Water-based binder for negative electrode of lithium ion battery as well as preparation method and application of water-based binder

Abstract

The invention provides a water-based binder for a negative electrode of a lithium ion battery as well as a preparation method and application of the water-based binder, and belongs to the technical field of binders and lithium ion battery auxiliary materials. The water-based adhesive is prepared from the following components in parts by mass: 0.1 to 10 parts of a water-based polymeric dispersant, 10 to 20 parts of a hydrophilic unsaturated carboxylic acid monomer I, 60 to 70 parts of a hydrophobic unsaturated nitrile monomer I, 5 to 35 parts of an acrylate monomer, 40 to 55 parts of a hydrophilic unsaturated carboxylic acid monomer II, 5 to 20 parts of an unsaturated amide monomer, 20 to 40 parts of a hydrophobic unsaturated nitrile monomer II, 0.1 to 1 part of an oil-soluble initiator and 180 to 300 parts of water. The water-based adhesive provided by the invention has relatively low swelling degree and relatively high peel strength. The negative plate prepared from the aqueous binder has good flexibility, and can reduce the internal resistance of the lithium ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical fields of adhesives and auxiliary materials for lithium - ion batteries, and particularly relates to an aqueous binder for the negative electrode of a lithium - ion battery, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium - ion batteries have been widely used in various industries due to their advantages such as high working voltage, small size, light weight, high energy, and no memory effect. The binder is one of the important auxiliary functional materials in lithium - ion batteries. It firmly combines the active material, conductive agent, and current collector to form the positive and negative electrode sheets. While maintaining the integrity of the electrode structure, it ensures the continuity of the electron or / and ion transport path. Commonly used aqueous binders for the negative electrode sheets of lithium - ion batteries are polyacrylic acid (PAA), styrene - butadiene rubber latex (SBR), and carboxymethyl cellulose (CMC). As a binder for the negative electrode of lithium - ion batteries, polyacrylic acid has broad application potential in the field of lithium - ion batteries due to its strong adhesion, electrochemical stability, environmental protection performance, electrochemical compatibility, and thermal stability. However, the high glass transition temperature of polyacrylic acid results in hard and brittle electrode sheets, which are prone to problems such as cracking and powder shedding, severely restricting its practical application in lithium - ion batteries. Although both SBR and CMC have good rheological and mechanical properties, when SBR is used alone, it is prone to swelling problems, and when CMC is used alone, the cycle performance of the battery is not ideal. Therefore, SBR and CMC are usually used in combination. However, to achieve the purpose of improving the adhesion strength of the binder, the amount of SBR needs to be increased, and increasing the amount of SBR will lead to a decline in the performance of the electrode sheet, cracking of the electrode sheet, or shedding of the active material, reducing the cycle service life of the electrode sheet. To address problems such as electrode sheet cracking, the main optimization and improvement methods are to use SBR, CMC, and PAA in combination. However, there are still problems such as SBR being prone to floating, CMC being difficult to dissolve, and increasing the process cost.

[0003] The above problems can be improved by copolymerizing PAA with soft monomers to lower the glass transition temperature of the binder. Commonly used soft monomers are acrylate monomers, and the copolymerization methods mostly use precipitation polymerization, emulsion polymerization, and solution polymerization. However, acrylate monomers have poor water solubility and poor migration ability in water, which easily leads to high residual monomers and low conversion rates in solution polymerization and precipitation polymerization reactions using water as a solvent, thereby increasing the post - treatment cost. Emulsion polymerization will introduce small - molecule emulsifiers, increasing the internal resistance of lithium - ion batteries and deteriorating the electrochemical performance of lithium - ion batteries. Summary of the Invention

[0004] Based on the above - mentioned technical problems, the present invention provides an aqueous binder for the negative electrode of a lithium - ion battery and a preparation method thereof. The aqueous binder has good adhesion performance, good flexibility, a simple preparation method, high monomer conversion rate, and pure products.

[0005] Specifically, to achieve the above object, the present invention adopts the following technical solutions: An aqueous binder for the negative electrode of a lithium-ion battery, comprising the following components in parts by mass: 0.1 to 10 parts of an aqueous polymer dispersant, 10 to 20 parts of a hydrophilic unsaturated carboxylic acid monomer I, 60 to 70 parts of a hydrophobic unsaturated nitrile monomer I, 5 to 35 parts of an acrylate monomer, 40 to 55 parts of a hydrophilic unsaturated carboxylic acid monomer II, 5 to 20 parts of an unsaturated amide monomer, 20 to 40 parts of a hydrophobic unsaturated nitrile monomer II, 0.1 to 1 part of an oil-soluble initiator, and 180 to 300 parts of water.

[0006] In a preferred embodiment, the aqueous polymer dispersant is at least one of sodium carboxymethyl cellulose, carboxyethyl cellulose, polyacrylic acid, sodium polyacrylate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene glycol.

[0007] In a preferred embodiment, the hydrophilic unsaturated carboxylic acid monomer I is at least one of β-acryloyloxypropionic acid, methacrylic acid, sodium methacrylate, lithium methacrylate, acrylic acid, sodium acrylate, lithium acrylate, itaconic acid, sodium itaconate, lithium itaconate, maleic acid, sodium maleate, lithium maleate, crotonic acid, sodium crotonate, lithium crotonate, fumaric acid, sodium fumarate, lithium fumarate, cinnamic acid, sodium cinnamate, and lithium cinnamate.

[0008] In a preferred embodiment, the hydrophobic unsaturated nitrile monomer I is acrylonitrile or / and methacrylonitrile.

[0009] In a preferred embodiment, the hydrophilic unsaturated carboxylic acid monomer II is at least one of β-acryloyloxypropionic acid, methacrylic acid, sodium methacrylate, lithium methacrylate, acrylic acid, sodium acrylate, lithium acrylate, itaconic acid, sodium itaconate, lithium itaconate, maleic acid, sodium maleate, lithium maleate, crotonic acid, sodium crotonate, lithium crotonate, fumaric acid, sodium fumarate, lithium fumarate, cinnamic acid, sodium cinnamate, and lithium cinnamate.

[0010] In a preferred embodiment, the unsaturated amide monomer is at least one of acrylamide, methacrylamide, and dimethylaminopropylacrylamide.

[0011] In a preferred embodiment, the hydrophobic unsaturated nitrile monomer II is acrylonitrile or / and methacrylonitrile.

[0012] In a preferred embodiment, the oil-soluble initiator is at least one of benzoyl peroxide, dicyclohexyl peroxydicarbonate, diisopropylbenzene peroxide, di-tert-butyl peroxide, lauroyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxy pivalate, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobisisobutyronitrile, dimethyl 2,2'-azobis(2-methylpropionate), and 1,1'-azobis(cyclohexanecarbonitrile).

[0013] The present invention also provides a method for preparing the aqueous binder, comprising the following steps: S1. Add the aqueous polymer dispersant to water, heat and stir to dissolve, obtaining dispersion liquid I; S2. Add the hydrophilic unsaturated carboxylic acid monomer I, the hydrophobic unsaturated nitrile monomer I and the acrylate monomer to the dispersion liquid I, mix evenly, obtaining dispersion liquid II; Mix the hydrophilic unsaturated carboxylic acid monomer II, the unsaturated amide monomer and the hydrophobic unsaturated nitrile monomer II evenly, obtaining a mixed monomer; S3. Heat the dispersion liquid II, add the oil-soluble initiator to react, obtaining a seed dispersion; S4. Dropwise add the mixed monomer to the seed dispersion, and keep the temperature for reaction after dropping; After the reaction ends, adjust the pH of the reaction system to 7-9, and adjust the solid content to 1 wt% - 20 wt%, obtaining the aqueous binder.

[0014] In a preferred embodiment, in step S1, heat to 40 - 100 °C.

[0015] In a preferred embodiment, in step S2, mix evenly by stirring, and the stirring speed is 400 - 2000 rpm.

[0016] In a preferred embodiment, in step S3, heat the dispersion liquid II to 60 - 90 °C, and the reaction time is 2 - 4 h.

[0017] In a preferred embodiment, in step S4, the dropping duration is 2 - 4 hours, and the holding reaction time is 3 - 5 hours.

[0018] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: (1) In the aqueous binder of the present invention, a hydrophilic unsaturated carboxylic acid monomer and an acrylate monomer with a low Tg (glass transition temperature) and capable of increasing viscosity are introduced. While reducing the Tg of the copolymer, a low swelling degree and a high peel strength are maintained.

[0019] (2) The aqueous polymer dispersant adopted in the present invention has a good dispersion effect on the negative electrode slurry of the lithium-ion battery. Compared with the binder prepared by using a small molecule surfactant (or emulsifier), it can significantly reduce the internal resistance of the lithium-ion battery.

[0020] (3) In the present invention, the suspension polymerization method is adopted, which greatly improves the conversion rate of the hydrophobic monomer. The prepared active ingredient does not need to be separated, and no additional post-treatment processes such as a large amount of water washing are required, greatly saving the production cost.

[0021] (4) The slurry prepared by using the water-based binder provided by the present invention is coated on the negative electrode sheet of the lithium-ion battery, which can significantly improve the flexibility of the electrode sheet and the affinity for the electrolyte, and at the same time can also reduce the internal resistance of the lithium-ion battery. Detailed implementation manners

[0022] The following content describes the technical solutions of the present application clearly and completely in combination with the embodiments, so that those skilled in the art can fully understand the present application. Obviously, the described embodiments are only some preferred embodiments of the present application, rather than all embodiments. Any equivalent transformation or substitution made to the following embodiments by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0023] The detailed implementation manner of the present invention provides a water-based binder, which includes the following components in parts by mass: 0.1-10 parts of a water-based polymer dispersant, 10-20 parts of a hydrophilic unsaturated carboxylic acid monomer I, 60-70 parts of a hydrophobic unsaturated nitrile monomer I, 5-35 parts of an acrylate monomer, 40-55 parts of a hydrophilic unsaturated carboxylic acid monomer II, 5-20 parts of an unsaturated amide monomer, 20-40 parts of a hydrophobic unsaturated nitrile monomer, 0.1-1 part of an oil-soluble initiator, and 180-300 parts of water.

[0024] In some specific embodiments, the water-based polymer dispersant is at least one of sodium carboxymethyl cellulose, carboxyethyl cellulose, polyacrylic acid, sodium polyacrylate, polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and polyethylene glycol (PEG). For example, the water-based polymer dispersant is a 2 wt% aqueous solution of sodium carboxymethyl cellulose with a viscosity of 200-3000 mPa·s; typically, the water-based polymer dispersant is a 2 wt% aqueous solution of sodium carboxymethyl cellulose with a viscosity of 200 mPa·s, 300 mPa·s, 500 mPa·s, 600 mPa·s, 800 mPa·s, 1000 mPa·s, 2000 mPa·s, or 3000 mPa·s. Or the water-based polymer dispersant is PVA0598, PVA1788, PVA2499, PVA2699, PVA6098, or PVA9588. Or the water-based polymer dispersant is PVP K15, PVP K30, or PVP K90. Or the water-based polymer dispersant is PEG400, PEG600, PEG1000, PEG1500, PEG2000, PEG3000, PEG4000, or PEG6000.

[0025] In some specific embodiments, the oil-soluble initiator is at least one of benzoyl peroxide, dicyclohexyl peroxydicarbonate, diisopropylbenzene peroxide, di-tert-butyl peroxide, lauroyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxy pivalate, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobisisobutyronitrile, dimethyl 2,2'-azobis(2-methylpropionate), and 1,1'-azobis(cyclohexanecarbonitrile).

[0026] The specific embodiments of the present invention further provide a method for preparing the aqueous binder, including the following steps: S1. Add the aqueous polymer dispersant to water, heat to 40-100°C and stir (the stirring speed is generally 200-400 rpm) to dissolve, and then cool to 20-40°C to obtain dispersion liquid I; S2. Add the hydrophilic unsaturated carboxylic acid monomer I, the hydrophobic unsaturated nitrile monomer I, and the acrylate monomer to the dispersion liquid I, and stir and disperse evenly at a speed of 400-2000 rpm (generally stir for 1-2 h) to obtain dispersion liquid II; Mix the hydrophilic unsaturated carboxylic acid monomer II, the unsaturated amide monomer, and the hydrophobic unsaturated nitrile monomer II evenly to obtain a mixed monomer; S3. Heat the dispersion liquid II to 60-90°C, add the oil-soluble initiator, and keep the reaction at a constant temperature for 2-4 hours under stirring (the stirring speed is generally 300-400 rpm) to obtain a seed dispersion; S4. Dropwise add the mixed monomer to the seed dispersion, and finish dropping within 2-4 hours; After dropping, keep the reaction at a constant temperature of 60-90°C for 3-5 hours; After the reaction is completed, adjust the pH of the reaction system to 7-9 with an alkaline solution to obtain the aqueous binder.

[0027] In some specific embodiments, the alkaline solution in step S4 is a 10 wt% - 20 wt% sodium carbonate aqueous solution or / and a 5 wt% - 15% lithium hydroxide aqueous solution.

[0028] In some specific embodiments, water is added in step S4 to adjust the solid content of the aqueous binder to 1 wt% - 20 wt%, so that the viscosity of the aqueous binder is 300 - 500000 mPa·s.

[0029] It can be understood that the specific types of the hydrophilic unsaturated carboxylic acid monomer I and the hydrophilic unsaturated carboxylic acid monomer II in step S2 may be the same or different; the specific types of the hydrophobic unsaturated nitrile monomer I and the hydrophobic unsaturated nitrile monomer II may be the same or different.

[0030] The sodium carboxymethylcellulose 2200 used in the following examples is an aqueous solution of sodium carboxymethylcellulose with a mass percentage content of 2% and a viscosity of 2200 mPa·s.

[0031] Example 1 An aqueous binder, comprising the following components in parts by mass: 0.05 part of sodium carboxymethyl cellulose 2200 (aqueous polymer dispersant), 2 parts of PVA 1788 (aqueous polymer dispersant), 5 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer I), 10 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer I), 60 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer I), 5 parts of dibutyl itaconate (acrylate monomer), 30 parts of isobutyl acrylate (acrylate monomer), 40 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of acrylamide (unsaturated amide monomer), 25 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer II), 0.3 part of dimethyl 2,2'-azobis(2-methylpropionate) (oil-soluble initiator), and 300 parts of water.

[0032] The preparation method of the aqueous binder comprises the following steps: S1. Add the aqueous polymer dispersants (sodium carboxymethyl cellulose 2200 and PVA 1788) into water, heat to 85°C and stir to dissolve, keep warm for 2 h under stirring at 300 rpm, then cool down and stabilize at 40°C to obtain dispersion liquid I.

[0033] S2. Add the hydrophilic unsaturated carboxylic acid monomer I (acrylic acid and β-acryloyloxypropionic acid), the hydrophobic unsaturated nitrile monomer I (acrylonitrile), and the acrylate monomers (dibutyl itaconate and isobutyl acrylate) into dispersion liquid I, stir and disperse at a speed of 800 rpm for 1 h to obtain dispersion liquid II. Mix the hydrophilic unsaturated carboxylic acid monomer II (acrylic acid and β-acryloyloxypropionic acid), the unsaturated amide monomer (acrylamide), and the hydrophobic unsaturated nitrile monomer II (acrylonitrile), stabilize the temperature at 25°C (the temperature should not be too high to avoid monomer polymerization), and stir at a speed of 200 rpm for 30 min to obtain a mixed monomer.

[0034] S3. Heat dispersion liquid II to 65°C, add the oil-soluble initiator (dimethyl 2,2'-azobis(2-methylpropionate)) under stirring at a speed of 300 rpm, keep warm and react for 4 h to obtain a seed dispersion liquid.

[0035] S4. Dropwise add the mixed monomer into the seed dispersion liquid, and finish dropping within 2 h; after dropping, keep warm and react at 65°C and under stirring at a speed of 300 rpm for 5 h. After the reaction ends, adjust the pH of the reaction system to 7 with an alkaline solution (10 wt% lithium hydroxide aqueous solution), add water to adjust the solid content of the reaction system to 6 wt% to obtain the aqueous binder with a viscosity of 18000 mPa·s.

[0036] Example 2 An aqueous binder, comprising the following components in parts by mass: 0.05 part of sodium carboxymethyl cellulose 2200, 2 parts of PVA1788, 15 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer I), 60 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer I), 5 parts of dibutyl itaconate, 30 parts of isobutyl acrylate, 40 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of acrylamide, 25 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer II), 0.3 part of dimethyl 2,2'-azobis(2-methylpropionate), and 300 parts of water.

[0037] The preparation method of this aqueous binder is basically the same as that in Example 1, except that in step S2, there is only 15 parts of β-acryloyloxypropionic acid as the hydrophilic unsaturated carboxylic acid monomer I, and the stirring speed during the preparation of dispersion liquid II is 1200 rpm. The viscosity of the prepared aqueous binder is 21000 mPa·s.

[0038] Example 3 An aqueous binder, comprising the following components in parts by mass: 1 part of PVA 1788, 0.5 part of PVP K30, 5 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer I), 10 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer I), 60 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer I), 5 parts of dibutyl itaconate, 30 parts of isobutyl acrylate, 40 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of acrylamide, 25 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer II), 0.3 part of azobisisobutyronitrile, and 300 parts of water.

[0039] The preparation method of this aqueous binder is the same as that in Example 1 except for the specific types and amounts of some components. The viscosity of the prepared aqueous binder is 21000 mPa·s.

[0040] Example 4 An aqueous binder, comprising the following components in parts by mass: 2 parts of PVA 1788, 15 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer I), 70 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer I), 5 parts of dibutyl itaconate, 30 parts of n-butyl acrylate, 40 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer II), 20 parts of acrylamide, 25 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer II), 0.3 part of dimethyl 2,2'-azobis(2-methylpropionate), and 300 parts of water.

[0041] The preparation method of this aqueous binder is the same as that in Example 1 except for the specific types and amounts of some components. The viscosity of the prepared aqueous binder is 20500 mPa·s.

[0042] Example 5 An aqueous binder, comprising the following components in parts by mass: 0.05 part of sodium carboxymethyl cellulose 2200, 2 parts of PEG4000, 5 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer I), 15 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer I), 65 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer I), 5 parts of dibutyl itaconate, 20 parts of n-butyl acrylate, 10 parts of isobutyl acrylate, 10 parts of β-acryloyloxypropionic acid (hydrophilic unsaturated carboxylic acid monomer II), 30 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of acrylamide, 35 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer II), 0.3 part of dimethyl 2,2'-azobis(2-methylpropionate), and 300 parts of water.

[0043] The preparation method of the aqueous binder is the same as that in Example 1 except for the specific types and amounts of some components. The viscosity of the prepared aqueous binder is 18000 mPa·s.

[0044] Example 6 An aqueous binder, comprising the following components in parts by mass: 0.05 part of sodium carboxymethyl cellulose 2200, 2 parts of PVA1788, 10 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer I), 10 parts of methacrylic acid (hydrophilic unsaturated carboxylic acid monomer I), 60 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer I), 30 parts of n-butyl acrylate, 5 parts of isobutyl acrylate, 20 parts of acrylic acid (hydrophilic unsaturated carboxylic acid monomer II), 20 parts of methacrylic acid (hydrophilic unsaturated carboxylic acid monomer II), 10 parts of methacrylamide, 35 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer II), 0.3 part of dimethyl 2,2'-azobis(2-methylpropionate), and 300 parts of water.

[0045] The preparation method of the aqueous binder is the same as that in Example 1 except for the specific types and amounts of some components. The viscosity of the prepared aqueous binder is 20000 mPa·s.

[0046] Example 7 An aqueous binder, comprising the following components in parts by mass: 0.1 part of hydroxyethyl cellulose, 10 parts of itaconic acid (hydrophilic unsaturated carboxylic acid monomer I), 60 parts of methylacrylonitrile (hydrophobic unsaturated nitrile monomer I), 5 parts of methyl acrylate, 48 parts of itaconic acid (hydrophilic unsaturated carboxylic acid monomer II), 5 parts of acrylamide, 20 parts of methylacrylonitrile (hydrophobic unsaturated nitrile monomer II), 0.1 part of benzoyl peroxide, and 180 parts of water.

[0047] The preparation method of the aqueous binder comprises the following steps: S1. Add hydroxyethyl cellulose to water, heat to 40 °C and stir to dissolve. Keep it at 200 rpm stirring rate for 2 h, then cool down and stabilize at 20 °C to obtain dispersion I.

[0048] S2. Add itaconic acid, methylacrylonitrile and methyl acrylate to dispersion I, stir and disperse at a speed of 400 rpm for 1 h to obtain dispersion II. Mix itaconic acid, acrylamide and methylacrylonitrile, stabilize the temperature at 25 °C, and stir at a speed of 200 rpm for 60 min to obtain a mixed monomer.

[0049] S3. Heat dispersion II to 60 °C, add benzoyl peroxide at a stirring speed of 400 rpm, and keep the reaction at a constant temperature for 3 h to obtain a seed dispersion.

[0050] S4. Dropwise add the mixed monomer to the seed dispersion within 3 h; after dropping, keep the reaction at a constant temperature at 60 °C and a stirring speed of 300 rpm for 4 h. After the reaction, adjust the pH of the reaction system to 8 with 15 wt% lithium hydroxide aqueous solution, and add water to adjust the solid content of the reaction system to 1 wt% to obtain an aqueous binder with a viscosity of 300 mPa·s.

[0051] Example 8 An aqueous binder, comprising the following components in parts by mass: 10 parts of polyacrylic acid, 8 parts of maleic acid (hydrophilic unsaturated carboxylic acid monomer I), 8 parts of fumaric acid (hydrophilic unsaturated carboxylic acid monomer I), 35 parts of acrylonitrile (hydrophobic unsaturated nitrile monomer I), 30 parts of methylacrylonitrile (hydrophobic unsaturated nitrile monomer I), 6 parts of isooctyl acrylate, 2 parts of dodecyl acrylate, 35 parts of maleic acid (hydrophilic unsaturated carboxylic acid monomer II), 20 parts of fumaric acid (hydrophilic unsaturated carboxylic acid monomer II), 15 parts of dimethylaminopropyl acrylamide, 40 parts of methylacrylonitrile (hydrophobic unsaturated nitrile monomer II), 1 part of dicyclohexyl peroxydicarbonate, and 240 parts of water.

[0052] The preparation method of the aqueous binder comprises the following steps: S1. Add polyacrylic acid to water, heat to 100 °C and stir to dissolve. Keep it at 400 rpm stirring rate for 2 h, then cool down and stabilize at 35 °C to obtain dispersion I.

[0053] S2. Add maleic acid, fumaric acid, acrylonitrile, methylacrylonitrile, isooctyl acrylate and dodecyl acrylate to dispersion I, stir and disperse at a speed of 2000 rpm for 2 h to obtain dispersion II. Mix hydrophilic unsaturated carboxylic acid monomer II, acrylamide and hydrophobic unsaturated nitrile monomer II, stabilize the temperature at 25 °C, and stir at a speed of 200 rpm for 60 min to obtain a mixed monomer.

[0054] S3. Heat the dispersion II to 90 °C, add dicyclohexyl peroxydicarbonate at a stirring speed of 300 rpm, keep the temperature for reaction for 2 hours to obtain a seed dispersion.

[0055] S4. Dropwise add the mixed monomers to the seed dispersion and finish dropping within 4 hours. After the dropping is completed, keep the temperature for reaction for 3 hours at 90 °C and a stirring speed of 300 rpm. After the reaction ends, adjust the pH of the reaction system to 9 with 15 wt% aqueous sodium carbonate solution to obtain an aqueous binder with a solid content of 20 wt% and a viscosity of 500000 mPa·s.

[0056] Comparative Example 1 A preparation method of an aqueous binder includes the following steps: P1. Mix 200 parts of deionized water, 15 parts of β-acryloyloxypropionic acid, 7 parts of sodium hydroxide, 60 parts of acrylonitrile, 5 parts of dibutyl itaconate, and 30 parts of isobutyl acrylate, set the temperature to 40 °C, set the stirring rate to 800 rpm, and stir and disperse for 1 h. After the dispersion is completed, set the stirring rate to 300 rpm, raise the temperature to 70 °C, add 0.6 part of initiator ammonium persulfate, and react for 4 h to obtain a seed dispersion A. Sequentially mix 100 parts of water, 40 parts of acrylic acid, 10 parts of β-acryloyloxypropionic acid, 20 parts of sodium hydroxide, 10 parts of acrylamide, and 25 parts of acrylonitrile, start stirring, set the stirring rate to 200 rpm, keep the temperature stable at 25 °C, and stir for 30 min to obtain a mixed monomer I.

[0057] P2. Drop the seed dispersion A into the mixed monomer I, with a dropping time of 2 h. After the dropping is completed, add an additional 0.6 part of ammonium persulfate, keep the temperature at 70 °C and 300 rpm for 5 h, and collect the precipitate particles after the heat preservation to test the conversion rate.

[0058] The scheme of this comparative example belongs to the solution polymerization method, and the unsaturated carboxylic acid is partially neutralized. Since the acrylate monomers have poor water solubility and cannot be completely introduced in the same dosage ratio as the acrylate in Example 2, a large amount of residual monomers will be caused. When the introduction amount of acrylate monomers is insufficient, it has no effect on improving the flexibility of the electrode sheet.

[0059] Comparative Example 2 A preparation method of an aqueous binder includes the following steps: Q1. Mix 300 parts of deionized water, 15 parts of β - acryloyloxypropionic acid, and 60 parts of acrylonitrile. Set the temperature to 40°C, the stirring rate to 800 rpm, and stir and disperse for 1 h. After the dispersion is completed, set the stirring rate to 300 rpm, heat up to 70°C, add 0.6 parts of initiator ammonium persulfate, and react for 4 h to obtain seed dispersion B. Mix 40 parts of acrylic acid, 10 parts of β - acryloyloxypropionic acid, 10 parts of acrylamide, and 25 parts of acrylonitrile, start stirring, with a stirring rate of 200 rpm, and keep the temperature stable at 25°C, and stir for 30 min to obtain mixed monomer II.

[0060] Q2. Drop seed dispersion B into mixed monomer II, with a dropping time of 2 h. After the dropping is completed, add an additional 0.6 parts of ammonium persulfate, keep warm at 70°C and 300 rpm for 5 h, and collect the precipitate particles after the heat preservation to test the conversion rate.

[0061] The scheme of this comparative example belongs to the precipitation polymerization method. Compared with the scheme of the example, it does not use an aqueous polymer dispersant and uses a water - soluble initiator, which is a commonly used preparation method and formula for the aqueous binder of the negative electrode of lithium - ion batteries. Since no acrylate monomer is introduced, problems such as cracking of the electrode sheet will occur.

[0062] Comparative Example 3 A preparation method of an aqueous binder includes the following steps: R1. Mix 300 parts of deionized water, 2 parts of PVA 1788, and 0.05 parts of sodium carboxymethylcellulose 2200. Start stirring (stirring speed is 200 rpm) and heat up to 85°C, and keep warm for 2 h. After the heat preservation is completed, cool down and stabilize at 40°C to obtain dispersion III.

[0063] R2. Add 15 parts of β - acryloyloxypropionic acid and 60 parts of acrylonitrile to dispersion III. Set the temperature to 40°C, the stirring rate to 800 rpm, and stir and disperse for 1 h. After the dispersion is completed, set the stirring rate to 300 rpm, heat up to 65°C, add 0.6 parts of initiator azobisisobutyronitrile, and react for 4 h to obtain seed dispersion C. Mix 40 parts of acrylic acid, 10 parts of β - acryloyloxypropionic acid, 10 parts of acrylamide, and 25 parts of acrylonitrile, start stirring, with a stirring rate of 200 rpm, and keep the temperature stable at 25°C, and stir for 30 min to obtain mixed monomer III.

[0064] R3. Drop seed dispersion C into mixed monomer III, with a dropping time of 2 h. After the dropping is completed, add an additional 0.6 parts of azobisisobutyronitrile, keep warm at 65°C for 5 h, with a stirring rate of 300 rpm, and collect the precipitate particles after the heat preservation to test the conversion rate.

[0065] The solution of this comparative example belongs to the precipitation polymerization method, and an aqueous polymer dispersant is added on the basis of Comparative Example 2.

[0066] Comparative Example 4 A method for preparing an aqueous binder, comprising the following steps: T1. Mix 300 parts of deionized water, 1.5 parts of sodium dodecyl sulfate (SDS), 0.5 part of sodium styrenesulfonate (SSS), and 0.05 part of sodium carboxymethyl cellulose 2200, start stirring, and heat up to 40 °C to obtain dispersion liquid IV.

[0067] T2. Add 25 parts of β-acryloyloxypropionic acid, 40 parts of acrylic acid, 85 parts of acrylonitrile, 10 parts of acrylamide, 5 parts of dibutyl itaconate, and 30 parts of isobutyl acrylate to dispersion liquid IV, set the temperature to 40 °C, set the stirring rate to 800 rpm, and stir and disperse for 1 h. After the dispersion is completed, set the stirring rate to 300 rpm, heat up to 75 °C, add 0.6 part of initiator ammonium persulfate, react for 4 h, add 1 part of ammonium persulfate, heat up to 80 °C and keep the temperature for reaction for 3 h, then cool down to 40 °C, add an alkaline solution (such as 10 wt% lithium hydroxide aqueous solution) to neutralize the product, and adjust the pH value to 7.

[0068] The solution of this comparative example belongs to emulsion polymerization.

[0069] Performance test of the aqueous binder The performance of the aqueous binders prepared in Examples 1 to 6 and Comparative Examples 1 to 4 was tested according to the following method, and the test results are shown in Table 1.

[0070] 1. Conversion rate determination: The total weight of all monomers and dispersants in the reaction feedstock is denoted as M0. After the reaction is completed, the product is filtered with a 400-mesh filter cloth, and the obtained precipitate particles are dried in a blast dryer at 160 °C for 4 h and weighed and denoted as M1. The conversion rate (%) = M1 / M0 × 100%.

[0071] 2. Preparation of slurry and negative electrode sheet: Mix artificial graphite as the negative electrode active material, conductive carbon black (super-P), styrene-butadiene rubber latex, sodium carboxymethyl cellulose, and the aqueous binder in a mass ratio of 97:1:0.25:0.25:1.5, add deionized water, and stir and mix evenly with a vacuum mixer to obtain a slurry. The aforementioned slurry is evenly coated on both sides of the negative electrode current collector copper foil, and after drying, cold pressing, and slitting, a negative electrode sheet is obtained.

[0072] 3. Determination of slurry dispersibility: Use a QXD scraping fineness meter, single groove 0 - 25 μm, scale 2.5 μm.

[0073] 4. Determination of the flexibility of the electrode sheet: Bend the prepared electrode sheet by a certain angle and observe whether the surface cracks or peels off. Record the maximum bending angle: the critical bending angle at which the electrode sheet cracks or peels off (the larger the angle, the better the flexibility). Record the number of bending times: the number of times the electrode sheet is repeatedly bent at a specific angle until the coating cracks or peels off.

[0074] 5. Determination of the peel strength: The test method refers to the method in ASTM D3330 of the American Society for Testing and Materials. The test equipment uses a universal testing machine, and the applicable range refers to the regulations in GB / T 2792-2014 "Test Method for Peel Strength of Adhesive Tapes".

[0075] 6. Determination of the swelling degree of the adhesive film: Place the binder in a normal-temperature, ventilated and dry environment to form an adhesive film, vacuum dry it at 60 °C for 72 h, cut out an adhesive film with a size of 2 mm × 6 mm, and weigh it as M2. Place the adhesive film in the electrolyte (a mixture of ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1), keep it in an oven at 70 °C for 72 h, dry the surface of the adhesive film with filter paper, and weigh it as M3. The swelling degree (%) = (M3 - M2) / M2 × 100%.

[0076] 7. Determination of the internal resistance of the battery: Assemble a conventional 2025 button battery, use polyolefin as the separator, the solvent is a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate with a volume ratio of 1:1:1, the content of electrolyte LiPF6 is 1 mol / L, and use a resistance tester to measure the resistance.

[0077] Table 1 Test results of the performance of the water-based binder

[0078] As can be seen from Table 1, in Comparative Example 1, due to the poor water solubility of the acrylate monomer, a large amount of monomer residues did not participate in the reaction, resulting in a significant decrease in the conversion rate. In Comparative Example 2, since no acrylate monomer was used, the conversion rate was significantly increased compared with Comparative Example 1. However, the flexibility of the negative electrode sheet prepared with the water-based binder of Comparative Example 2 decreased significantly. Compared with Comparative Example 2, in Comparative Example 3, a water-based polymer dispersant was added, which helped to disperse the monomer, significantly increased the conversion rate, and the flexibility of the negative electrode sheet also increased significantly. In Comparative Example 4, small molecule emulsifiers (SDS and SSS) were introduced, and the internal resistance increased significantly.

[0079] The above-described embodiments are only the preferred embodiments of the present application and are not used to limit the protection scope of the present application. For any person skilled in the art, various changes and modifications can be made to the present application. Any simple equivalent changes and modifications made based on the protection scope of the present application and the content of the specification shall be included in the protection scope of the present application.

Claims

1. An aqueous binder for the negative electrode of a lithium-ion battery, characterized in that, Comprising the following components in parts by mass: 0.1 to 10 parts of an aqueous polymer dispersant, 10 to 20 parts of a hydrophilic unsaturated carboxylic acid monomer I, 60 to 70 parts of a hydrophobic unsaturated nitrile monomer I, 5 to 35 parts of an acrylate monomer, 40 to 55 parts of a hydrophilic unsaturated carboxylic acid monomer II, 5 to 20 parts of an unsaturated amide monomer, 20 to 40 parts of a hydrophobic unsaturated nitrile monomer II, 0.1 to 1 part of an oil-soluble initiator, and 180 to 300 parts of water.

2. The aqueous binder according to claim 1, wherein The aqueous polymer dispersant is at least one of sodium carboxymethyl cellulose, carboxyethyl cellulose, polyacrylic acid, sodium polyacrylate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene glycol; and / or, the hydrophilic unsaturated carboxylic acid monomer I is at least one of β-acryloyloxypropionic acid, methacrylic acid, sodium methacrylate, lithium methacrylate, acrylic acid, sodium acrylate, lithium acrylate, itaconic acid, sodium itaconate, lithium itaconate, maleic acid, sodium maleate, lithium maleate, crotonic acid, sodium crotonate, lithium crotonate, fumaric acid, sodium fumarate, lithium fumarate, cinnamic acid, sodium cinnamate, and lithium cinnamate.

3. The aqueous binder according to claim 1, characterized in that, The hydrophobic unsaturated nitrile monomer I is acrylonitrile and / or methacrylonitrile; and / or, the hydrophilic unsaturated carboxylic acid monomer II is at least one of β-acryloyloxypropionic acid, methacrylic acid, sodium methacrylate, lithium methacrylate, acrylic acid, sodium acrylate, lithium acrylate, itaconic acid, sodium itaconate, lithium itaconate, maleic acid, sodium maleate, lithium maleate, crotonic acid, sodium crotonate, lithium crotonate, fumaric acid, sodium fumarate, lithium fumarate, cinnamic acid, sodium cinnamate, and lithium cinnamate.

4. The aqueous binder according to claim 1, wherein The unsaturated amide monomer is at least one of acrylamide, methacrylamide, and dimethylaminopropyl acrylamide; and / or, the hydrophobic unsaturated nitrile monomer II is acrylonitrile and / or methacrylonitrile.

5. The aqueous binder according to claim 1, wherein The oil-soluble initiator is at least one of benzoyl peroxide, dicyclohexyl peroxydicarbonate, diisopropylbenzene peroxide, di-tert-butyl peroxide, lauroyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxy pivalate, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobisisobutyronitrile, dimethyl 2,2'-azobis(2-methylpropionate), and 1,1'-azobis(cyclohexanecarbonitrile).

6. The preparation method of the aqueous binder according to any one of claims 1 to 5, characterized in that, Comprising the following steps: S1. Add the aqueous polymer dispersant to water, heat and stir to dissolve to obtain dispersion liquid I. S2. Add the hydrophilic unsaturated carboxylic acid monomer I, the hydrophobic unsaturated nitrile monomer I, and the acrylate monomer to the dispersion liquid I, mix evenly to obtain dispersion liquid II; mix the hydrophilic unsaturated carboxylic acid monomer II, the unsaturated amide monomer, and the hydrophobic unsaturated nitrile monomer II evenly to obtain a mixed monomer. S3. Heat the dispersion liquid II, add the oil-soluble initiator to react to obtain a seed dispersion liquid. S4. Dropwise add the mixed monomer to the seed dispersion liquid, and keep the temperature for reaction after the addition is complete. After the reaction is completed, adjust the pH of the reaction system to 7 to 9, and adjust the solid content to 1 wt% to 20 wt% to obtain the aqueous binder.

7. The preparation method according to claim 6, characterized in that, In step S1, heat to 40~100°C; or / and in step S2, mix evenly by stirring, and the stirring speed is 400~2000 rpm.

8. The preparation method according to claim 6, characterized in that, In step S3, heat the dispersion liquid II to 60~90°C, and the reaction time is 2~4 h.

9. The preparation method according to claim 6, characterized in that, In step S4, the dropping duration is 2~4 hours, and the holding reaction time is 3~5 hours.

10. Use of the aqueous binder according to any one of claims 1~5 in the preparation of a lithium-ion battery.

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