Negative electrode binder composition, negative electrode, and secondary battery

Abstract

To provide a negative electrode binder composition in which slurry stability is excellent and solvent swelling resistance is suppressed even without a thickener such as cellulose in a negative electrode active material of a LIB secondary battery, and a negative electrode and a secondary battery using the same.SOLUTION: A negative electrode binder composition comprises a copolymer containing a hydroxyl group-containing monomer (a) and an acid group-containing monomer (b) as an essential component. A weight average molecular weight of the copolymer is 700000 or more when being measured using an aqueous GPC measurement device. At the same time, a swelling rate is 0 to 10 wt% after a dried polymer film of the negative electrode binder composition is immersed in a carbonate-based mixed solvent at 45°C for 72 hours.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a negative electrode binder composition, a negative electrode using the same, and a secondary battery. [Background technology]

[0002] In lithium-ion secondary batteries, a common binder for the negative electrode is a combination of the water-soluble polymer carboxymethylcellulose sodium salt (CMC) and the aqueous latex resin styrene-butadiene copolymer (SBR). The main functions of the binder during the electrode fabrication process include: 1) uniformly dispersing components such as active materials and conductive additives; 2) adjusting the rheology of the electrode mixture slurry; 3) leveling the mixture layer during slurry coating and drying; and 4) binding the mixture components and the current collector. Furthermore, functions related to battery performance include: 5) suppressing electrode expansion due to volume changes of the active material that occur during charge-discharge cycles; 6) maintaining the binding between the active material and the current collector to ensure electronic conductivity; and 7) ensuring ionic conductivity by swelling appropriately when containing the electrolyte.

[0003] Lithium-ion rechargeable batteries are widely used as rechargeable power sources for devices such as laptops and mobile phones, but in recent years, their use has been rapidly expanding in medium- and large-scale equipment such as power tools, automobiles, and stationary energy storage systems. With the rapid expansion of their application range, the performance requirements for batteries vary across a wider temperature range, but capacity, output, and lifespan are the three most important performance aspects, and improvements in these areas are particularly desired.

[0004] In this context, various efforts are being made to meet the demand for higher battery performance. For example, in the case of anode materials, new anode active materials are being considered as an alternative to conventionally widely used carbon-based active materials (e.g., graphite) in order to increase capacity. Examples of new anode active materials include tin alloys, silicon alloys, and silicon oxides. These new anode active materials have a capacity several times larger than that of carbon-based active materials, and it is possible to increase the anode capacity even by adding only a small amount.

[0005] However, a problem with these new negative electrode active materials is that their capacity retention rate during charge-discharge cycles is inferior to that of carbon-based active materials. This is because the new negative electrode active materials undergo greater volume expansion and contraction during charging and discharging compared to carbon-based active materials. As a result, the active material layer of the electrode expands significantly, leading to destruction of the electrode structure, loss of active material, and a decrease in electronic conductivity.

[0006] Furthermore, if the SEI film formed on the active material surface is destroyed because it cannot keep up with the volume change, the active material surface not covered by the SEI film is exposed, and the decomposition of the electrolyte proceeds due to a new SEI film formation reaction. The SEI film is formed on the active material surface during the initial charge and is mainly composed of decomposition products of the electrolyte, but the binder resin in contact with the active material surface is also thought to be involved in film formation. This SEI film is thought to play a role in mediating lithium ion insertion and removal reactions, and at the same time contribute to improving battery performance by suppressing further electrolyte decomposition reactions. If the SEI film is too thin, the electrolyte decomposition reaction will not stop, and conversely, if it is too thick, the electrical resistance will increase, resulting in a negative impact on the battery's lifespan and efficiency.

[0007] To address the problems caused by volume changes in the novel anode active material, for example, Patent Document 1 proposes a method to suppress swelling of the electrode layer due to volume changes in the anode active material by using a high-strength aromatic polyimide as a binder. Patent Document 2 proposes a method to suppress volume changes in the anode active material by using partially crosslinked polyacrylic acid as a binder. Furthermore, Patent Document 3 proposes a method to suppress volume changes in the anode active material by using a copolymer of acrylic acid and polyvinyl alcohol as a binder. However, the binder in Patent Document 1 had poor initial charge-discharge efficiency and could not fully utilize the capacity of the active material. In addition, the binders in Patent Documents 2 and 3 did not necessarily exhibit sufficient high-temperature and low-temperature cycle characteristics. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 06648854 [Patent Document 2] Patent No. 06457043 [Patent Document 3] Patent No. 06888139 [Overview of the project] [Problems that the invention aims to solve]

[0009] As described above, conventional binders in the negative electrode of lithium-ion secondary batteries lacked the ability to suppress the deterioration of battery performance caused by volume expansion of novel active materials. Therefore, the object of the present invention is to provide a negative electrode and a secondary battery that can obtain good battery performance even when using novel active materials. [Means for solving the problem]

[0010] To solve these problems, the inventors diligently investigated and discovered that by adjusting the polymerization binder containing hydroxyl and acidic groups to an unprecedentedly high molecular weight, the slurry stability was extremely good even without thickeners such as cellulose, and solvent swelling resistance when used as a coating film was suppressed. Furthermore, the negative electrode fabricated using this binder exhibited strong peel strength and a low electrode expansion rate during charging. As a result, battery evaluations showed that it exhibited good charge-discharge characteristics even at high cycle counts, leading to the present invention.

[0011] In other words, the present invention relates to the following: [1] A negative electrode binder composition comprising a copolymer in which a hydroxyl group-containing monomer (a) and an acid group-containing monomer (b) are essential components, wherein the weight-average molecular weight of the copolymer, as measured using an aqueous GPC measuring device, is 700,000 or more, and the swelling rate after immersion of the dry polymer film of the negative electrode binder composition in a carbonate-based mixed solvent (EC (ethylene carbonate) / DEC (diethylene carbonate) = 50 / 50 (wt)) at 45°C for 72 hours is 0 to 10% by weight. [2] The negative electrode binder composition according to [1], wherein the hydroxyl group-containing monomer (a) is one or more from the group consisting of 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, and 4-hydroxybutyl methacrylate. [3] The negative electrode binder composition according to [1] or [2], wherein the acid group-containing monomer (b) is one or more from the group consisting of acrylic acid, methacrylic acid, maleic acid, monomethylmaleic acid, 2-carboxyethyl acrylate, 2-carboxyethyl methacrylate, maleic acid, and itaconic acid. [4] The negative electrode binder composition according to any one of [1] to [3], wherein the acid group-containing monomer (b) is neutralized with a basic composition or a light metal salt. [5] The negative electrode binder composition according to any one of [1] to [4], wherein the content of the hydroxyl group-containing monomer (a) is 10 to 80% by weight and the content of the acid group-containing monomer (b) is 10 to 80% by weight. [6] The negative electrode binder composition according to any one of [1] to [5], wherein the other monomer (c) contains at least one selected from the group consisting of acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethylacrylamide, and N-hydroxymethylacrylamide. [7] A negative electrode comprising, as a component, the negative electrode binder composition according to [6], wherein the content of the other monomer (c) is 0 to 80%. [8] A negative electrode comprising, as a component, the negative electrode binder composition according to any one of [1] to [7]. [9] A secondary battery composed of the negative electrode according to [8].

Advantages of the Invention

[0012] The negative electrode binder composition of the present invention has good slurry stability even without a thickener such as cellulose, and the negative electrode produced using this composition has suppressed electrode expansion during charging. Therefore, the negative electrode produced with the negative electrode binder composition of the present invention has high peel strength, and as a result, when battery evaluation is performed, it exhibits good charge-discharge characteristics even at a high cycle number, achieving the performance required for recent LIB negative electrodes.

Embodiments for Carrying Out the Invention

[0013] <Negative Electrode Binder Composition> The negative electrode binder composition of the present invention is a negative electrode binder composition containing a copolymer having a hydroxyl group-containing monomer (a) and an acid group-containing monomer (b) as essential components, wherein the weight average molecular weight of the copolymer when measured using an aqueous GPC measuring device is 700,000 or more, and the swelling ratio after immersing the dry polymer film of the negative electrode binder composition in a carbonate-based mixed solvent at 45°C for 72 hours is 0 to 10% by weight.

[0014] The weight average molecular weight of the copolymer when measured using an aqueous GPC measuring device is 700,000 or more, preferably 750,000 to 1,500,000, more preferably 800,000 to 1,200,000. When the weight average molecular weight is 700,000 or more, as described in the effects of the above invention, the slurry stability is good, and the negative electrode produced using this has suppressed electrode expansion during charging.

[0015] In the above aqueous GPC measuring device, polymer-based fillers such as general polyhydroxy methacrylate can be used as the filler for the column. As the column, for example, SB-806 HQ, SB-806M HQ of the Shodex OHpak series manufactured by Showa Denko KK can be used. As the eluent, neutral salt solutions such as sodium nitrate aqueous solution, sodium hydrogen chloride aqueous solution, sodium sulfate aqueous solution, and phosphate buffer solution can be used. The concentration of these eluents is preferably about 0.1 to 0.3 mol / L, for example. As the GPC measuring device, a Shimadzu / L20 system etc. can be used. As the standard substance in GPC measurement, polystyrene or pullulan can be used. Specifically, STANDARD P-82 (Pullulan) manufactured by Showa Denko KK etc. can be used as the standard substance.

[0016] As described above, the swelling rate of the dry polymer film of the negative electrode binder composition after immersion in a carbonate-based mixed solvent at 45°C for 72 hours is 0 to 10% by weight, but the swelling rate is preferably 0.1 to 6% by weight, and more preferably 0.1 to 4% by weight. A lower swelling rate is preferable, and when the swelling rate is within the above range, the peel strength when used as a negative electrode is strong, and as a result, good charge and discharge characteristics can be exhibited even with a high number of cycles when battery evaluation is performed.

[0017] The above swelling rate can be determined by, for example, drying the negative electrode binder composition at room temperature for 72 hours and then at 150°C for 30 minutes to produce a dry polymer film (dry coating) with a thickness of 150 μm, immersing this dry polymer film in a carbonate-based mixed solvent (for example, EC (ethylene carbonate) / DEC (diethylene carbonate) = 50 / 50 (wt.r)) at 60°C for 72 hours, measuring the weight of the film after immersion, and determining the rate of change in weight before and after immersion. A high swelling rate means that the negative electrode binder composition is easily plasticized by the solvent, and the adhesive strength is likely to decrease when used as a negative electrode.

[0018] Examples of hydroxyl group-containing monomers (a) in the above copolymer include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, and 2,3-dihydroxypropyl acrylate. Among these, hydroxyethyl acrylate (particularly 2-hydroxyethyl acrylate) is preferred as the hydroxyl group-containing monomer (a). The content of the hydroxyl group-containing monomer (a) relative to the total amount of monomers constituting the copolymer is, for example, 20 to 80% by weight, preferably 30 to 70% by weight. When the content of the hydroxyl group-containing monomer (a) is within the above range, both slurry stability and resistance to electrolyte swelling at high temperatures when used as a coating tend to be good.

[0019] Examples of acid group-containing monomers (b) in the above copolymer include acrylic acid, methacrylic acid, 2-carboxyethyl acrylate, maleic acid, and itaconic acid. A carboxylic acid is preferred as the acid group in the acid group-containing monomer (b). Among these, acrylic acid is particularly preferred as the acid group-containing monomer (b). The content of the acid group-containing monomer (b) relative to the total amount of monomers constituting the copolymer is, for example, 10 to 60% by weight, preferably 20 to 50% by weight. When the content of the acid group-containing monomer (b) is within the above range, both slurry stability and resistance to electrolyte swelling at high temperatures when formed as a coating tend to be good.

[0020] The monomers constituting the copolymer may include monomers other than the hydroxyl group-containing monomer (a) and the acid group-containing monomer (b) (hereinafter referred to as "other monomers (c)"). Examples of other monomers (c) include acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethylacrylamide, and N-hydroxymethylacrylamide. Among the other monomers (c), acrylamide is particularly preferred, as the inclusion of acrylamide has the effect of increasing the toughness when it forms a coating. When other monomers (c) are included, their content is, for example, 5 to 40% by weight, preferably 5 to 20% by weight. When the content of other monomers (c) is within the above range, there is a tendency for good resistance to electrolyte swelling at high temperatures.

[0021] The copolymer in the negative electrode binder composition of the present invention has constituent unit units derived from each of the above-mentioned hydroxyl group-containing monomer (a) and acid group-containing monomer (b), and other monomers (c) added as needed. The copolymer is obtained by appropriately charging the hydroxyl group-containing monomer (a) and acid group-containing monomer (b), and other monomers (c) added as needed, as described below, and copolymerizing them by known and conventional methods.

[0022] The negative electrode binder composition of the present invention contains, in addition to the copolymer described above, components necessary for constructing the negative electrode, such as SiO negative electrode material, graphite, acetylene black, and a solvent. In the negative electrode binder composition of the present invention, any type of SiO negative electrode material or graphite can be used. The proportion of copolymer (non-volatile content) in the negative electrode binder composition is, for example, 3 to 50% by weight, preferably 3 to 40% by weight.

[0023] The above-mentioned SiO anode material is a material containing SiO (silicon monoxide) as the main component that exhibits charge-discharge characteristics in the anode. In addition to the SiO anode material, it may also contain silicon particles, carbon, etc., which similarly exhibit charge-discharge characteristics. Furthermore, silicon oxycarbide (SiOC) may be included as the SiO anode material. These components may be present individually or in combination. The proportion of SiO anode material in the anode binder composition is, for example, 3 to 40% by weight, preferably 5 to 30% by weight.

[0024] The graphite mentioned above may be natural graphite or artificially synthesized synthetic graphite. Examples of graphite include natural graphite, synthetic graphite, hard carbon, soft carbon, and other carbon materials. Graphite, like SiO negative electrode material, is a component that exhibits charge-discharge characteristics. The proportion of graphite in the negative electrode binder composition is, for example, 3 to 40% by weight, preferably 5 to 30% by weight.

[0025] The acetylene black described above acts as a conductive additive in the negative electrode and may also be carbon black, Ketjen black, or other components besides acetylene black. The proportion of these conductive additive components in the negative electrode binder composition is, for example, 0.5 to 10% by weight, preferably 0.5 to 5% by weight.

[0026] The solvent is not particularly limited as long as it can disperse the components necessary for forming the negative electrode, but an aqueous solvent can be used, and ion-exchanged water is preferred. The proportion of the solvent in the negative electrode binder composition is, for example, 50 to 95% by weight, preferably 60 to 90% by weight. In addition, the proportion of non-volatile components excluding the solvent in the negative electrode binder composition of the present invention is, for example, 5 to 30% by weight, preferably 10 to 20% by weight.

[0027] The negative electrode binder composition of the present invention may contain conventionally used components as binder components other than those listed above, to the extent that they do not impair the effects of the present invention. Examples of such binder components include styrene-butadiene rubber copolymer (SBR); (meth)acrylic copolymers consisting of ethylenically unsaturated carboxylic acid esters (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylonitrile, and hydroxyethyl (meth)acrylate, etc.) and ethylenically unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, etc.); and polymer compounds such as polyvinylidene fluoride, polyethylene oxide, polyepicrohydrin, polyphosphazene, polyacrylonitrile, polyimide, polyamideimide, and carboxymethylcellulose (CMC).

[0028] The negative electrode binder composition of the present invention may contain an organic solvent such as N-methyl-2-pyrrolidone (NMP) to dissolve the above polymer compound. In addition, the negative electrode binder composition of the present invention may contain other components necessary as components of the negative electrode.

[0029] An example of a method for producing the negative electrode binder composition of the present invention is described below.

[0030] First, the copolymer, which is an essential component of the negative electrode binder composition, is synthesized. The copolymer is obtained by charging a solvent such as water into a reaction vessel, heating it to 50-80°C, and then adding a mixture of a hydroxyl group-containing monomer (a), an acid group-containing monomer (b), an optional other monomer (c), and a polymerization initiator such as ammonium persulfate, and proceeding with the polymerization reaction. The polymerization reaction may be carried out under an inert gas atmosphere such as nitrogen. The polymerization reaction can be carried out at a temperature of 50-80°C for 1-10 hours. After the reaction is complete, the mixture is cooled and the pH is adjusted.

[0031] Next, the obtained copolymer is mixed with other components necessary for forming the anode, such as SiO anode material, graphite, acetylene black, and a solvent, and dispersed in an aqueous solvent such as deionized water. Dispersion may be performed using dispersion equipment such as a stirrer, ball mill, super sand mill, or pressurized kneader. Alternatively, the mixture may be prepared by kneading in a kneading machine. Through such dispersion and kneading, an anode binder composition can be produced.

[0032] <Negative electrode> The negative electrode of the present invention contains the above-mentioned negative electrode binder composition as a component. The negative electrode of the present invention is obtained by applying the slurry, which is the negative electrode binder composition obtained above, onto the copper foil of a current collector to form a negative electrode layer as a thin film. Alternatively, as described later, the slurry, which is the negative electrode binder composition, may be molded into a sheet, pellet, or other shape, and the negative electrode may be obtained by integrating this with the current collector.

[0033] The material and shape of the current collector described above are not particularly limited; for example, copper, nickel, titanium, stainless steel, etc., can be used in the form of foil, perforated foil, mesh, or other strip-like material. Porous materials, such as porous metal (foamed metal) or carbon paper, can also be used.

[0034] The method for applying the above-mentioned negative electrode material slurry to the current collector is not particularly limited, but known methods include, for example, metal mask printing, electrostatic coating, dip coating, spray coating, roll coating, doctor blade coating, gravure coating, and screen printing. After application, it is preferable to perform rolling treatment using a flat plate press, calender roll, etc., as necessary.

[0035] Furthermore, the integration of the negative electrode material slurry, which is formed into a sheet, pellet, or other shape, with the current collector can be carried out by known methods such as rolling, pressing, or a combination thereof. The electrode density after integration is, for example, 1.0 to 1.8 g / cm³. 3 The concentration is preferably 1.1 to 1.7 g / cm³. 3 That is the case.

[0036] The negative electrode layer formed on the current collector and the negative electrode layer integrated with the current collector are preferably subjected to heat treatment. The heat treatment conditions are, for example, 80 to 150°C for 5 to 20 hours. This heat treatment removes the solvent, hardens the binder, and increases strength, improving the adhesion between particles and between particles and the current collector. Furthermore, these heat treatments are preferably carried out in an inert atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, in order to prevent oxidation of the current collector during the treatment.

[0037] <Secondary battery> The secondary battery of the present invention includes the negative electrode of the present invention described above. When used in a wet electrolyte secondary battery, for example, the secondary battery of the present invention can be constructed by arranging the positive electrode and the negative electrode of the present invention opposite each other with a separator in between, and injecting an electrolyte.

[0038] The positive electrode can be obtained by forming a positive electrode layer on the surface of the current collector, similar to the negative electrode. In this case, the current collector can be made of metals or alloys such as aluminum, titanium, or stainless steel, and can be in the form of foil, perforated foil, mesh, or other strip-like material.

[0039] The positive electrode material used in the positive electrode layer is not particularly limited. In the case of lithium-ion secondary batteries, for example, any metal compound, metal oxide, metal sulfide, or conductive polymer material capable of doping or intercalating lithium ions may be used, and is not particularly limited. Examples include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), and their composite oxides (LiCoxNiyMnzO2, x+y+z=1), lithium manganese spinel (LiMn2O4), lithium vanadium compounds, V2O5, V6O 13 Conductive polymers such as VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, olivine-type LiMPO4 (M:Co, Ni, Mn, Fe), polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc., can be used individually or in combination.

[0040] As a separator, for example, nonwoven fabrics, cloths, microporous films, or combinations thereof, mainly composed of polyolefins such as polyethylene and polypropylene can be used. However, if the structure of the non-aqueous electrolyte secondary battery being manufactured is such that the positive and negative electrodes do not come into direct contact, a separator is not required.

[0041] As the electrolyte, for example, a so-called organic electrolyte can be used, which is obtained by dissolving lithium salts such as LiClO4, LiPF6, LiAsF6, LiBF4, and LiSO3CF3 in a non-aqueous solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, cyclopentanone, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 3-methyl-1,3-oxazolidine-2-one, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, or ethyl acetate, either individually or in mixtures of two or more components.

[0042] The structure of the secondary battery of the present invention is not particularly limited, but it is common to have a structure in which a positive electrode, a negative electrode, and a separator, if necessary, are wound in a flat spiral shape to form a wound electrode plate group, or these are stacked as flat plates to form a stacked electrode plate group, and these electrode plate groups are sealed in an outer casing.

[0043] The secondary battery of the present invention is not particularly limited, but can be used as a paper battery, button battery, coin battery, stacked battery, cylindrical battery, prismatic battery, etc. The negative electrode active material of the present invention described above can also be applied to all electrochemical devices that use the insertion and removal of lithium ions as a charge and discharge mechanism, such as hybrid capacitors and solid lithium secondary batteries. [Examples]

[0044] The present invention will be described in detail below with reference to examples. Copolymers were synthesized by the methods of Synthesis Examples 1-7 below, and negative electrodes were prepared using the obtained copolymers by the methods of Examples 1-3 and Comparative Example 1-4. A secondary battery was prepared using the obtained negative electrode and a separately prepared positive electrode by the methods of Examples 4-6 and Comparative Example 5-10.

[0045] [Aqueous GPC measurement] For aqueous GPC measurements, a Shimadzu / L20 system was used as the HPLC system, and Shodex OHpak SB-806MHQ columns (8.0 mm I.D. × 300 mm L × 2) were used. A 0.2 mol / L sodium nitrate aqueous solution was used as the eluent, and the sample was dissolved to a 0.5% concentration. The solution was filtered through a φ0.45 filter before measurement. The weight-average molecular weight was determined using an RI detector while introducing 50 μL of the sample and flowing it at a flow rate of 0.70 mL / min. A calibration curve was created using Showa Denko STANDARD P-82 (Pullulan) as the standard substance.

[0046] "Synthesis of copolymers" [Synthesis Example 1] 500.0 parts by weight of deionized water was charged into a 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blowdown device. After 3 hours of N2 blowdown, the vessel was heated to 75°C. A mixture of 40.0 parts by weight of acrylic acid, 60.0 parts by weight of 2-hydroxyethyl acrylate, 0.367 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer), and 50.0 parts by weight of deionized water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition was complete, the mixture was kept at the same temperature for 2 hours, and then cooled. At a temperature below 40°C, a 5 mol / L aqueous sodium hydroxide solution and distilled water were added to adjust the pH to a range of 6.8 to 7.2 and the non-volatile content to a range of 14.8% to 15.2% by weight. The copolymer obtained had a non-volatile content of 14.8% by weight, a pH of 6.8, a viscosity of 3080 mPa·s, and a weight-average molecular weight of 850,000 as measured by aqueous GPC.

[0047] (Measurement of swelling degree in a carbonate-based mixed solvent at 45°C) The obtained copolymer solution was applied to a PET film and left to dry at room temperature for 3 days to form a copolymer coating. After peeling, the coating was cut into 1.0 cm × 1.0 cm squares and dried in an 80°C forced-air dryer for 1 hour, and then in a 110°C vacuum dryer for 10 hours. The thickness of the obtained coating was 100 to 150 μm. After measuring the weight of the coating, it was immersed in a carbonate-based mixed solvent (EC (ethylene carbonate) / DEC (diethylene carbonate) = 50 / 50 (wt)) at 45°C for 72 hours, and then the weight of the coating was measured again. The degree of swelling in the carbonate mixed solvent, calculated from the following formula (1), was 3.7%.

[0048] Swelling degree (%) in a carbonate mixed solvent = (Weight of film after immersion - Weight of film before immersion) / (Weight of film before immersion) × 100 (Equation 1)

[0049] [Synthesis Example 2] The polymerization reaction was carried out in the same manner as in Synthesis Example 1, except that 60.0 parts by weight of 4-hydroxybutyl acrylate and 0.332 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer) were used instead of 60.0 parts by weight of 2-hydroxyethyl acrylate. The copolymer obtained had a non-volatile content of 15.1% by weight, a pH of 6.9, a viscosity of 3000 mPa·s, and a weight-average molecular weight of 800,000 as measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 4.3%.

[0050] [Synthesis Example 3] The polymerization reaction was carried out in the same manner as in Synthesis Example 1, except that 40.0 parts by weight of 2-carboxyethyl acrylate and 0.272 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer) were used instead of 40.0 parts by weight of acrylic acid. The copolymer obtained had a non-volatile content of 14.9% by weight, a pH of 6.9, a viscosity of 3000 mPa·s, and a weight-average molecular weight of 840,000 as measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 3.9%.

[0051] [Synthesis Example 4] 500.0 parts by weight of deionized water was charged into a 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blower, and after 3 hours of N2 blowing, it was heated to 75°C. A mixture of 30.0 parts by weight of acrylic acid, 60.0 parts by weight of 2-hydroxyethyl acrylate, 10.0 parts by weight of acrylamide, 0.443 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer), and 50.0 parts by weight of deionized water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition was complete, the mixture was kept at the same temperature for 2 hours and then cooled. At a temperature of 40°C or below, a 5 mol / L aqueous sodium hydroxide solution and distilled water were added to adjust the pH to a range of 6.8 to 7.2 and the non-volatile content to a range of 14.8% to 15.2% by weight. The obtained copolymer had a non-volatile content of 15.1% by weight, a pH of 7.1, a viscosity of 3100 mPa·s, and a weight-average molecular weight of 730,000 as measured by aqueous GPC. Its degree of swelling in a carbonate mixed solvent was 2.9%.

[0052] [Synthesis Example 5] The polymerization reaction was carried out in the same manner as in Synthesis Example 4, except that 20.0 parts by weight of acrylic acid, 20.0 parts by weight of 2-hydroxyethyl acrylate, 60.0 parts by weight of acrylamide, 0.405 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer), and 25% aqueous ammonia instead of 5 mol / L aqueous sodium hydroxide solution were used. The obtained copolymer had a non-volatile content of 15.0% by weight, pH 7.0, viscosity of 12500 mPa·s, and a weight-average molecular weight of 780,000 measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 3.2%.

[0053] [Synthesis Example 6] The polymerization reaction was carried out in the same manner as in Synthesis Example 5, except that a 5 mol / L lithium hydroxide aqueous solution was used instead of a 5 mol / L sodium hydroxide aqueous solution. The obtained copolymer had a non-volatile content of 15.0 wt%, pH 7.0, viscosity of 13700 mPa·s, and a weight-average molecular weight of 780,000 measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 3.7%.

[0054] [Synthesis Example 7] The polymerization reaction was carried out in the same manner as in Synthesis Example 5, except that a 5 mol / L aqueous sodium hydroxide solution was used instead of 25% aqueous ammonia. The resulting copolymer had a non-volatile content of 15.0% by weight, a pH of 7.0, a viscosity of 13200 mPa·s, and a weight-average molecular weight of 780,000 as measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 3.7%.

[0055] [Synthesis Example 8] The polymerization reaction was carried out in the same manner as in Synthesis Example 4, except that 20.0 parts by weight of acrylic acid, 20.0 parts by weight of 2-hydroxyethyl acrylate, hydroxymethylacrylamide was used instead of acrylamide, 0.357 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomers), and 25% aqueous ammonia was used instead of 5 mol / L aqueous sodium hydroxide solution. The resulting copolymer had a non-volatile content of 15.0% by weight, pH 7.0, viscosity of 17100 mPa·s, and a weight-average molecular weight of 830,000 measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 3.2%.

[0056] [Synthesis Example 9] The polymerization reaction was carried out in the same manner as in Synthesis Example 4, except that 10.0 parts by weight of acrylic acid, 10.0 parts by weight of 2-hydroxyethyl acrylate, 80.0 parts by weight of acrylamide, 0.438 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer), and 25% aqueous ammonia instead of 5 mol / L aqueous sodium hydroxide solution were used. The obtained copolymer had a non-volatile content of 14.9% by weight, pH 7.0, viscosity of 15200 mPa·s, and a weight-average molecular weight of 860,000 measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 4.3%.

[0057] [Synthesis Example 10] The polymerization reaction was carried out in the same manner as in Synthesis Example 4, except that 35.0 parts by weight of acrylic acid, 35.0 parts by weight of 2-hydroxyethyl acrylate, 30.0 parts by weight of acrylamide, 0.414 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer), and 25% aqueous ammonia instead of 5 mol / L aqueous sodium hydroxide solution were used. The obtained copolymer had a non-volatile content of 15.0% by weight, pH 7.0, viscosity of 13300 mPa·s, and a weight-average molecular weight of 840,000 measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 4.2%.

[0058] [Synthesis Example 11] The polymerization reaction was carried out in the same manner as in Synthesis Example 4, except that 20.0 parts by weight of acrylic acid, 40.0 parts by weight of 2-hydroxyethyl acrylate, 40.0 parts by weight of acrylamide, 0.442 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomers), and 25% aqueous ammonia instead of 5 mol / L aqueous sodium hydroxide solution were used. The obtained copolymer had a non-volatile content of 15.1% by weight, pH 7.0, viscosity of 14500 mPa·s, and a weight-average molecular weight of 830,000 measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 5.8%.

[0059] [Synthesis Example 12] The polymerization reaction was carried out in the same manner as in Synthesis Example 4, except that 30.0 parts by weight of acrylic acid, 20.0 parts by weight of 2-hydroxyethyl acrylate, 50.0 parts by weight of acrylamide, 0.424 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer), and 25% aqueous ammonia instead of 5 mol / L aqueous sodium hydroxide solution were used. The obtained copolymer had a non-volatile content of 15.1% by weight, pH 7.0, viscosity of 13000 mPa·s, and a weight-average molecular weight of 840,000 measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 3.7%.

[0060] [Synthesis Example 13] The polymerization reaction was carried out in the same manner as in Synthesis Example 1, except that a 3-hour N2 blowing period was omitted. The copolymer obtained had a non-volatile content of 15.0% by weight, a pH of 6.8, a viscosity of 1380 mPa·s, and a weight-average molecular weight of 480,000 as measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 6.9%.

[0061] [Synthesis Example 14] The polymerization reaction was carried out in the same manner as in Synthesis Example 1, except that 20.0 parts by weight of acrylic acid, 80.0 parts by weight of 2-hydroxyethyl acrylate, and 0.340 parts by weight of ammonium persulfate (1500 ppm relative to the total number of moles of monomer) were used. The copolymer obtained had a non-volatile content of 14.9% by weight, a pH of 7.0, a viscosity of 2800 mPa·s, and a weight-average molecular weight of 750,000 as measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 18.8%.

[0062] [Synthesis Example 15] 500.0 parts by weight of deionized water was charged into a 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blower. After 3 hours of N2 blowing, the mixture was heated to 75°C. A mixture of 70.0 parts by weight of 2-hydroxyethyl acrylate, 30.0 parts by weight of acrylamide, 0.351 parts by weight of ammonium persulfate (1500 ppm relative to the total number of monomer moles), and 50.0 parts by weight of deionized water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition was complete, the mixture was kept at the same temperature for 2 hours and then cooled. The resulting copolymer had a non-volatile content of 15.0% by weight, a pH of 7.0, a viscosity of 4300 mPa·s, and a weight-average molecular weight of 750,000 as measured by aqueous GPC. The degree of swelling in a carbonate mixed solvent was 12.3%.

[0063] [Synthesis Example 16] 500.0 parts by weight of deionized water was charged into a 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blower. After 3 hours of N2 blowing, the mixture was heated to 75°C. A mixture of 70.0 parts by weight of 2-hydroxyethyl acrylate, 30.0 parts by weight of acrylamide, 0.351 parts by weight of ammonium persulfate (1500 ppm relative to the total number of monomer moles), and 50.0 parts by weight of deionized water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition was complete, the mixture was kept at the same temperature for 2 hours, and then cooled. At a temperature below 40°C, a 5 mol / L aqueous sodium hydroxide solution and distilled water were added to adjust the pH to a range of 6.8 to 7.2 and the non-volatile content to a range of 14.8% to 15.2% by weight. The resulting copolymer had a non-volatile content of 15.0% by weight, a pH of 7.0, a viscosity of 8900 mPa·s, and a weight-average molecular weight of 820,000 as measured by aqueous GPC. Furthermore, the degree of swelling in the carbonate mixed solvent was 3.5%.

[0064] The monomer composition, presence or absence of N2 blowing, type of neutralization salt, non-volatile content, pH, viscosity, weight-average molecular weight of the copolymer, and degree of swelling in a carbonate mixed solvent at 45°C for the above synthesis examples 1-16 are summarized in Table 1 below.

[0065] [Table 1]

[0066] "Fabrication of the negative electrode" [Example 1] (Preparation of negative electrode mixture slurry) 11.5 parts by weight of SiO negative electrode material (initial charge capacity 2062 mAh / g, initial discharge capacity 1631 mAh / g), 84.5 parts by weight of artificial graphite (initial charge capacity 371 mAh / g, initial discharge capacity 346 mAh / g), and 1.0 part by weight of acetylene black were weighed out and stirred for 30 seconds in a rotation / revolution mixer (Thinky ARE-310) at a rotation of 1000 rpm and a revolution of 2000 rpm. The polymer shown in Synthesis Example 1 above (non-volatile content 14.8% by weight) was diluted with distilled water to adjust the non-volatile content concentration to 8.0%, and 27.0 parts by weight (2.16 parts by weight in terms of solid content) of this aqueous solution was added to 19.0 parts by weight of distilled water and mixed until the mixture became a paste. Next, the mixture was stirred for 2 minutes in a rotation / revolution mixer (Thinky ARE-310) at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm. Since heat was generated by the stirring, it was cooled to room temperature with ice water. Again, the mixture was stirred for 2 minutes at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm, and then cooled to room temperature with ice water. 10.5 parts by weight (0.84 parts by weight in terms of non-volatile content) of the aqueous solution of the binder composition shown in Synthesis Example 1, which had been prepared to a non-volatile content concentration of 8%, was added and mixed until the mixture was homogeneous. Then, the mixture was stirred for 2 minutes in a rotation / revolution mixer (Thinky ARE-310) at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm, and then cooled to room temperature with ice water. 5 parts by weight of distilled water was added and mixed until the mixture was homogeneous. Next, to adjust the viscosity of the slurry, the viscosity was measured using a B-type viscometer, and distilled water was added as needed to bring it to a range of 2000-4000 Pa·s at 30 rpm. Finally, the mixture was stirred for 30 seconds in a rotation-revolution mixer (Thinky ARE-310) at 1000 rpm rotation and 2000 rpm revolution to prepare a negative electrode mixture slurry using the negative electrode binder composition of the present invention.

[0067] (Fabrication of the negative electrode) Next, the coating amount (surface density) of the negative electrode mixture after drying was 8.8 mg / cm². 2The gap of the bar coater was adjusted to achieve this, and the negative electrode mixture slurry was coated onto a copper foil serving as a current collector using this bar coater. Subsequently, it was dried for 8 minutes using a blowing dryer set at 80°C. The dried electrode was cut into strips with a width of 40 mm, and using a roll press machine (Mini Desktop Roll Press SA-602 manufactured by Tester Sangyo Co., Ltd.), it was pressed so that the density of the mixture layer became 1.55 g / cm 3 (with a thickness of the mixture layer of 66.7 μm). After vacuum drying at 110°C for 10 hours, when the density of the mixture layer was measured again, it was 1.50 g / cm 3 (with a thickness of the mixture layer of 68.6 μm). The initial charge capacity per unit area of this electrode was 4.95 mAh / cm 2 . Thus, the negative electrode of Example 1 (surface density 8.8 g / m 2 , mixture layer density 1.5 g / cm 3 , mixture layer thickness 68.6 μm, initial charge capacity per unit area of first cycle 4.95 mAh / cm 2 ) was obtained.

[0068] (Measurement of peel strength and confirmation of winding resistance of the electrode) [[ID=1⑧]]After leaving the negative electrode prepared above in a thermo-hygrostat chamber at a temperature of 25°C and a relative humidity of 50% for 6 hours, it was cut into strips with a width of 25 mm and a length of 100 mm. Then, using a double-sided tape (No. 5015 manufactured by Nitto Denko Corporation), it was bonded to a stainless steel plate with the active material surface as the adherent surface to obtain a sample for the peel strength test. The copper foil end was peeled off by about 10 mm, and a polyimide tape was attached thereto to serve as the attachment part to the peeling tester. A sample for the peel strength test was mounted on a peeling tester (Autograph AG-X Plus manufactured by Shimadzu Corporation), and a 180-degree peel test was performed. The peel strength was 34.5 N / m. Then, the peeling (failure) state of the negative electrode coating film was observed. Also, the negative electrode coating film was wound around a core with a diameter of φ5 mm, and visual observation was made to check whether cracks occurred in the coating film. No cracks occurred at this time.

[0069] [Example 2] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 2 was used. The peel strength at this time was 27.8 N / m. Furthermore, no cracks occurred.

[0070] [Example 3] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 3 was used. The peel strength at this time was 28.9 N / m. Furthermore, no cracks occurred.

[0071] [Example 4] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 4 was used. The peel strength at this time was 24.5 N / m. No cracks were observed.

[0072] [Example 5] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 5 was used. The peel strength at this time was 30.7 N / m. Furthermore, no cracks occurred.

[0073] [Example 6] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 6 was used. The peel strength at this time was 29.5 N / m. Furthermore, no cracks occurred.

[0074] [Example 7] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 7 was used. The peel strength at this time was 34.6 N / m. No cracks were observed.

[0075] [Example 8] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 8 was used. The peel strength at this time was 28.4 N / m. No cracks were observed.

[0076] [Example 9] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 9 was used. The peel strength at this time was 29.1 N / m. No cracks were observed.

[0077] [Example 10] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 10 was used. The peel strength at this time was 30.5 N / m. No cracks were observed.

[0078] [Example 11] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 11 was used. The peel strength at this time was 28.6 N / m. No cracks were observed.

[0079] [Example 12] The binder mixture used for slurry preparation was the same as in Example 1, except that the polymer shown in Synthesis Example 12 was used. The peel strength at this time was 28.4 N / m. No cracks were observed.

[0080] [Comparative Example 1] The binder mixture used to prepare the slurry was the same as in Example 1, except that the polymer shown in Synthesis Example 13 was used. The peel strength at this time was 17.4 N / m. No cracks were observed.

[0081] [Comparative Example 2] The binder mixture used to prepare the slurry was the same as in Example 1, except that the polymer shown in Synthesis Example 14 was used. The peel strength at this time was 14.5 N / m. No cracks were observed.

[0082] [Comparative Example 3] The binder mixture used to prepare the slurry was the same as in Example 1, except that the polymer shown in Synthesis Example 15 was used. The peel strength at this time was 15.2 N / m. No cracks were observed.

[0083] [Comparative Example 4] The binder mixture used to prepare the slurry was the same as in Example 1, except that the polymer shown in Synthesis Example 16 was used. The peel strength at this time was 14.5 N / m. No cracks were observed.

[0084] [Comparative Example 5] The binder mixture used to prepare the slurry was the same as in Example 1, except that sodium polyacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., degree of polymerization 22,000 to 70,000) was used. The peel strength in this case was 8.1 N / m. No cracks were observed.

[0085] [Comparative Example 6] (Preparation of negative electrode mixture slurry) 11.5 parts by weight of SiO negative electrode material (initial charge capacity 2062 mAh / g, initial discharge capacity 1631 mAh / g), 84.5 parts by weight of artificial graphite (initial charge capacity 371 mAh / g, initial discharge capacity 346 mAh / g), and 1.0 part by weight of acetylene black were weighed out and stirred for 30 seconds in a rotation / revolution mixer (Thinky ARE-310) at a rotation of 1000 rpm and a revolution of 2000 rpm. 48.0 parts by weight (0.96 parts by weight in terms of solid content) of an aqueous solution prepared by dissolving carboxymethylcellulose sodium salt (CMC, Nippon Paper Industries Sunrose MAC350HC) in distilled water and adjusting the non-volatile content to 2.0% was added and mixed until the mixture became a paste. Next, the mixture was stirred for 2 minutes in a rotation / revolution mixer (Thinky ARE-310) at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm. Since heat was generated by the stirring, it was cooled to room temperature with ice water. Again, the mixture was stirred for 2 minutes at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm, and then cooled to room temperature with ice water. 27.0 parts by weight (0.54 parts by weight in terms of non-volatile content) of the above CMC aqueous solution, which had been previously prepared to a non-volatile content concentration of 2%, was added and mixed until the mixture was uniform. Then, the mixture was stirred for 2 minutes in a rotation / revolution mixer (Thinky ARE-310) at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm, and then cooled to room temperature with ice water. 20 parts by weight of distilled water and 2.95 parts by weight (1.5 parts by weight in terms of non-volatile content) of styrene-butadiene copolymer (SBR) (DIC Corporation, DS407H, non-volatile content 50.8%) were added, and the mixture was stirred again in a rotating / revolving mixer (Thinky Corporation, ARE-310) for 30 seconds at a rotation of 1000 rpm and a revolution of 2000 rpm to prepare a negative electrode mixture slurry. The negative electrode was prepared, the peel strength was measured, and the winding resistance of the electrode was confirmed in the same manner as in Example 1. The peel strength at this time was 16.8 N / m. No cracks were observed.

[0086] The binder resin, peel strength, and 5φ bending crack resistance used in the negative electrodes prepared in Examples 1-12 and Comparative Examples 1-6 are shown in Table 2 below.

[0087] [Table 2]

[0088] Table 2 above shows that the example has higher peel strength than the comparative example.

[0089] "Fabrication of the positive electrode" (Preparation of positive electrode slurry) In a room where the humidity is adjusted to 30% or less, the positive electrode material LiMn 0.6 Co 0.2 Ni 0.2 94.0 parts by weight of O2 (initial charge capacity 191 mAh / g, initial discharge capacity 171 mAh / g) and 3.0 parts by weight of acetylene black were weighed out and stirred for 30 seconds in a rotation / revolution mixer (Thinky ARE-310) at a rotation of 1000 rpm and a revolution of 2000 rpm. 27.0 parts by weight (2.16 parts by weight in solid content) of an anhydrous N-methylpyrrolidone solution of polyvinylidene fluoride adjusted to a non-volatile content concentration of 8.0% and 19.0 parts by weight of anhydrous N-methylpyrrolidone were added and mixed until the mixture became a paste. Next, the mixture was stirred for 2 minutes in a rotation / revolution mixer (Thinky ARE-310) at a rotation of 1000 rpm and a revolution of 2000 rpm. Since heat was generated by the stirring, it was cooled to room temperature in ice water. The mixture was stirred again for 2 minutes at a rotation of 1000 rpm and a revolution of 2000 rpm, and then cooled to room temperature in ice water. 10.5 parts by weight (0.84 parts by weight in terms of non-volatile content) of an anhydrous N-methylpyrrolidone solution of polyvinylidene fluoride, which had been previously prepared to an 8% non-volatile content concentration, was added and mixed until homogeneous. Then, using a rotation / revolution mixer (Thinky ARE-310), the mixture was stirred for 2 minutes at a rotation of 1000 rpm and a revolution of 2000 rpm, and then cooled to room temperature in ice water. 5 parts by weight of anhydrous N-methylpyrrolidone was added and mixed until homogeneous. Next, to adjust the viscosity of the slurry, the viscosity was measured using a B-type viscometer, and anhydrous N-methylpyrrolidone was added as needed to achieve a viscosity in the range of 2000-4000 Pa·s at 30 rpm. Finally, the cathode mixture slurry was prepared by stirring for 30 seconds in a rotation / revolution mixer (Thinky ARE-310) under conditions of rotation at 1000 rpm and revolution at 2000 rpm.

[0090] (Fabrication of the positive electrode) First, the amount of the mixture applied after drying (surface density) was 25.0 mg / cm². 2 The gap of the bar coater was adjusted to achieve the desired result, and the positive electrode mixture slurry was coated onto the aluminum foil current collector using this bar coater. Next, it was dried for 10 minutes in a forced-air dryer set to 80°C. The dried electrodes were cut into strips 40 mm wide, and a roll press (Small Tabletop Roll Press SA-602, manufactured by Tester Industries Co., Ltd.) was used to obtain a mixture layer density of 3.4 g / cm³. 3 The electrode was pressed to a thickness of 73.3 μm (composite layer thickness) and then vacuum-dried at 110°C for 10 hours. The initial charge capacity per unit area of ​​this electrode was 4.49 mAh / cm². 2 Therefore, the positive electrode (surface density 25.0 gm / cm²) 2 The density of the combined layer is 3.4 g / cm³. 3 The combined layer thickness is 73.3 μm, and the initial charge capacity per unit area is 4.49 mAh / cm². 2 ) was obtained.

[0091] "Manufacturing of rechargeable batteries" [Example 13] First, the negative electrode shown in Example 1 was cut into a 24mm x 24mm square with a tab, and the positive electrode shown in the positive electrode fabrication example was cut into a 22mm x 22mm square with a tab using a die-cutting blade. Nickel tab leads were welded to the tab portion of the cut electrodes, and aluminum tab leads were welded to the tab portion of the positive electrode. Next, a separator (a 25-micron thick polyethylene microporous membrane) was cut into a 28mm x 3.8cm rectangle using a die-cutting blade. The positive and negative electrodes were placed facing each other with the separator in between, wrapped in laminate film, and the tab portion was fixed by heat sealing. Then, 300 μL of electrolyte (1.0 M LiPF6 ethylene carbonate / dimethyl carbonate / methyl ethyl carbonate = 30 / 30 / 40 mixed solution (volume ratio) + 1% vinyl carbonate + 5% fluoroethylene carbonate) was added, and the assembly was completely sealed by vacuum lamination to fabricate a laminate-type secondary battery.

[0092] (Initial charge / discharge efficiency and cycle characteristics) The secondary battery prepared as described above was mounted on a charge / discharge device and left at 25°C for 3 hours, after which it was charged and discharged once at 0.1C. The initial charge / discharge efficiency was 81.2%. Next, the battery was charged and discharged 50 times at 0.2C. The discharge capacity retention rate after 50 cycles, with the discharge capacity after the first cycle at 0.2C set to 100%, was 85.0%.

[0093] (Electrode swelling rate) After the initial charge and discharge, the battery was kept at 45°C and charged once at 0.5C. The secondary battery was then disassembled in a dry room, and the fully charged negative electrode was removed. After washing with dimethyl carbonate and air drying, the electrode thickness was measured using a micrometer. The electrode swelling rate was calculated using the following formula (Equation 2) and was found to be 32.0%.

[0094] Electrode swelling rate (%) = (Electrode thickness after disassembly - Electrode thickness during battery fabrication) / (Compound layer thickness during battery fabrication) × 100 (Equation 2)

[0095] [Example 14] Except for using the negative electrode prepared in Example 2, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.5%. The discharge capacity retention rate was 86.4%. Furthermore, the electrode expansion rate was 30.4%.

[0096] [Example 15] Except for using the negative electrode prepared in Example 3, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.4%. The discharge capacity retention rate was 84.9%. Furthermore, the electrode expansion rate was 30.2%.

[0097] [Example 16] The procedure was carried out in the same manner as in Example 13, except that the negative electrode prepared in Example 4 was used. The initial charge-discharge efficiency was 81.6%. The discharge capacity retention rate was 84.7%. Furthermore, the electrode expansion rate was 31.1%.

[0098] [Example 17] Except for using the negative electrode prepared in Example 5, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.6%. The discharge capacity retention rate was 85.3%. Furthermore, the electrode expansion rate was 30.7%.

[0099] [Example 18] Except for using the negative electrode prepared in Example 6, the procedure was carried out in the same manner as in Example 13. The initial charge-discharge efficiency was 82.7%. The discharge capacity retention rate was 85.3%. Furthermore, the electrode expansion rate was 30.6%.

[0100] [Example 19] The procedure was carried out in the same manner as in Example 13, except that the negative electrode prepared in Example 7 was used. The initial charge-discharge efficiency was 81.7%. The discharge capacity retention rate was 85.4%. Furthermore, the electrode expansion rate was 30.2%.

[0101] [Example 20] Except for using the negative electrode prepared in Example 8, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.5%. The discharge capacity retention rate was 85.0%. Furthermore, the electrode expansion rate was 30.0%.

[0102] [Example 21] Except for using the negative electrode prepared in Example 9, the procedure was carried out in the same manner as in Example 13. The initial charge-discharge efficiency was 81.4%. The discharge capacity retention rate was 84.7%. Furthermore, the electrode expansion rate was 30.6%.

[0103] [Example 22] Except for using the negative electrode prepared in Example 10, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.4%. The discharge capacity retention rate was 84.9%. Furthermore, the electrode expansion rate was 30.0%.

[0104] [Example 23] Except for using the negative electrode prepared in Example 11, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.5%. The discharge capacity retention rate was 84.7%. Furthermore, the electrode expansion rate was 29.8%.

[0105] [Example 24] Except for using the negative electrode prepared in Example 12, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.4%. The discharge capacity retention rate was 84.9%. Furthermore, the electrode expansion rate was 30.2%.

[0106] [Comparative Example 7] Except for using the negative electrode prepared in Comparative Example 1, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.4%. The discharge capacity retention rate was 80.4%. Furthermore, the electrode expansion rate was 35.2%.

[0107] [Comparative Example 8] The procedure was carried out in the same manner as in Example 13, except that the negative electrode prepared in Comparative Example 2 was used. The initial charge-discharge efficiency was 80.9%. The discharge capacity retention rate was 72.0%. Furthermore, the electrode expansion rate was 40.3%.

[0108] [Comparative Example 9] The procedure was carried out in the same manner as in Example 13, except that the negative electrode prepared in Comparative Example 3 was used. The initial charge-discharge efficiency was 81.3%. The discharge capacity retention rate was 74.0%. Furthermore, the electrode expansion rate was 45.9%.

[0109] [Comparative Example 10] Except for using the negative electrode prepared in Comparative Example 4, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.2%. The discharge capacity retention rate was 78.5%. Furthermore, the electrode expansion rate was 36.4%.

[0110] [Comparative Example 11] Except for using the negative electrode prepared in Comparative Example 5, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 80.9%. The discharge capacity retention rate was 76.0%. Furthermore, the electrode expansion rate was 36.8%.

[0111] [Comparative Example 12] Except for using the negative electrode prepared in Comparative Example 6, the procedure was the same as in Example 13. The initial charge-discharge efficiency was 81.1%. The discharge capacity retention rate was 79.0%. Furthermore, the electrode expansion rate was 39.3%.

[0112] The initial charge retention rate (%), capacity retention rate (%) after 100 cycles, and electrode swelling rate (%) of the secondary batteries prepared in Examples 13-24 and Comparative Example 7-12 are shown in Table 3 below.

[0113] [Table 3]

[0114] As can be seen from Table 3 above, Examples 13-24, which use the negative electrode binder composition of the present invention (Examples 1-12), show superior volume retention rate (%) at 100 cycles and electrode swelling rate (%) compared to Comparative Example 7-12, which does not use the negative electrode binder composition of the present invention.

Claims

1. A negative electrode binder composition comprising a copolymer consisting of a hydroxyl group-containing monomer (a), an acid group-containing monomer (b), and other monomers (c), The hydroxyl group-containing monomer (a) is selected from one or more of the group consisting of 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 2-hydroxybutyl methacrylate, and 4-hydroxybutyl methacrylate. The acid group-containing monomer (b) is selected from one or more of the group consisting of acrylic acid, methacrylic acid, maleic acid, monomethylmaleic acid, 2-carboxyethyl acrylate, 2-carboxyethyl methacrylate, and itaconic acid. As the other monomer (c) mentioned above, acrylamide is used. A negative electrode binder composition wherein the weight-average molecular weight of the copolymer, as measured using an aqueous GPC measuring device, is 700,000 or more and 1,200,000 or less, and the swelling rate after immersing the dry polymer film of the negative electrode binder composition in a carbonate-based mixed solvent (EC (ethylene carbonate) / DEC (diethylene carbonate) = 50 / 50 (wt)) at 45°C for 72 hours is 0 to 10% by weight.

2. The negative electrode binder composition according to claim 1, wherein the acid group-containing monomer (b) is neutralized with a basic composition or a light metal salt.

3. The negative electrode binder composition according to claim 1 or claim 2, wherein the content of the hydroxyl group-containing monomer (a) is 10 to 80% by weight relative to the total amount of monomers constituting the copolymer, and the content of the acid group-containing monomer (b) is 10 to 80% by weight relative to the total amount of monomers constituting the copolymer.

4. A negative electrode comprising the negative electrode binder composition according to any one of claims 1 to 3 as a component.

5. A secondary battery comprising the negative electrode described in claim 4.

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