Hydroxyl modified binder, preparation method thereof and secondary battery

Abstract

The invention provides a hydroxyl modified binder, a preparation method thereof and a secondary battery. The hydroxyl modified binder is a copolymer, monomers forming the copolymer comprise a first monomer and a second monomer, the first monomer comprises at least one of acrylic acid, acrylamide or acrylonitrile, and the second monomer comprises a compound as shown in a formula I; the first monomer and the second monomer respectively form a first monomer unit and a second monomer unit of the copolymer, based on the mass of the copolymer, the mass percentage content of the first monomer unit is A%, A is more than or equal to 5 and less than or equal to 60, the mass percentage content of the second monomer unit is B%, and B is more than or equal to 40 and less than or equal to 95. The hydroxyl modified binder has a good self-repairing characteristic, and the hydroxyl modified binder is applied to a negative pole piece of a secondary battery, so that the cyclic expansion performance and low-voltage storage safety performance of the secondary battery are improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technologies, and particularly to a hydroxyl-modified binder, a preparation method thereof, and a secondary battery. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, have been widely used in various small devices such as mobile phones, digital cameras, laptop computers, and drones due to advantages such as high energy density, high working voltage, and environmental friendliness, and occupy a dominant position in the field of power sources for current portable electronic devices.

[0003] High-capacity silicon anodes are the most promising new generation anode materials for lithium-ion batteries that can partially replace commercial graphite. However, during the lithium intercalation process, a huge volume expansion occurs, resulting in problems such as the rupture of the solid electrolyte interface and electrode pulverization, leading to a rapid capacity decay. The large volume expansion of silicon anodes limits their development. Binders with self-healing properties can regenerate their original structure and can cope with the continuous expansion and contraction of the silicon anode system during cycling, playing a good role in improving cyclic expansion. Binders such as polyvinyl alcohol (PVA), which have a large number of hydroxyl groups, thus have rich hydrogen bonds and therefore have good self-repair properties. However, the problem with PVA-type binders is that their hydroxyl groups have high activity, resulting in the generation of gas during the cycling of secondary batteries and during low-voltage storage, leading to bloating of secondary batteries and having a high failure risk in the market. Summary of the Invention

[0004] The purpose of the present application is to provide a hydroxyl-modified binder, a preparation method thereof, and a secondary battery, so as to balance the cyclic expansion performance and low-voltage storage safety performance of the secondary battery. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides a hydroxyl-modified binder. The hydroxyl-modified binder is a copolymer, and the monomers forming the copolymer include a first monomer and a second monomer. The first monomer includes at least one of acrylic acid, acrylamide, or acrylonitrile, and the second monomer includes a compound of formula I:

[0006]

[0007] Wherein, X is selected from C, an ester bond, an ether bond, a methyleneoxy group or an amide bond, R1 is selected from H or methyl, R2, R3 and R4 are each independently selected from H, methyl, hydroxy or hydroxymethyl, at least one of R2, R3 or R4 is selected from hydroxy or hydroxymethyl, and n is 0, 1 or 2; the first monomer and the second monomer respectively form the first monomer unit and the second monomer unit of the copolymer. Based on the mass of the copolymer, the mass percentage content of the first monomer unit is A%, 5 ≤ A ≤ 60, and the mass percentage content of the second monomer unit is B%, 40 ≤ B ≤ 95. The above-mentioned hydroxy-modified binder contains a hydrogen bond structure and has good self-healing properties, and can regenerate its original structure. When the above-mentioned hydroxy-modified binder is applied to the negative electrode sheet of a secondary battery, it can not only improve the continuous expansion and contraction of the silicon negative electrode system during cycling, but also improve the problem of gas swelling that occurs during cycling and low-voltage storage of the secondary battery, thereby simultaneously improving the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0008] In some embodiments of the present application, the second monomer includes at least one of β-hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 4-penten-1-ol, pentaerythritol allyl ether or N-(2-hydroxyethyl) acrylamide. The second monomer within the above range can provide hydroxyl groups with suitable activity for the hydroxy-modified binder, so that the hydroxy-modified binder has good self-healing performance, which can not only improve the continuous expansion and contraction of the silicon negative electrode system during cycling, but also improve the problem of gas swelling that occurs during cycling and low-voltage storage of the secondary battery, and further improve the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0009] In some embodiments of the present application, 5 ≤ A ≤ 40, 60 ≤ B ≤ 95. Adjusting the values of A and B within the above range is beneficial to further improve the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0010] In some embodiments of the present application, at 25 °C, with a stirring speed of 12 rpm and a stirring time of 2 min, when the mass percentage content of the aqueous dispersion of the hydroxy-modified binder is 5.5% to 6.5%, the viscosity range of the aqueous dispersion of the hydroxy-modified binder is 3000 mPa·s to 100000 mPa·s. Adjusting the viscosity range of the aqueous dispersion of the hydroxy-modified binder within the above range is beneficial to the full extension of the molecular chain and better combination with the negative electrode active material particles, thereby improving the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0011] In some embodiments of the present application, the infrared spectrum of the hydroxy-modified binder is at 3000 cm -1 to 3600 cm -1There is a stretching vibration peak of the hydroxyl group. At 100 °C, the peak intensity of this stretching vibration peak is I1, and at 30 °C, the peak intensity of this stretching vibration peak is I2, where I OH = I1 / I2, and I OH ≥ 1.1. When the value of I OH is within the above range, it indicates that the content of the hydroxyl group in the above-mentioned hydroxyl-modified binder changes with temperature, showing that the modified binder has good self-healing function, which is beneficial to improving the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0012] In some embodiments of the present application, 1.15 ≤ I OH ≤ 2. When the value of I OH is within the above range, it is beneficial to further improve the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0013] In some embodiments of the present application, using cyclic voltammetry, the weakest current signal value of hydrogen during the charge-discharge process of the hydroxyl-modified binder cycling at a voltage of 0 V to 3.3 V is Q1 A, and the strongest current signal value is Q2 A, where Q v = Q1 / Q2, and 0.8 ≤ Q v ≤ 0.9. When the value of Q v is within the above range, it indicates that less gas is generated during the cycling process and at low voltage of the secondary battery, showing that the secondary battery has good cycle expansion performance and low-voltage storage safety performance.

[0014] In some embodiments of the present application, the hydroxyl-modified binder satisfies at least one of the following characteristics: (1) The Young's modulus of the film of the hydroxyl-modified binder is 1.2 GPa to 7 GPa; (2) The elongation at break of the film of the hydroxyl-modified binder is 12% to 85%. The hydroxyl-modified binder satisfying at least one of the above characteristics indicates that the hydroxyl-modified binder has good toughness. Using it for the negative electrode sheet can improve the problem of interfacial pulverization during the cycling of the negative electrode sheet, thereby improving the cycle expansion performance of the secondary battery.

[0015] The second aspect of the present application provides a preparation method of the hydroxyl-modified binder provided in the first aspect of the present application, which includes the following steps:

[0016] (1) Dissolve the second monomer in a solvent and stir under the protection of an inert atmosphere to obtain a deoxygenated solution.

[0017] (2) Add a radical initiator to the deoxygenated solution, and then add the first monomer under the protection of an inert atmosphere to carry out a copolymerization reaction to obtain a mixture containing the hydroxyl-modified binder.

[0018] (3) Stir the mixture under negative pressure to remove the monomers and separate to obtain the hydroxyl-modified binder.

[0019] Among them, the solvent includes at least one of deionized water, ethanol or acetone. The mass ratio of the second monomer to the solvent is 1:(1000 to 5000), and the mass ratio of the second monomer to the radical initiator is 1:(0.1 to 2); based on the total mass of the first monomer and the second monomer, the mass percentage of the first monomer is 5% to 60%, and the mass percentage of the second monomer is 40% to 90%; the temperature of the copolymerization reaction is 50°C to 80°C, and the time of the copolymerization reaction is 2 h to 8 h; the pressure of the negative pressure is -0.1 MPa to -0.07 MPa, and the time for removing the monomer is 1 h to 5 h.

[0020] The hydroxyl-modified binder prepared by the above preparation method has good self-healing properties and can regenerate its original structure. Applying the above hydroxyl-modified binder to the negative electrode plate of a secondary battery can not only improve the continuous expansion and contraction of the silicon negative electrode system during the cycling process, but also improve the problem of gas swelling that occurs during the cycling process and low-voltage storage of the secondary battery, thereby improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0021] The third aspect of the present application provides a secondary battery, which includes a negative electrode plate. The negative electrode plate includes a negative current collector and a negative electrode material layer provided on at least one surface of the negative current collector. The negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-containing material. The negative electrode material layer further includes the hydroxyl-modified binder provided by the first aspect of the present application or the hydroxyl-modified binder prepared by the preparation method provided by the second aspect of the present application. The negative electrode material layer includes the above-mentioned hydroxyl-modified binder. This is not only beneficial to improving the continuous expansion and contraction of the silicon negative electrode system during the cycling process, but also can improve the problem of gas swelling that occurs during the cycling process and low-voltage storage of the secondary battery, thereby simultaneously improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0022] In some embodiments of the present application, the silicon-containing material includes at least one of silicon, silicon-carbon or silicon-oxygen. Based on the mass of the negative electrode material layer, the mass percentage of silicon element is M%, and 0.5 ≤ M ≤ 60. Selecting the silicon-containing material within the above range and regulating the mass percentage of silicon element within the above range can enable the secondary battery to have a high energy density. At the same time, it is beneficial to better exert the self-healing effect of the hydroxyl-modified binder in the silicon negative electrode system, thereby improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0023] In some embodiments of the present application, 5 ≤ M ≤ 40. Regulating the value of M within the above range is beneficial to further improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0024] In some embodiments of the present application, based on the mass of the negative electrode material layer, the mass percentage content of the hydroxyl-modified binder is N%, where 1.2 ≤ N ≤ 11.5. By adjusting the mass percentage content of the hydroxyl-modified binder within the above range, good adhesion can be achieved between the negative electrode active material particles, and at the same time, the self-healing effect of the hydroxyl-modified binder in the silicon negative electrode system can be exerted. In addition, the negative electrode material layer can have a high specific capacity, so that the secondary battery has a high energy density, good cyclic expansion performance, and low-voltage storage safety performance.

[0025] In some embodiments of the present application, 1.2 ≤ N ≤ 8. By adjusting the value of N within the above range, it is more beneficial to improve the cyclic expansion performance and low-voltage storage safety performance of the secondary battery.

[0026] In some embodiments of the present application, based on the mass of the negative electrode material layer, the mass percentage content of the hydroxyl-modified binder is N%, where 0.12 ≤ N / M ≤ 3.2. By adjusting the value of N / M within the above range, the synergistic effect between the hydroxyl-modified binder and the silicon element in the negative electrode material layer is good, which is beneficial to better exert the self-healing effect of the hydroxyl-modified binder in the silicon negative electrode system, so that the secondary battery has a high energy density, better cyclic expansion performance, and low-voltage storage safety performance.

[0027] In some embodiments of the present application, 0.24 ≤ N / M ≤ 1.6. By adjusting the value of N / M within the above range, the cyclic expansion performance and low-voltage storage safety performance of the secondary battery can be further improved.

[0028] Advantages of the present application:

[0029] The present application provides a hydroxyl-modified binder, a preparation method thereof, and a secondary battery. The hydroxyl-modified binder is a copolymer, and the monomers forming the copolymer include a first monomer and a second monomer. The first monomer includes at least one of acrylic acid, acrylamide, or acrylonitrile, and the second monomer includes a compound of formula I; the first monomer and the second monomer respectively constitute the first monomer unit and the second monomer unit of the copolymer. Based on the mass of the copolymer, the mass percentage content of the first monomer unit is A%, where 5 ≤ A ≤ 60, and the mass percentage content of the second monomer unit is B%, where 40 ≤ B ≤ 95. The above-mentioned hydroxyl-modified binder contains a hydrogen bond structure and has good self-healing characteristics, and can regenerate its original structure. Applying the above-mentioned hydroxyl-modified binder to the negative electrode sheet of the secondary battery can not only improve the continuous expansion and contraction of the silicon negative electrode system during the cycle, but also improve the problem of gas swelling during the cycle and low-voltage storage of the secondary battery, thereby improving the cyclic expansion performance and low-voltage storage safety performance of the secondary battery.

[0030] Of course, it is not necessary for any product or method implementing the present application to achieve all the above-mentioned advantages simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] To more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and those of ordinary skill in the art can also obtain other embodiments according to these drawings.

[0032] Figure 1 Infrared spectra of the hydroxyl-modified binder of Example 1-1 at 30 °C and 100 °C;

[0033] Figure 2 Infrared spectra of the binder of Comparative Example 2 at 30 °C and 100 °C;

[0034] Figure 3 Differential electrochemical mass spectrometry of the hydroxyl-modified binder of Example 1-2 measured by cyclic voltammetry;

[0035] Figure 4 Differential electrochemical mass spectrometry of the binder of Comparative Example 2 measured by cyclic voltammetry. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The following will clearly and completely describe the technical solutions in the present application in combination with the embodiments and drawings of the present application. Obviously, the described embodiments are only some embodiments of the present application, rather than all embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0037] It should be noted that in the specific embodiments of the present application, lithium-ion batteries are used as examples of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0038] The first aspect of the present application provides a hydroxyl-modified binder. The hydroxyl-modified binder is a copolymer, and the monomers forming the copolymer include a first monomer and a second monomer. The first monomer includes at least one of acrylic acid, acrylamide, or acrylonitrile, and the second monomer includes a compound of Formula I:

[0039]

[0040] wherein X is selected from C, an ester bond an ether bond (*-O-*), a methyleneoxy group or an amide bond R1 is selected from H or methyl, R2, R3 and R4 are each independently selected from H, methyl, hydroxy or hydroxymethyl, at least one of R2, R3 or R4 is selected from hydroxy or hydroxymethyl, and n is 0, 1 or 2; the first monomer and the second monomer respectively form the first monomer unit and the second monomer unit of the copolymer. Based on the mass of the copolymer, the mass percentage content of the first monomer unit is A%, 5 ≤ A ≤ 60, preferably 5 ≤ A ≤ 40, and the mass percentage content of the second monomer unit is B%, 40 ≤ B ≤ 95, preferably 60 ≤ B ≤ 95. For example, the value of A can be 5, 10, 14.2, 17, 20, 23, 26, 30, 33, 35, 37, 40, 48, 50, 55, 60 or a range composed of any two of these values, and the value of B can be 40, 48, 50, 55, 60, 62, 69, 70, 75, 80, 85, 90, 95 or a range composed of any two of these values.

[0041] The inventors found that when n in the molecular structure (Formula I) of the second monomer is too large, for example, the second monomer is 5 - hydroxypentyl acrylate n = 3, too many methylene groups in the main chain of its molecular structure will dilute the hydroxyl groups in the copolymer molecular chain, and the self - repair function of the hydroxyl - modified binder cannot be exerted. The silicon negative electrode system continuously expands during the cycling process, affecting the cycling expansion performance of the secondary battery. When the main chain in the molecular structure of the second monomer is too short and does not satisfy Formula I, for example, the second monomer is methyl 2 - (hydroxymethyl) acrylate It will cause the hydroxyl group concentration in the copolymer molecular chain to be too high, resulting in the agglomeration of the hydroxyl group-modified binder on the surface of the anode active material particles, unable to be discretely distributed, weakening the bonding strength of the binder to the surface of the anode active material particles. Moreover, when the hydroxyl group concentration in the copolymer molecular chain is too high, it will exacerbate the problem of gas swelling during the cycling process and low-voltage storage of the secondary battery, thus being unfavorable for improving the low-voltage storage safety performance of the secondary battery. Therefore, by selecting the first monomer and the second monomer within the above range to polymerize to form a copolymer, the first monomer unit in the polymer can provide hydrophilicity, improve the dispersion of the hydroxyl group-modified binder particles in the binder slurry, and is conducive to improving the discrete distribution and film formation of the hydroxyl group-modified binder on the surface of the anode active material particles; the second monomer unit provides hydroxyl groups, so that the above-mentioned hydroxyl group-modified binder contains a hydrogen bond structure and has self-healing properties, and can regenerate its original structure. Applying the above-mentioned hydroxyl group-modified binder to the anode electrode sheet of the secondary battery can not only improve the continuous expansion and contraction of the silicon anode system during the cycling process, but also improve the problem of gas swelling during the cycling process and low-voltage storage of the secondary battery, thus taking into account the cycling expansion performance and low-voltage storage safety performance of the secondary battery. At the same time, the above-mentioned first monomer unit can provide the copolymer with dispersion ability, and the second monomer unit can provide the copolymer with self-healing effect. When the value of A is too small, for example, less than 5, there are fewer functional groups providing dispersion ability in the copolymer, which is not conducive to improving the uniformity of the anode material in the anode electrode sheet; when the value of A is too large, for example, greater than 60, the copolymer has strong rigidity but large brittleness and insufficient toughness, resulting in insufficient binder life and easy pulverization during the cycling expansion process, and the bonding interface is prone to breakage. When the value of B is too small, for example, less than 40, the hydroxyl group content in the copolymer is too low, which is not conducive to exerting the self-healing effect of the hydroxyl group-modified binder, thus being not conducive to improving the cycling expansion performance of the secondary battery; when the value of B is too large, for example, greater than 95, the hydroxyl group content in the copolymer is too high, and gas is easily generated during the cycling process or low-voltage storage of the secondary battery, which will cause the gas swelling to intensify during the cycling process and low-voltage storage of the secondary battery, affecting the cycling expansion performance and low-voltage storage safety performance of the secondary battery. Therefore, regulating the values of A and B within the scope of this application is conducive to improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0042] In some embodiments of the present application, the second monomer includes at least one of β-hydroxypropyl acrylate (β-HPA), hydroxypropyl methacrylate, hydroxyethyl acrylate (HEA), 2-hydroxyethyl methacrylate (2HEMA), 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 4-penten-1-ol, pentaerythritol allyl ether, or N-(2-hydroxyethyl) acrylamide (HEAA). The structural formulas of the above substances are shown as follows:

[0043]

[0044] The second monomer within the above range contains polymerizable carbon-carbon double bonds and hydroxyl groups, has low chemical activity and high oxidation resistance characteristics. Selecting the second monomer within the above range can provide hydroxyl groups with appropriate activity for the hydroxyl-modified binder, enabling the hydroxyl-modified binder to have good self-healing performance, thereby not only improving the continuous expansion and contraction of the silicon negative electrode system during cycling, but also improving the problem of gas swelling during cycling and low-voltage storage of the secondary battery, and further improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0045] In some embodiments of the present application, at 25 °C, a stirring speed of 12 rpm, and a stirring time of 2 min, the hydroxyl-modified binder is dispersed in water to obtain an aqueous dispersion of the hydroxyl-modified binder with a mass percentage content of 5.5% to 6.5%. The viscosity range of the aqueous dispersion of the hydroxyl-modified binder is 3000 mPa·s to 100000 mPa·s. For example, the viscosity range of the aqueous dispersion of the hydroxyl-modified binder can be 3000 mPa·s, 5000 mPa·s, 8000 mPa·s, 10000 mPa·s, 50000 mPa·s, 75000 mPa·s, 100000 mPa·s, or a range composed of any two of these values. By controlling the viscosity range of the aqueous dispersion of the hydroxyl-modified binder within the above range, the particles of the hydroxyl-modified binder can have good dispersibility in its slurry. Applying the hydroxyl-modified binder with the above characteristics to the silicon-containing negative electrode material layer, the hydroxyl-modified binder has good dispersibility on the surface of the negative electrode active material particles, which is more conducive to exerting its self-healing effect, thereby improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0046] In some embodiments of the present application, the infrared spectrum of the hydroxyl-modified binder has a stretching vibration peak of hydroxyl groups at 3000 cm -1 to 3600 cm -1 The peak intensity of this stretching vibration peak is I1 at 100 °C and I2 at 30 °C. I OH = I1 / I2, and I OH ≥ 1.1. In some embodiments of the present application, 1.15 ≤ I OH ≤ 2. For example, the value of I OH can be 1.1, 1.15, 1.2, 1.3, 1.35, 1.4, 1.55, 1.68, 1.74, 1.85, 1.91, 2, or a range composed of any two of these values. The intensity of the stretching vibration peak of hydroxyl groups in the infrared spectrum of the above hydroxyl-modified binder at 3000 cm -1 to 3600 cm -1 can reflect the content of hydroxyl groups in the hydroxyl-modified binder. I OHThe value represents the change intensity of the hydroxyl peak with temperature, I OH The larger the value, the more hydroxyl groups that can provide repairable hydrogen bonds, indicating that the self-healing function of the hydroxyl-modified binder is stronger. Conversely, it indicates that its self-healing function is weaker. I OH When the value is within the above range, it indicates that the content of hydroxyl groups in the above-mentioned hydroxyl-modified binder changes with temperature, indicating that the modified binder has a good self-healing function, which is conducive to improving the cycle expansion performance and low-voltage storage safety performance of secondary batteries.

[0047] In some embodiments of the present application, using cyclic voltammetry, the weakest current signal value of hydrogen gas during the charge and discharge process of the hydroxyl-modified binder cycling at a voltage from 0 V to 3.3 V is Q1 A, and the strongest current signal value is Q2 A, Q v = Q1 / Q2, 0.8 ≤ Q v ≤ 0.9. For example, Q v The value can be 0.8, 0.82, 0.85, 0.86, 0.88, 0.9 or a range composed of any two of these values. Q v When the value is within the above range, it indicates that less gas is generated during the cycling process and at low voltage of the secondary battery, indicating that the secondary battery has good cycle expansion performance and low-voltage storage safety performance.

[0048] In the present application, the hydrogen current signal value of the above-mentioned hydroxyl-modified binder is measured by cyclic voltammetry. Specifically, the following steps can be used for testing: The hydroxyl-modified binder is coated on a copper foil to prepare a working electrode, and the coating thickness is 10 μm to 20 μm. A lithium sheet is used as the counter electrode, and a separator is set between the working electrode and the counter electrode. After injecting the electrolyte, a button cell is assembled. The above button cell is subjected to charge and discharge tests from 0 V to 3.3 V according to cyclic voltammetry. At the same time, differential electrochemical mass spectrometry (DEMS) is used to collect the gas generated during the charge and discharge process of cycling at a voltage from 0 V to 3.3 V and convert it into the current signal of the gas. Record that during the charge and discharge process of cycling at a voltage from 0 V to 3.3 V, the weakest current signal value of hydrogen gas is Q1 A, and the strongest current signal value is Q2 A. Q v = Q1 / Q2, Q v is used to represent the gas generation situation of the hydroxyl-modified binder during the charge and discharge process. Q v The larger Q is, the less gas is generated. Q v The smaller Q is, the more gas is generated.

[0049] In some embodiments of the present application, the Young's modulus of the film of the hydroxyl-modified binder is 1.2 GPa to 7 GPa. For example, the Young's modulus of the film of the hydroxyl-modified binder can be 1.2 GPa, 2 GPa, 3 GPa, 4 GPa, 5 GPa, 6 GPa, 7 GPa, or a range composed of any two of these values. The size of the film of the hydroxyl-modified binder can be 0.8 cm in width × 1 cm in length × 0.01 cm in thickness. The Young's modulus of the film of the hydroxyl-modified binder within the above range indicates that the hydroxyl-modified binder has good toughness, which is beneficial to the exertion of its self-healing property. When used in the negative electrode sheet, it can improve the problem of interfacial pulverization during the cycling of the negative electrode sheet, thereby improving the cycling expansion performance of the secondary battery.

[0050] In some embodiments of the present application, the elongation at break of the film of the hydroxyl-modified binder is 12% to 85%. For example, the elongation at break of the film of the hydroxyl-modified binder can be 12%, 14%, 16%, 20%, 30%, 40%, 50%, 60%, 75%, 85%, or a range composed of any two of these values. The size of the film of the hydroxyl-modified binder can be 1 cm in width × 2 cm in length × 0.01 cm in thickness. The elongation at break of the film of the hydroxyl-modified binder within the above range indicates that the hydroxyl-modified binder has good toughness, which is beneficial to the exertion of its self-healing property. When used in the negative electrode sheet, it can improve the problem of interfacial pulverization during the cycling of the negative electrode sheet, thereby improving the cycling expansion performance of the secondary battery.

[0051] In some embodiments of the present application, the weight-average molecular weight Mw of the hydroxyl-modified binder is 100,000 g / mol to 750,000 g / mol. For example, Mw can be 100,000 g / mol, 200,000 g / mol, 300,000 g / mol, 400,000 g / mol, 500,000 g / mol, 600,000 g / mol, 700,000 g / mol, 750,000 g / mol, or a range composed of any two of these values. Controlling Mw within the above range is beneficial to improving the tolerance of the binder to the electrolyte, while meeting good binding and dispersion properties; in addition, it can also provide good cohesion for the negative electrode material layer and improve the adhesion between the negative electrode material layer and the negative electrode current collector, reducing the risk of film peeling of the negative electrode material layer, thereby being beneficial to improving the cycling expansion performance of the secondary battery.

[0052] The second aspect of the present application provides a preparation method of the hydroxyl-modified binder provided in the first aspect of the present application, which includes the following steps:

[0053] (1) Dissolve the second monomer in a solvent and stir under the protection of an inert atmosphere to obtain a deoxygenated solution.

[0054] (2) Add a radical initiator to the deoxygenated solution, and then add the first monomer under the protection of an inert atmosphere to carry out a copolymerization reaction to obtain a mixed solution containing a hydroxyl-modified binder.

[0055] (3) Stir the mixed solution under negative pressure to remove the monomers and separate to obtain the hydroxyl-modified binder.

[0056] Among them, the solvent includes at least one of deionized water, ethanol, or acetone. The mass ratio of the second monomer to the solvent is 1:(1000 to 5000). For example, the mass ratio of the second monomer to the solvent can be 1:1000, 1:2000, 1:2500, 1:3000, 1:4000, 1:5000, or a range composed of any two of these values. The mass ratio of the second monomer to the radical initiator is 1:(0.1 to 2). For example, the mass ratio of the second monomer to the radical initiator can be 1:0.1, 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, or a range composed of any two of these values. Based on the total mass of the first monomer and the second monomer, the mass percentage of the first monomer is 5% to 60%, and the mass percentage of the second monomer is 40% to 90%. For example, the mass percentage of the first monomer can be 5%, 8%, 10%, 14.2%, 17%, 20%, 23%, 26%, 30%, 33%, 35%, 37%, 40%, 48%, 50%, 55%, 60%, or a range composed of any two of these values. The mass percentage of the second monomer can be 40%, 48%, 50%, 55%, 60%, 62%, 69%, 70%, 75%, 78%, 80%, 85%, 88%, 90%, or a range composed of any two of these values. The temperature of the copolymerization reaction is 50°C to 80°C, and the time of the copolymerization reaction is 2 hours (h) to 8 h. For example, the temperature of the copolymerization reaction can be 50°C, 60°C, 65°C, 70°C, 80°C, or a range composed of any two of these values. The time of the copolymerization reaction can be 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, or a range composed of any two of these values. The pressure of the negative pressure is -0.1 MPa to -0.07 MPa, and the time for removing the monomers is 1 h to 5 h. For example, the pressure of the negative pressure can be -0.1 MPa, -0.2 MPa, -0.3 MPa, -0.4 MPa, -0.5 MPa, -0.6 MPa, -0.07 MPa, or a range composed of any two of these values. The time for removing the monomers can be 1 h, 2 h, 3 h, 4 h, 5 h, or a range composed of any two of these values.

[0057] By using the above preparation method and sequentially adding the second monomer, free radical initiator, and first monomer according to the above steps, the proportion of block structure and random structure in the copolymer molecular chain can be reduced, and the proportion of alternating structure in the copolymer molecular chain can be increased, making the distribution of the second monomer units uniform in the polymer molecular chain, which is beneficial to improving the adhesion of the copolymer segment to the anode active material and can better exert the self-healing effect of the hydroxyl-modified binder. At the same time, by regulating the mass ratio of the second monomer to the solvent, the mass ratio of the second monomer to the free radical initiator, the mass percentage content of the first monomer and the second monomer, and the temperature and time of the copolymerization reaction within the scope of this application, the distribution of the second monomer units in the polymer molecular chain can be made more uniform, which is beneficial to improving the adhesion of the copolymer segment to the anode active material and can better exert the self-healing effect of the hydroxyl-modified binder. The hydroxyl-modified binder prepared by the above method has good self-healing characteristics and can regenerate its original structure. Applying the above hydroxyl-modified binder to the anode electrode sheet of a secondary battery can not only improve the continuous expansion and contraction of the silicon anode system during the cycling process, but also improve the problem of gas swelling during the cycling process and low-voltage storage of the secondary battery, thereby improving the cycling expansion performance and low-voltage storage safety performance of the secondary battery.

[0058] In step (1), the inert atmosphere includes at least one of argon or nitrogen. There are no special restrictions on the time, temperature, and rotation speed of stirring under the protection of the inert atmosphere, as long as the second monomer can be dispersed evenly. For example, the stirring time can be 3 h to 8 h, the stirring temperature can be room temperature, and the stirring rotation speed can be 10 rpm to 30 rpm.

[0059] In step (2), the free radical initiator can include but is not limited to at least one of the following: inorganic peroxides such as sodium persulfate, potassium persulfate, ammonium persulfate, ammonium persulfate and sodium bisulfite composite initiator, potassium peroxophosphate and hydrogen peroxide; organic peroxides such as tert-butyl peroxide, cumene hydroperoxide, terpinyl hydroperoxide, di-tert-butyl peroxide, tert-butyl cumyl peroxide, acetyl peroxide, isobutyl peroxide, octanoyl peroxide, benzoyl peroxide, 3,5,5-trimethylhexanol peroxide, tert-butyl isobutyrate peroxide; and azo compounds such as azobisisobutyronitrile, azo-bis-2,4-dimethylvaleronitrile, azodicyclohexanecarbonitrile and dimethyl azodicarboxylate. In this application, when adding the first monomer to the deoxygenated solution, in order to control the reaction rate and make the distribution of the second monomer units more uniform in the polymer molecular chain, the first monomer can be added dropwise. There are no special restrictions on the dropping rate in this application, as long as the purpose of this application can be achieved. For example, the dropping rate is 0.1 mL / s to 1 mL / s.

[0060] In step (3), the present application does not particularly limit the rotation speed of stirring, as long as the purpose of the present application can be achieved. For example, it can be 10 rpm to 30 rpm. The present application does not particularly limit the separation method, and the commonly used separation methods in the art can be adopted as long as the purpose of the present application can be achieved. For example, it can include but is not limited to separation by extraction. In step (3), after removing the monomer by negative pressure stirring, the obtained hydroxyl-modified binder is dispersed in the solvent remaining in the reaction and exists in the form of an emulsion of the hydroxyl-modified binder. It can be adjusted to an appropriate solid content and directly used for the preparation of the negative electrode paste. Those skilled in the art can also dry it according to actual needs to obtain solid particles of the hydroxyl-modified binder. In the preparation of the negative electrode plate, the above solvent is added to prepare a composition of the hydroxyl-modified binder and the solvent. The present application does not particularly limit this, as long as the purpose of the present application can be achieved. The temperature of the above drying can be 80°C to 120°C, and the drying time can be 2 h to 10 h.

[0061] The present application also provides a hydroxyl-modified binder composition, which includes the hydroxyl-modified binder provided in the first aspect of the present application and a solvent. Based on the mass of the hydroxyl-modified binder composition, the mass percentage content of the hydroxyl-modified binder is 5% to 7%, and the mass percentage content of the solvent is 93% to 95%. For example, the mass percentage content of the hydroxyl-modified binder can be 5%, 5.5%, 6%, 6.5%, 7% or a range composed of any two of these values, and the mass percentage content of the solvent can be 93%, 93.5%, 94%, 94.5%, 95% or a range composed of any two of these values. The solvent includes at least one of deionized water, ethanol or acetone. Using the hydroxyl-modified binder composition with the above characteristics to prepare the negative electrode plate can make the hydroxyl-modified binder more evenly dispersed in the negative electrode material layer, which is beneficial to better play the role of the hydroxyl-modified binder, thereby improving the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0062] The third aspect of the present application provides a secondary battery, which includes 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. The negative electrode material layer includes a negative electrode active material, the negative electrode active material includes a silicon-containing material, and the negative electrode material layer also includes the hydroxyl-modified binder provided in the first aspect of the present application or the hydroxyl-modified binder prepared by the preparation method provided in the second aspect of the present application. The secondary battery uses the above hydroxyl-modified binder as a binder in the negative electrode material and does not include components with too high hydroxyl activity such as polyvinyl alcohol. It can provide good adhesion for the negative electrode plate and also has a self-healing effect. Therefore, it is not only beneficial to improve the continuous expansion and contraction of the silicon negative electrode system during the cycle, but also can improve the problem of gas swelling during the cycle and low-voltage storage of the secondary battery, thereby improving the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0063] In some embodiments of the present application, the silicon-containing material includes at least one of silicon, silicon carbide, or silicon oxide. Based on the mass of the negative electrode material layer, the mass percentage of silicon element is M%, where 0.5 ≤ M ≤ 60, preferably 5 ≤ M ≤ 40. For example, the value of M can be 0.5, 3, 5, 8, 10, 12, 15, 17, 20, 30, 40, 50, 60, or a range composed of any two of these values. By selecting the silicon-containing material within the above range and controlling the mass percentage of silicon element within the above range, the secondary battery can have a high energy density. At the same time, it is beneficial to better exert the self-healing effect of the hydroxyl-modified binder in the silicon negative electrode system, thereby improving the cycle expansion performance and low-voltage storage safety performance of the secondary battery.

[0064] Generally, the mass percentage M% of silicon element in the negative electrode material layer can be adjusted by adjusting the mass percentage of the silicon-containing material in the negative electrode material layer or the mass ratio of silicon element in the silicon-containing material. Keeping the mass ratio of silicon element in the silicon-containing material unchanged, increasing the mass percentage of the silicon-containing material in the negative electrode material layer, M% increases; decreasing the mass percentage of the silicon-containing material in the negative electrode material layer, M% decreases. Keeping the mass percentage of the silicon-containing material in the negative electrode material layer unchanged, increasing the mass ratio of silicon element in the silicon-containing material, M% increases; decreasing the mass ratio of silicon element in the silicon-containing material, M% decreases.

[0065] In some embodiments of the present application, based on the mass of the negative electrode material layer, the mass percentage of the hydroxyl-modified binder is N%, where 1.2 ≤ N ≤ 11.5, preferably 1.2 ≤ N ≤ 8. For example, the value of N can be 1.2, 2, 4, 5, 6, 8, 10, 11.5, or a range composed of any two of these values. Controlling the mass percentage of the hydroxyl-modified binder within the above range can make the negative electrode active material particles have good adhesion between particles. At the same time, the self-healing effect of the hydroxyl-modified binder in the silicon negative electrode system can be exerted. In addition, the negative electrode material layer can have a high specific capacity, so that the secondary battery has a high energy density, good cycle expansion performance, and low-voltage storage safety performance.

[0066] In some embodiments of the present application, based on the mass of the negative electrode material layer, the mass percentage content of the hydroxyl-modified binder is N%, and 0.12 ≤ N / M ≤ 3.2, preferably 0.24 ≤ N / M ≤ 1.6. For example, the value of N / M can be 0.12, 0.24, 0.4, 0.5, 1, 1.3, 1.6, 1.75, 2.1, 2.3, 2.5, 3.2 or a range composed of any two of these numerical values. By regulating the value of N / M within the above range, the synergistic effect between the hydroxyl-modified binder and the silicon element in the negative electrode material layer is good, which is beneficial to better exert the self-repairing effect of the hydroxyl-modified binder in the silicon negative electrode system, so that the secondary battery has a higher energy density, better cycle expansion performance and low-voltage storage safety performance.

[0067] In the present application, the negative electrode material layer can be disposed on one surface in the thickness direction of the negative electrode current collector, or can be disposed on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of the negative electrode current collector or a partial area of the negative electrode current collector, and there is no particular limitation in the present application as long as the object of the present application can be achieved. In the present application, there is no particular limitation on the thickness of the negative electrode current collector and the negative electrode material layer as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the single-sided negative electrode material layer is 30 μm to 160 μm.

[0068] In the present application, the negative electrode material layer may further contain graphite, a conductive agent, and a dispersant. There is no particular limitation on the conductive agent in the present application as long as the object 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), graphite, carbon fiber, carbon nanowire, graphene, metal material, or conductive polymer. The above carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above carbon fiber may include, but is not limited to, vapor-grown carbon fiber (VGCF) and / or nanofiber. The above metal material may include, but is not limited to, metal powder and / or metal fiber. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above conductive polymer may include, but is not limited to, at least one of poly(phenylene) derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. The dispersant may include sodium carboxymethyl cellulose. Based on the mass of the negative electrode material layer, the mass percentage content of graphite is 0% to 88%, the mass percentage content of the silicon-containing material is 10% to 98%, the mass percentage content of the conductive agent is 0.1% to 1.5%, and the mass percentage content of the dispersant is 0.3% to 1.5%.

[0069] In this application, there is no particular limitation on the preparation method of the negative electrode sheet, as long as the purpose of this application can be achieved. For example, it can be prepared by the following method: Disperse the hydroxyl-modified binder in a solvent to obtain a binder slurry with a solid content of 5% to 7%. Mix the negative electrode active material, conductive agent, and dispersant, first add the binder slurry, and then add it to the solvent and stir evenly to obtain a negative electrode slurry with a solid content of 15wt% to 45wt%. Coat the negative electrode slurry evenly on one surface of the negative electrode current collector, and after drying, obtain a negative electrode sheet with a single-sided coated negative electrode material layer. Then repeat the above coating steps on the other surface of the negative electrode current collector, and after drying, obtain a negative electrode sheet with a double-sided coated negative electrode material layer. After the coating is completed, perform cold pressing and cutting to obtain the negative electrode sheet.

[0070] The secondary battery of this application further includes a positive electrode sheet. There is no particular limitation on the positive electrode sheet in this application, as long as the purpose of this application can be achieved. For example, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. In this application, the positive electrode material layer can be provided on one surface in the thickness direction of the positive electrode current collector, or on two surfaces in the thickness direction of the positive electrode current collector. It should be noted that the "surface" here can be the entire area of the positive electrode current collector surface or a partial area of the positive electrode current collector surface. There is no particular limitation in this application, as long as the purpose of this application can be achieved. There is no particular limitation on the positive electrode current collector in this application, as long as the purpose of this application can be achieved. For example, the positive electrode current collector can include aluminum foil, aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc. There is no particular limitation on the thickness of the positive electrode current collector and the positive electrode material layer in this application, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm, and the thickness of the single-sided positive electrode material layer is 30μm to 120μm.

[0071] In this application, the positive electrode material layer includes a positive electrode active material. There is no particular limitation on the type of the positive electrode active material in this application, as long as the purpose of this application can be achieved. The positive electrode active material may include at least one of lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide, lithium manganese oxide, or lithium manganese iron phosphate, etc. The positive electrode material layer of this application may also contain a conductive agent and a binder. There is no particular limitation on the conductive agent in this application, as long as the purpose of this 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, flake graphite, carbon fiber, graphene, amorphous carbon, hard carbon, soft carbon, glassy carbon, carbon nanofiber, metal material, or conductive polymer. 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 fiber may include, but is not limited to, vapor-grown carbon fiber (VGCF) and / or nanofiber. The above-mentioned metal material may include, but is not limited to, metal powder and / or metal fiber. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. There is no particular limitation on the binder in this application, as long as the purpose of this application can be achieved. For example, the binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyamideimide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0072] In this application, there is no particular limitation on the preparation method of the positive electrode plate, as long as the purpose of this application can be achieved. For example, it can be prepared by the following method: Mix the positive electrode active material, the conductive agent, and the binder, add N-methylpyrrolidone (NMP), and stir evenly to obtain a positive electrode slurry with a solid content of 65 wt% to 85 wt%. Uniformly coat the positive electrode slurry on one surface of the positive electrode current collector, and after drying, a positive electrode plate with a single-sided coated positive electrode material layer is obtained. Then repeat the above coating step on the other surface of the positive electrode current collector, and after drying, a positive electrode plate with a double-sided coated positive electrode material layer is obtained. After the coating is completed, it is cold-pressed and cut to obtain the positive electrode plate. There is no particular limitation on the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode material layer in this application, as long as the purpose of this application can be achieved.

[0073] The secondary battery of the present application further includes an electrolyte. The electrolyte of the present application may include a lithium salt and a non-aqueous solvent. The present application has no particular limitation on the type of the lithium salt, as long as the object of the present application can be achieved. For example, the lithium salt may include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), or lithium difluoro(oxalato)borate (LiDFOB). The present application has no particular limitation on the type of the above non-aqueous solvent, as long as the object of the present application can be achieved. For example, it may include, but is not limited to, at least one of a carbonate compound, a carboxylate compound, an ether compound, or other non-aqueous solvents. The above carbonate compound may include, but is not limited to, at least one of a linear carbonate compound or a cyclic carbonate compound. The above linear carbonate compound may include, but is not limited to, at least one of dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or methyl ethyl carbonate. The above cyclic carbonate compound may include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, or vinylene carbonate. The above carboxylate compound may include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate (EP), propyl propionate, γ-butyrolactone, decanolide, valerolactone, or caprolactone. The above ether compound may include, but is not limited to, at least one of ethylene glycol dimethyl ether, 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 other non-aqueous solvents may include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0074] The secondary battery of the present application further includes a separator. There is no particular limitation on the separator in the present application, as long as the object of the present application can be achieved. For example, the material of the separator may include, but is not limited to, at least one of polyolefins (PO) mainly composed of polyethylene (PE) and polypropylene (PP), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator may include at least one of a woven film, a non-woven film, a microporous film, a composite film, a rolled film, or a spun film. In some embodiments, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film, or a composite film having 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 film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer, an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. In some embodiments, the inorganic layer includes inorganic particles and a binder. There is no particular limitation on the inorganic particles in the present application. For example, the inorganic particles may include at least one of alumina, silica, magnesia, titania, hafnium dioxide, tin dioxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. There is no particular limitation on the binder in the present application. For example, the binder may be at least one of the above-mentioned binders. In some embodiments of the present application, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene). In the present application, there is no particular limitation on the thickness of the separator, as long as the object of the present application can be achieved. For example, the thickness of the separator may be 3 μm to 30 μm.

[0075] The secondary battery further includes a housing for accommodating the positive electrode plate, the separator, the negative electrode plate, and the electrolyte, as well as other components known in the field of secondary batteries. The present application does not limit the above-mentioned other components. There is no particular limitation on the housing in the present application, and it may be a housing well-known in the art, as long as the object of the present application can be achieved. For example, the housing may be a hard shell housing or a flexible housing. The material of the hard shell housing may be metal, and the present application does not limit the type of the metal. Any metal hard shell housing known in the art may be used, as long as the object of the present application can be achieved. The flexible housing may be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0076] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and there is no special limitation 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 sheet, the separator and the negative electrode sheet in sequence, and winding, folding, etc. according to needs to obtain a wound structure electrode assembly, placing the electrode assembly into the casing, injecting the electrolyte into the casing and sealing it to obtain the secondary battery. Alternatively, stack the positive electrode sheet, the separator and the negative electrode sheet in sequence, and then fix the four corners of the entire laminated structure with tape to obtain a laminated structure electrode assembly, place the electrode assembly into the casing, inject the electrolyte into the casing and seal it to obtain the secondary battery. In addition, an overcurrent protection element, a guide plate, etc. may be placed in the casing as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0077] The secondary battery of the present application is not particularly limited, and it may include any device that generates an electrochemical reaction. For example, the secondary battery may include, but is not limited to: lithium metal secondary battery, lithium ion secondary battery (lithium ion battery), lithium polymer secondary battery or lithium ion polymer secondary battery.

[0078] The fourth aspect of the present application provides an electronic device, which includes the secondary battery provided by the third aspect of the present application. Thus, the electronic device of the present application has a long service life.

[0079] The present application does not particularly limit the type of the electronic device, and it may be any electronic device known in the prior art. In some embodiments of the present application, the electronic device may include, but is not limited to: laptop computer, pen input computer, mobile computer, e-book player, portable phone, portable fax machine, portable copier, portable printer, head-mounted stereo headphones, video recorder, liquid crystal TV, hand-held cleaner, portable CD player, minidisc, transceiver, electronic notepad, calculator, memory card, portable recorder, radio, backup power supply, motor, automobile, motorcycle, moped, bicycle, lighting fixture, toy, game console, clock, power tool, flashlight, camera, large household battery and lithium ion capacitor, etc.

[0080] Examples

[0081] Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0082] Test methods and equipment:

[0083] Test for the mass percentage content of monomer units in the polymer:

[0084] The monomer composition after pyrolysis of the polymer was tested by pyrolysis gas chromatography-mass spectrometry (py GC-MS), and the mass percentage content of monomer units in the polymer was analyzed. The hydroxy-modified binder was dried in an oven at 80 °C for 8 h, and the dried hydroxy-modified binder was sent to a pyrolysis gas chromatography-mass spectrometry instrument for testing. The instrument model was Agilent 7890B / 5977B, the detector was a mass selective detector MSD, the injector was a pyrolysis gas EGA / PY-3030D, the column oven temperature was 40 °C to 320 °C, and the pyrolyzer temperature was 40 °C to 800 °C.

[0085] Viscosity test:

[0086] At 25 °C, with a stirring speed of 12 rpm and a stirring time of 2 min, the hydroxy-modified binder was dispersed in water to obtain an aqueous dispersion of the hydroxy-modified binder with a mass percentage content of 6%. A digital rotational viscometer (Shanghai Jingtian Electronic Instrument Co., Ltd., LVDV-1) was used to test the viscosity of the aqueous dispersion of the hydroxy-modified binder. The appropriate rotor and rotation speed were selected according to the binder aqueous dispersion to be measured. The rotor was slowly tilted and immersed in the aqueous dispersion of the binder to prevent air bubbles from forming at the bottom of the rotor. The liquid level of the aqueous dispersion of the binder was at the middle position of the rotor groove. The rotational viscometer was started, and the reading was taken after three minutes when the reading remained unchanged. This was recorded as the viscosity of the aqueous dispersion of the binder, with the unit of mPa·s.

[0087] Infrared spectroscopy test:

[0088] The Fourier transform infrared spectrometer was used to test the infrared spectra of the hydroxy-modified binder at 30 °C and 100 °C respectively.

[0089] Q v Test:

[0090] The binder of the example or comparative example was coated on a 6-μm-thick copper foil to prepare a working electrode, and the coating thickness was 15 μm. A negative electrode steel shell, a spring piece, and a positive electrode steel shell were provided. A lithium sheet was used as the counter electrode, and a polyethylene film was used as the separator. First, they were stacked in the order of the negative electrode steel shell, the spring piece, the working electrode, the separator, the counter electrode, and the positive electrode steel shell. After injecting the electrolyte, a button cell was assembled. The electrolyte, copper foil, and separator used above were the same as those in Example 1-1.

[0091] The above-mentioned button cell was subjected to charge-discharge tests according to the cyclic voltammetry method using an electrochemical workstation (model 1470E / 1455, manufacturer Solartron). The voltage window was set from 0 V to 3.3 V, the scanning rate was 0.1 mV / s, and the number of cycles was 3. At the same time, differential electrochemical mass spectrometry (DEMS, model HPR-40DEMS, manufacturer Hiden Analytical) was used to collect the gases generated during the above cycle and convert them into current signals of the gases, and the current signal of hydrogen was collected. The strongest hydrogen signal value was recorded as Q1 A, and the weakest hydrogen signal value was recorded as Q2 A. Q v = Q1 / Q2.

[0092] Test of Young's modulus and elongation at break:

[0093] The hydroxyl-modified binder slurry was cast in a mold, and then the hydroxyl-modified binder slurry was dried at 80 °C for 4 h to obtain a film. The Young's modulus (E) and elongation at break of the film of the hydroxyl-modified binder were tested by a universal testing machine.

[0094] The Young's modulus was tested according to the international standard ASTM E111-04(2010) Standard Test Methods for Young's Modulus, Tangent Modulus, and Chord Modulus. The film of the hydroxyl-modified binder was cut into specimens with dimensions of 0.8 cm × 1 cm × 0.01 cm. The specimens were clamped by the upper and lower fixtures of a universal testing machine. The test conditions included: amplitude 10 μm, frequency 1 Hz, adaptation time 2 min, test time 5 min, preload force 0.1 N, tensile angle 180°. The stress σ and strain ε of the specimens were measured, and the Young's modulus was calculated according to Hooke's law E = σ / ε, with the unit of GPa.

[0095] The elongation at break was tested according to the national standard GB / T 30776-2014 Test Method for Tensile Strength and Elongation at Break of Adhesive Tape. The film of the hydroxyl-modified binder was cut into specimens with dimensions of 1 cm × 2 cm × 0.01 cm (length × width × height). The specimens were clamped by the upper and lower fixtures of a universal testing machine. The initial distance between the upper and lower fixtures was fixed as L0, and the initial length of the sample was L0. The upper and lower fixtures were stretched at a constant rate of 50 mm / min until the sample was broken. The distance between the upper and lower fixtures at the moment of sample breakage was recorded as L1. Then the elongation at break L = (L1 - L0) / L0 × 100%.

[0096] Test of weight-average molecular weight:

[0097] The gel permeation chromatography (GPC) test was carried out using an Agilent 1260 HPLC gel chromatograph equipped with a refractive index detector (RID). The chromatographic column model was 79911GP-502 to characterize the weight-average molecular weight (Mw) of the hydroxyl-modified binder. The specific operation steps were as follows: 5 mg of the hydroxyl-modified binder sample was dispersed in 2 mL of deionized aqueous solution (HPLC grade), the column temperature was 30 °C, the mobile phase was HPLC grade water, and the flow rate was 1.0 mL·min -1 .

[0098] Test for mass percentage content of silicon element:

[0099] The lithium-ion battery was disassembled and the negative electrode sheet was taken out. The negative electrode sheet was fixed on the sample stage using conductive carbon paste, and the sample stage was placed on the bracket. The bracket and the sample stage were placed in a cross-section polisher (CP), and after evacuating to 10 -4 Pa, argon gas was used to cut along the thickness direction of the negative electrode sheet. The cross-section of the sample after cutting by CP was placed face up on the SEM sample stage, and a Philips XL-30 field emission scanning electron microscope (SEM) was used for observation. The acceleration voltage for the test was 10 kV, the emission current was 10 mA, the magnification was 3000 times, and the energy dispersive spectrometer (EDS) equipped with the SEM was used to test the mass percentage content M% of silicon element in the negative electrode material layer.

[0100] Test for mass percentage content of hydroxyl-modified binder:

[0101] The lithium-ion battery was discharged to 3.0 V at a current of 0.2 C, and this step was repeated three times to prevent voltage rebound. The lithium-ion battery was disassembled in a drying room (water content < 2%), the negative electrode sheet was taken out, and the negative electrode sheet was cleaned with dimethyl carbonate (DMC) solvent. The cleaning steps were as follows: The negative electrode sheet was soaked in DMC for 12 h, and the liquid completely submerged the negative electrode sheet. After soaking, the solvent was poured out, and the above cleaning steps were repeated 2 times. After cleaning, the negative electrode sheet was placed in an oven at 60 °C and dried for 8 h. Then, the negative electrode material layer on the surface of the negative electrode sheet was scraped off, and the sample was sent to a pyrolysis gas chromatography-mass spectrometry instrument for testing. The instrument parameters were Agilent 7890B / 5977B, the detector was a mass selective detector MSD, the injector was a pyrolysis gas EGA / PY-3030D, the column oven temperature was 40 °C to 320 °C, and the pyrolyzer temperature was 40 °C to 800 °C.

[0102] Cyclic swelling performance:

[0103] The cycle expansion performance of a lithium-ion battery is evaluated by the expansion rate after 500 cycles. The lower the expansion rate of the lithium-ion battery after 500 cycles, the better the cycle expansion performance of the lithium-ion battery. The higher the expansion rate of the lithium-ion battery after 500 cycles, the worse the cycle expansion performance of the lithium-ion battery. At 25 °C, the lithium-ion battery is charged at a constant current of 1.0C to a full charge voltage of 4.53V, and then charged at a constant voltage of 4.53V until the cut-off current is 0.05C; then the lithium-ion battery is discharged at a constant current of 0.7C to 3.0V, which is a cycle of charge and discharge process. Repeat the above operation for 500 cycles. Use a micrometer to measure the thickness H0 of the lithium-ion battery when it is charged to 4.53V for the first time, and measure the thickness H1 of the lithium-ion battery when it is charged in the 500th cycle; the same position of each lithium-ion battery is fixed for testing, and the thickness of each position is tested three times, and the average value is taken as the test result. The expansion rate (%) of the lithium-ion battery after 500 cycles = (H1 - H0) / H0 × 100%.

[0104] Low voltage storage safety performance:

[0105] The low voltage storage safety performance of a lithium-ion battery is evaluated by the over-discharge gas generation voltage of the lithium-ion battery. The lower the over-discharge gas generation voltage, the better the low voltage storage safety performance; the higher the over-discharge gas generation voltage, the worse the low voltage storage safety performance. At 25 °C, the lithium-ion battery is charged at a constant current of 1.0C to a full charge voltage of 4.53V, and then charged at a constant voltage of 4.53V until the cut-off current is 0.05C; then the lithium-ion battery is discharged at a constant current of 0.7C to 3.0V, which is a cycle of charge and discharge process. Repeat the above operation for 100 cycles. After 100 cycles of charge and discharge, the lithium-ion battery is charged at a constant current of 1.0C to a full charge voltage of 4.53V, and then connect an instrument that can monitor the thickness and voltage in real time, and discharge it to 0V at a constant current of 0.2C at the same time; monitor the voltage when the thickness change of the lithium-ion battery is greater than 10μm, and record it as the over-discharge gas generation voltage, with the unit of V.

[0106] It can be understood that when the voltage range marked on the outer package of the factory lithium-ion battery is 3.0V to 4.53V, the charge cut-off voltage is 4.53V and the discharge cut-off voltage is 3.0V. It should be noted that the charge cut-off voltage of the lithium-ion battery in this application as an example is 4.53V, and the discharge cut-off voltage is 3.0V, but this application is not limited thereto.

[0107] Example 1-1

[0108] <Preparation of Hydroxyl-Modified Binder>

[0109] (1) Dissolve the second monomer N-(2-hydroxyethyl) acrylamide (HEAA) in deionized water as the solvent, add it to a flask, and stir for 5 h at 25 °C under argon protection to obtain a deoxygenated solution. Among them, the mass ratio of the second monomer to the solvent is 1:2500.

[0110] (2) Add the radical initiator (NH4)2S2O8 / NaHSO3 to the deoxygenated solution and stir for 10 minutes. Then, transfer the first monomer acrylic acid to a funnel under argon protection and dropwise add it to the flask. After the addition of the first monomer is completed, transfer the flask to a constant temperature oil bath at 60 °C and stir at a speed of 15 rpm for 2.5 h for copolymerization reaction to obtain a mixed solution containing a hydroxyl-modified binder. The mass ratio of the second monomer to the radical initiator is 1:0.1, the temperature of the copolymerization reaction is 60 °C, and the time is 2.5 h. Based on the total mass of the first monomer and the second monomer, the mass percentage content of the first monomer is 30%, and the mass percentage content of the second monomer is 70%. In the above (NH4)2S2O8 / NaHSO3, the mass ratio of (NH4)2S2O8 to NaHSO3 is 1:0.3.

[0111] (3) Stir the above mixed solution at 15 rpm under a negative pressure of -0.5 MPa for 2.5 h to remove the monomers and obtain an aqueous dispersion of the hydroxyl-modified binder.

[0112] <Preparation of the negative electrode plate>

[0113] Using deionized water as the solvent, prepare the hydroxyl-modified binder obtained above into a negative electrode binder slurry with a solid content of 6 wt%.

[0114] Mix silicon carbon (mass ratio of silicon to carbon is 50:50), graphite, conductive agent conductive carbon black, and dispersant sodium carboxymethyl cellulose (Mw = 350000). First, add the binder slurry and stir evenly, then add deionized water as the solvent to prepare a slurry with a solid content of 45 wt%. After stirring evenly with a vacuum mixer, obtain a negative electrode slurry. Uniformly coat the negative electrode slurry on one surface of a negative electrode current collector copper foil with a thickness of 6 μm and dry it at 120 °C to obtain a negative electrode plate with a single-sided coated negative electrode material layer, and the coating weight of the negative electrode material layer is 100 mg / 1540 mm 2。Then repeat the above steps on the other surface of the copper foil to obtain a negative electrode plate with a negative electrode material layer coated on both sides. After drying at 120°C and cold pressing, and then through cutting and welding the tab, a negative electrode plate with a specification of 78mm×875mm is obtained for use. Among them, the thickness of the single-sided negative electrode material layer is 35μm. Based on the mass of the negative electrode material layer, the mass percentage content of the silicon-containing material is 10%, the mass percentage content of the conductive agent is 0.1%, the mass percentage content of the dispersant is 0.5%, the mass percentage content N% of the hydroxyl-modified binder is 2%, and the balance is graphite. The mass percentage content M% of silicon element is 5%.

[0115] <Preparation of the positive electrode plate>

[0116] Mix the positive electrode active material lithium cobaltate, the conductive agent conductive carbon black, and the binder polyvinylidene fluoride according to a mass ratio of 97.9:0.9:1.2, add N-methylpyrrolidone (NMP) as a solvent, and formulate it into a slurry with a solid content of 75wt%. After vacuum stirring evenly, a positive electrode slurry is obtained. Coat the positive electrode slurry evenly on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm, and dry it at 120°C to obtain a positive electrode plate with a positive electrode material layer coated on one side. The coating weight of the positive electrode material layer is 267.8mg / 1540mm 2 。Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode plate with a positive electrode material layer coated on both sides. After drying at 120°C and cold pressing, and then through cutting and welding the tab, a positive electrode plate with a specification of 74mm×867mm is obtained for use. Among them, the thickness of the single-sided positive electrode material layer is 42μm.

[0117] <Separator>

[0118] Use a polyethylene film with a thickness of 15μm (provided by Celgard).

[0119] <Preparation of the electrolyte>

[0120] In an environment with a water content of less than 10ppm, mix dimethyl carbonate, diethyl carbonate, and ethylene carbonate according to a mass ratio of 1:1:1 to obtain an organic solvent, and then add the electrolyte salt LiPF6 to the organic solvent and mix evenly to obtain the electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of the electrolyte salt is 12.5%, and the rest is the organic solvent.

[0121] <Preparation of the lithium-ion battery>

[0122] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, and then wound to obtain an electrode assembly. After welding the electrode tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag, dried in a vacuum oven at 85°C for 12 hours to remove moisture, the above-prepared electrolyte is injected, and after vacuum packaging, standing, formation (constant current charging at 0.02C to 3.5V, and then constant current charging at 0.1C to 3.9V), shaping, capacity testing, secondary packaging and other processes, a lithium-ion battery is obtained.

[0123] Examples 1-2 to 1-27

[0124] Except for adjusting the relevant parameters according to Tables 1 and 2, the rest is the same as Example 1-1.

[0125] Examples 2-1 to 2-15

[0126] Except for adjusting the silicon-carbon mass ratio and / or its mass percentage content in the silicon-containing material so that the mass percentage content M% of silicon element is as shown in Table 3, and adjusting the mass percentage content N% of the hydroxyl-modified binder as shown in Table 3, the rest is the same as Example 1-1. When the mass percentage content of the silicon-containing material and / or the mass percentage content of the hydroxyl-modified binder change, the mass percentage content of graphite changes accordingly, and the mass percentage contents of the conductive agent and the dispersant remain unchanged.

[0127] Comparative Example 1

[0128] Except for using polyacrylic acid (PAA, weight average molecular weight of 300000 g / mol) as the binder to replace the hydroxyl-modified binder in the <preparation of the negative electrode sheet>, the rest is the same as Example 1-1.

[0129] Comparative Example 2

[0130] Except for using a mixture of polyvinyl alcohol (PVA, weight average molecular weight of 80000 g / mol) and polyacrylic acid (PAA, weight average molecular weight of 300000 g / mol) as the binder to replace the hydroxyl-modified binder in the <preparation of the negative electrode sheet>, the rest is the same as Example 1-1. Among them, based on the mass of the binder, the mass percentage content of PVA is 80%, and the mass percentage content of PAA is 20%.

[0131] Comparative Examples 3 to 6

[0132] Except for adjusting the relevant parameters according to Tables 1 and 2, the rest is the same as Example 1-1.

[0133] The relevant parameters and performance tests of each example and each comparative example are shown in Tables 1 to 3.

[0134] Table 1

[0135]

[0136]

[0137] Table 2

[0138]

[0139]

[0140] Note: " / " in Table 2 indicates no relevant parameters or substances.

[0141] Referring to Table 1 and Table 2, it can be seen from Examples 1-1 to 1-27 and Comparative Examples 1 to 6 that the hydroxyl-modified binder is a copolymer, and the monomers forming the copolymer include the first monomer and the second monomer within the scope of the present application. By regulating the mass percentage content A% of the first monomer unit and the mass percentage content B% of the second monomer unit within the scope of the present application, Mw, viscosity, I OH , Q v The hydroxyl-modified binder within the scope of the present application can be obtained, and the adhesive film of the hydroxyl-modified binder has appropriate Young's modulus and elongation at break. The lithium-ion battery including the hydroxyl-modified binder of the present application has a low expansion rate and over-discharge gas generation voltage, indicating that the lithium-ion battery has good cycle expansion performance and low-voltage storage safety performance at the same time. The binder of Comparative Example 1 is PAA, the binder of Comparative Example 2 is a combination of 80% PVA and 20% PAA, the mass percentage content A% of the first monomer unit and the mass percentage content B% of the second monomer unit in Comparative Examples 3 and 4 are not within the scope of the present application, and the types of the second monomers in Comparative Examples 5 and 6 are not within the scope of the present application. At least one of the viscosity, I OH and Q v And at least one of the Young's modulus and elongation at break of the binder adhesive film is not within the scope of the present application. The lithium-ion battery has a high expansion rate or a high over-discharge gas generation voltage, indicating that the cycle expansion performance and low-voltage storage safety performance of the lithium-ion battery cannot be taken into account at the same time.

[0142] The values of A and B usually affect the cyclic swelling performance and low-voltage storage safety performance of lithium-ion batteries. It can be seen from Examples 1-1 to 1-5, Comparative Example 3 and Comparative Example 4 that when the value of A is too small and the value of B is too large, such as in Comparative Example 3, the lithium-ion battery has a lower swelling rate and an excessive over-discharge gas generation voltage. When the value of A is too large and the value of B is too small, such as in Comparative Example 4, the lithium-ion battery has an excessive swelling rate and a lower over-discharge gas generation voltage, indicating that the cyclic swelling performance and low-voltage storage safety performance of the lithium-ion battery cannot be balanced. Therefore, by adjusting the values of A and B within the scope of this application, the lithium-ion battery can simultaneously have good cyclic swelling performance and low-voltage storage safety performance.

[0143] The preparation parameters of the hydroxyl-modified binder usually affect the cyclic swelling performance and low-voltage storage safety performance of lithium-ion batteries. It can be seen from Examples 1-1, 1-6 to 1-14 that by adjusting the mass ratio of the second monomer to the solvent, the mass ratio of the second monomer to the free radical initiator, the mass percentage content of the first monomer, the mass percentage content of the second monomer, the temperature and time of the copolymerization reaction within the scope of this application, Mw, viscosity, I OH and Q v of the hydroxyl-modified binder within the scope of this application, and the gel film of the hydroxyl-modified binder has appropriate Young's modulus and elongation at break, and the lithium-ion battery has a lower swelling rate and over-discharge gas generation voltage, indicating that the lithium-ion battery has good cyclic swelling performance and low-voltage storage safety performance.

[0144] The types of the first monomer and the second monomer usually affect the cyclic swelling performance and low-voltage storage safety performance of lithium-ion batteries. It can be seen from Examples 1-15 to 1-22 that by selecting the first monomer and the second monomer within the scope of this application, the gel film of the hydroxyl-modified binder can have appropriate Young's modulus and elongation at break, and the lithium-ion battery has a lower swelling rate and over-discharge gas generation voltage, indicating that the lithium-ion battery has good cyclic swelling performance and low-voltage storage safety performance.

[0145] The values of Mw, viscosity, I OH and Q v of the hydroxyl-modified binder, Young's modulus and elongation at break usually affect the cyclic swelling performance and low-voltage storage safety performance of lithium-ion batteries. It can be seen from Examples 1-1 to 1-27 that by adjusting the above parameters within the scope of this application, the lithium-ion battery has a lower swelling rate and over-discharge gas generation voltage, indicating that the lithium-ion battery has good cyclic swelling performance and low-voltage storage safety performance.

[0146] Figure 1 shows the infrared spectrum of the hydroxyl-modified binder of Example 1-1. From Figure 1It can be seen that the infrared spectrum of the hydroxyl-modified binder in Example 1-1 has a stretching vibration peak of hydroxyl groups at 3000 cm -1 to 3600 cm -1 . At 100 °C, the peak intensity I1 of this stretching vibration peak is 0.947, and at 30 °C, the peak intensity I2 of this stretching vibration peak is 0.700. I OH = I1 / I2 = 1.34, indicating that the hydroxyl-modified binder of the present application has good self-healing ability. Figure 2 The infrared spectrum of the binder polyvinyl alcohol in Comparative Example 2 is shown. It can be seen from Figure 2 that the infrared spectrum of the binder in Comparative Example 2 has a stretching vibration peak of hydroxyl groups at 3000 cm -1 to 3600 cm -1 . At 100 °C, the peak intensity I1 of this stretching vibration peak is 0.900, and at 30 °C, the peak intensity I2 of this stretching vibration peak is 0.844. I OH = I1 / I2 = 1.07, indicating that the polyvinyl alcohol binder also has self-healing ability. Comparing Figure 1 and Figure 2 , the I OH value of Example 1-1 is greater than that of Comparative Example 2, indicating that the hydroxyl-modified binder of Example 1-1 of the present application has better self-healing characteristics.

[0147] Figure 3 The differential electrochemical mass spectrometry diagram measured by cyclic voltammetry for the button battery prepared with the hydroxyl-modified binder of Example 1-2 is shown. Figure 4 The differential electrochemical mass spectrometry diagram measured by cyclic voltammetry for the button battery prepared with the binder of Comparative Example 2 is shown. Figure 3 Among them, during the charge-discharge cycle from 0 V to 3.3 V, the weakest current signal value of hydrogen gas generated by the hydroxyl-modified binder of Example 1-2 is Q1 = 2.38×10 -13 A, and the strongest current signal value is Q2 = 2.98×10 -13 A. Q v = Q1 / Q2 = 0.8. Figure 4 Among them, during the charge-discharge cycle from 0 V to 3.3 V, the weakest current signal value of hydrogen gas generated by the binder of Comparative Example 2 is Q1 = 2.58×10 -13 A, and the strongest current signal value is Q2 = 3.44×10 -13 A. Q v = Q1 / Q2 = 0.75. Combining Figure 3 and Figure 4 it can be seen that the Q v value of Example 1-2 is greater than that of Comparative Example 2, indicating that the hydroxyl-modified binder of Example 1-2 of the present application generates less gas during the charge-discharge cycle.

[0148] Table 3

[0149]

[0150]

[0151] The mass percentage content M% of silicon element usually affects the cycle expansion performance and low-voltage storage safety performance of lithium-ion batteries. It can be seen from Examples 1-1, 2-1 to 2-4 that by adjusting the value of M within the scope of this application, the lithium-ion battery can have a lower expansion rate and over-discharge gas generation voltage, indicating that the lithium-ion battery has good cycle expansion performance and low-voltage storage safety performance.

[0152] The mass percentage content N% of the hydroxyl-modified binder usually affects the cycle expansion performance and low-voltage storage safety performance of lithium-ion batteries. It can be seen from Examples 1-1, 2-5 to 2-10 that by adjusting the value of N within the scope of this application, the lithium-ion battery can have a lower expansion rate and over-discharge gas generation voltage, indicating that the lithium-ion battery has good cycle expansion performance and low-voltage storage safety performance.

[0153] The value of N / M usually affects the cycle expansion performance and low-voltage storage safety performance of lithium-ion batteries. It can be seen from Examples 1-1, 2-1 to 2-15 that by adjusting the value of N / M within the scope of this application, the lithium-ion battery can have a lower expansion rate and over-discharge gas generation voltage, indicating that the lithium-ion battery has good cycle expansion performance and low-voltage storage safety performance.

[0154] It should be noted that in this article, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method or article including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or further includes elements inherent to such process, method or article.

[0155] The term "one of", "a", "a kind of" or other similar terms connecting elements refers to any one of the listed elements. For example, "one of A or B" means only A or only B; for another example, "one of A, B and C" means only A, only B or only C. The term "at least one of", "at least a", "at least a kind of" or other similar terms connecting elements refers to any combination of the listed elements. For example, "at least one of A or B" means only A, only B, A and B. For another example, "at least one of A, B or C" means only A, only B, only C, only A and B, only A and C, only B and C, A and B and C.

[0156] Each embodiment in this specification is described in a related manner. For the same or similar parts among the embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments.

[0157] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the scope of protection of the present application.

Claims

1. A hydroxyl-modified binder, wherein the hydroxyl-modified binder is a copolymer, and the monomers forming the copolymer include a first monomer and a second monomer. The first monomer includes at least one of acrylic acid, acrylamide or acrylonitrile, and the second monomer includes a compound of formula I: Among them, X is selected from C, an ester bond, an ether bond, a methyleneoxy group or an amide bond, R1 is selected from H or methyl, R2, R3 and R4 are each independently selected from H, methyl, hydroxyl or hydroxymethyl, and at least one of R2, R3 or R4 is selected from hydroxyl or hydroxymethyl, and n is 0, 1 or 2; The first monomer and the second monomer respectively form a first monomer unit and a second monomer unit of the copolymer. Based on the mass of the copolymer, the mass percentage content of the first monomer unit is A%, 5 ≤ A ≤ 60, and the mass percentage content of the second monomer unit is B%, 40 ≤ B ≤ 95.

2. The hydroxyl-modified binder according to claim 1, wherein, The second monomer includes at least one of β-hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 4-penten-1-ol, pentaerythritol allyl ether or N-(2-hydroxyethyl) acrylamide.

3. The hydroxyl-modified binder according to claim 1, wherein, 5 ≤ A ≤ 40, 60 ≤ B ≤ 95.

4. The hydroxyl-modified binder according to claim 1, wherein, At 25°C, with a stirring speed of 12 rpm and a stirring time of 2 min, when the mass percentage content of the aqueous dispersion of the hydroxyl-modified binder is 5.5% to 6.5%, the viscosity range of the aqueous dispersion of the hydroxyl-modified binder is 3000 mPa·s to 100000 mPa·s.

5. The hydroxyl-modified binder according to any one of claims 1 to 4, wherein, The infrared spectrum of the hydroxyl-modified binder has a stretching vibration peak of hydroxyl groups in the range of 3000 cm -1 to 3600 cm -1 The peak intensity of the stretching vibration peak at 100 °C is I1, and the peak intensity of the stretching vibration peak at 30 °C is I2. I OH = I1 / I2, and I OH ≥ 1.

1.

6. The hydroxyl-modified binder according to claim 5, wherein, 1.15≤I OH ≤2。 7. The hydroxyl-modified binder according to any one of claims 1 to 4, wherein, The weakest current signal value of hydrogen during the charge-discharge process of the hydroxyl-modified binder cycled at a voltage from 0 V to 3.3 V is Q1 A, and the strongest current signal value is Q2 A, where Q v = Q1 / Q2, and 0.8 ≤ Q v ≤ 0.

9.

8. The hydroxyl-modified binder according to any one of claims 1 to 4, which satisfies at least one of the following characteristics: (1) The Young's modulus of the adhesive film of the hydroxyl-modified binder is 1.2 GPa to 7 GPa; (2) The elongation at break of the adhesive film of the hydroxyl-modified binder is 12% to 85%; (3) The weight average molecular weight of the hydroxyl-modified binder is 100000 g / mol to 750000 g / mol.

9. A method for preparing the hydroxyl-modified binder according to any one of claims 1 to 8, which includes the following steps: (1) Dissolve the second monomer in a solvent, and stir under the protection of an inert atmosphere to obtain a deoxygenated solution; (2) Add a radical initiator to the deoxygenated solution, and then add the first monomer under the protection of an inert atmosphere to carry out a copolymerization reaction to obtain a mixture containing the hydroxyl-modified binder; (3) Stir the mixture under negative pressure to remove the monomers, and separate to obtain the hydroxyl-modified binder; wherein, the solvent includes at least one of deionized water, ethanol or acetone, the mass ratio of the second monomer to the solvent is 1:(1000 to 5000), and the mass ratio of the second monomer to the radical initiator is 1:(0.1 to 2); based on the total mass of the first monomer and the second monomer, the mass percentage content of the first monomer is 5% to 60%, and the mass percentage content of the second monomer is 40% to 95%; The temperature of the copolymerization reaction is 50°C to 80°C, and the time of the copolymerization reaction is 2 h to 8 h; the pressure of the negative pressure is -0.1 MPa to -0.07 MPa, and the time for removing the monomer is 1 h to 5 h.

10. A secondary battery, which includes a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material includes a silicon-containing material. The negative electrode material layer further includes the hydroxyl group-modified binder according to any one of claims 1 to 8 or the hydroxyl group-modified binder prepared by the preparation method according to claim 9.

11. The secondary battery according to claim 10, wherein, The silicon-containing material includes at least one of silicon, silicon carbide, or silicon oxide. Based on the mass of the negative electrode material layer, the mass percentage content of silicon element is M%, and 0.5 ≤ M ≤ 60.

12. The secondary battery according to claim 10, wherein, Based on the mass of the negative electrode material layer, the mass percentage content of the hydroxyl group-modified binder is N%, and 1.2 ≤ N ≤ 11.

5.

13. The secondary battery according to claim 11, wherein, Based on the mass of the negative electrode material layer, the mass percentage content of the hydroxyl group-modified binder is N%, and 0.12 ≤ N / M ≤ 3.

2.

14. An electronic device, which includes the secondary battery according to any one of claims 10 to 13.

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