Thermally crosslinkable binder composition for secondary battery, slurry composition for secondary battery, electrode for secondary battery, and secondary battery

Abstract

The present invention provides a thermally crosslinkable binder composition for a secondary battery, a slurry composition for a secondary battery, an electrode for a secondary battery, and a secondary battery. The thermally crosslinkable binder composition for a secondary battery comprises a first polymer (A), a second polymer (B), and water, wherein the first polymer (A) comprises at least a structural unit derived from an acrylonitrile monomer and a structural unit derived from a hydroxyl-containing (meth)acrylate monomer, the second polymer (A) comprises a structural unit derived from an N-vinyl-2-pyrrolidone monomer and a structural unit derived from a sulfonic acid group-containing monomer, and a weight ratio of the first polymer (A) to the second polymer (B) is 50:50 or more and 98:2 or less, and a gel fraction of a cured product is 90% or more.

Description

Thermally crosslinking binder composition for secondary batteries, slurry composition for secondary batteries, electrodes for secondary batteries, and secondary batteries. Technical Field

[0001] This invention relates to a thermally crosslinking binder composition for a secondary battery negative electrode, a slurry composition for a secondary battery negative electrode, a negative electrode for a secondary battery, and a secondary battery. Background Technology

[0002] Lithium-ion batteries are widely used due to their small size, light weight, high energy density, and ability to be repeatedly charged and discharged. Therefore, improvements are being made to battery components to further enhance the performance and capacity of lithium-ion batteries.

[0003] Lithium-ion batteries utilize electrode active materials capable of intercalating and deintercalating lithium (Li) ions as the positive and negative electrodes, with lithium ions moving between them to function. In particular, carbon materials such as graphite, which possess high charge-discharge capacity per unit weight at low potentials close to that of lithium (Li), are commonly used as negative electrode materials. However, the energy density per unit weight of these carbon materials as batteries has reached its limit. Therefore, to achieve higher capacity and durability, it is useful to utilize silicon (Si)-based negative electrode active materials, which have a higher theoretical capacity than graphite and can alloy with lithium (Li). On the other hand, silicon (Si)-based negative electrode active materials suffer from a sharp decrease in energy capacity during charge-discharge due to volume changes such as expansion and contraction associated with the alloying and dealloying of Si and Li, resulting in a short cycle life.

[0004] The application of conductive additives is being promoted to reduce electrode resistance and improve battery load resistance. Carbon nanotubes (CNTs) are promising conductive additives, as even small amounts can effectively form conductive networks, reduce electrode resistance after charge-discharge cycles, and suppress capacity decline. In particular, the use of single-walled carbon nanotubes (SWCNTs) is effective, but SWCNTs have strong aggregation and are difficult to disperse uniformly, resulting in high viscosity dispersions that are difficult to handle.

[0005] To improve the capacity of lithium-ion batteries, it is extremely important to optimize the structure of binder resin to eliminate the problems caused by the volume change of electrode active material during secondary battery charging and discharging, and to solve various problems caused by the dispersibility with conductive additives such as CNT.

[0006] The binder resin serves the following functions: dispersing electrode active materials and CNTs in the slurry for secondary batteries; bonding between electrode active materials, bonding between electrode active materials and CNTs, and bonding between electrode active materials and current collectors after drying. Furthermore, through repeated charging and discharging of the secondary battery, if the adhesive force of the binder resin decreases due to volume changes in the electrode active materials, electrode active materials may detach, leading to a reduction in battery capacity.

[0007] To address these various problems, techniques have been proposed, such as using multiple different water-soluble polymers to improve dispersibility and adhesion with negative electrode active materials. Patent Document 1 discloses a technique comprising multiple water-soluble polymers with weight-average molecular weights differing by more than four times. Furthermore, Patent Document 2 discloses a technique comprising multiple materials with varying viscosities in a 5% aqueous solution containing water-soluble polymers. On the other hand, Patent Document 3 discloses a technique that improves the problem of volume changes in active materials during charging and discharging through thermal crosslinking of the constituent materials in the binder composition. Patent Document 4 discloses a secondary battery technology that utilizes carboxymethyl cellulose (CMC)-based binder resins to achieve a high-concentration and highly dispersible CNT dispersion with good cycle life in order to form an effective conductive network.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2021 / 200350

[0011] Patent Document 2: International Publication No. 2014 / 024967

[0012] Patent Document 3: Japanese Patent Application Publication No. 2020-24896

[0013] Patent Document 4: Japanese Patent Application Publication No. 2021-175699 Summary of the Invention

[0014] The problem that the invention aims to solve

[0015] In Patent Document 1, the charge-discharge cycle characteristics are superior compared to conventional water-based binder resins such as carboxymethyl cellulose (CMC) / styrene-butadiene rubber (SBR). However, when using Si-based anode active materials, the dispersibility of CNTs becomes a problem, resulting in insufficient cycle performance, which needs improvement. Furthermore, cracking and curling easily occur during the process of coating the slurry composition consisting of binder resin and electrode active material onto the current collector and then firing it, posing a manufacturing problem. In Patent Document 2, compared to conventional CMC / SBR binders, it improves the charge-discharge cycle characteristics, but the stability of the slurry composition consisting of binder resin and electrode active material, as well as the dispersibility with CNTs, become issues. Therefore, when using high-density active materials, springback (increased film thickness during cycling) occurs. In Patent Document 3, the CNT dispersibility is insufficient, resulting in high resistance; in systems with a high concentration of Si-based anode active materials, there is still room for improvement in cycle performance. Patent document 4 discloses a modified CMC-based raw material for improving the dispersibility of CNTs. However, the CNT dispersion in the CMC binder is insufficient, and improvements are needed in the cycle performance of the secondary battery.

[0016] The problem this invention aims to solve is to achieve a high-performance negative electrode for high-capacity lithium-ion batteries. The purpose of this invention is to resolve various problems arising from the use of silicon (Si)-based active materials (such as SiO-based or SiC-based raw materials) in conjunction with conductive additives (CNTs), including poor CNT dispersibility, high viscosity of the dispersion, and drawbacks such as reduced solids content in binder resins, low adhesion to current collectors, and poor tolerance to electrolytes.

[0017] In particular, CNTs can form a conductive network in the electrode active material and mitigate the disruption of conductive paths caused by volume changes in high-capacity silicon (Si)-based electrode active materials such as SiO-based or SiC-based materials, thereby suppressing capacity reduction. On the other hand, the purpose of this invention is to realize a high-performance binder resin that can overcome the problems of low dispersibility and the rebound of high-capacity batteries (increased film thickness during cycling).

[0018] Technical solutions for solving the problem

[0019] In order to solve the above-mentioned problems, the inventors conducted in-depth research and found that various problems can be solved by using a water-soluble polymer composed of specific components, thereby completing the present invention.

[0020] The essence of this invention is provided by the following items.

[0021] (Project 1)

[0022] A thermally crosslinkable adhesive composition for secondary batteries, comprising:

[0023] First polymer (A), second polymer (B), and water,

[0024] The first polymer (A) comprises at least structural units derived from acrylonitrile monomers and structural units derived from hydroxyl-containing (meth)acrylate monomers.

[0025] The second polymer (B) comprises structural units derived from N-vinyl-2-pyrrolidone monomers and structural units derived from sulfonic acid-containing monomers.

[0026] The weight ratio of the first polymer (A) to the second polymer (B) is 50:50 or more and 98:2 or less, and the gel fraction of the cured product is 90% or more.

[0027] (Project 2)

[0028] According to the thermally crosslinkable binder composition for secondary batteries described in Project 1 above, the sulfonic acid-containing monomer comprises tert-butylacrylamide sulfonic acid and / or an alkali metal salt of tert-butylacrylamide sulfonic acid.

[0029] (Project 3)

[0030] According to the thermally crosslinkable adhesive composition for secondary batteries described in Project 2 above, the first polymer (A) further comprises structural units derived from sulfonic acid monomers.

[0031] (Project 4)

[0032] According to any one of the above items 1 to 3, the weight-average molecular weight of the first polymer (A) is 300,000 or more and 2,000,000 or less, and the weight-average molecular weight of the second polymer (B) is 3,000 or more and 200,000 or less.

[0033] (Project 5)

[0034] According to any one of the above items 1 to 4, the thermal crosslinking adhesive composition for secondary batteries comprises a content of 10% to 75% by weight of the structural unit derived from the sulfonic acid monomer in 100% by weight of the second polymer (B).

[0035] (Project 6)

[0036] A slurry composition for secondary batteries, in addition to water, the first polymer (A), and the second polymer (B) of the thermal crosslinking binder composition for secondary batteries according to any one of items 1 to 5 above, also includes an electrode active material containing a silicon-based active material and carbon nanotubes.

[0037] (Project 7)

[0038] An electrode for a secondary battery, wherein a layer of a slurry composition for a secondary battery as described in item 6 is formed on a current collector.

[0039] (Project 8)

[0040] A secondary battery comprising the electrodes for a secondary battery as described in item 7 above.

[0041] Invention Effects

[0042] The thermally crosslinkable binder composition for secondary batteries of the present invention comprises two polymers: a first polymer (A) and a second polymer (B). The first polymer (A) is a copolymer containing structural units derived from a specific monomer, and the second polymer (B) is a copolymer containing structural units derived from a monomer different from the specific monomer. In this thermally crosslinkable binder composition for secondary batteries, the first polymer (A) maintains good adhesion to the electrode active material or current collector, and the second polymer (B) can efficiently disperse the conductive additive CNT. Both contain structural units of monomers that can be thermally crosslinked between polymers. When the two polymers contain structural units of common monomers, resin compatibility is improved, phase separation inhibition effect is significant, and material stability is enhanced. Therefore, the thermally crosslinkable binder composition for secondary batteries of the present invention can both effectively disperse silicon-based active materials and conductive additive CNT, and can prepare stable slurries with high solids content. The above-mentioned slurry, composed of water, a first polymer (A), and a second polymer (B), is coated onto a current collector and dried to obtain a negative electrode. It has excellent adhesion to the current collector and can form a CNT conductive network, thereby obtaining a high-stability, high-capacity secondary battery with small electrode volume expansion and low film thickness increase. Attached Figure Description

[0043] Figure 1 is a cross-sectional schematic diagram showing the internal structure of a lithium-ion secondary battery.

[0044] Explanation of symbols: 1. Lithium-ion secondary battery; 10. Negative electrode; 11. Negative electrode active material; 12. Conductive additive (CNT, etc.); 13. Negative electrode binder; 14. Negative electrode current collector (copper foil); 20. Positive electrode; 21. Positive electrode active material; 22. Conductive additive (CNT, etc.); 23. Positive electrode binder; 24. Positive electrode current collector (aluminum foil); 30. Separator; 40. Electrolyte solution (electrolyte). Detailed Implementation

[0045] The following is an overview of the subject matter of this invention described in detail. This overview is not intended to limit the scope of the claims.

[0046] Thermally crosslinking adhesive composition for secondary batteries

[0047] The thermally crosslinkable binder composition for secondary batteries of the present invention (hereinafter referred to simply as "binder composition") comprises a first polymer (A), a second polymer (B), and water, wherein the first polymer (A) and the second polymer (B) are water-soluble copolymers.

[0048] In addition, in this disclosure, a "water-soluble" compound means that when 0.5 g of the compound is dissolved in 100 g of water at 25°C, the insoluble component is less than 0.5% by weight (less than 2.5 mg).

[0049] <First Polymer (A)>

[0050] The first polymer (A) of the present invention comprises at least structural units derived from acrylonitrile monomers and structural units derived from hydroxyl-containing (meth)acrylate monomers, and may, if desired, also comprise structural units derived from sulfonic acid monomers or unsaturated carboxylic acid monomers such as (meth)acrylic acid. In particular, when the sulfonic acid monomer constituting the second polymer (B) described later comprises tert-butylacrylamide sulfonic acid and / or an alkali metal salt of tert-butylacrylamide sulfonic acid, preferably, the first polymer (A) comprises structural units derived from sulfonic acid monomers.

[0051] In addition, in this disclosure, "(meth)propene" means propylene and / or methpropylene.

[0052] (Polymer of acrylonitrile (AN) monomer)

[0053] Polyacrylonitrile (PAN) is a polymer using acrylonitrile (AN) monomers. It possesses high mechanical strength, preventing electrode materials from peeling off from the current collector and improving the long-term stability of the battery. PAN exhibits high heat resistance and excellent tolerance to various electrolytes. On the other hand, it has low water solubility. Therefore, in this invention, it is suitable for copolymerization with monomers that have other functions to form copolymers.

[0054] The content of structural units derived from AN monomers in 100% by weight of the first polymer (A) is preferably 10% by weight or more and 50% by weight or less, more preferably 15% by weight or more and 40% by weight or less.

[0055] Furthermore, the content of structural units derived from AN monomers in 100% by weight of the first polymer (A) can be regarded as the content of AN monomers in 100% by weight of the monomer group constituting the first polymer (A) (hereinafter, the same applies to other monomers constituting the first polymer (A) and the second polymer (B).

[0056] (Hydroxy-containing (meth)acrylate monomers)

[0057] The hydroxyl-containing (meth)acrylate monomer is a (meth)acrylate having one or more hydroxyl groups, preferably a (meth)acrylate having a monohydroxyalkyl group having 1 to 8 carbon atoms. Specific examples include 2-hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, etc.

[0058] Starting from the structural unit derived from the sulfonic acid group monomer in the second polymer (B), and obtaining a cross-linked structure through esterification of the sulfonic acid group with an alcohol (hydroxyl group), it is preferable that the hydroxyl-containing (meth)acrylate monomer contains a hydroxyl group at its end. Among the hydroxyl-containing (meth)acrylate monomers, 2-hydroxyethyl acrylate (2HEA) and 2-hydroxyethyl methacrylate (2HEMA) are preferred due to their high water solubility.

[0059] The structural unit derived from the hydroxyl-containing (meth)acrylate monomer is preferably 5% or more and 50% or less in 100% by weight of the first polymer (A), more preferably 10% or more and 45% or less by weight.

[0060] (A copolymer of unsaturated carboxylic acids and monomers containing sulfonic acid groups)

[0061] As monomers constituting the first polymer (A), unsaturated carboxylic acids such as (meth)acrylic acid and unsaturated sulfonic acid monomers can be used simultaneously to improve water solubility. Specific examples of unsaturated carboxylic acid monomers include carboxyl-containing olefinic unsaturated monomers such as (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, (meth)acrylamidoic acid, and (meth)acrylamidodecanoic acid, as well as ethyl succinate (meth)acrylate and other carboxyl-containing olefinic unsaturated monomers or their base neutralization products. Specific examples of unsaturated sulfonic acid monomers (monomers containing sulfonic acid groups) include α,β-olefinic unsaturated sulfonic acids such as vinyl sulfonic acid or (meth)allyl sulfonic acid, (meth)tert-butylacrylamide sulfonic acid (ATBS), 2-(meth)acrylamide-2-methylpropanesulfonic acid, 2-(meth)acrylamide-2-hydroxypropanesulfonic acid, 3-sulfopropane (meth)acrylate, bis-(3-sulfopropyl)itaconate, etc., or their neutralizations.

[0062] In the first polymer (A) of the present invention, it is preferable to use monomers that improve water solubility, have excellent copolymerization properties, and are well compatible with the second polymer (B) for copolymerization. For example, acrylic acid or its inorganic salt, tert-butylacrylamide sulfonic acid (ATBS) or its inorganic salt ATBS-Na (including partially neutralized compounds) are preferred.

[0063] Examples of inorganic salts mentioned above include alkali metal salts and alkaline earth metal salts. Examples of alkali metals include lithium, sodium, and potassium, while examples of alkaline earth metals include magnesium and calcium.

[0064] The content of structural units derived from unsaturated carboxylic acid monomers in 100% by weight of the first polymer (A) is preferably 20% by weight or more and 50% by weight or less, more preferably 20% by weight or more and 40% by weight or less. Similarly, the content of structural units derived from sulfonic acid monomers is preferably 3% by weight or more and 35% by weight or less, more preferably 5% by weight or more and 30% by weight or less.

[0065] Without impairing the effects of the present invention, other monomers besides those described above may be used as monomers constituting the first polymer (A).

[0066] <Second Polymer (B)>

[0067] The second polymer (B) of the present invention comprises structural units derived from N-vinyl-2-pyrrolidone monomers and structural units derived from sulfonic acid-containing monomers.

[0068] (Polymer of N-vinyl-2-pyrrolidone (VP) monomer)

[0069] Polyvinylpyrrolidone (PVP) is a polymer using N-vinyl-2-pyrrolidone (VP) monomers. It exhibits excellent adhesion, chemical stability, and good performance characteristics during charge and discharge. Furthermore, due to its high hydrophilicity and lipophilicity, it functions as a stabilizer to uniformly disperse CNTs in water or organic solvents. Specifically, the PVP polymer chains adsorb onto the surface of CNTs, preventing aggregation. In addition, PVP acts as a surfactant, reducing the surface tension of CNTs and facilitating uniform dispersion in liquids. From these perspectives, using N-vinyl-2-pyrrolidone (VP) monomers is advantageous.

[0070] The content of structural units derived from VP monomers in 100% by weight of the second polymer (B) is preferably 25% by weight or more and 90% by weight or less, more preferably 30% by weight or more and 80% by weight or less.

[0071] (Contains sulfonic acid group monomers)

[0072] The sulfonic acid-containing monomer imparts electrolyte resistance to polymer (B), preventing aggregation between CNTs through ion repulsion and improving the stability of the slurry. Specific examples of the sulfonic acid-containing monomer are the same as those described above for the unsaturated sulfonic acid monomer. From the viewpoint of improving copolymerization with VP and water solubility, tert-butylacrylamide sulfonic acid (ATBS) or its inorganic salt (sodium salt, ATBS-Na, including a partially neutralized form) is preferably used as the monomer constituting the second polymer (B). In particular, when ATBS is included as a common monomer in the monomers constituting the first polymer (A) and the second polymer (B), the compatibility of the first polymer (A) and the second polymer (B) can be improved, and thermal crosslinking is easier, thus making it preferable.

[0073] The structural units derived from sulfonic acid monomers (including their inorganic salts) in 100% by weight of the second polymer (B) are preferably 10% by weight or more and 75% by weight or less. From the viewpoint of maintaining good dispersibility and adhesion with CNTs, etc., in the slurry composition described later, and exhibiting resistance to electrolytes, it is more preferably 25% by weight or more and 67% by weight or less.

[0074] Without impairing the effects of the present invention, other monomers besides those described above may be used as monomers constituting the second polymer (B).

[0075] <Water>

[0076] Examples of water used in this invention include water treated with ion exchange resin (ion-exchange water) and water treated with a reverse osmosis membrane purification system (ultrapure water). The conductivity of the water used is preferably 0.5 mS / m or less. If the conductivity of the water exceeds this range, it may affect the adsorption capacity of the binder composition for the negative electrode active material, resulting in poor dispersibility and reduced uniformity of the negative electrode. Furthermore, in this invention, a hydrophilic solvent can be mixed with the water without compromising the dispersion stability of the binder composition of the first polymer (A) and the second polymer (B). Examples of hydrophilic solvents include methanol, ethanol, and N-methylpyrrolidone (NMP). The amount added is preferably 5% by weight or less relative to water.

[0077] <Preparation of Adhesive Compositions>

[0078] The first polymer (A) and the second polymer (B) have different molecular chain lengths and molecular numbers, and play different roles according to their respective characteristics. In particular, the first polymer (A) is responsible for improving film properties such as adhesion to negative electrode active materials or current collectors, while the second polymer (B) is responsible for improving dispersibility with CNTs. In this invention, an adhesive composition is disclosed in which the weight ratio of the first polymer (A) and the second polymer (B) is 50:50 or more and 98:2 or less, and the gel fraction of the cured product is 90% or more.

[0079] The gel fraction (%) of the cured adhesive refers to the weight proportion of the thermally cross-linked polymer contained in the cured adhesive. Specifically, it is determined according to the following method: An adhesive composition containing a first polymer (A) and a second polymer (B) is applied to a substrate to obtain a film thickness of 30 to 100 μm after drying. The film is then dried at 120°C for 1 hour to obtain a cured adhesive. Approximately 1 g of the cured adhesive (weighing value α) is taken and immersed in 100 g of water at room temperature. After 10 minutes, the swollen cured adhesive is removed and dried at 120°C for 1 hour. The weight of the insoluble component is then measured (weighing value β). The gel fraction (%) is calculated based on the obtained weighing values ​​using the following formula.

[0080] (Formula) Gel fraction (%) = (Weighing value β / Weighing value α) × 100

[0081] In addition, the gel fraction can be adjusted by adjusting the content of structural units derived from hydroxyl-containing (meth)acrylate monomers constituting the first polymer (A) and the content of structural units derived from sulfonic acid monomers constituting the second polymer (B).

[0082] When preparing the binder composition, if the weight ratio of the polymer and the gel fraction of the cured product deviate from the above-mentioned specified range, the capacitance during charge-discharge cycles will decrease significantly, and the variation in electrode film thickness will increase, which is therefore undesirable. As described above, in this invention, the weight ratio of the first polymer (A) and the second polymer (B) is 50:50 or more and 98:2 or less, and the gel fraction of the cured product is 90% or more. Furthermore, a more preferred range is that the weight ratio of the first polymer (A) and the second polymer (B) is 65:35 or more and 97:3 or less, and the gel fraction of the cured product is 95% or more.

[0083] <Molecular weight of polymers, etc.>

[0084] The weight-average molecular weight (Mw) of the first polymer (A) is not particularly limited, but from the viewpoint of the dispersion stability of the slurry composition for lithium-ion battery anodes, it is preferably 300,000 or more and 2,000,000 or less, and more preferably 350,000 or more and 1,500,000 or less.

[0085] The weight-average molecular weight (Mw) of the second polymer (B) is not particularly limited, but from the viewpoint of dispersibility of the slurry composition for secondary batteries, it is preferably 0.3 million or more and 200,000 or less, more preferably 0.5 million or more and 50,000 or less. If the weight-average molecular weight (Mw) is too low, the electrolyte tolerance may be insufficient; if it is too high, the dispersibility of CNTs may be reduced.

[0086] The weight-average molecular weight can be determined by gel permeation chromatography (GPC) in a suitable solvent and converted using polyethylene oxide as a reference.

[0087] From the viewpoint of solution stability, the pH value of the thermally crosslinkable binder composition for secondary batteries, comprising the first polymer (A) and the second polymer (B), is preferably 5 or higher and 8 or lower. If the pH value is too low, the initial battery capacity may decrease; if the pH value is too high, ester hydrolysis may easily occur, which is unsuitable. The pH value of the binder composition can be measured at 25°C using a glass electrode pH meter. In particular, when neutralizing (including partially neutralizing) a monomer containing an unsaturated acid group or its inorganic salt, the pH value of its aqueous solution can be used for evaluation.

[0088] In the polymer-containing thermally crosslinkable adhesive composition for secondary batteries of the present invention, thermal crosslinking occurs due to the esterification reaction between the hydroxyl groups of the hydroxyl-containing (meth)acrylate monomer in the first polymer (A) and the alkyl sulfonic acid groups of the sulfonic acid monomer in the second polymer (B). This crosslinking reaction occurs, for example, when a slurry composition using this adhesive composition is applied to a current collector and dried. As a result, it exhibits a gel fraction due to its insolubility in water. This mechanism is merely illustrative, and the application of the present invention is not limited thereto. From the viewpoint of suppressing volume changes (resilience) accompanying charge-discharge cycles, the gel fraction of the cured product of the thermally crosslinkable adhesive composition for secondary batteries of the present invention is not particularly limited, but is preferably 90% or more, more preferably 95% or more.

[0089] In the polymer-containing thermally crosslinked binder composition for secondary batteries of the present invention, the molar ratio (sulfonic acid group / hydroxyl group) of acidic groups (sulfonic acid group / hydroxyl group) is preferably 0.1 or more and 10.0 or less, more preferably 0.2 or more and 5.0 or less. This results in a densely formed crosslinked structure, effectively suppressing volume changes (resilience) during charge-discharge cycles, but the present invention is not limited thereto. When the molar ratio of acidic groups to hydroxyl groups in the thermally crosslinked binder composition for secondary batteries is too small, the available second polymer (B) is limited, and the crosslinked structure cannot be sufficiently formed during heating, failing to achieve the desired effect. On the other hand, when the molar ratio of acidic groups to hydroxyl groups in the binder composition is too large, unreacted second polymer (B) tends to remain during heating, exhibiting a trend of reduced electrolyte tolerance, and failing to achieve the desired effect.

[0090] The above molar ratio can be calculated using the following formula.

[0091] (Formula) aaa / (AAA+BBB)

[0092] Wherein, aaa is the value obtained by multiplying the molar amount (aa) of the hydroxyl-containing monomer contained in 100% by weight of the monomer group constituting the first polymer (A) by the weight ratio of the first polymer (A) contained in the binder composition (aaa = weight ratio of polymer (A) × aa).

[0093] AAA is the value obtained by multiplying the molar amount (AA) of the sulfonic acid-containing monomers contained in 100% by weight of the monomer group constituting the first polymer (A) by the weight ratio of the first polymer (A) contained in the binder composition (AAA = weight ratio of polymer (A) × AA).

[0094] BBB is the value obtained by multiplying the molar amount (BB) of the sulfonic acid-containing monomers contained in 100% by weight of the monomer group constituting the second polymer (B) by the weight ratio of the second polymer (B) contained in the binder composition (BBB = weight ratio of polymer (B) × BB).

[0095] <Method for manufacturing a thermally crosslinking adhesive composition for secondary batteries>

[0096] (Example of manufacturing the first polymer (A))

[0097] In one embodiment, the first polymer (A) is obtained, for example, by the following method. Based on 100% by weight of the monomer group constituting the first polymer (A), a monomer group comprising at least the following components is copolymerized: preferably 10 to 50% by weight of acrylonitrile monomer, preferably 5 to 45% by weight of hydroxyethyl 2-acrylate (2HEA) monomer, preferably 20 to 50% by weight of acrylic acid monomer, and preferably 3 to 25% by weight of sodium salt of tert-butylacrylamide sulfonic acid (ATBS-Na) monomer. Furthermore, the neutralization treatment of acrylic acid can be a monomer neutralization treatment before polymerization or a neutralization treatment after polymerization.

[0098] The first polymer (A) can be synthesized using a known polymerization method (preferably free radical polymerization). In the polymerization reaction, a free radical polymerization initiator is added to a monomer mixture, and a chain transfer agent or crosslinking agent is added as needed. The mixture is stirred and carried out at a reaction temperature of approximately 50 to 100°C to obtain the desired polymer. The reaction time is preferably, for example, approximately 1 to 10 hours.

[0099] (Example of manufacturing the second polymer (B))

[0100] In one embodiment, the second polymer (B) is obtained, for example, by the following method: a monomer group comprising at least the following components is copolymerized, based on 100% by weight of the monomer group constituting the second polymer (B): preferably 25 to 90% by weight of N-vinyl-2-pyrrolidone monomer, and preferably 10 to 75% by weight of sodium salt of tert-butylacrylamide sulfonic acid (ATBS-Na) monomer.

[0101] The second polymer (B) can be synthesized using a known polymerization method (preferably free radical polymerization). In the polymerization reaction, a free radical polymerization initiator is added to the monomer mixture, and a chain transfer agent or crosslinking agent is added as needed. The mixture is stirred and carried out at a reaction temperature of approximately 50 to 100°C to obtain the desired polymer. The reaction time is preferably, for example, approximately 1 to 10 hours.

[0102] The free radical polymerization initiator used for the polymerization of the first polymer (A) and the second polymer (B) described above can be a known free radical polymerization initiator. Examples of free radical polymerization initiators include persulfates such as potassium persulfate and ammonium persulfate, redox polymerization initiators composed of the above persulfates and reducing agents such as sodium bisulfite, and azo initiators such as 2,2-azobis-2-amidinylpropane dihydrochloride. The amount of free radical polymerization initiator used is not particularly limited, but it is preferably 0.05% by weight or more and 12.0% by weight or less, more preferably 0.1% by weight or more and 6.0% by weight or less, based on 100% by weight of the monomer group.

[0103] To improve manufacturing stability, the pH of the reaction solution can be adjusted using common neutralizing agents such as potassium hydroxide, sodium hydroxide, and lithium hydroxide before the free radical polymerization reaction and / or during the water solubilization of the copolymer.

[0104] When the first polymer (A) has acid groups, the neutralization rate during dispersion of the electrode active material is not particularly limited. From the viewpoint of preventing a decrease in initial capacity, the neutralization rate is preferably 70% or more, and from the viewpoint of preventing hydrolysis, it is 95% or less.

[0105] Furthermore, a neutralization rate of 100% means neutralization is performed using the same number of moles of base as the acid component contained in each polymer. A neutralization rate of 50% means neutralization is performed using half the number of moles of base as the acid component contained in each polymer.

[0106] In one embodiment, the first polymer (A) and the second polymer (B) are preferably inorganic salts.

[0107] <Additives>

[0108] In the thermally crosslinking binder composition (aqueous solution) for secondary batteries of the present invention, additives such as dispersants, leveling agents, preservatives, antioxidants, thickeners, and particulate polymers may be appropriately used.

[0109] Secondary battery slurry composition

[0110] This disclosure provides a slurry composition for secondary batteries, which, in addition to water, the first polymer (A), and the second polymer (B) comprising the aforementioned thermally crosslinking binder composition for secondary batteries, also includes an electrode active material containing a silicon (Si)-based active material and carbon nanotubes. The electrode active material is, for example, a negative electrode active material that simultaneously uses both silicon (Si)-based and carbon-based active materials. Furthermore, the pH value of the slurry composition for secondary batteries is preferably 5 to 7. In the slurry composition (aqueous solution) for secondary batteries of the present invention, conductive additives, leveling agents, preservatives, thickeners, and binding agents other than CNTs may also be appropriately used simultaneously. Additionally, the slurry composition for secondary batteries containing a negative electrode active material is also described as a slurry composition for the negative electrode of a secondary battery.

[0111] <Negative Electrode Active Material>

[0112] The electrode active material of a lithium-ion battery realizes the charge and discharge process of the battery by receiving or releasing lithium ions at the positive and negative electrodes. Specifically, the negative electrode active material stores and releases energy by receiving lithium ions during charging and releasing lithium ions during discharging. In the present invention, in order to increase the capacity of the lithium-ion battery, as the negative electrode active material, it is preferably at least a silicon (Si)-based material. As the high-capacity silicon (Si)-based active material material of the present invention, nanostructured silicon (Nano-Si) materials, silicon oxide (SiO) -based materials, Si-C porous composite (SiC) -based materials, etc. can be cited. On the other hand, silicon (Si)-based materials have the problem of significant volume changes such as expansion or contraction during charge and discharge cycles. Therefore, it is also effective to use carbon materials, etc. simultaneously.

[0113] Silicon (Si) exhibits an extremely high theoretical capacity of about 4200 mAh / g by alloying with lithium. On the other hand, due to the large volume change accompanying the insertion and extraction of lithium ions, the decomposition is aggravated, and there is a problem of short cycle life. Therefore, a nanostructured silicon (Nano-Si) material that nano-sizes the active material particle size has been proposed. Nano-Si (nanoparticle) materials are not easily cracked due to large stresses and are expected to improve the cycle life. In addition, the high-density grain boundaries of nanomaterials are generally considered to enable rapid diffusion of Li ions and can be used in the present invention.

[0114] As the above-mentioned silicon oxide (SiO) -based material, a silicon oxide (SiO) -based negative electrode active material represented by the compositional formula SiOx (0 < x < 2, preferably 0.1 ≤ x ≤ 1) is preferred. This SiO-based active material has an amorphous structure and has an extremely high theoretical capacity of about 1200 mAh / g compared with the conventional graphite negative electrode active material (about 372 mAh / g), and is useful for the purpose of improving the energy density of secondary batteries.

[0115] As other silicon-based materials, Si-C porous composite (SiC) -based materials are preferably used. Such SiC-based active materials can be obtained, for example, by exposing porous carbon particles to silane gas at high temperature by chemical vapor deposition (CVD) method to generate silicon in the small holes of the porous carbon. The theoretical capacity of SiC is about 1800 mAh / g, which is much higher than that of graphite negative electrode active materials, and can be used in combination with other active material materials, etc.

[0116] To achieve stable charge-discharge cycle performance while utilizing the high energy density of silicon (Si)-based materials, it is preferable to use them simultaneously with highly conductive carbon materials. Examples of carbon materials include highly crystalline carbon such as graphite (also known as black lead, natural graphite, artificial graphite, etc.), low-crystalline carbon (soft carbon, hard carbon), carbon black (Ketjen black, acetylene black, channel black, lamp black, furnace black, pyrolysis carbon black, etc.), fullerenes, carbon nanotubes (CNTs), carbon nanofibers, mesophase carbon microspheres (MCMB), and pitch-carbon fibers.

[0117] To significantly enhance the effects of this invention, from the viewpoint of improving the battery capacity of lithium-ion batteries, the content of silicon (Si)-based material, based on 100% by weight of the negative electrode active material, is preferably 5% by weight or more and 50% by weight or less. More preferably, it is in the range of 15% by weight or more and 45% by weight or less. Balancing the improvement of energy density and cycle life is important. Increasing the content of silicon (Si)-based material increases the theoretical capacity, but it also leads to the disadvantage of accelerated electrode degradation due to volume changes during charging and discharging.

[0118] <Conductive additives>

[0119] In this invention, carbon nanotubes (CNTs) are included as conductive additives. Examples of CNTs include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). SWCNTs are preferred for use in slurry compositions for the negative electrode of secondary batteries, but MWCNTs may also be partially included. CNTs have a shape formed by winding planar graphite into a cylindrical form. CNTs can be obtained from relatively homogeneous raw materials using methods such as chemical vapor deposition (CVD). To achieve high-capacity secondary batteries, CNTs, through their excellent conductivity and high mechanical strength, can suppress the adverse effects caused by volume changes in the electrode active material during charge-discharge cycles, thereby improving battery output density and cycle life.

[0120] In this embodiment, the suitable average diameter of the CNTs is 0.4 nm or more and 15 nm or less, more preferably 0.4 nm or more and 10 nm or less. Furthermore, the suitable length of the CNTs is 1 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less. Fine CNTs have a large surface area and high conductivity, thus improving electron mobility. However, if the length is too short, it is difficult to form a CNT network; if it is too long, the viscosity increases, making dispersion extremely difficult.

[0121] The appropriate amount of conductive additive is, based on 100 parts by weight of negative electrode active material, 0.01% to less than 0.2% by weight of CNT. Other conductive additives that can be used simultaneously with CNT include carbon black such as acetylene black, Ketjen black, and furnace black.

[0122] In the negative electrode slurry composition for secondary batteries, the content of the binder composition is preferably 0.5% to 10% by weight or more, based on 100% by weight of the negative electrode active material. By using this range of content, an electrode with better adhesion, lower resistance, and better charge-discharge characteristics can be provided.

[0123] In the above slurry composition, the water content is preferably 50% or more and 150% or less, based on 100% by weight of the negative electrode active material.

[0124] <Slurry viscosity adjusting solvent>

[0125] The thermally crosslinking binder composition for secondary batteries of the present invention is an aqueous binder resin, and the main component of the slurry viscosity adjusting solvent is water. To improve the flowability and uniformity of the slurry, optimize the manufacturing process, and enhance battery performance, solvents other than water may also be used simultaneously. For example, ethanol, isopropanol, ethylene glycol, or N-methyl-2-pyrrolidone (NMP) may be used in part.

[0126] The method for manufacturing the secondary battery slurry composition of the present invention can include: a method of mixing an aqueous solution of a binder composition with a negative electrode active material, a conductive additive, etc.; or a method of separately mixing an aqueous solution of the binder composition, a negative electrode active material, a conductive additive, and water. Furthermore, the mixing order is not particularly limited in this method. As mixing means for the slurry, ball mills, sand mills, air jet mills, pigment dispersers, pulverizers, ultrasonic dispersers, homogenizers, planetary mixers, Hobart mixers, etc., can be used.

[0127] While not specifically limited, wet air jet milling is a practical method for dispersing CNTs. Wet air jet milling utilizes the impact energy of a high-pressure liquid jet ejected at supersonic speeds from a micro-nozzle to pulverize and disperse solid particles. The following steps are used to disperse CNTs.

[0128] 1) Supply of high-pressure liquid: The slurry composition is supplied to the micro-nozzles inside the mill via a high-pressure pump.

[0129] 2) Supersonic jetting and collision: The liquid is jetted from the nozzle at supersonic speed and collides at high speed in the collision zone. At this time, particles such as CNTs contained in the liquid are dispersed and pulverized.

[0130] 3) Application of shear stress: The strong shear stress generated during the collision breaks the bond between easily aggregated CNT particles, achieving uniform dispersion.

[0131] 4) Circulation process: The design allows the slurry to circulate multiple times within the device to achieve the desired dispersion state.

[0132] Wet air jet mills can utilize extremely high shear stress to minimize damage to CNT and other long fiber nanomaterials and disperse them uniformly, thus making them usable.

[0133] Electrodes for secondary batteries

[0134] In this disclosure, a negative electrode for a secondary battery, particularly a negative electrode for a lithium-ion secondary battery, is provided by coating the aforementioned slurry composition for a secondary battery onto a current collector and then drying and curing it. The electrode is obtained by forming a dried and cured product of the aforementioned slurry composition for a secondary battery on the current collector.

[0135] There are no particular limitations on the coating method. Well-known film-forming devices such as comma coaters, gravure coaters, micro-gravure coaters, die coaters, and doctor blade coaters can be listed.

[0136] There are no particular limitations on the drying method, but the temperature is preferably around 60 to 160°C, more preferably around 70 to 140°C, and the atmosphere can be dry air or an inert atmosphere.

[0137] The thickness of the electrode (cured coating) is not limited, but is preferably about 5 to 300 μm, more preferably about 10 to 250 μm. By setting it within this range, sufficient Li insertion and extraction capability of the electrode can be ensured at high current densities.

[0138] As the current collector, any known current collector can be used. The material of the current collector is not particularly limited; examples include metallic materials such as copper, iron, aluminum, nickel, stainless steel, and nickel-plated steel, or carbon materials such as carbon cloth and carbon paper. The form of the current collector is also not particularly limited; in the case of metallic materials, examples include metal foil, metal cylinder, metal coil, and metal plate; in the case of carbon materials, examples include carbon plate, carbon film, and carbon pillar. When used as the negative electrode, copper foil is preferred as the current collector.

[0139] Secondary batteries

[0140] This disclosure provides a secondary battery having the aforementioned electrodes for a secondary battery, particularly a lithium-ion secondary battery.

[0141] Referring to FIG1, a suitable embodiment of the lithium-ion secondary battery of the present invention will be described. The lithium-ion secondary battery 1 of the present invention comprises a negative electrode 10, a positive electrode 20, a separator 30, and an electrolyte solution 40. The negative electrode is formed by forming a film of a negative electrode active material 11, a conductive additive 12 such as CNTs, and a negative electrode binder 13 (the subject of the present invention) on a negative electrode current collector 14 and effectively bonding them together. On the other hand, the positive electrode is formed by forming a film of a positive electrode active material 21, a conductive additive 22, and a positive electrode binder 23 on a positive electrode current collector 24.

[0142] Electrolyte solution 40 can be a non-aqueous electrolyte that supports the electrolyte and can be dissolved in a non-aqueous solvent. Furthermore, the aforementioned non-aqueous electrolyte may also contain a film-forming agent.

[0143] For non-aqueous solvents, known non-aqueous solvents can be used alone or in combination. Examples of non-aqueous solvents include chain carbonate solvents such as diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate; cyclic carbonate solvents such as ethylene carbonate, propylene carbonate, and butyl carbonate; chain ether solvents such as 1,2-dimethoxyethane; cyclic ether solvents such as tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, and 1,3-dioxolane; chain ester solvents such as methyl formate, methyl acetate, and methyl propionate; cyclic ester solvents such as γ-butyrolactone and γ-valerolactone; and acetonitrile. In particular, combinations of mixed solvents comprising cyclic carbonate solvents and chain carbonate solvents are preferred.

[0144] Lithium salts are used as the supporting electrolyte. Known lithium salts can be used alone or in combination. Examples of supporting electrolytes include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, C4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. In particular, LiPF6, LiClO4, and CF3SO3Li are preferred for their ease of solubility in solvents and high degree of dissociation. The higher the degree of dissociation of the supporting electrolyte used, the higher the lithium-ion conductivity, and the more effectively the ion conductivity can be tuned.

[0145] For film-forming agents, known film-forming agents can be used alone or in combination. Examples of film-forming agents include carbonate compounds such as vinylene carbonate, vinyl ethylene carbonate, vinyl ethyl carbonate, methyl phenyl carbonate, fluoroethylene carbonate, and difluoroethylene carbonate; olefin sulfides such as ethylene sulfides and propylene sulfides; sulpholactone compounds such as 1,3-propane sulpholactone and 1,4-butane sulpholactone; and acid anhydrides such as maleic anhydride and succinic anhydride. By setting the content of the film-forming agent in the electrolyte solution to 10% by weight or less, effects such as suppressing initial irreversible capacity and improving low-temperature and rate performance can be expected.

[0146] The separator is a structural component located between the positive and negative electrodes, preventing short circuits between them. Specifically, a porous separator, such as a porous membrane or non-woven fabric, is preferred. A non-aqueous electrolyte is impregnated in the separator. The raw materials for the separator can include polyolefins such as polyethylene and polypropylene, and polyethersulfone, with polyolefins being preferred.

[0147] For the positive electrode, known positive electrodes can be used without restriction. First, a positive electrode slurry is prepared by mixing the positive electrode active material, conductive additive, and positive electrode binder with an organic solvent. The prepared positive electrode slurry is then coated onto a positive electrode current collector, dried, and pressed to fabricate the positive electrode.

[0148] Examples of positive electrode active materials include inorganic and organic positive electrode active materials. Inorganic positive electrode active materials include transition metal oxides, lithium-transition metal composite oxides, and transition metal sulfides. Transition metals include Fe, Co, Ni, Mn, and Al. Inorganic compounds used as positive electrode active materials include lithium-containing composite metal oxides such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiFePO4, and LiFeVO4; transition metal sulfides such as TiS2, TiS3, and amorphous MoS2; and Cu2V2O3, amorphous V2O-P2O5, MoO3, V2O5, and V6O3. 13 Transition metal oxides, etc. These compounds can be obtained by partial elemental substitution. Examples of organic cathode active materials include conductive polymers such as polyacetylene and poly(p-phenylene oxide). Iron oxides with poor conductivity can be used as carbon-coated electrode active materials by adding carbon source materials during reduction calcination. Among these, LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and LiFePO4 are preferred in terms of practicality, electrical properties, and long lifespan.

[0149] To compensate for the conductivity of the positive electrode active material, conductive additives can be used. Examples of conductive additives include fibrous carbon such as vapor-grown carbon fiber (VGCF), carbon nanotubes (CNT), and carbon nanofibers (CNF), graphite particles, and carbon black such as acetylene black and Ketjen black.

[0150] For the binder used in the positive electrode, known binders can be used alone or in combination. Specifically, examples include fluorinated resins (polyvinylidene fluoride, polytetrafluoroethylene, etc.), polyolefins (polyethylene, polypropylene, etc.), and acrylic polymers (acrylic acid copolymers, methacrylic acid copolymers), etc.

[0151] Aluminum foil, stainless steel foil, etc. can be used as positive current collectors.

[0152] There are no particular limitations on the form of the lithium-ion battery described above. Examples of lithium-ion battery forms include cylindrical batteries where sheet electrodes and separators are wound into a spiral shape; cylindrical batteries with an inner and outer structure consisting of granular electrodes and separators; and coin-shaped batteries where granular electrodes and separators are stacked. Furthermore, by packaging these battery forms into any outer casing, they can be manufactured into any shape, such as coin-shaped, cylindrical, or square.

[0153] There are no particular limitations on the manufacturing method of lithium-ion batteries; they can be assembled according to the appropriate steps based on the battery structure. The negative electrode is placed on the outer casing, followed by the electrolyte and separator, and then the positive electrode, opposite the negative electrode, is placed on top. The batteries are then secured with gaskets and sealing plates to complete the process.

[0154] There are no specific restrictions on the manufacturing process of lithium-ion batteries, but it is mainly completed through the following stages.

[0155] 1) Material preparation and electrode fabrication stage (preparation of negative electrode material and positive electrode material); For the negative electrode and positive electrode, a slurry containing active material, conductive additive, binder composition and solvent (water) is prepared respectively, and the electrode is made after coating and drying.

[0156] 2) Calendering stage: The dried electrode sheets are compressed using a roller press to homogenize the thickness and density of the electrodes. This process improves the energy density of the battery.

[0157] 3) Electrode cutting and assembly stage: After cutting the electrode sheets into the specified shapes, the positive electrode, negative electrode and separator are stacked to form a battery cell.

[0158] 4) Electrolyte injection and sealing stage; After injecting electrolyte into the battery cell, it is sealed to prevent electrolyte leakage.

[0159] 5) Absolute Drying (Oven Drying) Stage: The oven drying stage aims to remove trace amounts of moisture and other volatile components from the lithium-ion battery cell to the greatest extent possible. Removing trace amounts of moisture prevents it from reacting with lithium to form lithium hydroxide and hydrogen gas, which would cause an increase in internal battery pressure. It also prevents moisture from reacting with the electrolyte to produce decomposition products, thereby improving the battery's cycle life. This stage is extremely important for both safety and battery performance. For example, in this stage, a vacuum drying device is used under high vacuum, including pre-drying, and treatment at temperatures between 60 and 150°C to ensure complete drying.

[0160] The binder composition of the present invention is designed to significantly improve the cycle characteristics of secondary batteries by performing a thermal crosslinking reaction between the first polymer (A) and the second polymer (B) through the oven-drying stage, resulting in a crosslinked structure with an infinitely large molecular weight.

[0161] 6) Final Assembly and Formation Stage: The battery cells are assembled into their final form and encapsulated in a container to safely protect them and minimize the impact of the external environment. Next, the formation stage involves the battery's first charge / discharge, activating the internal electrode materials and forming the solid electrolyte interface (SEI) layer. This process establishes the battery's initial performance, ensuring long-term performance and safety.

[0162] Example

[0163] The present invention will be described below by way of examples and comparative examples, but is not limited thereto. In addition, unless otherwise specified in the examples, "%" means "weight %" and "parts" means "parts by weight".

[0164] (Example 1)

[0165] 1. Example of manufacturing the first polymer (A) component

[0166] (Synthesis of polymer (A-1))

[0167] Add the following to a reaction apparatus equipped with a stirrer, thermometer, reflux cooling pipe, and nitrogen inlet pipe: 1800g of ion-exchanged water, 100g of acrylonitrile (AN) (Tokyo Chemical Industries Co., Ltd., "Acrylonitrile (stabilized with MEHQ)") (40% by weight of 100% by weight of monomers), 25g of hydroxyethyl 2-acrylate (2HEA) (Tokyo Chemical Industries Co., Ltd., "2-Hydroxyethyl Acrylate (stabilized with MEHQ)") (10% by weight of 100% by weight of monomers), 75g of acrylic acid (AA) (Fujifilm Wako Pure Chemicals Co., Ltd., "Acrylic Acid, Wako Premium Grade") (30% by weight of 100% by weight of monomers), and a 50% aqueous solution of sodium tert-butylacrylamide sulfonic acid (ATBS-Na) (Tokyo Chemical Industries Co., Ltd., "Sodium 2-Acrylamido-2-methylpropane-1-sulfonate (ca. 50% in Water) (stabilized with MEHQ)"). 100g of MEHQ (20% by weight of monomers out of 100% by weight), 20.8g of sodium hydroxide (sodium hydroxide, reagent grade, manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) (amount with a neutralization rate of 50% in AA units), were added. After removing oxygen from the reaction system by purging with nitrogen, the temperature was raised to 60°C. 2.6g of polymerization initiator 2,2-azobis(2-methylpropanedihydrochloride) dihydrochloride (2,2-azobis(2-methylpropanediamine) dihydrochloride V-50, manufactured by Fujifilm and Koko Pure Chemical Industries Co., Ltd.) and 2.6g of deionized water were added, and the temperature was raised to 72°C and the reaction was carried out for 30 minutes. Then, an aqueous solution of the polymerization initiator was added dropwise, and the reaction was continued for 3 hours. Then, 18.7g of sodium hydroxide was added to achieve a neutralization rate of 95% in AA units. The concentration was adjusted with deionized water to obtain an aqueous solution of the copolymer with a solid Nv content of 12% by weight and a weight-average molecular weight (Mw) of 1,100,000 (1.1 million).

[0168] In this invention, as the first polymer (A), according to the synthesis example of the polymer (A-1) described above, a copolymer having the monomer composition ratio, molecular weight, and solid content shown in Table 1 below is obtained. (A-1) to (A-5) are examples corresponding to the first polymer (A) in this invention, and (a-1) to (a-5) represent examples corresponding to the same comparative examples.

[0169] [Table 1]

[0170] [Table 1] Composition ratio, molecular weight and solid content of the first polymer (A)

[0171] AN: Acrylonitrile

[0172] ·2HEA: Hydroxyethyl 2-acrylate

[0173] AA: Acrylic acid

[0174] ·ATBS-Na: Sodium salt of tert-butylacrylamide sulfonic acid

[0175] 2. Example of manufacturing the second polymer (B) component

[0176] (Example of polymer (B-1) synthesis)

[0177] 514 g of deionized water was added to a reaction apparatus equipped with a stirrer, thermometer, reflux cooling pipe, and nitrogen inlet pipe. After purging with nitrogen to remove oxygen from the reaction system, the temperature was raised to 72°C. 6.0 g of polymerization initiator V-50 and 6.0 g of deionized water were added, followed by the dropwise addition of 100 g of N-vinyl-2-pyrrolidone (VP) (manufactured by Tokyo Chemical Industry Co., Ltd., "1-Vinyl-2-pyrrolidone (stabilized with N,N-Di-sec-butyl-p-phenylenediamine)") and 400 g of a 50% aqueous solution of ATBS-Na. The reaction was then carried out at 90°C for 3 hours to obtain an aqueous solution of the copolymer.

[0178] In this invention, as the second polymer (B), a copolymer having the monomer composition ratios, molecular weights, and solid content shown in Table 2 below is obtained according to the synthesis example of the polymer (B-1) described above. (B-1) to (B-5) are examples corresponding to the second polymer (B) of this invention, and (b-1) to (b-3) represent examples corresponding to the same comparative examples.

[0179] [Table 2]

[0180] [Table 2] Composition ratio, molecular weight and solid content of the second polymer (B) component

[0181] ·VP: N-vinyl-2-pyrrolidone

[0182] ·ATBS-Na: Sodium salt of tert-butylacrylamide sulfonic acid

[0183] 3. Example of preparation of thermally crosslinkable binder composition for secondary batteries

[0184] <Preparation of Thermally Crosslinked Adhesive Compositions for Secondary Batteries>

[0185] When acrylic monomers are used in the first polymer (A), the polymer is neutralized (including partial neutralization) with sodium hydroxide to prepare an aqueous polymer solution (A-1). In the second polymer (B), an aqueous polymer solution (B-1) with a copolymerization solids content of 30% by weight is prepared.

[0186] Next, the above-mentioned polymer aqueous solution (A-1) and polymer aqueous solution (B-1) are mixed and diluted with water to obtain a thermally crosslinkable binder composition for secondary batteries with a specified solid content.

[0187] The initial evaluation results of the binder compositions obtained by blending polymers (A) and (B) according to the solid weight ratio are shown in Table 3.

[0188] The following describes the preparation of thermally crosslinkable binder compositions for secondary batteries using various polymers with different manufacturing conditions, corresponding to each embodiment and comparative example.

[0189] [Table 3]

[0190] 4. Evaluation of thermally crosslinkable adhesive compositions for secondary batteries

[0191] <Compatibility>

[0192] In this disclosure, "compatibility" during mixing refers to the visual evaluation of whether turbidity or other phenomena caused by phase separation occurs when water-soluble polymers (A) and (B) are mixed in a glass container to prepare an adhesive composition.

[0193] "Good compatibility" means that the aqueous solutions of various copolymers have mutual affinity at 25°C, and there is no phase separation or turbidity when they are mixed, and they can form a solution or a homogeneous composition.

[0194] In addition, the "compatibility" after drying refers to whether solidification, phase separation, and turbidity occur after the adhesive composition is dropped onto a glass plate and dried at 120°C for 5 minutes by observing its appearance.

[0195] The compatibility of the adhesive composition during mixing should be visually evaluated according to the following criteria.

[0196] ◎: A state that is completely transparent and has good compatibility.

[0197] ○: Slightly cloudy, but the state on the other side of the glass container can still be seen visually.

[0198] △: Noticeably cloudy, making it impossible to visually see the state on the other side of the glass container.

[0199] ×: Incompatible, exhibiting a state of delamination.

[0200] The compatibility of the adhesive composition after drying should be visually evaluated according to the following criteria.

[0201] ◎: A state that is completely transparent and has good compatibility.

[0202] ○: Slightly cloudy, but the condition of the solidified material on the back can be visually seen.

[0203] △: Noticeably cloudy, making it impossible to visually see the condition of the back of the solidified material.

[0204] ×: Incompatible; cured product appears as spots.

[0205] <Gel fraction>

[0206] The change in water resistance caused by the crosslinking reaction (thermal crosslinking) of the adhesive composition after drying and heat treatment was evaluated by the weight change rate after immersion in water at room temperature and drying, and by the gel fraction of the cured product.

[0207] The above evaluation results of the gel fraction show that the gel fraction in each embodiment is above 95%. This is because the hydroxyl groups in the water-soluble polymer (A) derived from the hydroxyl-containing (meth)acrylate monomer and the acidic groups (sulfonic acid groups) in the water-soluble polymer (B) derived from the sulfonic acid-containing monomer undergo sufficient cross-linking reaction.

[0208] On the other hand, in Comparative Examples 2, 4, and 7, the gelation rate was less than 90%. This is because the cross-linking reaction between the water-soluble polymer (A) and the water-soluble polymer (B) was not sufficiently carried out. Specifically, in Comparative Example 2, the water-soluble polymer (A) did not contain structural units derived from (meth)acrylate monomers (such as 2HEA) having hydroxyl groups. Furthermore, in Comparative Example 7, the water-soluble polymer (B) did not contain structural units derived from monomers having sulfonic acid groups (such as ATBS-Na). In addition, in Comparative Example 4, the compatibility between the water-soluble polymer (A) and the water-soluble polymer (B) was poor. Due to these factors, the cross-linking reaction was not sufficiently carried out, resulting in a gelation rate of less than 90%.

[0209] Electrolyte tolerance

[0210] The electrolyte resistance of an adhesive composition is evaluated by measuring the weight change rate (dissolution rate) before and after immersion in the electrolyte, based on the effect of the crosslinking reaction (thermal crosslinking) on ​​the adhesive composition after drying and heat treatment.

[0211] Specifically, a sample of the adhesive composition was immersed in an electrolyte solution prepared by mixing 50 mL of ethylene carbonate (hereinafter referred to as EC) (manufactured by Fujifilm and Koichi Pure Chemical Industries Co., Ltd.) and 50 mL of ethyl methyl carbonate (hereinafter referred to as EMC) (manufactured by Tokyo Chemical Industry Co., Ltd.) at 40°C for 22 hours, and then dried at 150°C for 3 hours. The rate of change (dissolution rate) of the weight of the adhesive composition sample after immersion in the electrolyte solution and drying compared with its weight before immersion in the electrolyte solution was calculated using the following formula.

[0212] (Mode)

[0213] Weight change rate (dissolution rate) = {1 - (weight after electrolyte impregnation and drying) / (weight before electrolyte impregnation)} × 100 (%)

[0214] The evaluation results of the electrolyte tolerance mentioned above show that the weight change rate in each embodiment is within 2%. This is because the cross-linking reaction is sufficient between the hydroxyl groups derived from the hydroxyl-containing (meth)acrylate monomer in the water-soluble polymer (A) and the acidic groups (sulfonic acid groups) derived from the sulfonic acid monomer in the water-soluble polymer (B).

[0215] On the other hand, in Comparative Examples 2, 4, and 7, the weight change rate was 2% or more (including those that could not be determined). This was because the cross-linking reaction between the water-soluble polymer (A) and the water-soluble polymer (B) was not sufficiently carried out, resulting in unreacted water-soluble polymer (B) remaining and dissolving into the electrolyte. Specifically, in Comparative Example 2, the water-soluble polymer (A) did not contain hydroxyl-containing (meth)acrylate monomers (such as 2HEA). Furthermore, in Comparative Example 7, the water-soluble polymer (B) did not contain sulfonic acid monomers (such as ATBS-Na). In addition, in Comparative Example 4, the compatibility between the water-soluble polymer (A) and the water-soluble polymer (B) was poor. Due to these factors, the cross-linking reaction was not sufficiently carried out, resulting in poor electrolyte tolerance.

[0216] 5. Manufacturing of slurry compositions for secondary batteries and fabrication of electrodes.

[0217] <Example of manufacturing a slurry composition for the negative electrode of a secondary battery>

[0218] 30 parts by weight of the binder composition prepared in Example 1, 0.03 parts by weight of single-walled carbon nanotubes (manufactured by OCSiAl, trade name "TUBALL"), and 57 parts by weight of ion-exchanged water were dispersed using a disperser. In addition, 70 parts by weight of carbon-based active material (manufactured by Carbon One New Energys, trade name "C ONE-A-P1B") and 30 parts by weight of silicon (Si) oxide (manufactured by DAEJOO ELECTRONIC MATERIALS, trade name "DMSO") were added and dispersed using a commercially available planetary mixer (manufactured by Thinky, trade name "Awatori") to obtain the electrode slurry composition of Example 14 with a solid content of 55% by weight and a viscosity of 4,500 mPa·s.

[0219] Similarly, the electrode slurry compositions of Examples 15 to 26 and Comparative Examples 8 to 14 were obtained with the composition ratios shown in Table 4.

[0220] Furthermore, Examples 27 and 28 illustrate examples of using a SiC-based active material with a higher theoretical capacity instead of silicon oxide (SiO) as the silicon (Si)-based material. Specifically, an electrode paste composition was obtained using a SiC-based active material (manufactured by Carbon One New Energy Co., Ltd., trade name "C ONE-LSC-2"), following the same method. Example 27 is an example of preparing an electrode paste with the same theoretical capacity as that of a silicon oxide (SiO)-based material. On the other hand, Example 28 is an example prepared using the same composition ratio as the previous Example 20, and is expected to have a high theoretical capacity.

[0221] Table 4 shows the composition examples and property evaluation results of slurry compositions prepared using various binder compositions and raw materials through a series of methods. The viscosity of the slurry composition was adjusted to range from 3,000 to 5,000 mPa·s by adjusting the amount of ion-exchange water added. The results indicate that the slurry concentration varies depending on the amount of ion-exchange water used.

[0222] In addition, as Comparative Example 15, 4.5 parts by weight of an aqueous dispersion (SBR concentration: 40% by weight) of an emulsion aqueous solution of styrene-butadiene copolymer latex (SBR) (manufactured by Nippon Zeon Co., Ltd., trade name "BM400-B") and 60 parts by weight of a 2% by weight aqueous solution of carboxymethyl cellulose (CMC) (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., trade name "Cellogen WS-C"), which has the same function as the water-soluble polymer (B), were used to replace 30 parts by weight of the above binder composition; 7.5 parts by weight of a 0.4% aqueous dispersion of single-walled carbon nanotubes (SWCNT) (manufactured by OCSiAl, trade name "TUBALL BATT H2O 0.4%") was used to replace 0.03 parts by weight of single-walled carbon nanotubes (SWCNT), and an electrode paste composition with a solid content of 45% by weight and a viscosity of 4300 mPa·s was obtained.

[0223] [Table 4]

[0224] <Manufacture of electrode>

[0225] The above slurry composition for secondary batteries was uniformly coated on the surface of a copper foil current collector by a doctor blade method so that the dried film thickness was 80 μm, and dried at 80°C for 6 minutes to obtain an electrode. Then, roll pressing was performed so that the density of the film (electrode active material layer) was 1.6 to 1.7 g / cm 3 to obtain an electrode.

[0226] 6. Evaluation of the characteristics of the slurry composition for secondary batteries

[0227] <Adhesion>

[0228] The slurry composition was coated on a copper foil and dried at 80°C for 6 minutes. Then, the slurry side was fixed with double-sided tape, and a cut was made between the slurry / copper foil for peeling, and the 180° peel strength was measured.

[0229] The evaluation results of the above adhesion showed that the peel strength was 8 N / m or more in each of the examples, and the adhesion was high. On the other hand, in Comparative Example 8 and Comparative Example 10, the peel strength was less than 8 N / m, and the adhesion was insufficient. This is because, in Comparative Example 8, the molecular weight of polymer a-1 contained in the binder composition of Comparative Example 1 was low, and during drying, the binder material and the solvent moved to the electrode surface layer together (binder migration phenomenon), resulting in a reduction in the binder near the copper foil interface. In addition, in Comparative Example 10, the AN usage amount of polymer a-3 contained in the binder composition of Comparative Example 3 was small, so sufficient adhesion to the copper foil could not be exhibited.

[0230] <CNT dispersibility>

[0231] The slurry composition was separated into liquid and solid fractions using a centrifuge. The coloration of the liquid fraction in the tubes immediately after separation and after one day of standing was evaluated by visual observation. When dispersibility was poor, CNTs agglomerated and settled, and the liquid fraction became transparent.

[0232] The dispersion of CNTs should be visually evaluated according to the following criteria.

[0233] 〇: The liquid is colored black, and the condition on the other side of the tube cannot be seen visually.

[0234] △: The liquid is light black, but the state on the other side of the tube can be seen visually.

[0235] ×: The liquid becomes almost transparent.

[0236] Based on the results of this evaluation, the CNT dispersibility in each embodiment was confirmed to be good or high. On the other hand, in Comparative Examples 12 to 15, the CNT dispersibility was insufficient. This was because, in Comparative Example 12, the polymer b-1 contained in the binder composition of Comparative Example 5 had a low molecular weight, resulting in insufficient CNT dispersibility. Additionally, in Comparative Example 13, the polymer b-2 contained in the binder composition of Comparative Example 6 had a low amount of VP monomer, resulting in insufficient CNT dispersibility. In Comparative Example 14, the polymer b-3 contained in the binder composition of Comparative Example 7 had a low amount of ATBS, resulting in insufficient dispersion stability of the initially dispersed CNTs, leading to sedimentation. In Comparative Example 15, sedimentation occurred due to insufficient dispersion stability of the CNTs used in the 0.4% aqueous dispersion of single-walled carbon nanotubes (SWCNTs).

[0237] Evaluation of Electrode Adhesion (Peel Strength)

[0238] Test pieces measuring 2.5 cm wide and 15 cm long were cut from the electrodes obtained in the examples and comparative examples and fixed with the coated side facing upwards. Next, a 25 mm wide double-sided adhesive tape (“Multi-purpose Double-sided Tape No. 512”, manufactured by NITOMS) (according to JIS Z 1528) was pressed and adhered to the surface of the active material layer of the test piece. Then, the other side of the double-sided tape was adhered to a steel plate. Using a tensile testing machine (A&D Corporation “MCT-2150W”) at 25°C, the stress when the copper foil was peeled off at a speed of 300 mm / min in a 180° direction from one end of the test piece was measured. The value converted to per 1 m width was calculated as the peel strength. A higher peel strength indicates a stronger adhesion between the current collector and the active material layer, or a higher bond between the active materials, indicating that it is difficult to peel the active material layer from the current collector or that the active materials are difficult to peel off from each other.

[0239] <Electrode Condition Evaluation>

[0240] The surface condition of the electrodes obtained in the examples and comparative examples was visually evaluated according to the following criteria.

[0241] A: The surface is homogeneous overall, and no aggregates are observed.

[0242] B: The surface is generally homogeneous, with slight irregularities on the electrode layer, but no condensates are observed.

[0243] C: A large amount of condensate is visible overall, and there are many irregularities on the electrode layer. In addition, cracks or abnormalities such as current collector peeling off from the electrode layer are visible.

[0244] The results of this evaluation show that, in all embodiments, the electrode surface condition is good, and the coating properties and uniformity of the slurry composition are excellent. In particular, good initial properties can be confirmed in both silicon oxide (SiO)-based and SiC-based materials as silicon (Si)-based materials in the negative electrode active material.

[0245] On the other hand, in Comparative Example 11, the surface condition of the electrode was uneven. This was due to the poor compatibility of the binder composition of Comparative Example 4 used in Comparative Example 11.

[0246] 7. Assembly and evaluation of lithium half-cells

[0247] <Assembly of Lithium Half-Batteries>

[0248] Inside an argon-filled glove box, the electrodes, punched to a diameter of 16 mm, were placed inside the sealing gasket on the stainless steel lower cover of the test battery. Next, a separator made of a 24 mm diameter polypropylene porous membrane was placed, and 500 μL of electrolyte was injected to prevent air contamination. Then, a commercially available 16 mm diameter lithium foil was placed, and the casing of a bipolar coin cell was sealed to assemble the lithium half-cell. The electrolyte used was a solution of LiPF6 dissolved at a concentration of 1 mol / L in a solvent of ethylene carbonate / dimethyl carbonate = 1 / 1 (molar ratio).

[0249] Batteries made with various binder compositions and raw materials were produced using the same method.

[0250] The battery characteristics evaluation results of lithium half-cells are shown in Table 5.

[0251] [Table 5]

[0252] [Table 5] Evaluation results of lithium half-cell characteristics

[0253] Evaluation of Initial Charge-Discharge Efficiency (Initial Efficiency)

[0254] The initial charge-discharge efficiency (initial efficiency) is calculated using the following formula.

[0255] (Formula) Initial efficiency = (First discharge capacity / First charge capacity) × 100 (%)

[0256] <Charge-Discharge Measurement>

[0257] The lithium half-cell manufactured above was placed in a constant temperature bath at 25°C and charged at a constant current (0.5C) until the voltage reached 0.01V, at which point charging was complete (cut off). Then, it was discharged at a constant current (0.1C) until the voltage reached 1.0V, at which point discharging was complete (cut off). This charging and discharging cycle was repeated 30 times.

[0258] <Evaluation of Discharge Capacity Retention>

[0259] The discharge capacity retention rate is calculated using the following formula.

[0260] (Mode)

[0261] Discharge capacity retention rate = {(Discharge capacity at 30th cycle) / (Discharge capacity at 1st cycle)} × 100 (%)

[0262] In addition, in the above measurement conditions, "1C" represents the current value of a battery with a certain capacitance that is discharged at a constant current and the discharge ends within 1 hour. For example, "0.1C" refers to the current value that is discharged in 10 hours, and "10C" refers to the current value that is discharged in 0.1 hours.

[0263] Evaluation of Direct Current Resistance (DCR)

[0264] The DC resistance of a lithium-ion secondary battery was measured using a constant current at a charging rate of 0.5C to adjust the depth of charge (SOC) to 50% (50%) at an ambient temperature of 0°C. At room temperature (25°C), for the lithium-ion secondary battery adjusted to 50% SOC, the charging and discharging sides were alternately energized for 10 seconds each at current rates of 1C, 3C, 5C, and 10C. A graph was plotted with the current value after 10 seconds at each current rate on the x-axis and the voltage value on the y-axis. The slope of the approximate straight line based on the least squares method was used as "charging side = input DCR" and "discharging side = output DCR". Furthermore, a 10-minute pause was set when the energizing direction and the energizing current changed at each current level.

[0265] 8. Fabrication of pouch lithium-ion batteries

[0266] The following describes the fabrication of a pouch lithium-ion battery.

[0267] (1) Fabrication of the negative electrode

[0268] The electrode slurry for secondary batteries from Example 14 was placed on a current collector made of copper foil and coated into a film using a doctor blade. The product obtained by coating the electrode slurry onto the current collector was dried at 80°C for 3 minutes to evaporate and remove moisture, and then tightly bonded using a roller press. At this point, the density of the electrode active material layer was 1.7 g / cm³. 2 The conjugate was heated at 100°C in a vacuum dryer for 36 hours to produce a negative electrode with an active material layer thickness of 15 μm.

[0269] (2) Production of the positive electrode

[0270] NMC811 as the positive electrode active material, acetylene black and carbon nanotubes as conductive additives, and polyvinylidene fluoride (PVDF) as a binder were mixed in 97 parts by weight, 1.5 parts by weight, 0.5 parts by weight, and 1.2 parts by weight, respectively. This mixture was then dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a slurry for the positive electrode of a secondary battery. Next, aluminum foil was prepared as the positive electrode current collector. The slurry for the positive electrode of a secondary battery was placed on the aluminum foil and coated into a film using a doctor blade. The aluminum foil coated with the slurry was dried at 80°C for 2 minutes to evaporate and remove NMP. Then, a roller press was used to tightly bond the layers. At this point, the density of the positive electrode active material layer was 3.4 g / cm³. 2 The conjugate was heated at 120°C in a vacuum dryer for 6 hours to produce a positive electrode with a positive active material layer thickness of about 120 μm.

[0271] (3) Fabrication of soft-pack lithium-ion batteries

[0272] Using the aforementioned positive and negative electrodes, a pouch lithium-ion secondary battery is fabricated. Specifically, a rectangular sheet of membrane composed of a porous polypropylene membrane is sandwiched between the positive and negative electrodes, serving as an electrode assembly. This electrode assembly is covered with an aluminum-plastic film consisting of two sheets, and after sealing three sides, an electrolyte is injected into the pouch-like aluminum-plastic film. The electrolyte is a solution obtained by dissolving LiPF6 in a solvent with a 1 / 1 (weight ratio) of ethylene carbonate and methyl ethyl carbonate at a concentration of 1 mol / L. Then, by sealing the remaining side, a pouch secondary battery is obtained where all four sides are airtight, and the electrode assembly and electrolyte are sealed. Furthermore, the positive and negative electrodes have tabs that allow for external electrical connection, with a portion of these tabs extending to the outside of the pouch secondary battery. No malfunctions were observed when the pouch secondary battery fabricated using the above process was powered on.

[0273] The slurry composition of Comparative Example 15, in addition to the composition of the slurry composition of Example 14, was used to prepare a pouch lithium-ion battery and evaluated in the same manner.

[0274] <Charge-Discharge Measurement>

[0275] The pouch lithium-ion battery manufactured as described above is placed in a constant temperature bath at 25°C and charged at a constant current (0.5C) until the voltage reaches 0.01V, at which point charging is complete (cutoff). Then, it is discharged at a constant current (1.0C) until the voltage reaches 1.0V, at which point discharging is complete (cutoff). This charge-discharge cycle is repeated 500 times.

[0276] Evaluation of Initial Charge-Discharge Efficiency (Initial Efficiency)

[0277] The initial charge-discharge efficiency (initial efficiency) is calculated using the following formula.

[0278] (Formula) Initial efficiency = (First discharge capacity / First charge capacity) × 100 (%)

[0279] <Evaluation of Discharge Capacity Retention>

[0280] The discharge capacity retention rate is calculated using the following formula.

[0281] (Mode)

[0282] Discharge capacity retention rate = {(Discharge capacity at 500th cycle) / (Discharge capacity at 1st cycle)} × 100 (%)

[0283] <Determination of Rebound Rate>

[0284] The lithium-ion battery was disassembled after charge-discharge testing, and the thickness of the electrodes was measured.

[0285] The rebound rate of the electrode is calculated by the following formula.

[0286] (Mode)

[0287] Rebound rate = {(Electrode thickness after 500 cycles - Current collector thickness) / (Electrode thickness before charge / discharge - Current collector thickness)} × 100 (%)

[0288] The battery characteristic evaluation results of the soft-pack lithium-ion batteries produced by the above method are shown in Table 6.

[0289] [Table 6]

[0290] [Table 6] Evaluation results of battery characteristics of soft-pack lithium-ion batteries

[0291] As shown in Table 5, the discharge capacity retention of lithium half-cells containing electrodes made using the electrode slurry for secondary batteries described in Example 14 was consistently good. Furthermore, as shown in Table 6, the rebound performance and discharge capacity retention of the pouch lithium-ion batteries were confirmed to be excellent.

[0292] On the other hand, in lithium half-cells containing electrodes made using electrode slurries for secondary batteries (Comparative Example 15) with CMC / SBR-type resins as binders, the discharge capacity retention rate was worse than that of the examples. Furthermore, in this comparative example, the evaluation results for the resilience and discharge capacity retention rate of the pouch lithium-ion battery were also worse than those of the examples. This is because the use of CMC / SBR-type binders, which lack thermal crosslinking properties, prevents the binder composition from adapting to the deformation of the silicon-based active material during charging and discharging, causing the binder composition's properties to gradually deteriorate and the discharge capacity retention rate to decrease. Furthermore, the lack of crosslinking properties prevents the suppression of deformation of the silicon-based active material, resulting in poor resilience.

[0293] Industrial utilization potential

[0294] According to the present invention, a thermally crosslinkable binder composition for secondary batteries can be provided. When applied to silicon (Si)-based anode active materials and CNTs, this composition exhibits both excellent dispersibility and fully utilizes the characteristics of a high-performance anode, thereby achieving a high-capacity lithium-ion secondary battery. This thermally crosslinkable binder composition for secondary batteries, the slurry composition for secondary batteries, and the electrodes and secondary batteries made from these raw materials can overcome the rebound problem (increased film thickness and volume change during cycling) during charging and discharging of secondary batteries, thereby providing a high-capacity secondary battery with significantly improved cycle life.

[0295] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A thermally crosslinkable adhesive composition for secondary batteries, comprising: First polymer (A), second polymer (B), and water, The first polymer (A) comprises at least structural units derived from acrylonitrile monomers and structural units derived from hydroxyl-containing (meth)acrylate monomers. The second polymer (B) comprises structural units derived from N-vinyl-2-pyrrolidone monomers and structural units derived from sulfonic acid-containing monomers. The weight ratio of the first polymer (A) to the second polymer (B) is 50:50 or more and 98:2 or less, and the gel fraction of the cured product is 90% or more.

2. The thermally crosslinkable binder composition for secondary batteries according to claim 1, wherein the sulfonic acid-containing monomer comprises tert-butylacrylamide sulfonic acid and / or an alkali metal salt of tert-butylacrylamide sulfonic acid.

3. The thermally crosslinkable binder composition for secondary batteries according to claim 2, wherein the first polymer (A) further comprises structural units derived from sulfonic acid monomers.

4. The thermally crosslinkable adhesive composition for secondary batteries according to any one of claims 1 to 3, wherein the weight-average molecular weight of the first polymer (A) is 300,000 or more and 2,000,000 or less, and the weight-average molecular weight of the second polymer (B) is 3,000 or more and 200,000 or less.

5. The thermally crosslinkable adhesive composition for secondary batteries according to any one of claims 1 to 3, wherein the structural unit derived from the sulfonic acid monomer accounts for 10% to 75% by weight of 100% by weight of the second polymer (B).

6. A slurry composition for secondary batteries, comprising, in addition to water, the first polymer (A), and the second polymer (B) of the thermal crosslinking binder composition for secondary batteries according to any one of claims 1 to 3, an electrode active material containing a silicon-based active material, and carbon nanotubes.

7. An electrode for a secondary battery, wherein a layer of a slurry composition for a secondary battery according to claim 6 is formed on a current collector.

8. A secondary battery comprising the electrode for a secondary battery according to claim 7.

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