A water-based binder and its preparation method and application

Abstract

The present invention belongs to the technical field of binder material preparation, and specifically relates to a water-based binder, its preparation method and application. The preparation method comprises (1) preparing a protective glue solution; adding a first polymerizable monomer and an initiator to the protective glue solution, and carrying out a polymerization reaction; the first polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, and an acrylamide monomer; the content of the unsaturated nitrile monomer in the first polymerizable monomer is not less than 15wt%; (2) adding a second polymerizable monomer and an initiator to the product of step (1), and carrying out a polymerization reaction; the second polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, an acrylamide monomer and a post-crosslinking monomer. When the water-based binder prepared by the preparation method is applied to the negative electrode plate, it not only has a good bonding effect but also leaves sufficient lithium ion transmission channels between the negative electrode active material and the conductive agent, effectively reducing the internal resistance of the plate and improving the electrochemical performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of adhesive material preparation, and in particular relates to a water-based adhesive and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries are widely used in energy storage, electric vehicles, and consumer electronics due to their high energy density, long cycle life, and low self-discharge rate. Lithium-ion batteries are mainly composed of a positive electrode, a negative electrode, an electrolyte, a separator, and various auxiliary materials (conductive agents, binders). Among them, the binder is crucial in lithium batteries. It can firmly adhere the electrode active material, conductive agent, and dispersant to the current collector to form a complete electrode sheet. At the same time, the binder effectively limits the volume expansion and shedding of the active material during the charge and discharge process, and can also reduce the interface impedance between the current collector and the electrode material.

[0003] Lithium-ion battery binders can be divided into two categories, oily and water-based, depending on the dispersion medium. The positive electrode often uses the oily binder PVDF, but its use requires the addition of large amounts of N-methylpyrrolidone (NMP), which is toxic and harmful to the environment. The negative electrode, on the other hand, often uses more environmentally friendly water-based binders, such as polyvinyl alcohol (PVA), sodium carboxymethyl cellulose (CMC), styrene-butadiene latex (SBR), polyacrylic acid (PAA), and polyacrylonitrile (PAN). SBR is currently the most widely used negative electrode binder. Because it lacks suspension and dispersion capabilities, it is usually used with the thickener CMC to improve the dispersibility of the slurry. However, the addition of CMC can introduce new problems. Adding too much can increase the battery's impedance; adding too little can lead to problems such as low peel strength and high swelling.

[0004] Polyacrylic acid binder (PAA) is an emerging mainstream negative electrode binder, which usually contains a large number of carboxyl groups. On the one hand, it can form hydrogen bonds between the negative electrode active material and the current collector, thereby exhibiting excellent bonding performance; on the other hand, it can form intramolecular hydrogen bonds to exhibit strong cohesive force. Combined with its oleophobic structure, it will not be easily swollen by the electrolyte.

[0005] Although the solution-type PAA binder provided by the existing technology has good resistance to electrolyte swelling and bonding properties, PAA usually coats the active material and conductive agent more densely, and the polymer itself does not have conductive ability, resulting in high impedance of lithium-ion batteries, which has an adverse effect on the battery's charge and discharge performance, cycle performance and temperature characteristics. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the defects of the solution-type PAA binder in the prior art, such as causing high battery impedance, affecting the battery's charge and discharge performance, cycle performance and temperature characteristics, thereby providing a water-based binder and its preparation method and application.

[0007] To this end, the present invention provides the following technical solutions.

[0008] The present invention provides a method for preparing a water-based binder, comprising the following steps:

[0009] (1) preparing a protective colloid solution; adding a first polymerizable monomer and an initiator to the protective colloid solution to initiate a polymerization reaction; the first polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, and an acrylamide monomer; and the content of the unsaturated nitrile monomer in the first polymerizable monomer is not less than 15 wt %;

[0010] (2) adding a second polymerizable monomer and an initiator to the product of step (1) to carry out a polymerization reaction; the second polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, an acrylamide monomer and a post-crosslinking monomer;

[0011] Based on the total weight of the first polymerized monomer and the second polymerized monomer, the total content of the unsaturated nitrile monomer in the first polymerized monomer and the second polymerized monomer is 65-75 wt %.

[0012] The first polymerized monomer accounts for 10-25 wt% of the total weight of the first polymerized monomer and the second polymerized monomer.

[0013] Furthermore, the preparation method satisfies at least one of (1)-(2):

[0014] (1) The mass of the unsaturated carboxylic acid monomer in the first polymerized monomer is not higher than 60 wt % of the total mass of the unsaturated carboxylic acid monomer in the first polymerized monomer and the second polymerized monomer;

[0015] (2) The total content of the unsaturated carboxylic acid monomer and the acrylamide monomer in the first polymerizable monomer is less than 85 wt %.

[0016] Furthermore, based on the total mass of the first polymerized monomer and the second polymerized monomer, the total content of the unsaturated carboxylic acid monomer is 15-30 wt%, and / or the total content of the acrylamide monomer is 1-15 wt%, and / or the content of the post-crosslinking monomer is not higher than 5 wt%.

[0017] Furthermore, the preparation method satisfies at least one of (1) to (6):

[0018] (1) The unsaturated carboxylic acid monomer is at least one of acrylic acid, methacrylic acid, maleic acid and itaconic acid;

[0019] (2) the unsaturated carboxylic acid monomer is at least one of acrylic acid and methacrylic acid;

[0020] (3) The unsaturated nitrile monomer is at least one of acrylonitrile, α-haloacrylonitrile, and α-alkylacrylonitrile;

[0021] (4) The unsaturated nitrile monomer is acrylonitrile;

[0022] (5) The post-crosslinking monomer is a compound containing an acetoacetyl group;

[0023] (6) The post-crosslinking monomer is at least one of allyl acetoacetate and acetoacetoxyethyl methacrylate.

[0024] Furthermore, the preparation method satisfies at least one of (1)-(2):

[0025] (1) The protective colloid is at least one of polyvinyl alcohol, polyvinyl pyrrolidone and sodium carboxymethyl cellulose;

[0026] (2) The amount of the protective glue added is not higher than 1 wt% of the total mass of the first polymerized monomer and the second polymerized monomer.

[0027] Furthermore, the preparation method satisfies at least one of (1)-(2):

[0028] (1) The polymerization reaction temperature in step (1) is 30-90° C. and the reaction time is no more than 2 hours;

[0029] (2) The polymerization reaction temperature in step (2) is 30-90° C. and the polymerization time is 1.5-5 h.

[0030] The present invention also provides a water-based adhesive prepared by the above preparation method.

[0031] Furthermore, the aqueous binder satisfies at least one of (1)-(2):

[0032] (1) The solid content of the aqueous binder is 5-20%;

[0033] (2) The viscosity of the aqueous binder is 5000-50000 mPa·s.

[0034] The present invention also provides an aqueous binder prepared by the above preparation method or the use of the above aqueous binder as a binder for negative electrode active materials.

[0035] The technical solution of the present invention has the following advantages:

[0036] 1. The present invention provides a method for preparing an aqueous binder, comprising: (1) preparing a protective colloid solution; adding a first polymerizable monomer and an initiator to the protective colloid solution, and carrying out a polymerization reaction; the first polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, and an acrylamide monomer; the content of the unsaturated nitrile monomer in the first polymerizable monomer is not less than 15 wt%; (2) adding a second polymerizable monomer and an initiator to the product of step (1), and carrying out a polymerization reaction; the second polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, an acrylamide monomer, and a post-crosslinking monomer; based on the total mass of the first polymerizable monomer and the second polymerizable monomer, the total content of the unsaturated nitrile monomer in the first polymerizable monomer and the second polymerizable monomer is 65-75 wt%. When the aqueous binder prepared by the preparation method is applied to a negative electrode plate, it not only has a good bonding effect but also leaves sufficient lithium ion transmission channels between the negative electrode active material and the conductive agent, effectively reducing the internal resistance of the plate and improving the electrochemical performance of the battery.

[0037] 2. The aqueous adhesive provided by the present invention uses water as a dispersion medium and does not require the use of additional organic solvents during the synthesis and subsequent electrode processing processes, making it an environmentally friendly adhesive. DETAILED DESCRIPTION

[0038] The following examples are provided for a better understanding of the present invention and are not intended to limit the best mode of implementation. They do not limit the content and scope of protection of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0039] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0040] Existing solution-based polyacrylic acid binders offer good resistance to electrolyte swelling and excellent bonding properties. However, PAA binders typically provide a dense coating of active materials and conductive agents, while the polymer itself lacks conductivity. This results in high impedance in lithium-ion batteries, adversely affecting the battery's charge-discharge performance, cycling performance, and temperature characteristics. To address these issues, the present invention provides the following technical solutions.

[0041] In a first aspect, the present invention provides a method for preparing a water-based binder, comprising:

[0042] (1) preparing a protective gel solution; adding the protective gel solution to a reactor, introducing an inert gas, stirring, heating, adding a first polymerization monomer and an initiator, and performing a polymerization reaction; after the reaction is completed, adding a neutralizer to adjust the pH;

[0043] The first polymerizable monomers include unsaturated carboxylic acid monomers, unsaturated nitrile monomers and acrylamide monomers; the content of the unsaturated nitrile monomers in the first polymerizable monomers is not less than 15 wt %;

[0044] (2) adding a second polymerization monomer and an initiator to the product of step (1) to initiate a polymerization reaction; after the reaction is completed, adding and eliminating an initiator aqueous solution and keeping the temperature; after the insulation is completed, adjusting the temperature in the kettle to 60-90° C., adding a neutralizer to adjust the pH, and obtaining a dispersed water-based binder.

[0045] The second polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, an acrylamide monomer and a post-crosslinking monomer; based on the total mass of the first polymerizable monomer and the second polymerizable monomer, the content of the unsaturated nitrile monomer in the first polymerizable monomer and the second polymerizable monomer is 65-75 wt %.

[0046] When the binder prepared by the present invention is applied to the negative electrode plate, it not only has a good bonding effect but also leaves sufficient lithium ion channels between the negative electrode active material and the conductive agent, effectively reduces the internal resistance of the plate, and improves the electrochemical performance of the battery. In step (1) of the present invention, the proportion of the polymerized monomers is precisely regulated to ensure that the product of step (1) has good water solubility while also having a certain hydrophobicity, and acts as a solubilizing agent, so that the intermediate product obtained in this step has a good solubilizing effect on the polymerized monomers and polymers in step (2) (the polymer includes the polymerization intermediate product, polymerization final product, etc. of step (2)). In step (1), the content of the unsaturated nitrile monomer is regulated to be not less than 15wt%, so that the product of this step has a certain hydrophobicity. The binder prepared by the present invention has a hydrophilic-hydrophobic structure and is a water-dispersible binder. When used as a binder for negative electrode active materials, it exhibits low internal resistance performance.

[0047] Furthermore, the intermediate product obtained in step (1) provides a uniform polymerization environment for the polymerization monomers in step (2), does not participate in the polymerization reaction in step (2), and plays a solubilizing agent role, which can fully ensure that each monomer in step (2) is uniformly copolymerized; as the molecular weight of the polymerization product gradually increases, due to its relatively hydrophobic structure, it associates with each other, driving the intermediate product in step (1) to precipitate from water together, and forming an aqueous dispersion under stirring. Since the intermediate product in step (1) has a large number of carboxyl units, it has excellent stability in water by virtue of the ion repulsion effect after alkali neutralization, providing a necessary condition for the stable existence of the final microscopic form of the binder. The present invention regulates the total addition amount of the unsaturated nitrile monomer to be 65-75wt% of the total mass of the first polymerization monomer and the second polymerization monomer, and controls the content of the unsaturated nitrile monomer in the first polymerization monomer in step (1) to be not less than 15wt%, so that the polymer can be soluble in water while also having a certain hydrophobic structure; in step (2), a large amount of unsaturated nitrile monomer is added, and the polymer component with a unique hydrophilic and hydrophobic structure is obtained in combination with the intermediate product in step (1), thereby further forming an aqueous dispersion.

[0048] The aqueous binder produced by the present invention is a water-dispersible binder. When applied to the negative electrode, this water-dispersible binder forms a point bond with the active material, unlike the surface bond between the solution-based PAA binder and the active material in the prior art. Compared to the prior art, the aqueous-dispersible binder of the present invention provides a smoother conduction path for lithium ions between the active material and the conductive agent, resulting in a lower internal resistance of the electrode sheet. The binder provided by the present invention has good affinity with the active material and provides good suspension stability for the active material, making the prepared slurry less prone to sedimentation.

[0049] In an optional embodiment, the first polymerized monomer accounts for 10-25wt% of the total weight of the first polymerized monomer and the second polymerized monomer. Alternatively, the first polymerized monomer accounts for 13-20% of the total weight of the first polymerized monomer and the second polymerized monomer. Controlling the content of the first polymerized monomer in the present invention helps to ensure that the molecular weight of the intermediate product of step (1) is less than the molecular weight of the product of step (2), and the product of step (1) has no cross-linked structure. If the content of the first polymerized monomer is too high, it will affect the bonding performance of the adhesive.

[0050] In an optional embodiment, the amount of the unsaturated carboxylic acid monomer in the first polymerized monomer is not more than 60 wt% of the total mass of the unsaturated carboxylic acid monomers in the first polymerized monomer and the second polymerized monomer. If the content of the unsaturated carboxylic acid monomer in the second polymerized monomer is too low, the hydrophobicity of the product of step (2) will become stronger, and it will be easy to precipitate from the system, which is not conducive to stability.

[0051] In an optional embodiment, the total content of the unsaturated carboxylic acid monomer and the acrylamide monomer in the first polymerization monomer is less than 85 wt %, which can further ensure the hydrophilicity and hydrophobicity of the product of step (1) and have a solubilizing effect on the polymerization process of step (2).

[0052] In an optional embodiment, based on the total mass of the first polymerized monomer and the second polymerized monomer, the total content of the unsaturated carboxylic acid monomer is 15-30 wt%, and / or the total content of the acrylamide monomer is 1-15 wt%, and / or the content of the post-crosslinking monomer is not higher than 5 wt%.

[0053] In an optional embodiment, the unsaturated carboxylic acid monomer is at least one of acrylic acid, methacrylic acid, maleic acid and itaconic acid;

[0054] The unsaturated carboxylic acid monomer is at least one of acrylic acid and methacrylic acid;

[0055] In an optional embodiment, the unsaturated nitrile-based monomer is at least one of acrylonitrile, α-haloacrylonitrile, and α-alkylacrylonitrile; for example, methacrylonitrile, ethacrylonitrile, and the like.

[0056] The post-crosslinking monomer is an acetoacetyl-containing compound. It has a high-temperature self-crosslinking structure, chemically crosslinking during the baking process of the electrode, giving the binder excellent bonding strength and resistance to electrolyte swelling. The diketone structure in the post-crosslinking monomer chelates with the metal, further enhancing the binder's adhesion to the current collector.

[0057] In an optional embodiment, the protective colloid is at least one of polyvinyl alcohol, polyvinyl pyrrolidone and sodium carboxymethyl cellulose;

[0058] The amount of the protective glue added is not higher than 1 wt % of the total weight of the first polymerized monomer and the second polymerized monomer.

[0059] In an optional embodiment, the polymerization reaction temperature in step (1) is 30-90°C and the time is no more than 2 hours; alternatively, the polymerization reaction temperature in step (1) is 35-85°C and the time is 0.5-1.5 hours. As an example, the polymerization reaction temperature is 30°C, 40°C, 50°C, 60°C, 70°C, 90°C, or any two values ​​within the range.

[0060] In an optional embodiment, the polymerization reaction temperature in step (2) is 30-90°C and the time is 1.5-5 hours; alternatively, the polymerization reaction temperature in step (2) is 35-85°C and the time is 2-4 hours. As an example, the polymerization reaction temperature is 30°C, 40°C, 50°C, 60°C, 70°C, 90°C, or any two values ​​within the range.

[0061] In an optional embodiment, the time of holding the aqueous solution of the initiator after adding and eliminating the initiator in step (2) is 1-3 hours, preferably 1.5-2 hours. The present invention can further improve the conversion rate of the polymerization monomer and reduce the residual monomer in the polymer by eliminating the initiator after adding.

[0062] In an optional embodiment, in step (1) and step (2), the neutralizing agent is selected from at least one of Na2CO3, K2CO3, Li2CO3, NaOH, KOH, and LiOH aqueous solutions.

[0063] In an optional embodiment, the initiator is a type of initiator known in the art, and the initiator is a thermal initiator or a redox system initiator. The oxidizing agent in the redox system initiator is at least one of ammonium persulfate, potassium persulfate, sodium persulfate, tert-butyl hydroperoxide, and hydrogen peroxide, preferably ammonium persulfate and potassium persulfate. The reducing agent is at least one of isoascorbic acid, sodium metabisulfite, sodium bisulfite, and sodium hydrosulfite, preferably isoascorbic acid and sodium metabisulfite. The thermal initiator can be ammonium persulfate (APS), etc., and the amount of the thermal initiator added is an amount known in the art.

[0064] In an optional embodiment, the total amount of the oxidant in step (1) and step (2) is 0.11 wt% of the total amount of the first polymerized monomer and the second polymerized monomer, preferably 0.20.8 wt%; the total amount of the reducing agent in step (1) and step (2) is 0.11 wt% of the total amount of the first polymerized monomer and the second polymerized monomer, preferably 0.20.8 wt%.

[0065] In step (1), the amount of the oxidant is 1-25 wt %, preferably 5-20 wt %, of the total amount of the oxidants in step (1) and step (2); the amount of the reducing agent is 1-25 wt %, preferably 5-20 wt % of the total amount of the reducing agents in step (1) and step (2). In step (2), the amount of the oxidant is 75-99 wt %, preferably 80-95 wt % of the total amount of the oxidants in step (1) and step (2); the amount of the reducing agent is 75-99 wt %, preferably 80-95 wt % of the total amount of the reducing agents in step (1) and step (2).

[0066] Secondly, the present invention provides a water-based binder in which the polymer molecular chains tend to cluster rather than stretch in water. Compared with solution-type PAA binders, it still has a suitable viscosity at a higher solid content, which facilitates the subsequent slurry configuration and electrode coating process.

[0067] The aqueous binder satisfies at least one of (1)-(2):

[0068] (1) The solid content of the aqueous binder is 5-20%;

[0069] (2) The viscosity of the aqueous binder is 5000-50000 mPa·s.

[0070] In a third aspect, the present invention provides the use of an aqueous binder as a binder for a negative electrode active material, wherein the negative electrode active material is a common material in the art, such as artificial graphite, natural graphite, hard carbon, silicon carbon, etc.

[0071] The present application is further described in detail below with reference to specific examples. These examples are not to be construed as limiting the scope of protection claimed in this application. Unless otherwise specified, the chemical reagents used in each example and comparative example are commercially available and are of analytical grade. The main raw materials are as follows:

[0072] Acrylic acid (AA), manufacturer: Wanhua Chemical; methacrylic acid (MAA), manufacturer: Wanhua Chemical; acrylamide (AM), manufacturer: Wanhua Chemical; acrylonitrile (AN), manufacturer: Wanhua Chemical; allyl acetoacetate (AAA), manufacturer: Myrel; ammonium persulfate (APS), manufacturer: Jinan Jinhao Chemical Co., Ltd.; sodium persulfate (SPS), manufacturer: Jinan Jinhao Chemical Co., Ltd.; sodium metabisulfite (Na2S2O5), manufacturer: Wanhua Chemical; sodium carboxymethyl cellulose (CMC), manufacturer: Sinopec Sichuan Wei Co., Ltd.; polyvinyl alcohol (PVA), manufacturer: Sinopharm Chemical; polyvinyl pyrrolidone (PVP), manufacturer: Sinopharm Chemical; sodium hydroxide (NaOH), manufacturer: Inotech; conductive carbon black (Super-P), manufacturer: Yiruishi Group; graphite (G), manufacturer: Shanshan Co., Ltd.

[0073] Example 1

[0074] This embodiment provides a method for preparing a water-based binder, comprising the following steps:

[0075] (1) Mix 60g AA, 5g AM and 25g AN to obtain a first polymerization monomer, which is set aside. Mix 80g AA, 20g AM, 300g AN and 10g AAA to obtain a second polymerization monomer, which is set aside. Dissolve 0.5g SPS (sodium persulfate) in 30g deionized water to obtain an oxidant solution, and dissolve 0.25g Na2S2O5 in 30g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as the first initiator, which is set aside. Dissolve 2.5g SPS in 50g deionized water to obtain an oxidant solution, and dissolve 1.25g Na2S2O5 in 50g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as the second initiator, which is set aside. Dissolve 3g SPS in 50g deionized water to obtain an oxidant, and dissolve 1.5g Na2S2O5 in 50g deionized water to obtain a reducing agent, which is used as a post-elimination initiator, which is set aside.

[0076] Add 3g of CMC and 2000g of deionized water to a reactor under a nitrogen atmosphere. Stir and heat to 45°C until dissolved to obtain a protective gel solution. Add the first polymerization monomer to the reactor, followed by the first initiator, dropwise over 15 minutes. After the addition is complete, incubate for 45 minutes. Add a 15wt% aqueous sodium hydroxide solution to adjust the pH to 7-8.

[0077] (2) Keeping the set temperature at 45°C constant, the second polymerization monomer and the second initiator are simultaneously added dropwise to the reactor for 3 hours. After the addition is completed, the temperature is kept at 1 hour. Then, the initiator is eliminated after the addition. The addition time is 1 hour. After the addition is completed, the temperature is kept at 1 hour. Then, the temperature is raised to 70°C. After the temperature stabilizes, a sodium hydroxide aqueous solution (15 wt%) is added to adjust the pH to 7-8. After the pH stabilizes, the temperature is kept at 2 hours. The material is filtered to obtain the target binder.

[0078] Example 2

[0079] This embodiment provides a preparation method of a water-based binder, which is basically the same as that of Example 1, except that: ① the heating temperature of the reactor is set to 80°C instead of 45°C; ② the first initiator, the second initiator, and the post-elimination initiator are different; in this embodiment, the first initiator uses APS instead of SPS and Na2S2O5, and the mass of APS is the total amount of SPS and Na2S2O5; the second initiator uses APS instead of SPS and Na2S2O5, and the mass of APS is the total amount of SPS and Na2S2O5; the post-elimination initiator uses APS instead of SPS and Na2S2O5, and the mass of APS is the total amount of SPS and Na2S2O5.

[0080] Example 3

[0081] This embodiment provides a method for preparing a water-based binder, which is basically the same as that of Example 1, except that polyvinyl alcohol or the like is used instead of CMC.

[0082] Example 4

[0083] This embodiment provides a method for preparing a water-based binder, comprising the following steps:

[0084] (1) 80g AA, 15g AM and 20g AN are mixed to obtain a first polymerization monomer, which is set aside. 60g AA, 15g AM, 305g AN and 5g AAA are mixed to obtain a second polymerization monomer, which is set aside. 0.6g SPS is dissolved in 30g deionized water to obtain an oxidant solution, and 0.3g Na2S2O5 is dissolved in 30g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a first initiator, which is set aside. 2.4g SPS is dissolved in 50g deionized water to obtain an oxidant solution, and 1.2g Na2S2O5 is dissolved in 50g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a second initiator, which is set aside. 3g SPS is dissolved in 50g deionized water to obtain an oxidant solution, and 1.5g Na2S2O5 is dissolved in 50g deionized water to obtain a reducing agent solution, which is used as a post-elimination initiator, which is set aside.

[0085] Add 3g of CMC and 2000g of deionized water to a reactor under a nitrogen atmosphere. Stir and heat to 45°C until dissolved to obtain a protective gel solution. Add the first polymerization monomer to the reactor, followed by the first initiator, dropwise over 15 minutes. After the addition is complete, incubate for 45 minutes. Add a 15wt% aqueous sodium hydroxide solution to adjust the pH to 7-8.

[0086] (2) Keeping the set temperature at 45°C constant, the second polymerization monomer and the second initiator are simultaneously added dropwise to the reactor for 3 hours. After the addition is completed, the temperature is kept at 1 hour. Then, the initiator is eliminated after the addition. The addition time is 1 hour. After the addition is completed, the temperature is kept at 1 hour. Then, the temperature is raised to 70°C. After the temperature stabilizes, a sodium hydroxide aqueous solution (15 wt%) is added to adjust the pH to 7-8. After the pH stabilizes, the temperature is kept at 2 hours. The material is filtered to obtain the target binder.

[0087] Example 5

[0088] This embodiment provides a method for preparing a water-based binder, comprising the following steps:

[0089] (1) Mix 30g AA, 5g AM and 25g AN to obtain a first polymerization monomer, which is set aside. Mix 90g AA, 5g AM, 330g AN and 15g AAA to obtain a second polymerization monomer, which is set aside. Dissolve 0.3g SPS in 30g deionized water to obtain an oxidant solution, and dissolve 0.15g Na2S2O5 in 30g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a first initiator, which is set aside. Dissolve 2.7g SPS in 50g deionized water to obtain an oxidant solution, and dissolve 1.35g Na2S2O5 in 50g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a second initiator, which is set aside. Dissolve 3g SPS in 50g deionized water to obtain an oxidant solution, and dissolve 1.5g Na2S2O5 in 50g deionized water to obtain a reducing agent solution, which is used as a post-elimination initiator, which is set aside.

[0090] Add 3g of CMC and 2000g of deionized water to a reactor under a nitrogen atmosphere. Stir and heat at 45°C until dissolved to obtain a protective gel solution. Add the first polymerization monomer to the reactor, followed by the first initiator, dropwise over 15 minutes. After the addition is complete, incubate for 45 minutes. Add a 15wt% aqueous sodium hydroxide solution to adjust the pH to 7-8.

[0091] (2) Keeping the set temperature at 45°C constant, the second polymerization monomer and the second initiator are simultaneously added dropwise to the reactor for 3 hours. After the addition is completed, the temperature is kept at 1 hour. Then, the initiator is eliminated after the addition. The addition time is 1 hour. After the addition is completed, the temperature is kept at 1 hour. Then, the temperature is raised to 70°C. After the temperature stabilizes, a sodium hydroxide aqueous solution (15 wt%) is added to adjust the pH to 7-8. After the pH stabilizes, the temperature is kept at 2 hours. The material is filtered to obtain the target binder.

[0092] Example 6

[0093] This embodiment provides a method for preparing a water-based binder, comprising the following steps:

[0094] (1) Mix 60g AA, 5g AM and 25g AN to obtain a first polymerization monomer, which is set aside. Mix 80g AA, 20g AM, 305g AN and 5g AAA to obtain a second polymerization monomer, which is set aside. Dissolve 0.5g SPS in 30g deionized water to obtain an oxidant solution, and dissolve 0.25g Na2S2O5 in 30g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a first initiator, which is set aside. Dissolve 2.5g SPS in 50g deionized water to obtain an oxidant solution, and dissolve 1.25g Na2S2O5 in 50g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a second initiator, which is set aside. Dissolve 3g SPS in 50g deionized water to obtain an oxidant solution, and dissolve 1.5g Na2S2O5 in 50g deionized water to obtain a reducing agent solution, which is used as a post-elimination initiator, which is set aside.

[0095] Add 3g of CMC and 2000g of deionized water to a reactor under a nitrogen atmosphere. Stir and heat to 30°C until dissolved to obtain a protective gel solution. Add the first polymerization monomer to the reactor, followed by the first initiator, dropwise over 15 minutes. After the addition is complete, incubate for 45 minutes. Add the solution (wt%) to adjust the pH to 7-8.

[0096] (2) Keeping the set temperature at 45°C constant, the second polymerization monomer and the second initiator were simultaneously added dropwise to the reactor for 1.5 hours. After the addition was completed, the temperature was kept at 1 hour. Then, the initiator was eliminated after the addition was completed. The addition time was 30 minutes. After the addition was completed, the temperature was kept at 1 hour. Then, the temperature was raised to 70°C. After the temperature stabilized, a sodium carbonate aqueous solution (15 wt%) was added to adjust the pH to 7-8. After the pH stabilized, the temperature was kept at 2 hours. The material was filtered to obtain the target binder.

[0097] Comparative Example 1

[0098] This comparative example provides a method for preparing a water-based binder, comprising the following steps:

[0099] (1) 140 g of AA, 25 g of AM, 325 g of AN, and 10 g of AAA were mixed to obtain a polymerization monomer, which was set aside. 3.0 g of SPS was dissolved in 80 g of deionized water to obtain an oxidant solution, and 1.5 g of Na2S2O5 was dissolved in 80 g of deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which was used as an initiator and set aside. 3 g of SPS was dissolved in 50 g of deionized water to obtain an oxidant solution, and 1.5 g of Na2S2O5 was dissolved in 50 g of deionized water to obtain a reducing agent solution, which was used as a post-elimination initiator and set aside.

[0100] 3g of CMC and 2000g of deionized water were added to a reactor, stirred under a nitrogen atmosphere, and heated to 45°C. The mixture was stirred and dissolved to obtain a protective adhesive solution. The polymerization monomer and initiator were simultaneously added dropwise to the reactor over a 3-hour period, followed by an incubation period of 1 hour. The initiator was then removed after the addition, with a 1-hour incubation period followed by an incubation period of 1 hour. This comparative example lacked the solubilization and protection of a macromolecular solubilizing agent during the reaction process, resulting in the relatively hydrophobic polymer product being unable to stably and uniformly exist in water and subsequently precipitating and lumping from the water. The polymer product precipitated in large quantities, exhibiting severe lumps and preventing the neutralization step, resulting in no binder.

[0101] Comparative Example 2

[0102] This comparative example provides a method for preparing a water-based binder, comprising the following steps:

[0103] (1) Mix 60g of AA and 5g of AM to obtain a first polymerization monomer, which is set aside. Mix 80g of AA, 20g of AM, 325g of AN, and 10g of AAA to obtain a second polymerization monomer, which is set aside. Dissolve 0.5g of SPS in 30g of deionized water to obtain an oxidant solution, and dissolve 0.25g of Na2S2O5 in 30g of deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a first initiator, which is set aside. Dissolve 2.5g of SPS in 50g of deionized water to obtain an oxidant solution, and dissolve 1.25g of Na2S2O5 in 50g of deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a second initiator, which is set aside. Dissolve 3g of SPS in 50g of deionized water to obtain an oxidant solution, and dissolve 1.5g of Na2S2O5 in 50g of deionized water to obtain a reducing agent solution, which is used as a post-elimination initiator, which is set aside.

[0104] Add 3g of CMC and 2000g of deionized water to a reactor under a nitrogen atmosphere. Stir and heat to 45°C until dissolved to obtain a protective gel solution. Add the first polymerization monomer to the reactor, followed by the first initiator, dropwise over 15 minutes. After the addition is complete, incubate for 45 minutes. Add a 15wt% aqueous sodium hydroxide solution to adjust the pH to 7-8.

[0105] (2) The second polymerization monomer and the second initiator were simultaneously added dropwise to the reactor for 3 hours, and the temperature was kept at 1 hour after the addition was completed. The initiator was then eliminated after the addition was completed, and the temperature was kept at 1 hour after the addition was completed. In this comparative example, no acrylonitrile was added in step (1). The solubilization effect of the intermediate product in step (1) was limited, and the polymerization product in step (2) precipitated in large quantities and coagulated severely, making it impossible to proceed to the neutralization step, and no binder was obtained.

[0106] Comparative Example 3

[0107] This comparative example provides a preparation method of a water-based binder, which is basically the same as Example 1, except that the second polymerizable monomer does not contain the post-crosslinking monomer AAA.

[0108] Comparative Example 4

[0109] This comparative example provides a method for preparing a water-based binder, which is basically the same as Example 1, except that the cross-linking monomer in the second polymerizable monomer is divinylbenzene.

[0110] Comparative Example 5

[0111] This comparative example provides a method for preparing a water-based binder, which is substantially the same as that of Example 1, except that the total amount of acrylonitrile used is 49% of the total amount of the first and second polymerizable monomers. The first and second polymerizable monomers have the following compositions and are prepared in the same manner as in Example 1.

[0112] (1) 80g AA, 20g AM and 50g AN are mixed to obtain a first polymerization monomer, which is set aside. 120g AA, 30g AM, 200g AN and 10g AAA are mixed to obtain a second polymerization monomer, which is set aside. 0.8g SPS is dissolved in 30g deionized water to obtain an oxidant solution, and 0.4g Na2S2O5 is dissolved in 30g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a first initiator, which is set aside. 2.2g SPS is dissolved in 50g deionized water to obtain an oxidant solution, and 1.1g Na2S2O5 is dissolved in 50g deionized water to obtain a reducing agent solution, thereby obtaining a redox system initiator, which is used as a second initiator, which is set aside. 3g SPS is dissolved in 50g deionized water to obtain an oxidant solution, and 1.5g Na2S2O5 is dissolved in 50g deionized water to obtain a reducing agent solution, which is used as a post-elimination initiator, which is set aside.

[0113] Add 3g of CMC and 2000g of deionized water to a reactor under a nitrogen atmosphere. Stir and heat to 45°C until dissolved to obtain a protective gel solution. Add the first polymerization monomer to the reactor, followed by the first initiator, dropwise over 15 minutes. After the addition is complete, incubate for 45 minutes. Add a 15wt% aqueous sodium hydroxide solution to adjust the pH to 7-8.

[0114] (2) Keeping the set temperature at 45°C constant, the second polymerization monomer and the second initiator are simultaneously added dropwise to the reactor for 3 hours. After the addition is completed, the temperature is kept at 1 hour. Then, the initiator is eliminated after the addition. The addition time is 1 hour. After the addition is completed, the temperature is kept at 1 hour. Then, the temperature is raised to 70°C. After the temperature stabilizes, a sodium hydroxide aqueous solution (15 wt%) is added to adjust the pH to 7-8. After the pH stabilizes, the temperature is kept at 2 hours. The material is filtered to obtain the target binder.

[0115] Test Case

[0116] This test example provides the performance of the adhesive, using the commercially available SBR3001A / BCQ15-2CMC adhesive system as control group 1 and the commercially available Indole LA136D PAA adhesive as control group 2. The specific results are as follows:

[0117] (1) Electrolyte swelling evaluation method: The adhesive is dried to form a film to test the swelling of the adhesive in the electrolyte. Specifically, the adhesive solution is dried in a 50°C oven to form a film, and a membrane with a mass of M1 is taken; the membrane is immersed in the electrolyte (the volume ratio of EC, EMC and DEC is 3:5:2) and placed in a 60°C oven for 48 hours. The electrolyte on the surface of the membrane is wiped dry with filter paper and weighed and recorded as M2. The electrolyte swelling degree is calculated using the following formula:

[0118]

[0119] (2) Preparation of lithium-ion battery negative electrode: Add 0.1 parts of CMC to 150 parts of deionized water at room temperature, stir at high speed for 20 minutes, then add 2 parts of conductive carbon black sp, 96 parts of negative electrode active material (graphite) and 1.9 parts of binder (from each embodiment, comparative example and control group) respectively, and filter through a 100-mesh filter to obtain a negative electrode slurry. It should be noted that in the process of preparing the slurry, each chemical substance added must be stirred at high speed for 15 minutes to mix evenly, and then continue to add the next chemical substance. The filtered slurry is coated on a copper foil with a thickness of 10μm. The coating thickness of the slurry is 100μm, and then placed in 135℃ for drying for 25min. It is naturally cooled to room temperature in the furnace and rolled by a roller press to obtain an electrode plate. The electrode plates are evaluated for slurry dispersion, peel strength and electrochemical performance.

[0120] Peel strength evaluation method: The instrument uses a Kejian tensile testing machine. Specifically, the electrode sheet prepared above is cut into pieces 200mm long and 25mm wide along the rolling direction. The electrode sheet is flatly affixed to a smooth steel plate using 3M double-sided tape. The stainless steel plate is then secured to the tensile testing machine. The electrode sheet not affixed to the tape is reversely clamped to the tensile testing machine probe. The tensile testing machine base pulls the electrode sheet at a speed of 10cm / min, and the sensor measures the peel force during this process. Five parallel samples are tested in each group, and the average value is the final peel strength, expressed in N / m.

[0121] Binder slurry viscosity evaluation method: The instrument uses a Brookfield DV2T tester. Specifically, place the pre-temperature (40°C) slurry (the above-mentioned negative electrode slurry) under the instrument, select the 64# rotor, and test the test conditions at 12rpm. Keep the rotor groove level with the liquid surface, start the test, and read the viscosity reading after it stabilizes. Test three times and take the average value. The negative electrode slurry viscosity test method is the same as the binder slurry.

[0122] Slurry stability evaluation method: The prepared negative electrode slurry is left at room temperature for 2 days. The slurry is observed for stratification and the solid content of the upper and lower layers of the slurry is measured to determine the slurry stability. The upper and lower layers refer to the upper and lower surfaces of the slurry container, respectively.

[0123] (3) Electrochemical performance evaluation method: The positive electrode sheet is prepared using lithium iron phosphate as the positive electrode active material. The preparation method is the same as that of the negative electrode sheet. The negative electrode sheet and the positive electrode sheet made of a binder are assembled into an aluminum-plastic soft-package battery, and then the electrochemical performance of the battery is tested.

[0124] DCR: Under normal temperature conditions, the battery cell is discharged to 2.75V at a constant current of 1C, allowed to stand, and then charged to 4.2V at a constant current of 1C and a constant voltage of 4.2V; then the battery cell is discharged to 2.8V at a current rate of 1C, and the capacity is recorded. After 2-4 cycles, the last discharge capacity is used as the 100% capacity value of the battery cell for subsequent DCR tests. At the target temperature, the battery cell is charged to 4.2V at a current rate of 1C and a constant voltage of 4.2V; then the battery cell is discharged to 50% SOC at a current rate of 1C and allowed to stand. The battery cell is discharged at 3C for 30s, and then discharged to 2.75V at a current rate of 1C. The DCR value, that is, the DC resistance, is calculated using Ohm's law formula, as follows:

[0125]

[0126] Capacity retention: Charge the battery to 4.2V at a 1C rate and maintain a constant voltage at 4.2V. Then, discharge the battery at a 1C rate to a cutoff voltage of 2.75V, completing one cycle. Test capacity retention after 1000 cycles.

[0127] Charging rate: Under normal temperature conditions, discharge the battery cell at a current rate of 1C to 2.5V and maintain a constant voltage of 2.5V; then discharge the battery cell at a current rate of 1C, with a cut-off voltage of 4.2V.

[0128] Discharge rate: Under normal temperature conditions, charge the battery cell at a current rate of 1C to 4.75V and maintain a constant voltage of 4.75V; then discharge the battery cell at a current rate of 1C, with a cut-off voltage of 2.5V.

[0129] Table 1 Physical properties test results

[0130]

[0131]

[0132] Note: “ / ” in the table means there is no corresponding data.

[0133] Table 2 Electrochemical performance data Note: “ / ” in the table means there is no corresponding data.

[0134] Combined with the above results, when the binder prepared by the present invention is applied to the negative electrode, the internal resistance of the battery is lower and more excellent. Compared with commercially available binders, the negative electrode plate prepared by the binder of the present invention has less swelling in the electrolyte and higher peel strength, and can also enable the battery to have both low internal resistance and high cycle performance. In comparative example 3, no post-crosslinking monomer is added, which makes the swelling degree of the plate greater and the peeling performance worse, affecting the rate performance of the battery, etc. In comparative example 4, the internal crosslinking agent divinylbenzene is added, and crosslinking occurs during the reaction process, the peeling performance deteriorates, and the beneficial effects brought by crosslinking are not significant, which further affects the electrochemical performance of the battery. In comparative example 5, the amount of acrylonitrile added is reduced. During the test, the internal resistance of the battery under low temperature conditions increases, indicating that the present invention regulates the total amount of unsaturated nitrile monomer added to 65-75wt% of the total mass of the first polymerized monomer and the second polymerized monomer, and controls the content of the unsaturated nitrile monomer in the first polymerized monomer in step (1) to be not less than 15wt%, so that the polymer can be soluble in water while also having a certain hydrophobic structure; in step (2), a large amount of unsaturated nitrile monomer is added, and combined with the intermediate product of step (1), a polymer component with a unique hydrophilic and hydrophobic structure is obtained to further form an aqueous dispersion. When the binder is applied to the negative electrode of the battery, the internal resistance of the battery at low temperatures can be significantly reduced.

[0135] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing a water-based binder, characterized in that: The following steps are involved: (1) preparing a protective colloid solution; adding a first polymerizable monomer and an initiator to the protective colloid solution to initiate a polymerization reaction; the first polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, and an acrylamide monomer; and the content of the unsaturated nitrile monomer in the first polymerizable monomer is not less than 15 wt %; (2) adding a second polymerizable monomer and an initiator to the product of step (1) to carry out a polymerization reaction; the second polymerizable monomer comprises an unsaturated carboxylic acid monomer, an unsaturated nitrile monomer, an acrylamide monomer and a post-crosslinking monomer; the post-crosslinking monomer is a compound containing an acetoacetyl group; Based on the total weight of the first polymerized monomer and the second polymerized monomer, the total content of the unsaturated nitrile monomer in the first polymerized monomer and the second polymerized monomer is 65-75 wt %.

2. The preparation method according to claim 1, characterized in that The first polymerized monomer accounts for 10-25 wt % of the total weight of the first polymerized monomer and the second polymerized monomer.

3. The preparation method according to claim 1 or 2, characterized in that The preparation method satisfies at least one of (1)-(2): (1) The mass of the unsaturated carboxylic acid monomer in the first polymerized monomer is not higher than 60 wt % of the total mass of the unsaturated carboxylic acid monomer in the first polymerized monomer and the second polymerized monomer; (2) The total content of the unsaturated carboxylic acid monomer and the acrylamide monomer in the first polymerizable monomer is less than 85 wt %.

4. The preparation method according to claim 1 or 2, characterized in that Based on the total mass of the first polymerized monomer and the second polymerized monomer, the total content of the unsaturated carboxylic acid monomer is 15-30 wt%, and / or the total content of the acrylamide monomer is 1-15 wt%, and / or the content of the post-crosslinking monomer is not higher than 5 wt%.

5. The preparation method according to claim 1 or 2, characterized in that The preparation method satisfies at least one of (1) to (5): (1) The unsaturated carboxylic acid monomer is at least one of acrylic acid, methacrylic acid, maleic acid and itaconic acid; (2) The unsaturated carboxylic acid monomer is at least one of acrylic acid and methacrylic acid; (3) The unsaturated nitrile monomer is at least one of acrylonitrile, α-haloacrylonitrile, and α-alkylacrylonitrile; (4) The unsaturated nitrile monomer is acrylonitrile; (5) The post-crosslinking monomer is at least one of allyl acetoacetate and acetoacetoxyethyl methacrylate.

6. The preparation method according to claim 1 or 2, characterized in that The preparation method satisfies at least one of (1)-(2): (1) The protective colloid is at least one of polyvinyl alcohol, polyvinyl pyrrolidone and sodium carboxymethyl cellulose; (2) The amount of the protective glue added is not higher than 1 wt% of the total mass of the first polymerized monomer and the second polymerized monomer.

7. The preparation method according to claim 1 or 2, characterized in that The preparation method satisfies at least one of (1)-(2): (1) The polymerization reaction temperature in step (1) is 30-90°C and the time is no more than 2 hours; (2) The polymerization reaction temperature in step (2) is 30-90°C and the reaction time is 1.5-5h.

8. A water-based binder obtained by the preparation method according to any one of claims 1 to 7.

9. The aqueous adhesive according to claim 8, characterized in that The aqueous binder satisfies at least one of (1)-(2): (1) The solid content of the aqueous binder is 5-20%; (2) The viscosity of the aqueous binder is 5000-50000 mPa·s.

10. Use of the aqueous binder prepared by the preparation method according to any one of claims 1 to 7 or the aqueous binder according to any one of claims 8 to 9 as a binder for negative electrode active materials.

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