Negative electrode binder composition, negative electrode, and secondary battery

Abstract

To provide a negative electrode binder composition and a negative electrode and a secondary battery using the same, the negative electrode binder composition providing excellent slurry stability without a thickener such as cellulose in a negative electrode active material of an LIB secondary battery and suppressing solvent-resistant swelling characteristics.SOLUTION: There is provided a negative electrode binder composition including a copolymer containing a hydroxyl group-containing monomer (a) and an acid group-containing monomer (b) as essential components. Weight-average molecular weight of the copolymer is equal to or larger than 700,000 when measured using a water system GPC measurement device. A swelling rate of a dried polymer film of the negative electrode binder composition is 0 to 10 weight % after the film is dipped in carbonate-based mixed solvent for 72 hours at 45°C.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] The binder used in the negative electrode of lithium-ion secondary batteries is typically a combination of the water-soluble polymer carboxymethylcellulose sodium salt (CMC) and the water-based latex resin styrene-butadiene copolymer (SBR). The binder's main functions during the electrode fabrication process include: 1) uniformly dispersing active materials and conductive additives, 2) adjusting the rheology of the electrode mixture slurry, 3) leveling the mixture layer during application and drying, and 4) binding the mixture components and the current collector. Other functions that contribute to battery performance include: 5) suppressing electrode expansion due to volumetric changes in the active material during charge-discharge cycles, 6) maintaining the bond between the active material and the current collector to ensure electronic conductivity, and 7) ensuring ionic conductivity by appropriately swelling with the electrolyte.

[0003] Lithium-ion secondary batteries are widely used as rechargeable power sources for devices such as laptops and mobile phones, but in recent years their use has expanded rapidly in power tools, as well as in medium- to large-sized devices such as automobiles and stationary energy storage facilities. As the range of applications expands rapidly, the performance required of batteries over a wider temperature range is also becoming more diverse, but the three most important performance factors are capacity, output, and lifespan, and improvements in these are particularly desired.

[0004] Under these circumstances, various efforts are being made to meet the demand for higher battery performance. For example, in the case of negative electrode materials, new negative electrode active materials are being considered to replace the conventionally widely used carbon-based active materials (e.g., graphite) in an effort to increase capacity. Examples of new negative electrode active materials include tin alloys, silicon alloys, and silicon oxides. These new negative electrode active materials have a capacity several times larger than that of carbon-based active materials, and even adding a small amount can increase the negative electrode capacity.

[0005] However, these new negative electrode active materials have a problem in that their capacity retention rate during charge-discharge cycles is inferior to that of carbon-based active materials. The reason for this is that the new negative electrode active materials undergo greater volume expansion and contraction during charge-discharge cycles than carbon-based active materials, which causes the active material layer of the electrode to expand significantly, resulting in destruction of the electrode structure, loss of the active material, and a decrease in electronic conductivity.

[0006] Furthermore, if the SEI film formed on the active material surface is unable to keep up with volume changes and is destroyed, the active material surface not covered by the SEI film is exposed, and electrolyte decomposition due to a new SEI film formation reaction can proceed. The SEI film forms on the active material surface during the first charge and is primarily composed of electrolyte decomposition products, but the binder resin in contact with the active material surface is also thought to be involved in the film formation. This SEI film is thought to contribute to improving battery performance by mediating the lithium ion insertion / extraction reaction while suppressing further electrolyte decomposition reactions. If the SEI film is too thin, the electrolyte decomposition reaction will not stop, while if it is too thick, electrical resistance will increase, adversely affecting the battery's lifespan and efficiency.

[0007] As an effort to solve the problem of volume change in the novel negative electrode active material, for example, Patent Document 1 below proposes a method of suppressing swelling of the electrode layer due to volume change of the negative electrode active material by using a high-strength aromatic polyimide as a binder. Patent Document 2 also proposes a method of suppressing volume change of the negative electrode active material by using a partially cross-linked polyacrylic acid as a binder. Patent Document 3 also proposes a method of suppressing volume change of the negative electrode 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 was unable to fully utilize the capacity of the active material. Furthermore, the binders in Patent Documents 2 and 3 did not necessarily have 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 Summary of the Invention [Problem to be solved by the invention]

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

[0010] To solve these problems, the inventors conducted extensive research and discovered that by adjusting the molecular weight of a polymeric binder containing hydroxyl groups and acid groups to an unprecedentedly high level, the slurry stability was excellent even without the need for thickeners such as cellulose, and the solvent swelling resistance of the coating film was suppressed. Furthermore, the negative electrode fabricated using this binder exhibited high peel strength and a low electrode expansion rate upon charging, and as a result, when battery evaluation was performed, it was found to exhibit good charge-discharge characteristics even at high cycle counts, leading to the present invention.

[0011] That is, the present invention relates to the following. [1] A negative electrode binder composition comprising a copolymer essentially comprising a hydroxyl group-containing monomer (a) and an acid group-containing monomer (b), wherein the weight-average molecular weight of the copolymer is 700,000 or more as measured using an aqueous GPC measuring device, and the swelling ratio of a dried polymer film of the negative electrode binder composition after immersion 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 wt%. [2] The negative electrode binder composition according to [1], wherein the hydroxyl group-containing monomer (a) is at least one selected 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 at least one selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, monomethyl maleic 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) is at least one selected from the group consisting of acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethylacrylamide, and N-hydroxymethylacrylamide. [7] A negative electrode comprising 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 the negative electrode binder composition according to any one of [1] to [7] as a component. [9] A secondary battery comprising the negative electrode according to [8]. [Effects 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 negative electrodes prepared using this composition exhibit reduced electrode expansion during charging. Therefore, negative electrodes prepared using the negative electrode binder composition of the present invention have high peel strength, and as a result, when battery evaluation is performed, they exhibit good charge-discharge characteristics even over a high number of cycles, achieving the performance required for recent LIB negative electrodes. DETAILED DESCRIPTION OF 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, as essential components, a hydroxyl group-containing monomer (a) and an acid group-containing monomer (b), wherein the weight-average molecular weight of the copolymer is 700,000 or more as measured using an aqueous GPC measuring device, and the swelling ratio of a dried 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.

[0014] The weight-average molecular weight of the copolymer, as measured using an aqueous GPC measuring device, is 700,000 or more, preferably 750,000 to 1,500,000, and more preferably 800,000 to 1,200,000. As described above in the effects of the invention, a weight-average molecular weight of 700,000 or more provides good slurry stability, and a negative electrode produced using the same is prevented from swelling during charging.

[0015] In the aqueous GPC measurement device, a typical polymeric filler such as polyhydroxymethacrylate can be used as the column packing. Examples of columns that can be used include SB-806 HQ and SB-806M HQ from the Shodex OHpak series manufactured by Showa Denko K.K. Furthermore, neutral salt solutions such as aqueous sodium nitrate, aqueous sodium hydrogen chloride, aqueous sodium sulfate, and phosphate buffer can be used as the eluent. The concentration of these eluents is preferably about 0.1 to 0.3 mol / L. Examples of GPC measurement devices that can be used include the Shimadzu / L20 system. Polystyrene or pullulan can be used as the standard substance in GPC measurement. Specifically, the standard substance can be STANDARD P-82 (Pullulan) manufactured by Showa Denko K.K.

[0016] As described above, the swelling ratio of a dried 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 ratio is preferably 0.1 to 6% by weight, and more preferably 0.1 to 4% by weight. A lower swelling ratio is preferable, and if the swelling ratio is in the above range, the peel strength when formed into a negative electrode is strong, and as a result, when a battery evaluation is performed, good charge / discharge characteristics can be exhibited even at a high number of cycles.

[0017] The swelling ratio can be determined by, for example, drying the negative electrode binder composition for 72 hours at room temperature and then for 30 minutes at 150°C to produce a dried polymer film (dried coating) with a film thickness of 150µm, immersing this dried 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 ratio means that the negative electrode binder composition contains a solvent and is easily plasticized, and that the adhesive strength is easily reduced when used as a negative electrode.

[0018] Examples of the hydroxyl group-containing monomer (a) in the copolymer include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, and 2,3-dihydroxypropyl acrylate. Among these, hydroxyl group-containing monomer (a) is preferably hydroxyethyl acrylate (particularly 2-hydroxyethyl acrylate). 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 the slurry stability and the electrolyte swelling resistance at high temperatures when formed into a coating tend to be good.

[0019] Examples of the acid group-containing monomer (b) in the copolymer include acrylic acid, methacrylic acid, 2-carboxyethyl acrylate, maleic acid, and itaconic acid. The acid group in the acid group-containing monomer (b) is preferably a carboxylic acid. Of 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 the slurry stability and the electrolyte swelling resistance at high temperatures when formed into a coating tend to be good.

[0020] The monomers constituting the copolymer may contain a monomer other than the hydroxyl group-containing monomer (a) and the acid group-containing monomer (b) (hereinafter referred to as "other monomer (c)"). Examples of the other monomer (c) include acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethylacrylamide, and N-hydroxymethylacrylamide. Of these, acrylamide is preferred as the other monomer (c), and the inclusion of acrylamide has the effect of increasing the toughness of the resulting coating. When the other monomer (c) is contained, its content is, for example, 5 to 40% by weight, preferably 5 to 20% by weight. When the content of the other monomer (c) is within the above range, resistance to electrolyte swelling at high temperatures tends to be good.

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

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

[0023] The SiO negative electrode material is a material containing SiO (silicon monoxide) as a main component that exhibits charge-discharge characteristics in the negative electrode. In addition to the SiO negative electrode material, silicon particles, carbon, etc. that also exhibit charge-discharge characteristics may be contained. The SiO negative electrode material may also contain silicon oxycarbide (SiOC). These components may be contained alone or in combination. The proportion of the SiO negative electrode material in the negative electrode binder composition is, for example, 3 to 40 wt %, preferably 5 to 30 wt %.

[0024] The graphite may be natural graphite or artificially synthesized graphite, and examples of graphite include carbon materials such as natural graphite, artificial graphite, hard carbon, and soft carbon. Graphite is also a component that exhibits charge-discharge characteristics, similar to SiO negative electrode materials. 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 acts as a conductive aid in the negative electrode, and may be a component other than acetylene black, such as carbon black, ketjen black, etc. The proportion of these components acting as conductive aids in the negative electrode binder composition is, for example, 0.5 to 10% by weight, and preferably 0.5 to 5% by weight.

[0026] The solvent is not particularly limited as long as it can disperse the components necessary for constituting 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. 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 other conventionally used components as binder (binding agent) components, as long as the effects of the present invention are not impaired. Examples of such binder (binding agent) components include styrene-butadiene rubber copolymers (SBR), (meth)acrylic copolymers composed of ethylenically unsaturated carboxylic acid esters (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, (meth)acrylonitrile, and hydroxyethyl (meth)acrylate) and ethylenically unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid), and polymeric compounds such as polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin, 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 polymer compound. In addition, the negative electrode binder composition of the present invention may contain other components necessary as components of the negative electrode in addition to the above.

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

[0030] First, a 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 to 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 allowing the polymerization reaction to proceed. The polymerization reaction may be carried out in an inert gas atmosphere such as nitrogen. The polymerization reaction can be carried out at a temperature of 50 to 80°C for 1 to 10 hours. After the reaction is complete, the mixture is cooled and the pH is adjusted.

[0031] Next, the components necessary for constructing the negative electrode, such as SiO negative electrode material, graphite, acetylene black, and a solvent, are added to the obtained copolymer, and the mixture is dispersed in an aqueous solvent such as ion-exchanged water. Dispersion may be performed using a dispersing device such as a stirrer, ball mill, super sand mill, or pressure kneader. Alternatively, the mixture may be kneaded in a kneader. A negative electrode binder composition can be produced through such dispersion and kneading.

[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 can be obtained by applying the above-obtained negative electrode binder composition slurry onto a current collector copper foil to form a thin film negative electrode layer. Alternatively, as described below, the negative electrode binder composition slurry may be formed into a shape such as a sheet or pellet, and then integrated with a current collector to obtain a negative electrode.

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

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

[0035] The negative electrode material slurry formed into a sheet, pellet, or other shape can be integrated with the current collector by a known method, such as rolling, pressing, or a combination thereof. The electrode density after integration is, for example, 1.0 to 1.8 g / cm. 3 and preferably 1.1 to 1.7 g / cm 3 is.

[0036] The negative electrode layer formed on the current collector and the negative electrode layer integrated with the current collector are preferably subjected to a heat treatment. The heat treatment conditions are, for example, 80 to 150°C for 5 to 20 hours. This heat treatment removes the solvent and hardens the binder, increasing strength and improving adhesion between particles and between the particles and the current collector. It is preferable that this heat treatment be performed in an inert atmosphere such as helium, argon, or nitrogen, or in a vacuum atmosphere, to prevent oxidation of the current collector during treatment.

[0037] <Secondary battery> The secondary battery of the present invention includes the negative electrode of the present invention. When the secondary battery of the present invention is used as a wet electrolyte secondary battery, for example, the secondary battery can be constructed by disposing a positive electrode and the negative electrode of the present invention opposite each other with a separator interposed therebetween and injecting an electrolytic solution.

[0038] The positive electrode can be obtained by forming a positive electrode layer on the surface of a current collector in the same manner as the negative electrode. In this case, the current collector can be a strip of metal or alloy such as aluminum, titanium, or stainless steel, in the form of foil, perforated foil, mesh, or the like.

[0039] The positive electrode material used in the positive electrode layer is not particularly limited. When fabricating a lithium ion secondary battery, for example, a 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, VO5, VO 13 , VO2, MnO2, TiO2, MoV2O8, TiS2, V2S5, VS2, MoS2, MoS3, Cr3O8, Cr2O5, olivine-type LiMPO4 (M: Co, Ni, Mn, Fe), conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, polyacene, porous carbon, etc. can be used alone or in combination.

[0040] The separator may be, for example, a nonwoven fabric, cloth, or microporous film primarily composed of a polyolefin such as polyethylene or polypropylene, or a combination thereof. Note that if the nonaqueous electrolyte secondary battery to be fabricated has a structure in which the positive electrode and the negative electrode are not in direct contact with each other, it is not necessary to use a separator.

[0041] As the electrolyte, for example, a so-called organic electrolyte can be used, which is obtained by dissolving a lithium salt such as LiClO4, LiPF6, LiAsF6, LiBF4, or 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-oxazolidin-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 alone or as a mixture of two or more components.

[0042] The structure of the secondary battery of the present invention is not particularly limited, but typically, a positive electrode, a negative electrode, and a separator, which is provided as needed, are wound into a flat spiral shape to form a wound electrode plate group, or these are stacked in the form of flat plates to form a stacked electrode plate group, and these electrode plate groups are enclosed in an exterior body.

[0043] The secondary battery of the present invention is not particularly limited, and can be used as a paper battery, a button battery, a coin battery, a laminated battery, a cylindrical battery, a prismatic battery, etc. The above-mentioned negative electrode active material of the present invention can also be applied to general electrochemical devices that use the insertion and desorption of lithium ions as a charge / discharge mechanism, such as hybrid capacitors and solid-state lithium secondary batteries. [Example]

[0044] The present invention will be described in more detail below with reference to the following examples. Copolymers were synthesized according to the methods of Synthesis Examples 1-7 below, and negative electrodes were fabricated using the obtained copolymers according to the methods of Examples 1-3 and Comparative Examples 1-4. Secondary batteries were fabricated using the obtained negative electrodes and separately fabricated positive electrodes according to the methods of Examples 4-6 and Comparative Examples 5-10.

[0045] [Aqueous GPC measurement] Aqueous GPC measurements were performed using a Shimadzu / L20 HPLC system and a Shodex OHpak SB-806MHQ column (8.0 mm ID x 300 mm L x 2). The eluent was a 0.2 mol / L aqueous solution of sodium nitrate. The sample was dissolved at 0.5% and filtered through a φ0.45 filter before measurement. 50 μL of the sample was added and flowed at a flow rate of 0.70 mL / min. The weight-average molecular weight was determined using an RI detector. A calibration curve was created using a Showa Denko STANDARD P-82 (Pullulan) standard.

[0046] "Synthesis of copolymers" [Synthesis Example 1] A 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blower was charged with 500.0 parts by weight of ion-exchanged water and heated to 75°C after 3 hours of N2 blowing. 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 moles of monomers), and 50.0 parts by weight of ion-exchanged water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition, the mixture was maintained at the same temperature for 2 hours and then cooled. At a temperature below 40°C, 5 mol / L aqueous sodium hydroxide solution and distilled water were added to adjust the pH to 6.8-7.2 and the nonvolatile content to 14.8-15.2% by weight. The resulting copolymer had a nonvolatile 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 carbonate-based mixed solvent at 45°C) The resulting copolymer solution was applied to a PET film and left to dry at room temperature for three days to form a copolymer coating. After peeling, the film was cut into a 1.0 cm x 1.0 cm square and dried in a ventilated oven at 80°C for one hour and then in a vacuum oven at 110°C for 10 hours. The resulting coating had a thickness of 100 to 150 μm. The weight of this coating was measured, followed by immersion in a carbonate-based mixed solvent (EC (ethylene carbonate) / DEC (diethylene carbonate) = 50 / 50 (wt)) at 45°C for 72 hours, after which the weight of the coating was measured again. The swelling ratio in the carbonate mixed solvent calculated using the following formula (1) was 3.7%.

[0048] Swelling degree in carbonate mixed solvent (%) = (coating weight after immersion - coating weight before immersion) / (coating weight 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 was used instead of 60.0 parts by weight of 2-hydroxyethyl acrylate, and 0.332 parts by weight of ammonium persulfate (1500 ppm relative to the total moles of monomers). The copolymer obtained had a nonvolatile 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 moles of monomers) were used instead of 40.0 parts by weight of acrylic acid. The copolymer obtained had a nonvolatile 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] A 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blower was charged with 500.0 parts by weight of ion-exchanged water. After 3 hours of N2 blowing, the vessel 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 moles of monomers), and 50.0 parts by weight of ion-exchanged water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition, the mixture was maintained at the same temperature for 2 hours and then cooled. At a temperature of 40°C or below, 5 mol / L aqueous sodium hydroxide solution and distilled water were added to adjust the pH to 6.8-7.2 and the nonvolatile content to 14.8-15.2% by weight. The copolymer obtained had a nonvolatile content of 15.1 wt%, pH of 7.1, a viscosity of 3100 mPa·s, and a weight-average molecular weight of 730,000 as measured by aqueous GPC. The 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 moles of monomers), and 25% aqueous ammonia were used instead of 5 mol / L aqueous sodium hydroxide. The resulting copolymer had a nonvolatile content of 15.0% by weight, a pH of 7.0, a viscosity of 12,500 mPa·s, and a weight-average molecular weight of 780,000 as measured by aqueous GPC. The swelling index 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 aqueous solution of lithium hydroxide was used instead of a 5 mol / L aqueous solution of sodium hydroxide. The resulting copolymer had a nonvolatile content of 15.0 wt%, a pH of 7.0, a viscosity of 13,700 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%.

[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 solution of sodium hydroxide was used instead of 25% aqueous ammonia. The resulting copolymer had a nonvolatile content of 15.0 wt%, a pH of 7.0, a viscosity of 13,200 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 instead of acrylamide, 0.357 parts by weight of ammonium persulfate (1500 ppm relative to the total moles of monomers), and 25% aqueous ammonia instead of 5 mol / L aqueous sodium hydroxide were used. The resulting copolymer had a nonvolatile content of 15.0% by weight, a pH of 7.0, a viscosity of 17,100 mPa·s, and a weight-average molecular weight of 830,000 as measured by aqueous GPC. The swelling index 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 moles of monomers), and 25% aqueous ammonia were used instead of 5 mol / L aqueous sodium hydroxide. The resulting copolymer had a nonvolatile content of 14.9% by weight, a pH of 7.0, a viscosity of 15,200 mPa·s, and a weight-average molecular weight of 860,000 as measured by aqueous GPC. The swelling index 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 moles of monomers), and 25% aqueous ammonia were used instead of 5 mol / L aqueous sodium hydroxide. The resulting copolymer had a nonvolatile content of 15.0% by weight, a pH of 7.0, a viscosity of 13,300 mPa·s, and a weight-average molecular weight of 840,000 as measured by aqueous GPC. The swelling index 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 moles of monomers), and 25% aqueous ammonia were used instead of 5 mol / L aqueous sodium hydroxide. The resulting copolymer had a nonvolatile content of 15.1% by weight, a pH of 7.0, a viscosity of 14,500 mPa·s, and a weight-average molecular weight of 830,000 as measured by aqueous GPC. The swelling index 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 moles of monomers), and 25% aqueous ammonia were used instead of 5 mol / L aqueous sodium hydroxide. The resulting copolymer had a nonvolatile content of 15.1% by weight, a pH of 7.0, a viscosity of 13,000 mPa·s, and a weight-average molecular weight of 840,000 as measured by aqueous GPC. The swelling index 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 the 3-hour N2 blowing was omitted. The copolymer thus obtained had a nonvolatile content of 15.0 wt%, pH 6.8, 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 moles of monomers) were used. The copolymer obtained had a nonvolatile 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] A 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blower was charged with 500.0 parts by weight of ion-exchanged water and heated to 75°C after 3 hours of N2 blowing. 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 moles of monomers), and 50.0 parts by weight of ion-exchanged water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition, the mixture was maintained at the same temperature for 2 hours and then cooled. The resulting copolymer had a nonvolatile 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 swelling index in a carbonate mixed solvent was 12.3%.

[0063] [Synthesis Example 16] A 1.0 L reaction vessel equipped with a stirrer, thermometer, condenser, and nitrogen blower was charged with 500.0 parts by weight of ion-exchanged water and heated to 75°C after 3 hours of N2 blowing. 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 moles of monomers), and 50.0 parts by weight of ion-exchanged water was added dropwise over 3 hours to carry out the polymerization reaction. After the dropwise addition, the mixture was maintained at the same temperature for 2 hours and then cooled. At a temperature below 40°C, 5 mol / L aqueous sodium hydroxide solution and distilled water were added to adjust the pH to 6.8-7.2 and the nonvolatile content to 14.8-15.2% by weight. The resulting copolymer had a nonvolatile 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. 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 swelling degree in a carbonate mixed solvent at 45°C in the above Synthesis Examples 1-16 are summarized in Table 1 below.

[0065] [Table 1]

[0066] "Preparation of negative electrode" [Example 1] (Preparation of negative electrode mixture slurry) 11.5 parts by weight of SiO2 anode 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 and stirred for 30 seconds in a centrifugal mixer (ARE-310 manufactured by Thinky) at 1000 rpm and 2000 rpm. 27.0 parts by weight (2.16 parts by weight in terms of solids) of an aqueous solution prepared by diluting the polymer (non-volatile content 14.8% by weight) shown in Synthesis Example 1 above with distilled water and adjusting the non-volatile content to 8.0% were added to the solution, and the mixture was mixed until a paste was formed. The mixture was then stirred for 2 minutes in a planetary centrifugal mixer (Thinky ARE-310) at 1000 rpm and 2000 rpm. Because stirring generated heat, the mixture was cooled to room temperature with ice water. The mixture was again stirred for 2 minutes at 1000 rpm and 2000 rpm, and then cooled to room temperature with ice water. 10.5 parts by weight (0.84 parts by weight in terms of nonvolatile content) of the aqueous binder composition shown in Synthesis Example 1, previously prepared to a nonvolatile content of 8%, was added and mixed until uniform. The mixture was then stirred for 2 minutes in a planetary centrifugal mixer (Thinky ARE-310) at 1000 rpm and 2000 rpm, and cooled to room temperature with ice water. Five parts by weight of distilled water was added and mixed until uniform. Next, to adjust the viscosity of the slurry, the viscosity was measured using a Brookfield viscometer, and distilled water was added as needed to adjust the viscosity to within the range of 2000 to 4000 Pa s at 30 rpm. Finally, the mixture was stirred for 30 seconds using a centrifugal mixer (ARE-310, manufactured by Thinky) at a centrifugal speed of 1000 rpm and a centrifugal speed of 2000 rpm to prepare a negative electrode mixture slurry using the negative electrode binder composition of the present invention.

[0067] (Preparation of negative electrode) Next, the coating amount (area density) of the negative electrode mixture after drying was 8.8 mg / cm 2The gap of the bar coater was adjusted so that the negative electrode mixture slurry was applied to a copper foil current collector using the bar coater. The negative electrode mixture slurry was then dried for 8 minutes in a fan-type dryer set at 80°C. The dried electrode was cut into strips with a width of 40 mm and pressed into a roll press (Small tabletop roll press SA-602 manufactured by Tester Sangyo Co., Ltd.) until the mixture layer density became 1.55 g / cm. 3 After vacuum drying at 110°C for 10 hours, the density of the mixture layer was measured again and found to be 1.50 g / cm 3 (The thickness of the mixture layer was 68.6 μm.) The initial charge capacity per unit area of this electrode was 4.95 mAh / cm 2 As a result, the negative electrode of Example 1 (area density 8.8 gm / cm 2 , composite layer density 1.5g / cm 3 , composite layer thickness 68.6 μm, initial charge capacity per unit area 4.95 mAh / cm 2 ) was obtained.

[0068] (Measurement of peel strength and confirmation of electrode winding resistance) The negative electrode fabricated as described above was left in a constant temperature and humidity chamber at 25°C and 50% relative humidity for 6 hours and then cut into strips measuring 25 mm wide and 100 mm long. The active material side was then attached to a stainless steel plate using double-sided tape (Nitto Denko Corporation, No. 5015) to prepare a peel strength test sample. Approximately 10 mm of the copper foil edge was peeled off, and polyimide tape was applied to the peel tester. The peel strength test sample was attached to a peel tester (Shimadzu Corporation, Autograph AG-X Plus) and subjected to a 180° peel test. The peel strength was 34.5 N / m. The peeling (fracture) state of the negative electrode coating was then observed. The negative electrode coating was also wrapped around a 5 mm diameter core and visually inspected for cracks. No cracks were observed.

[0069] [Example 2] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 2. The peel strength was 27.8 N / m, and no cracks were observed.

[0070] [Example 3] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 3. The peel strength at this time was 28.9 N / m, and no cracks were observed.

[0071] [Example 4] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 4. The peel strength was 24.5 N / m, and no cracks were observed.

[0072] [Example 5] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 5. The peel strength was 30.7 N / m, and no cracks were observed.

[0073] [Example 6] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 6. The peel strength at this time was 29.5 N / m, and no cracks were observed.

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

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

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

[0077] [Example 10] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 10. The peel strength was 30.5 N / m, and no cracks were observed.

[0078] [Example 11] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 11. The peel strength was 28.6 N / m, and no cracks were observed.

[0079] [Example 12] The procedure was the same as in Example 1, except that the binder mixture used to prepare the slurry was the polymer shown in Synthesis Example 12. The peel strength was 28.4 N / m, and no cracks were observed.

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

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

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

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

[0084] Comparative Example 5 The same procedure as in Example 1 was carried out except that the binder mixture used to prepare the slurry was sodium polyacrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., polymerization degree 22,000 to 70,000). The peel strength at this time 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 SiO2 anode 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 and mixed in a centrifugal mixer (ARE-310, manufactured by Thinky) at 1000 rpm and 2000 rpm for 30 seconds. 48.0 parts by weight (0.96 parts by weight in terms of solids) of an aqueous solution prepared by dissolving carboxymethylcellulose sodium salt (CMC, Sunrose MAC350HC, manufactured by Nippon Paper Industries Co., Ltd.) in distilled water and adjusting the nonvolatile content to 2.0% were added and mixed until the mixture became a paste. The mixture was then stirred for 2 minutes in a planetary centrifugal mixer (Thinky ARE-310) at 1000 rpm and 2000 rpm. Because stirring generated heat, the mixture was cooled to room temperature with ice water. The mixture was then stirred again for 2 minutes at 1000 rpm and 2000 rpm, after which it was cooled to room temperature with ice water. 27.0 parts by weight (0.54 parts by weight in terms of nonvolatile content) of the above CMC aqueous solution, previously adjusted to a nonvolatile content of 2%, was added and mixed until uniform. The mixture was then stirred for 2 minutes in a planetary centrifugal mixer (Thinky ARE-310) at 1000 rpm and 2000 rpm, and 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 nonvolatile content) of styrene butadiene copolymer (SBR) (DS407H manufactured by DIC Corporation, nonvolatile content concentration 50.8%) were added, and the mixture was again stirred for 30 seconds using a centrifugal mixer (ARE-310 manufactured by Thinky Corporation) at a centrifugal speed of 1000 rpm and a centrifugal speed of 2000 rpm to prepare a negative electrode mixture slurry. The negative electrode was fabricated, the peel strength was measured, and the electrode's winding resistance 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 resins used in the negative electrodes produced in Examples 1 to 12 and Comparative Examples 1 to 6, their peel strengths, and 5φ bending crack resistances are shown in Table 2 below.

[0087] [Table 2]

[0088] From Table 2 above, it can be seen that the peel strength of the examples is higher than that of the comparative examples.

[0089] "Preparation of positive electrode" (Preparation of Positive Electrode Slurry) In a room with humidity adjusted to 30% or less, the cathode 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 and mixed in a centrifugal mixer (Thinky ARE-310) at 1000 rpm and 2000 rpm for 30 seconds. 27.0 parts by weight (2.16 parts by weight in terms of solids) of an anhydrous N-methylpyrrolidone solution of polyvinylidene fluoride adjusted to a nonvolatile content of 8.0% and 19.0 parts by weight of anhydrous N-methylpyrrolidone were added and mixed until the mixture became a paste. The mixture was then stirred in a centrifugal mixer (Thinky ARE-310) at 1000 rpm and 2000 rpm for 2 minutes. Since the mixture generated heat during mixing, it was cooled to room temperature with ice water. The mixture was stirred again for 2 minutes at 1000 rpm and 2000 rpm, then cooled to room temperature in ice water. 10.5 parts by weight (0.84 parts by weight of nonvolatile content equivalent) of a polyvinylidene fluoride anhydrous N-methylpyrrolidone solution previously prepared to an 8% nonvolatile content was added and mixed until uniform. The mixture was then stirred for 2 minutes in a centrifugal mixer (Thinky ARE-310) at 1000 rpm and 2000 rpm, and cooled to room temperature in ice water. Five parts by weight of anhydrous N-methylpyrrolidone was added and mixed until uniform. The viscosity of the slurry was then measured using a Brookfield viscometer, and anhydrous N-methylpyrrolidone was added as needed to adjust the viscosity to a range of 2000–4000 Pa·s at 30 rpm. Finally, the mixture was stirred for 30 seconds using a rotation / revolution mixer (ARE-310 manufactured by Thinky) at a rotation speed of 1000 rpm and a revolution speed of 2000 rpm to prepare a positive electrode mixture slurry.

[0090] (Preparation of positive electrode) First, the coating amount (area density) of the mixture after drying was 25.0 mg / cm 2 The gap of the bar coater was adjusted so that the positive electrode mixture slurry was applied onto an aluminum foil current collector using the bar coater. The positive electrode mixture slurry was then dried for 10 minutes in a fan-type dryer set at 80°C. The dried electrode was cut into strips with a width of 40 mm and then rolled using a roll press (Small tabletop roll press SA-602 manufactured by Tester Sangyo Co., Ltd.) until the mixture layer density became 3.4 g / cm. 3 After pressing to a thickness of 73.3 μm (mixture layer thickness), it was vacuum dried at 110°C for 10 hours. The initial charge capacity per unit area of this electrode was 4.49 mAh / cm 2 This resulted in a positive electrode (areal density 25.0 gm / cm 2 , composite layer density 3.4g / cm 3 , composite layer thickness 73.3 μm, initial charge capacity per unit area 4.49 mAh / cm 2 ) was obtained.

[0091] "Making secondary batteries" [Example 13] First, the negative electrode shown in Example 1 was cut into a 24 mm x 24 mm square with a tab, and the positive electrode shown in the positive electrode preparation example was cut into a 22 mm x 22 mm square with a tab using a Thomson blade. Nickel tab leads were welded to the tabs of the cut electrodes for the negative electrode and aluminum tab leads were welded to the tabs of the cut electrodes. Next, a separator (a 25-micron-thick polyethylene microporous film) was cut into a 28 mm x 3.8 cm rectangle using a Thomson blade. The positive and negative electrodes were placed opposite each other with the separator interposed between them, packaged in a laminate film, and the tabs were fixed by thermocompression. 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 battery was completely sealed by vacuum lamination to prepare a laminated secondary battery.

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

[0093] (electrode swelling rate) After the initial charge / discharge, the battery was held 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 with a micrometer. The electrode swelling ratio was calculated using the following formula (2) and was found to be 32.0%.

[0094] Electrode swelling rate (%) = (disassembled electrode thickness - electrode thickness when battery is fabricated) / (mixture layer thickness when battery is fabricated) × 100 (Equation 2)

[0095] [Example 14] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Example 2 was used. The initial charge-discharge efficiency was 81.5%, the discharge capacity retention rate was 86.4%, and the electrode expansion rate was 30.4%.

[0096] [Example 15] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Example 3 was used. The initial charge-discharge efficiency was 81.4%, the discharge capacity retention rate was 84.9%, and the electrode expansion rate was 30.2%.

[0097] [Example 16] The same procedures as in Example 13 were carried out 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%, and the electrode expansion rate was 31.1%.

[0098] [Example 17] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Example 5 was used. The initial charge-discharge efficiency was 81.6%, the discharge capacity retention rate was 85.3%, and the electrode expansion rate was 30.7%.

[0099] [Example 18] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Example 6 was used. The initial charge-discharge efficiency was 82.7%, the discharge capacity retention rate was 85.3%, and the electrode expansion rate was 30.6%.

[0100] [Example 19] The same procedure as in Example 13 was carried out 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%, and the electrode expansion rate was 30.2%.

[0101] [Example 20] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Example 8 was used. The initial charge-discharge efficiency was 81.5%, the discharge capacity retention rate was 85.0%, and the electrode expansion rate was 30.0%.

[0102] [Example 21] The same procedure as in Example 13 was carried out except that the negative electrode prepared in Example 9 was used. The initial charge-discharge efficiency was 81.4%, the discharge capacity retention rate was 84.7%, and the electrode expansion rate was 30.6%.

[0103] [Example 22] The same procedure as in Example 13 was carried out except that the negative electrode prepared in Example 10 was used. The initial charge-discharge efficiency was 81.4%, the discharge capacity retention rate was 84.9%, and the electrode expansion rate was 30.0%.

[0104] [Example 23] The same procedure as in Example 13 was carried out except that the negative electrode prepared in Example 11 was used. The initial charge-discharge efficiency was 81.5%, the discharge capacity retention rate was 84.7%, and the electrode expansion rate was 29.8%.

[0105] [Example 24] The same procedure as in Example 13 was carried out except that the negative electrode prepared in Example 12 was used. The initial charge-discharge efficiency was 81.4%, the discharge capacity retention rate was 84.9%, and the electrode expansion rate was 30.2%.

[0106] Comparative Example 7 The same procedures as in Example 13 were carried out except that the negative electrode prepared in Comparative Example 1 was used. The initial charge-discharge efficiency was 81.4%, the discharge capacity retention rate was 80.4%, and the electrode expansion rate was 35.2%.

[0107] [Comparative Example 8] The same procedures as in Example 13 were carried out 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%, and the electrode expansion rate was 40.3%.

[0108] Comparative Example 9 The same procedures as in Example 13 were carried out 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%, and the electrode expansion rate was 45.9%.

[0109] [Comparative Example 10] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Comparative Example 4 was used. The initial charge-discharge efficiency was 81.2%, the discharge capacity retention rate was 78.5%, and the electrode expansion rate was 36.4%.

[0110] [Comparative Example 11] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Comparative Example 5 was used. The initial charge-discharge efficiency was 80.9%, the discharge capacity retention rate was 76.0%, and the electrode expansion rate was 36.8%.

[0111] [Comparative Example 12] The same procedures as in Example 13 were carried out except that the negative electrode prepared in Comparative Example 6 was used. The initial charge-discharge efficiency was 81.1%, the discharge capacity retention rate was 79.0%, and the electrode expansion rate was 39.3%.

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

[0113] [Table 3]

[0114] From Table 3 above, it can be seen that Examples 13-24, which used the negative electrode binder compositions of the present invention (Examples 1-12), were superior in both the 100th cycle capacity retention rate (%) and the electrode swelling rate (%) to Comparative Examples 7-12, which did not use the negative electrode binder compositions of the present invention.