Aqueous negative electrode binder, method for preparing the same, and use thereof

Abstract

The application relates to a water-based negative electrode binder and a preparation method and application thereof, the preparation monomer of the water-based negative electrode binder comprises an ethylenically unsaturated polyhydroxy monomer, an ethylenically unsaturated carboxyl monomer, an ethylenically unsaturated alkyl monomer and an acrylamide monomer; the structural formula of the ethylenically unsaturated polyhydroxy monomer is shown in formula (1): wherein R1, R2 and R3 are independently selected from H, COOH or a C1-C6 saturated aliphatic hydrocarbon group, A is selected from N or O, is a saturated branched structure, R4 is selected from a C1-C5 saturated aliphatic hydrocarbon group, and n is an integer greater than or equal to 2. The water-based negative electrode binder can effectively inhibit the volume expansion of a silicon material and improve the cycle life of a secondary battery.

Description

Technical Field

[0001] This application relates to the field of functional polymer materials technology for lithium-ion batteries, and in particular to an aqueous negative electrode binder, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, with their high energy density and high power characteristics, are the core power source for portable electronic devices. However, limited by the relatively low theoretical specific capacity of current graphite anodes, the energy density of commercial batteries is nearing its limit, making it difficult to meet the growing demand for long battery life in consumer electronics. Based on this, silicon-based materials, with their superior theoretical specific capacity, have emerged as a strong candidate for the next generation of high-energy-density lithium-ion battery anodes.

[0003] However, in practical applications, in order to enhance the overall energy density of the battery, it is necessary to increase the amount of silicon material added. This will cause the electrode to produce a more severe volume expansion effect during the lithium ion insertion and extraction process, which will cause the solid electrolyte interphase (SEI) film to repeatedly break and regenerate, thereby consuming a large amount of active lithium and ultimately causing a sharp decline in the cycle performance of the cell. Summary of the Invention

[0004] Therefore, it is necessary to provide an aqueous negative electrode binder, its preparation method, and its application to address the above problems; the aqueous negative electrode binder can effectively suppress the volume expansion of silicon materials and improve the cycle life of secondary batteries.

[0005] Through long-term and in-depth research, the applicant discovered that conventional high molecular polymers such as polyacrylic acid (PAA) have strong bonding force with silicon particles as negative electrode binders, and the resulting adhesive film has high strength and can provide strong initial binding force. Although suitable for silicon negative electrode systems, they lack ductility and will break due to stress concentration during cycling, thus losing their ability to inhibit the volume expansion of silicon particles.

[0006] Based on this, this application proposes an aqueous negative electrode binder, the monomers of which include olefinic unsaturated polyhydroxy monomers, olefinic unsaturated carboxyl monomers, olefinic unsaturated alkyl monomers and acrylamide monomers.

[0007] The structural formula of the olefinic unsaturated polyhydroxy monomer is shown in formula (1):

[0008] ,

[0009] Wherein, R1, R2, and R3 are independently selected from H, COOH, or C1~C6 saturated aliphatic hydrocarbon groups, and A is selected from N or O. It is a saturated branched structure, R4 is selected from saturated aliphatic hydrocarbon groups of C1~C5, and n is an integer ≥2.

[0010] The aqueous negative electrode binder described in this application uses olefinic unsaturated polyhydroxy monomers with special terminal hydroxyl branched structures, olefinic unsaturated carboxyl monomers, olefinic unsaturated alkyl monomers, and acrylamide monomers as raw materials. The binder polymer is formed through unsaturated bond polymerization. The binder structural unit has a high hydroxyl concentration, which can reduce the surface tension of the aqueous negative electrode slurry. While effectively improving the dispersibility of silicon-based materials, it can also further form a stable elastic cross-linked network during the heating process of preparing the electrode sheet. This suppresses the system expansion of the silicon-based negative electrode during the lithium insertion / extraction process. In this way, while avoiding secondary agglomeration of silicon-based materials and improving the adhesion of the binder, the structural integrity of the electrode sheet is guaranteed, and the cycle life of the secondary battery is improved.

[0011] It is understood that this application uses olefinic unsaturated polyhydroxy monomers with special terminal hydroxyl branching structures, olefinic unsaturated carboxyl monomers, olefinic unsaturated alkyl monomers, and acrylamide monomers as raw materials. In the process of preparing the binder, the binder polymer is first formed through a first reaction of unsaturated bonds. This binder polymer not only has a special hydroxyl branching structure but also contains abundant carboxyl groups. Then, during the heating process of preparing the electrode sheet, a second reaction is carried out to tightly crosslink the hydroxyl and carboxyl groups, forming a stable elastic crosslinking network. This allows the silicon in the high-silicon anode cell to be bound by the binder crosslinking network after lithium insertion during charge-discharge cycles. At the same time, after delithiation, the silicon is prevented from pulverizing due to the rebound of the binder crosslinking network. This avoids problems such as continuous SEI film formation, continuous lithium ion consumption, impedance increase, and deterioration of kinetic performance.

[0012] It should be noted that, for the structural formula of the olefinic unsaturated polyhydroxy monomer shown in formula (1), when A is selected from O, the divalent oxygen forms two covalent bonds, and one of the covalent bonds is connected to G, as shown in formula (1'). When A is selected from N, the trivalent nitrogen forms three covalent bonds, and two of these covalent bonds are connected to G, as shown in equation (1). The sum of a and b equals n.

[0013] In one embodiment of this application, R1, R2, and R3 in the structural formula of the olefinic unsaturated polyhydroxy monomer are independently selected from H, COOH, CH3, CH3CH2, and CH3CH2CH2, respectively.

[0014] In one embodiment of this application, R4 is selected from CH2, CH2CH2, CH2CH2CH2, CH(CH3)CH2, and CH2CH2CH2CH2.

[0015] In one embodiment of this application, the n in the structure of the olefinic unsaturated polyhydroxy monomer in the aqueous negative electrode binder is an integer ≥2, preferably an integer between 2 and 10. By obtaining monomers with different n values, the hydroxyl density in the aqueous negative electrode binder can be controlled, which is beneficial to forming a controllable and high-strength elastic cross-linking network during electrode processing. At the same time, it can also avoid side reactions caused by excessive absorption of water and electrolyte due to excessive hydrophilic hydroxyl content.

[0016] In one embodiment of this application, the G of the saturated branched structure includes at least one carbonyl group or ester group, and the number of carbonyl groups or ester groups is an integer ≥1. This not only helps to control the polarity and flexibility of the main chain, making the chain segment length distribution between crosslinking points more suitable for forming an elastic crosslinking network, but also enables the formation of abundant intermolecular and intramolecular hydrogen bonds, making the crosslinking network more stable while maintaining elasticity, thereby improving cohesive strength and resistance to deformation.

[0017] In one embodiment of this application, the branching points of the saturated branched structure are selected from N or C, more preferably N, which is beneficial to increase the density of branched end groups, thereby increasing the crosslinking density of the elastic crosslinking network and improving the stability, toughness and fatigue resistance of the elastic crosslinking network.

[0018] In one embodiment of this application, the structural formula of the olefinic unsaturated polyhydroxy monomer is shown in formula (2):

[0019] ,

[0020] Among them, R1, R2, and R3 are independently selected from H, COOH, CH3, CH3CH2, and CH3CH2CH2, respectively;

[0021] R4 is selected from CH2, CH2CH2, CH2CH2CH2, CH(CH3)CH2, and CH2CH2CH2CH2;

[0022] G1 is selected from C1 to C5 alkylene, oxaalkylene, or carbonyl-containing oxaalkylene; or G1 is selected from a carbon chain containing at least one branching point;

[0023] G2 is m can be 0, 1 or 2. It is understandable that when m is 0, the carbonyl group in G2 is directly linked to the branching point N.

[0024] A is selected from N or O, and n is an integer from 2 to 10.

[0025] It should be noted that when G1 has no branching point, G1 is selected from C1 to C5 alkylene, oxaalkylene, or carbonyl-containing oxaalkylene; when G1 is selected from carbon chains containing at least one branching point, the olefinic unsaturated polyhydroxy monomer has a multi-level branched structure. For example, when G1 is selected from carbon chains containing one branching point N, the N branching point in G1 is the first-level branch, and the N branching point in G2 is the second-level branch; when G1 is selected from carbon chains containing two branching points C, the first C branching point closest to A in G1 is the first-level branch, the second C branching point is the second-level branch, and the N branching point in G2 is the third-level branch.

[0026] In one embodiment of this application, the structural formulas of the olefinic unsaturated polyhydroxy monomer are shown in formulas (1-1), (1-2), (1-3), (1-4), and (1-5):

[0027] , , , , .

[0028] It should be noted that this application does not limit the preparation method of the olefinic unsaturated polyhydroxy monomer. Different preparation methods can be used based on different monomer structures. For example, for the olefinic unsaturated polyhydroxy monomer with the structure of formula (1-1), a stepwise reaction method can be used. First, dicarboxymethylamine is reacted with acrylic acid in the presence of a catalyst, and then excess diethanolamine is added for a second reaction. Finally, the olefinic unsaturated polyhydroxy monomer with the structure of formula (1-1) can be separated from the product. For the olefinic unsaturated polyhydroxy monomer with the structure of formula (1-2), a stepwise reaction method can also be used. First, maleic anhydride is reacted with diethanolamine, and then succinic anhydride and the remaining diethanolamine are added for a second reaction. Finally, the olefinic unsaturated polyhydroxy monomer with the structure of formula (1-2) can be separated from the product. For the olefinic unsaturated polyhydroxy monomer with the structure of formula (1-3), it can be prepared by directly reacting dipentaerythritol hexaacrylate with excess diethanolamine.

[0029] In one embodiment of this application, the aqueous negative electrode binder, after being treated at 90℃~120℃ for 1h~3h, has an elastic modulus of 1500MPa~3500MPa and a swelling rate of 10%~50% in the electrolyte. It should be noted that after being treated at the electrode heating temperature, the aqueous negative electrode binder forms an elastic cross-linked network, achieving a certain elastic modulus and swelling rate, which is more conducive to ensuring that the aqueous negative electrode binder, when used on the electrode, can effectively suppress the volume expansion of the silicon-based negative electrode material.

[0030] It is understandable that the swelling rate indicates the degree of volume expansion of a polymer in a solution; the electrolyte can use conventional components, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1, containing 1 mol / L of LiPF6.

[0031] In one embodiment of this application, based on the total mass of monomers as 100%, the olefinic unsaturated polyhydroxy monomer accounts for 5% to 15%, the olefinic unsaturated carboxyl monomer accounts for 50% to 70%, the olefinic unsaturated alkyl monomer accounts for 5% to 15%, and the acrylamide monomer accounts for 15% to 25%.

[0032] In one embodiment of this application, the olefinic unsaturated carboxyl monomer is selected from at least one of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, β-acryloyloxypropionic acid, 4-vinylbenzoic acid, and crotonic acid.

[0033] The olefinic unsaturated carboxyl monomer described in this application contains carboxyl groups. On the one hand, it can participate in the reaction of hydroxyl groups in the olefinic unsaturated polyhydroxy monomer during the heating process of the electrode coating to form a cross-linked network with a certain elasticity. On the other hand, the carboxyl groups that do not participate in the reaction can also form a strong adhesive effect with the current collector through hydrogen bonding, which helps to avoid problems such as coating peeling off and cell cycle failure caused by volume changes during charging and discharging.

[0034] In one embodiment of this application, the olefinically unsaturated alkyl monomer is selected from CH2=CR5-COO-R6, CH2=CR5-OCOR6, CH2=CR5-O-R6, and CH2=CR5-COO-(CH2CH2O). n -R7;

[0035] Wherein, R5 is selected from H or CH3; R6 is selected from C6 to C30 straight-chain or branched saturated alkyl groups; R7 is selected from H or C6 to C30 straight-chain or branched saturated alkyl groups; n is an integer from 10 to 30.

[0036] Preferably, the olefinically unsaturated alkyl monomer is selected from isooctyl acrylate, isooctyl methacrylate, dodecyl acrylate, dodecyl methacrylate, octadecyl acrylate, octadecyl methacrylate, docosyl acrylate, docosyl methacrylate, isobornyl methacrylate, vinyl dodecanoate, vinyl hexadecanoate, vinyl octadecanoate, vinyl docosyl ether, dodecyl vinyl ether, hexadecyl vinyl ether, octadecyl vinyl ether, docosyl vinyl ether, and poly(ethylene glycol) methacrylate (CH2=C(CH3)COO-(CH2CH2O)). n-H), polyethylene glycol octadecyl methacrylate (CH2=C(CH3)COO-(CH2CH2O) n -C 18 H 37 ), polyethylene glycol docosyl methacrylate (CH2=C(CH3)COO-(CH2CH2O) n -C 22 H 45 At least one of the following.

[0037] The alkyl-linked unsaturated monomers described in this application contain alkyl chains with a large number of carbon atoms. On the one hand, they can adjust the overall mechanical strength of the binder, reduce the rigidity of the crosslinking network of the binder, and give the crosslinking network of the binder a certain degree of resilience. On the other hand, as a non-polar hydrophobic part, they react with the polar hydrophilic parts formed by the reaction of alkyl-linked unsaturated polyhydroxy monomers, alkyl-linked unsaturated carboxyl monomers, and acrylamide monomers to form a hydrophilic-hydrophobic amphiphilic structure. By directional adsorption at the interface, they reduce the surface tension of the silicon anode slurry and improve the leveling and smoothness of the coating.

[0038] In one embodiment of this application, the acrylamide monomer is selected from at least one of acrylamide, N-methylacrylamide, N-ethylacrylamide, and N-butylacrylamide.

[0039] The acrylamide monomers described in this application are highly polar monomers, which can provide certain water solubility and cohesive forces formed by hydrogen bonds, thereby improving the overall structural stability of the coating.

[0040] It should be noted that the water-based negative electrode binder described in this application is usually dispersed in water to form a solution. It is understood that this application does not limit the water content of the binder.

[0041] This application provides a method for preparing the aqueous negative electrode binder as described above, comprising the following steps:

[0042] An initiator solution is prepared by dissolving olefinically unsaturated polyhydroxy monomers, olefinically unsaturated carboxyl monomers, olefinically unsaturated alkyl monomers, and acrylamide monomers in water.

[0043] The initiator solution is added to the monomer mixture solution to carry out the reaction, and an intermediate solution is obtained.

[0044] A pH adjuster is added to the intermediate solution to obtain an aqueous negative electrode binder.

[0045] In one embodiment of this application, the initiator is selected from at least one of ammonium persulfate, potassium persulfate, and sodium persulfate. The free radicals generated by the decomposition of the initiator in aqueous solution are used to efficiently initiate the polymerization of unsaturated monomers.

[0046] In one embodiment of this application, the reaction temperature is 55℃~85℃, including but not limited to any one of 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃ or any range between two; the total reaction time is 5h~12h, including but not limited to any one of 5h, 8h, 10h, 12h or any range between two.

[0047] In one embodiment of this application, the reaction is carried out in a protective gas selected from at least one of nitrogen, argon, and helium.

[0048] In one embodiment of this application, the pH adjuster is selected from at least one of ammonia, sodium hydroxide, lithium hydroxide, and potassium hydroxide.

[0049] In one embodiment of this application, the pH value of the aqueous negative electrode binder is 5 to 7, including but not limited to any one of 5, 5.5, 6, 6.5, 7 or any range between two of them.

[0050] This application provides an electrode sheet comprising the aqueous negative electrode binder as described above or an aqueous negative electrode binder prepared by the method described above.

[0051] In one embodiment of this application, the electrode sheet is a negative electrode sheet with a silicon mass content of ≥10%.

[0052] This application also provides a secondary battery, including the electrode plates described above.

[0053] Using the aqueous negative electrode binder provided in this application in the electrode sheet to make a secondary battery can significantly improve the cycle life of the secondary battery while maintaining low internal resistance. In particular, the capacity can still be maintained at more than 89% after 100 cycles.

[0054] It should be noted that this application does not impose any restrictions on the preparation process of the electrode sheets and the secondary battery; conventional processes in the field can be used, and this application will not elaborate further on these aspects. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0056] Figure 1 The infrared spectrum of the olefinic unsaturated polyhydroxy monomer used in Example 1;

[0057] Figure 2 The infrared spectrum of the olefinic unsaturated polyhydroxy monomer used in Example 2;

[0058] Figure 3 The image shows the infrared spectrum of the olefinic unsaturated polyhydroxy monomer used in Example 3. Detailed Implementation

[0059] To facilitate understanding of this application, it will be described in more detail below. However, it should be understood that this application can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0060] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular implementations or embodiments only and is not intended to be limiting of this application. In this application, when numerical ranges are involved, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Furthermore, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0061] The following specific embodiments will further illustrate the aqueous negative electrode binder, its preparation method, and its application. However, those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used without specified manufacturers are all commercially available conventional products.

[0062] Example 1

[0063] Dicarboxymethylamine and 1 wt% dimethylaminopyridine catalyst were added to a reactor. The mixture was stirred and heated to 120°C under a nitrogen atmosphere. Methacrylic acid, in an equimolar ratio with dicarboxymethylamine, was gradually added. After reacting for 5 hours, the temperature was lowered to 80°C, and diethanolamine, 20 wt% toluene, and 1 wt% p-toluenesulfonic acid, in a ratio twice that of dicarboxymethylamine, were added. The mixture was stirred and heated to 120°C under a nitrogen atmosphere for another 6 hours. Toluene and water generated during the reaction were continuously separated during the process, yielding an olefinically unsaturated polyhydroxy monomer. Infrared spectroscopy analysis of this monomer yielded the following results: Figure 1As shown, it can be proven that the monomer has the structure of formula (1-1). .

[0064] Five parts of the olefinic unsaturated polyhydroxy monomer shown in formula (1-1), 60 parts of methacrylic acid, 10 parts of dodecyl acrylate, and 25 parts of acrylamide were added to 900 parts of water to form a monomer mixed solution; one part of ammonium persulfate was dissolved in 100 parts of water to form an initiator solution.

[0065] The monomer mixture was heated to 60°C under a nitrogen atmosphere, and the initiator solution was gradually added dropwise to the monomer mixture for reaction over 3 hours. After the addition was complete, the temperature was raised to 75°C and the reaction was maintained for 4 hours to obtain the intermediate solution.

[0066] Sodium hydroxide was added to the intermediate solution to adjust the pH to 6.2, resulting in a solution of an aqueous negative electrode binder.

[0067] Example 2

[0068] Maleic anhydride, diethanolamine, and anhydrous methanol were added to a reaction vessel in an equimolar ratio. The mixture was stirred in an ice bath at 0°C for 6 hours. Succinic anhydride and diethanolamine in a ratio twice that of dicarboxymethylamine, along with 20 wt% toluene and 1 wt% p-toluenesulfonic acid, were then added. The mixture was stirred and heated to 120°C under a nitrogen atmosphere for another 6 hours. Toluene and water generated during the reaction were continuously separated, yielding an olefinically unsaturated polyhydroxy monomer. Infrared spectroscopy analysis of this monomer yielded the following results: Figure 2 As shown, it can be proven that the monomer has the structure of formula (1-2). .

[0069] A solution of an aqueous negative electrode binder was prepared using an olefinic unsaturated polyhydroxy monomer as shown in formula (1-2) with the same proportions and methods as in Example 1.

[0070] Example 3

[0071] Dipentaerythritol hexaacrylate and an equal mass of N-methylpyrrolidone were added to a reaction vessel, and the mixture was stirred and heated to 30°C. Diethanolamine was gradually added dropwise to the mixture at a molar ratio of five times that of dipentaerythritol hexaacrylate, and the reaction was continued for 12 hours. After maintaining the temperature for another 12 hours, N-methylpyrrolidone was removed using a rotary evaporator to obtain an olefinically unsaturated polyhydroxy monomer. Infrared spectroscopy analysis of this monomer yielded the following results: Figure 3 As shown, it can be proven that the monomer has the structure of formula (1-3). .

[0072] A solution of an aqueous negative electrode binder was prepared using the olefinic unsaturated polyhydroxy monomers shown in formulas (1-3) with the same proportions and methods as in Example 1.

[0073] Example 4

[0074] Triethanolamine and toluene in a three-molar ratio were stirred and heated to 120°C in a reaction vessel. 1 wt% p-toluenesulfonic acid was added, and maleic anhydride in an equimolar ratio with triethanolamine was gradually added dropwise. After the addition was complete, the reaction proceeded for 5 hours. Then, succinic anhydride and diethanolamine in a two-molar ratio with triethanolamine, along with 1 wt% p-toluenesulfonic acid, were added, and the reaction was continued at 120°C with stirring for another 6 hours. Toluene and the water produced during the reaction were continuously separated during this process, yielding an olefinically bonded unsaturated polyhydroxy monomer. Infrared spectroscopy analysis of this monomer confirmed that it possesses the structure of formula (1-4). .

[0075] A solution of an aqueous negative electrode binder was prepared using the olefinic unsaturated polyhydroxy monomers shown in formulas (1-4) with the same proportions and methods as in Example 1.

[0076] Example 5

[0077] Diethanolamine and 1 wt% dimethylaminopyridine catalyst were added to a reactor. The mixture was stirred and heated to 120°C under a nitrogen atmosphere. Methacrylic acid, in an equimolar ratio with diethanolamine, was gradually added. After reacting for 6 hours, the temperature was lowered to 80°C. Succinic anhydride and dicarboxymethylamine, 20 wt% toluene, and 1 wt% p-toluenesulfonic acid, in an equimolar ratio with diethanolamine, were slowly added. The mixture was stirred and heated to 120°C under a nitrogen atmosphere. After reacting for 6 hours, the temperature was lowered to 80°C. o C. Diethanolamine was added in a molar ratio twice that of dicarboxymethylamine, along with 1 wt% p-toluenesulfonic acid. The mixture was stirred at 120 °C for 6 h, during which toluene and water were continuously separated, yielding an olefinically unsaturated polyhydroxy monomer. Infrared spectroscopy analysis confirmed that the monomer possesses the structure of formula (1-5). .

[0078] A solution of an aqueous negative electrode binder was prepared using an olefinic unsaturated polyhydroxy monomer as shown in formula (1-5) with the same proportions and methods as in Example 1.

[0079] Example 6

[0080] 10 parts of the olefinic unsaturated polyhydroxy monomer shown in formula (1-1), 50 parts of acrylic acid, 15 parts of isooctyl methacrylate, and 25 parts of acrylamide were added to 900 parts of water to form a monomer mixed solution; 1 part of ammonium persulfate was dissolved in 100 parts of water to form an initiator solution.

[0081] The monomer mixture was heated to 55°C under a nitrogen atmosphere, and the initiator solution was gradually added dropwise to the monomer mixture for 2 hours. After the addition was completed, the temperature was raised to 80°C and the reaction was maintained for 6 hours to obtain the intermediate solution.

[0082] Sodium hydroxide was added to the intermediate solution to adjust the pH to 6.9, resulting in a solution of an aqueous negative electrode binder.

[0083] Example 7

[0084] 15 parts of the olefinic unsaturated polyhydroxy monomer shown in formula (1-1), 50 parts of maleic acid, 15 parts of dodecyl methacrylate, and 20 parts of N-methacrylamide were added to 900 parts of water to form a monomer mixed solution; 1 part of potassium persulfate was dissolved in 100 parts of water to form an initiator solution.

[0085] The monomer mixture was heated to 70°C under a nitrogen atmosphere, and the initiator solution was gradually added dropwise to the monomer mixture for reaction over 3 hours. After the addition was complete, the temperature was raised to 85°C and the reaction was maintained for 9 hours to obtain the intermediate solution.

[0086] Sodium hydroxide was added to the intermediate solution to adjust the pH to 5.1, resulting in a solution of an aqueous negative electrode binder.

[0087] Example 8

[0088] 20 parts of the olefinic unsaturated polyhydroxy monomer shown in formula (1-1), 50 parts of methacrylic acid, 15 parts of dodecyl acrylate, and 15 parts of acrylamide were added to 900 parts of water to form a monomer mixed solution; 1 part of ammonium persulfate was dissolved in 100 parts of water to form an initiator solution.

[0089] The monomer mixture was heated to 60°C under a nitrogen atmosphere, and the initiator solution was gradually added dropwise to the monomer mixture for reaction over 3 hours. After the addition was complete, the temperature was raised to 75°C and the reaction was maintained for 4 hours to obtain the intermediate solution.

[0090] Sodium hydroxide was added to the intermediate solution to adjust the pH to 6.2, resulting in a solution of an aqueous negative electrode binder.

[0091] Comparative Example 1

[0092] 65 parts of methacrylic acid, 10 parts of dodecyl acrylate, and 25 parts of acrylamide were added to 900 parts of water to form a monomer mixed solution; 1 part of ammonium persulfate was dissolved in 100 parts of water to form an initiator solution.

[0093] The monomer mixture was heated to 60°C under a nitrogen atmosphere, and the initiator solution was gradually added dropwise to the monomer mixture for reaction over 3 hours. After the addition was complete, the temperature was raised to 75°C and the reaction was maintained for 4 hours to obtain the intermediate solution.

[0094] Sodium hydroxide was added to the intermediate solution to adjust the pH to 6.2, thus obtaining the binder solution.

[0095] Comparative Example 2

[0096] Five parts of hydroxyethyl acrylate, 60 parts of methacrylic acid, 10 parts of dodecyl acrylate, and 25 parts of acrylamide were added to 900 parts of water to form a monomer mixed solution; one part of ammonium persulfate was dissolved in 100 parts of water to form an initiator solution.

[0097] The monomer mixture was heated to 60°C under a nitrogen atmosphere, and the initiator solution was gradually added dropwise to the monomer mixture for reaction over 3 hours. After the addition was complete, the temperature was raised to 75°C and the reaction was maintained for 4 hours to obtain the intermediate solution.

[0098] Sodium hydroxide was added to the intermediate solution to adjust the pH to 6.2, thus obtaining the binder solution.

[0099] Comparative Example 3

[0100] Five parts of the olefinic unsaturated polyhydroxy monomer of formula (1-1) were directly added to the binder solution obtained in Comparative Example 1.

[0101] The adhesives prepared in all examples and comparative examples were characterized, and the results are shown in Table 1.

[0102] Table 1

[0103]

[0104] The binders prepared in all examples and comparative examples were used to prepare lithium-ion batteries, and the preparation methods are as follows:

[0105] (1) Preparation of negative electrode sheet: Based on solid weight, 86 parts silicon-carbon (15% pure silicon ratio), 4 parts conductive carbon black (Super P), 1 part single-walled carbon nanotubes, 0.5 parts sodium carboxymethyl cellulose (Dow CMC, CRT30000PA), and 8.5 parts of the aforementioned aqueous silicon negative electrode binder were mixed and dispersed evenly with water as a solvent to form a negative electrode slurry. This slurry was then coated onto both the front and back surfaces of a copper foil and dried in a forced-air drying process at 95°C to remove the solvent. The negative electrode sheet was then formed by calendering at room temperature.

[0106] (2) Preparation of positive electrode sheet: Based on solid weight, 97 parts of lithium iron phosphate (Hunan Yuneng, Y9C), 1 part of conductive carbon black (Super P) and 2 parts of polyvinylidene fluoride (PVDF) were mixed and dispersed evenly with N-methylpyrrolidone as solvent to form a positive electrode slurry. The slurry was then coated on both sides of the aluminum foil and dried at 105°C to remove the solvent. Finally, the slurry was rolled at room temperature to form a positive electrode sheet.

[0107] (3) Cell preparation: Weld conductive tabs onto the obtained positive and negative electrode sheets, place a polyethylene separator between the positive and negative electrode sheets, wind it into a bare cell, wrap it in an aluminum-plastic film, and inject an electrolyte composed of EC:EMC:DEC volume ratio = 1:1:1 (containing 1.0M LiPF6). After encapsulation, the battery is formed to obtain the soft-pack battery.

[0108] The prepared lithium-ion batteries were subjected to electrical performance tests, and the results are shown in Table 2.

[0109] (1) Internal resistance change rate: The internal resistance of the pouch cell before and after 100 cycles was tested using a Solartron electrochemical workstation. The internal resistance change rate (%) = (internal resistance after the 100th cycle - internal resistance before the cycle) / internal resistance before the cycle.

[0110] (2) Cycle performance: The prepared pouch cells were formed and capacity tested using a battery testing cabinet. The formation process involved charging at a constant current of 0.05C for 2.0 hours, followed by charging at a constant current of 0.15C for 2.5 hours. The capacity testing process involved charging at a constant current of 0.33C to 4.2V, then charging at a constant voltage of 4.2V to the cutoff current of 0.02C, and discharging at 0.33C to 2.5V. At 25°C, the formed and capacity-tested cells were charged at a constant current of 0.33C to 4.2V, then charged at a constant voltage to the cutoff current of 0.02C, rested for 5 minutes, and then discharged at 0.33C. Charge the battery to 2.5V, let it rest for 5 minutes, and record the discharge capacity after the first cycle. Then, charge it to 4.2V at a constant current of 0.5C, then charge it to the cutoff current of 0.02C at a constant voltage, let it rest for 5 minutes, discharge it to 2.5V at 0.5C, let it rest for 5 minutes, and repeat this cycle. After 100 charge / discharge cycles, record the discharge capacity after the 100th cycle. Calculate the capacity retention rate of the battery after 100 cycles using the following formula: Capacity retention rate after 100 cycles (%) = Discharge capacity after the 100th cycle / Discharge capacity after the first cycle.

[0111] Table 2

[0112]

[0113] As shown in Tables 1 and 2, the embodiments provided in this application use a specific aqueous negative electrode binder. During the heating process in preparing the electrode sheet, a stable elastic cross-linked network is further formed, thereby suppressing the volume expansion of the silicon-based negative electrode during the lithium insertion / extraction process, which causes system pulverization, deterioration of interface contact, and damage to ion and electron transport channels, leading to a sharp increase in battery internal resistance. This ensures the structural integrity of the electrode sheet and improves the cycle life of the secondary battery while avoiding secondary agglomeration of silicon-based materials and improving the adhesive strength. The capacity retention rate after 100 cycles can reach over 89%. However, in Example 8, due to the excessive amount of the olefinic unsaturated polyhydroxy monomer shown in formula (1-1), the elastic cross-linked network formed is more elastic, and the binder is too rigid. It cannot buffer the volume expansion of the silicon-based negative electrode during the lithium insertion / extraction process through deformation, making the electrode sheet prone to cracking and powdering, affecting the interface contact between particles, increasing the battery internal resistance, causing a rapid drop in capacity, and a decrease in capacity retention rate after 100 cycles.

[0114] The binder in Comparative Example 1 does not contain olefinic unsaturated polyhydroxy monomers and cannot form an "elastic cross-linked network." Therefore, it cannot suppress the volume expansion of the silicon-based anode during lithium insertion / extraction, easily causing system pulverization, deterioration of interfacial contact, and damage to ion and electron transport channels, leading to a sharp increase in battery internal resistance and a greater rate of change in internal resistance. The binder in Comparative Example 2 uses hydroxyethyl acrylate, which can form a cross-linked network. However, because hydroxyethyl acrylate lacks aliphatic branched segments for buffering, the cross-linked network formed has a high modulus and a low density. This results in insufficient ability to suppress the volume expansion of the silicon-based anode during lithium insertion / extraction, leading to an increase in the rate of change in internal resistance and a decrease in capacity during battery cycling. Comparative Example 3 adds polyhydroxy monomers through physical blending, which can also form a cross-linked network during heating. However, because the polyhydroxy monomers added through physical blending are relatively unevenly dispersed in the system and easily dissolve and become free in the electrolyte, participating in electrochemical reactions, this leads to an increase in the rate of change in internal resistance and a decrease in the cell cycle retention rate.

[0115] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0116] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A water-based negative electrode binder, characterized in that, The monomers used in the preparation of the waterborne negative electrode binder include olefinic unsaturated polyhydroxy monomers, olefinic unsaturated carboxyl monomers, olefinic unsaturated alkyl monomers, and acrylamide monomers. The structural formula of the olefinic unsaturated polyhydroxy monomer is shown in formula (1): ， Wherein, R1, R2, and R3 are independently selected from H, COOH, or C1~C6 saturated aliphatic hydrocarbon groups, and A is selected from N or O. The structure is a saturated branched structure, wherein G in the saturated branched structure includes at least one carbonyl group or ester group, and the number of carbonyl groups or ester groups is an integer ≥1; R4 is selected from C1~C5 saturated aliphatic hydrocarbon groups, and n is an integer ≥2; Based on the total mass of monomers as 100%, the olefinic unsaturated polyhydroxy monomers account for 5% to 15%, the olefinic unsaturated carboxyl monomers account for 50% to 70%, the olefinic unsaturated alkyl monomers account for 5% to 15%, and the acrylamide monomers account for 15% to 25%. First, a binder polymer is formed through a first reaction via unsaturated bonds. This binder polymer not only has a special hydroxyl branched structure but also contains abundant carboxyl groups. Then, during the heating process of preparing the electrode sheet, a second reaction is carried out to tightly crosslink the hydroxyl and carboxyl groups, forming a stable elastic crosslinked network.

2. The water-based negative electrode binder according to claim 1, characterized in that, The structural formula of the olefinic unsaturated polyhydroxy monomer satisfies at least one of the following conditions: (1) R1, R2, and R3 are independently selected from H, COOH, CH3, CH3CH2, and CH3CH2CH2, respectively; (2) R4 is selected from CH2, CH2CH2, CH2CH2CH2, CH(CH3)CH2, CH2CH2CH2CH2; (3) n is an integer from 2 to 10; (4) The branching point of the saturated branched structure is selected from N or C.

3. The water-based negative electrode binder according to claim 2, characterized in that, The structural formula of the olefinic unsaturated polyhydroxy monomer is shown in formula (2): ， Among them, R1, R2, and R3 are independently selected from H, COOH, CH3, CH3CH2, and CH3CH2CH2, respectively; R4 is selected from CH2, CH2CH2, CH2CH2CH2, CH(CH3)CH2, and CH2CH2CH2CH2; G1 is selected from C1 to C5 alkylene, oxaalkylene, or carbonyl-containing oxaalkylene; or G1 is selected from a carbon chain containing at least one branching point; G2 is m is 0, 1, or 2; A is selected from N or O, and n is an integer from 2 to 10.

4. The aqueous negative electrode binder according to any one of claims 1 to 3, characterized in that, The structural formulas of the olefinic unsaturated polyhydroxy monomers are shown in formulas (1-1), (1-2), (1-3), (1-4), and (1-5): 、 、 、 、 。 5. The water-based negative electrode binder according to claim 1, characterized in that, The aqueous negative electrode binder, after being treated at 90℃~120℃ for 1h~3h, has an elastic modulus of 1500MPa~3500MPa and a swelling rate of 10%~50% in the electrolyte.

6. The water-based negative electrode binder according to claim 1, characterized in that, The olefinic unsaturated carboxyl monomer is selected from at least one of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, β-acryloyloxypropionic acid, 4-vinylbenzoic acid, and crotonic acid.

7. The water-based negative electrode binder according to claim 1, characterized in that, The olefinically unsaturated alkyl monomer is selected from CH2=CR5-COO-R6, CH2=CR5-OCOR6, CH2=CR5-O-R6, and CH2=CR5-COO-(CH2CH2O). n -R7; Wherein, R5 is selected from H or CH3; R6 is selected from C6 to C30 straight-chain or branched saturated alkyl groups; R7 is selected from H or C6 to C30 straight-chain or branched saturated alkyl groups; n is an integer from 10 to 30.

8. The aqueous negative electrode binder according to claim 7, characterized in that, The olefinic unsaturated alkyl monomer is selected from at least one of the following: isooctyl acrylate, isooctyl methacrylate, dodecyl acrylate, dodecyl methacrylate, octadecyl acrylate, octadecyl methacrylate, docosyl acrylate, docosyl methacrylate, isobornyl methacrylate, vinyl dodecanoate, vinyl hexadecanoate, vinyl octadecanoate, vinyl docosyl ether, dodecyl vinyl ether, hexadecyl vinyl ether, octadecyl vinyl ether, docosyl vinyl ether, poly(ethylene glycol) methacrylate, polyethylene glycol octadecyl methacrylate, and polyethylene glycol docosyl methacrylate.

9. The water-based negative electrode binder according to claim 1, characterized in that, The acrylamide monomer is selected from at least one of acrylamide, N-methylacrylamide, N-ethylacrylamide, and N-butylacrylamide.

10. A method for preparing an aqueous negative electrode binder as described in any one of claims 1 to 9, characterized in that, Includes the following steps: An initiator solution is prepared by dissolving olefinically unsaturated polyhydroxy monomers, olefinically unsaturated carboxyl monomers, olefinically unsaturated alkyl monomers, and acrylamide monomers in water. The initiator solution is added to the monomer mixture solution to carry out the reaction, and an intermediate solution is obtained. A pH adjuster is added to the intermediate solution to obtain an aqueous negative electrode binder.

11. The method for preparing the aqueous negative electrode binder according to claim 10, characterized in that, The preparation method satisfies at least one of the following conditions: (1) The reaction temperature is 55℃~85℃, and the total reaction time is 5h~12h; (2) The reaction is carried out in a protective gas, which is selected from at least one of nitrogen, argon and helium; (3) The pH adjuster is selected from at least one of ammonia, sodium hydroxide, lithium hydroxide, and potassium hydroxide; (4) The initiator is selected from at least one of ammonium persulfate, potassium persulfate, and sodium persulfate; (5) The pH value of the water-based negative electrode binder is 5~7.

12. An electrode sheet, characterized in that, Includes the aqueous negative electrode binder according to any one of claims 1 to 9, or the aqueous negative electrode binder prepared by the preparation method of the aqueous negative electrode binder according to any one of claims 10 to 11.

13. The electrode sheet according to claim 12, characterized in that, The electrode sheet is a negative electrode sheet with a silicon mass content of ≥10%.

14. A secondary battery, characterized in that, Includes the electrode sheet as described in claim 12 or claim 13.

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