / min, and the temperature range is increased from room temperature to an upper limit of 500°C. After the heating is completed, a thermal gravimetric curve of the binder is obtained. The thermal gravimetric curve is analyzed, and the mass loss between 300°C and 450°C represents the mass loss of the binder. The mass percentage of the binder is calculated.

[0086] Cyclic performance test:

[0087] The cycle performance of lithium-ion batteries is evaluated by their capacity retention rate. The higher the capacity retention rate, the better the cycle performance of the lithium-ion battery, and vice versa. The test process is as follows: place the lithium-ion battery in a 25±2℃ constant temperature box and let it stand for 30 minutes. Then charge the lithium-ion battery at a constant current of 0.5C to a charge cut-off voltage of 4.45V, then charge it at a constant voltage to a current of 0.05C, let it stand for 10 minutes, and then discharge it at 0.5C to a discharge cut-off voltage of 3.0V. This is one charge and discharge cycle. Record the first discharge capacity as C1, repeat the charge and discharge cycle 500 times and stop the test. Record the 500th discharge capacity as C2, then the capacity retention rate of the lithium-ion battery % = C2 / C1×100%.

[0088] Dynamic performance test:

[0089] The kinetic performance of secondary batteries is evaluated using direct current resistance (DCR). A larger DCR value indicates better kinetic performance, and vice versa. The lithium-ion battery is placed in a 10±2°C incubator for 2 hours to allow the battery to reach a constant temperature. The battery is then charged at a constant current of 0.5C to 4.2V, then at a constant current of 0.3C to 4.45V. At 4.45V, the battery is charged at a constant voltage of 0.02C and allowed to rest for 30 minutes. The battery is then discharged at a constant current of 0.1C to 3.4V and allowed to rest for 60 minutes, allowing the battery to reach a 50% state of charge (SOC). The voltage at this point is recorded as V1. The battery is then discharged at a constant current of I = 1C for 10 seconds, with the voltage change recorded at high speed 10 times per second. The voltage at the end of discharge is recorded as V2. The DC resistance (DCR) is then calculated as |V1 - V2| / I. This test is repeated three times, and the average value is calculated to obtain the DCR of the lithium-ion battery in mΩ.

[0090] The lithium-ion battery used as an example in this application has a charge cut-off voltage of 4.45 V and a discharge cut-off voltage of 3.0 V. The charge and discharge cut-off voltages of the lithium-ion battery can be based on the information printed on the battery's factory packaging.

[0091] Example 1-1

[0092] <Preparation of Adhesive>

[0093] (1) Deionized water, pH regulator lithium carbonate, and initiator ammonium persulfate were added to a three-necked flask and mixed uniformly to obtain a mixed solution E.

[0094] (2) The first monomer, acrylic acid, and the second monomer, isooctyl acrylate (2-EHA), are mixed evenly to obtain a monomer mixed solution 1. 40% by mass of the monomer mixed solution 1 is added to the mixed solution E, and the mixture is stirred evenly at room temperature. Then, a first reaction is carried out. The temperature T1 of the first reaction is 80° C., and the reaction time t1 is 4 h to obtain a mixed solution H.

[0095] (3) Add the crosslinking monomer pentaerythritol allyl ether (PEAE) and the functional monomer 3-aminopropyltriethoxysilane (APTES) to the remaining monomer mixture 1 and stir evenly to prepare monomer mixture 2. The monomer mixture 2 is added dropwise to the above mixture H for a time of t a The temperature of the mixture H is kept at T during the addition process. a After the addition is completed, the second reaction is carried out. The temperature T2 of the second reaction is 80°C and the time t2 is 1h. After the reaction is completed, a mixed solution K containing a binder is obtained.

[0096] (4) Adding a pH regulator, lithium carbonate, to the mixed solution K to adjust the pH to 7, filtering, and then vacuum treating the mixture to reduce the vacuum degree in the reactor to -70 kPa. Stirring was continued at room temperature at a dispersion speed of 250 rpm for 2 h to remove residual monomers, thereby obtaining an aqueous dispersion of the binder.

[0097] Wherein, based on the total mass of the monomers, the mass percentage of the first monomer is 80.6%, the mass percentage of the second monomer is 17%, the mass percentage of the cross-linking monomer is 0.4%, and the mass percentage of the functional monomer is 2%. The mass ratio of the total mass of the monomers to deionized water is 1:10, the mass ratio of the total mass of the monomers to the initiator is 1:0.06, and the mass ratio of the total mass of the monomers to the pH adjuster is 1:0.06. Based on the mass of the copolymer, the mass percentage A of the first monomer unit, the mass percentage B of the second monomer unit, the mass percentage C of the cross-linking monomer unit, and the mass percentage D of the functional monomer unit are shown in Table 2.

[0098] <Preparation of negative electrode sheet>

[0099] The aqueous dispersion of the binder prepared above was prepared into a binder slurry with a solid content of 15% using deionized water as a solvent.

[0100] Silicon carbon (Si:C = 50:50), graphite, and dispersant carboxymethyl cellulose were mixed, and the binder slurry was added first 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, the 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-sided negative electrode material layer. The coating weight of the negative electrode material layer was 142mg / 1540mm 2. Then repeat the above steps on the other surface of the copper foil to obtain a negative electrode pole piece coated with a negative electrode material layer on both sides. After drying at 120°C, cold pressing, cutting and welding the pole ears, a negative electrode pole piece with a specification of 78mm×875mm is obtained for standby use. Among them, the thickness of the single-sided negative electrode material layer is 54.5μm. Based on the mass of the negative electrode material layer, the mass percentage of silicon in the silicon-containing material is 30%, the mass percentage of the dispersant is 0.5%, the mass percentage of the binder is 3%, and the balance is graphite.

[0101] <Preparation of positive electrode sheet>

[0102] 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.

[0103] <Preparation of Electrolyte>

[0104] 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.

[0105] <Diaphragm>

[0106] A porous polyethylene film with a thickness of 7 μm (supplied by Celgard) was used as the separator.

[0107] <Preparation of lithium-ion batteries>

[0108] 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.

[0109] Example 1-2 to Example 1-23

[0110] Except for adjusting the relevant parameters according to Table 2, the rest is the same as Example 1-1.

[0111] Example 1-24 to Example 1-27

[0112] Except for adjusting the time and / or temperature of the first reaction according to Table 1 so that the binder-related parameters are as shown in Table 2, the rest is the same as Example 1-1.

[0113] Example 2-1 to Example 2-6

[0114] The same procedures as in Example 1-1 were followed except that the binder weight percentage was adjusted according to Table 3. When the binder weight percentage was changed, the silicon-containing material and dispersant weight percentages remained unchanged, and the graphite weight percentage was changed accordingly.

[0115] Example 2-7 to Example 2-8

[0116] The same procedures as in Example 1-1 were followed, except that the mass percentage of the silicon-containing material was adjusted so that the mass percentage of silicon was as shown in Table 3. When the mass percentage of the silicon-containing material was changed, the mass percentages of the binder and dispersant remained unchanged, and the mass percentage of the graphite changed accordingly.

[0117] Comparative Examples 1 to 5

[0118] Except for adjusting the relevant parameters according to Table 2, the rest is the same as Example 1-1.

[0119] Comparative Example 6 to Comparative Example 7

[0120] Except for adjusting the time and temperature of the first reaction according to Table 1 so that the binder-related parameters are as shown in Table 2, the rest is the same as Example 1-1.

[0121] The relevant parameters and performance tests of each embodiment and each comparative example are shown in Tables 1 to 3.

[0122] Table 1

[0123] First reaction temperature T1 (℃) First reaction time t1(h) Example 1-1 80 4 Examples 1-24 70 4 Examples 1-25 80 5 Examples 1-26 80 3 Examples 1-27 90 4 Comparative Example 6 95 4 Comparative Example 7 65 4

[0124] Table 2

[0125]

[0126]

[0127] Note: “ / ” in Table 2 indicates no relevant parameters or substances.

[0128] From Examples 1-1 to 1-27 and Comparative Examples 1 to 7, it can be seen that selecting a copolymer formed by polymerization of a first monomer, a second monomer, a cross-linking monomer, and a functional monomer within the scope of this application as a binder, and regulating the phase angle enhancement rate α% of the binder within the scope of this application, facilitates uniform adhesion of the binder to the surface of the negative electrode active material particles. The binder of this application is applied to the negative electrode plate, which can enable the lithium-ion battery to have a high capacity retention rate and a low DCR, indicating that the lithium-ion battery has good cycle performance and kinetic performance. In the binders of Comparative Examples 1 to 5, the monomers forming the copolymer do not include at least one of the first monomer, the second monomer, the cross-linking monomer, and the functional monomer, and the phase angle enhancement rate α% is not within the scope of this application. The phase angle enhancement rate α% of the binders of Comparative Examples 6 and 7 is not within the scope of this application, and the lithium-ion battery has a low capacity retention rate and a high DCR, indicating that the cycle performance and kinetic performance of the lithium-ion battery are poor.

[0129] The phase angle increase rate α% of the binder usually affects the cycle performance and kinetic performance of the lithium-ion battery. From Examples 1-1 to 1-27 and Comparative Examples 6 to 7, it can be seen that when the value of α is too large, such as in Comparative Example 6, and when the value of α is too small, such as in Comparative Example 7, the lithium-ion battery has a lower capacity retention rate and a higher DCR, indicating that the cycle performance and kinetic performance of the lithium-ion battery are poor. Therefore, by regulating the value of α within the scope of this application, the lithium-ion battery can have a higher capacity retention rate and a lower DCR, indicating that the lithium-ion battery has good cycle performance and kinetic performance.

[0130] The phase angle θ0° of the binder generally affects the cycling and kinetic performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-27, regulating the value of θ0 within the range of this application can result in lithium-ion batteries with higher capacity retention and lower DCR, indicating that the lithium-ion batteries have good cycling and kinetic performance.

[0131] The mass percentage A of the first monomer unit, the mass percentage B of the second monomer unit, the mass percentage C of the cross-linking monomer unit, and the mass percentage D of the functional monomer unit generally affect the cycling performance and kinetic performance of the lithium-ion battery. As can be seen from Examples 1-1 to 1-17, by adjusting the values of A, B, C, and D within the range of this application, the lithium-ion battery can have a high capacity retention rate and a low DCR, indicating that the lithium-ion battery has good cycling performance and kinetic performance.

[0132] The types of first monomer, second monomer, cross-linking monomer, and functional monomer generally affect the cycling and kinetic performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-18, and 1-23, selecting first monomer, second monomer, cross-linking monomer, and functional monomer within the scope of this application can result in lithium-ion batteries with high capacity retention and low DCR, indicating that the lithium-ion batteries have good cycling and kinetic performance.

[0133] The number of carbon atoms in the second monomer typically affects the cycling and kinetic performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-19, and 1-21, regulating the number of carbon atoms in the second monomer within the scope of this application can result in lithium-ion batteries with higher capacity retention and lower DCR, indicating that the lithium-ion batteries have good cycling and kinetic performance.

[0134] The mass ratio X of the functional monomer unit to the second monomer unit generally affects the cycling and kinetic performance of a lithium-ion battery. As can be seen from Examples 1-1 to 1-7 and 1-14 to 1-17, regulating the value of X within the range of this application can result in a lithium-ion battery with a high capacity retention rate and a low DCR, indicating that the lithium-ion battery has good cycling and kinetic performance.

[0135] The weight-average molecular weight of the copolymer, the swelling degree of the binder in the electrolyte, and the dissolution rate generally affect the cycling and kinetic performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-27, regulating these parameters within the range of this application can result in lithium-ion batteries with high capacity retention and low DCR, indicating that the lithium-ion batteries have good cycling and kinetic performance.

[0136] Figure 1 A graph showing changes in the phase angle of the binder of Example 1-1 and the phase angle of the mixture of the binder and the negative electrode active material with shear stress is shown. Figure 1In Example 1-1 of the present application, the phase angle θ0° of the binder is 15.0° (the phase angle under shear stress of 0.1 Pa, the same below), the phase angle θ1° of the mixture of the binder and the negative electrode active material is 22.5°, and the phase angle improvement rate α% of the binder is 50%. Figure 2 A graph showing the phase angle of the binder of Comparative Example 1 and the phase angle of the mixture of the binder and the negative electrode active material as a function of shear stress is shown, Figure 2 In the comparative example 5, the phase angle θ0° of the binder is 5.3°, the phase angle θ1° of the mixture of the binder and the negative electrode active material is 5.8°, and the phase angle improvement rate α% of the binder is 11.5%. By comparison, it can be seen that the binder of Example 1-1 of the present application has a higher phase angle improvement rate.

[0137] Table 3

[0138]

[0139] Note: “ / ” in Table 3 indicates no relevant parameters or substances.

[0140] The weight percentage of the binder generally affects the cycling and kinetic performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1, and 2-6, regulating the weight percentage of the binder within the range of this application can result in lithium-ion batteries with higher capacity retention and lower DCR, indicating that the lithium-ion batteries have good cycling and kinetic performance.

[0141] The mass percentage of silicon typically affects the cycling and kinetic performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-7, and 2-8, regulating the mass percentage of silicon within the range of this application can result in lithium-ion batteries with higher capacity retention and lower DCR, indicating that the lithium-ion batteries have good cycling and kinetic performance.

[0142] 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.

[0143] 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.

[0144] 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 for a negative electrode sheet, wherein: The binder comprises a copolymer, and the monomers forming the copolymer include a first monomer, a second monomer, a cross-linking monomer and a functional monomer, wherein the first monomer comprises at least one of acrylic acid or methacrylic acid, and the second monomer is selected from C7 to C 19 An acrylate compound, wherein the functional monomer includes at least one of allyltriethoxysilane, N-propyltriethoxysilane or 3-aminopropyltriethoxysilane; The phase angle improvement rate of the adhesive is α%, 25≤α≤75; Under the conditions of an ambient temperature of 25±5°C, a shear stress of 0.1 Pa, and an applied frequency of the shear stress of 1 Hz, the phase angle of the binder is θ0°, the phase angle of the mixture of the binder and the negative electrode active material is θ1°, the mass ratio of the binder to the negative electrode active material in the mixture is 7:3, and the 2. The adhesive according to claim 1, wherein 40≤α≤60。 3. The adhesive according to claim 1, wherein 12≤θ0≤18.

4. The adhesive according to claim 1, wherein The crosslinking monomer includes at least one of pentaerythritol allyl ether, trimethylolpropane triacrylate, divinylbenzene, ethoxylated trimethylolpropane triacrylate or pentaerythritol triallyl ether.

5. The adhesive according to claim 1, wherein The first monomer, the second monomer, the cross-linking monomer and the functional monomer constitute the first monomer unit, the second monomer unit, the cross-linking monomer unit and the functional monomer unit of the copolymer respectively. Based on the mass of the copolymer, the mass percentage of the first monomer unit is 67.6% to 87.6%, the mass percentage of the second monomer unit is 10% to 30%, the mass percentage of the cross-linking monomer unit is 0.2% to 1%, and the mass percentage of the functional monomer unit is 1% to 3%. The adhesive according to claim 5 , wherein: Based on the mass of the copolymer, the mass percentage of the second monomer unit is 15% to 20%, and the mass percentage of the cross-linking monomer unit is 0.3% to 0.5%; and / or, The mass ratio X of the functional monomer unit to the second monomer unit is 0.06 to 0.

2.

7. The adhesive according to claim 1, wherein The second monomer is selected from C 10 to C 12 of acrylate compounds.

8. The adhesive according to claim 1, wherein The second monomer includes at least one of butyl acrylate, isooctyl acrylate, butyl methacrylate, lauryl methacrylate, hexadecyl acrylate, or lauryl acrylate.

9. The adhesive according to any one of claims 1 to 8, wherein The weight average molecular weight Mw of the copolymer is 50,000 g / mol to 100,000 g / mol.

10. The adhesive according to any one of claims 1 to 8, wherein The swelling degree of the binder in the electrolyte is 10% to 30%, and / or the dissolution rate of the binder in the electrolyte is 2% to 3%.

11. A secondary battery comprising a positive electrode sheet 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 the binder according to any one of claims 1 to 10.

12. The secondary battery according to claim 11, wherein The negative electrode material layer includes a negative electrode active material, which includes graphite and a silicon-containing material. Based on the mass of the negative electrode material layer, the mass percentage of silicon element is 1.5% to 30%.

13. The secondary battery according to claim 11, wherein Based on the mass of the negative electrode material layer, the mass percentage of the binder is 1% to 10%.

14. The secondary battery according to claim 11, wherein Based on the mass of the negative electrode material layer, the mass percentage of the binder is 2% to 5%. 15 . An electronic device comprising the secondary battery according to claim 11 .

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