Binder for Anode of Secondary Battery, Anode of Secondary Battery and Secondary Battery

Abstract

This invention relates to a binder for an anode of a secondary battery, an anode of a secondary battery, and a secondary battery. Specifically, this invention provides a binder for an anode of a secondary battery that can not only improve anode adhesion, but also minimize resistance in a secondary battery, and ultimately, improve the life of a secondary battery.

Claims

1. A binder for an anode of a secondary battery comprising: a copolymer comprising, based on the total weight (100 wt %) of repeat units, a) 40 to 55 wt % of first repeat units derived from aliphatic conjugated diene-based first monomers, b) 40 to 55 wt % of second repeat units derived from aromatic vinyl-based second monomers, c) 0.1 to 3 wt % of third repeat units derived from unsaturated carboxylic acid-based third monomers, and d) 1 to 11 wt % of fourth repeat units derived from polyethylene glycol mono(meth)acrylate.

2. The binder for an anode of a secondary battery according to claim 1, wherein a weight ratio of the first repeat units to the fourth repeat units (first repeat units/fourth repeat units) is 3.64 to 55.

3. The binder for an anode of a secondary battery according to claim 1, wherein a weight ratio of the second repeat units to the fourth repeat units (second repeat units/fourth repeat units) is 3.64 to 55.

4. The binder for an anode of a secondary battery according to claim 1, wherein a weight ratio of the third repeat units to the fourth repeat units (third repeat units/fourth repeat units) is 0.01 to 3.

5. The binder for an anode of a secondary battery according to claim 1, wherein a weight ratio of the first repeat units to the second repeat units (first repeat units/second repeat units) is 0.73 to 1.38.

6. The binder for an anode of a secondary battery according to claim 1, wherein the first repeat units are included in a content of 41 to 54 wt %, based on the total weight (100 wt %) of the repeat units.

7. The binder for an anode of a secondary battery according to claim 1, wherein the second repeat units are included in a content of 41 to 54 wt %, based on the total weight (100 wt %) of the repeat units.

8. The binder for an anode of a secondary battery according to claim 1, wherein the third repeat units are included in a content of 1.5 to 2.3 wt %, based on the total weight (100 wt %) of the repeat units.

9. The binder for an anode of a secondary battery according to claim 1, wherein the fourth repeat units are included in a content of 1 to 10 wt %, based on the total weight (100 wt %) of the repeat units.

10. The binder for an anode of a secondary battery according to claim 1, wherein the aliphatic conjugated diene-based first monomers are one or more selected from the group consisting of 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1,2-dimethyl-1,3-butadiene, 1,4-dimethyl-1,3-butadiene, 1-ethyl-1,3-butadiene, 2-phenyl-1,3-butadiene, 1,3-pentadiene, 1,4-pentadiene, 3-methyl-1,3-pentadiene, 4-methyl-1,3-pentadiene, 2,4-dimethyl-1,3-pentadiene, 3-ethyl-1,3-pentadiene, 1,3-hexadiene, 1,4-hexadiene, 1,5-hexadiene, 2-methyl-1,5-hexadiene, 1,6-heptadiene, 6-methyl-1,5-heptadiene, 1,6-octadiene, 1,7-octadiene and 7-methyl-1,6-octadiene.

11. The binder for an anode of a secondary battery according to claim 1, wherein the aromatic vinyl-based second monomers are one or more selected from the group consisting of styrene, α-methylstyrene, β-methylstyrene, p-t-butylstyrene, chlorostyrene, vinyl benzoate, methyl vinyl benzoate, vinyl naphthalene, chloromethylstyrene, hydroxymethylstyrene and divinyl benzene.

12. The binder for an anode of a secondary battery according to claim 1, wherein the unsaturated carboxylic acid-based third monomers are one or more selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, fumaric acid, glutaric acid, itaconic acid, tetrahydrophthalic acid, crotonic acid, isocrotonic acid, and nadic acid.

13. The binder for an anode of a secondary battery according to claim 1, wherein the copolymer is latex particles having an average particle diameter of 50 nm to 500 nm.

14. The binder for an anode of a secondary battery according to claim 1, further comprising an aqueous solvent.

15. The binder for an anode of a secondary battery according to claim 14, wherein the aqueous solvent is included in he a content of 50 to 1,000 parts by weight, based on 100 parts by weight of the copolymer.

16. A method for preparing a binder for an anode of a secondary battery, comprising a step of conducting emulsion polymerization of a monomer mixture in the presence of an emulsifier and a polymerization initiator to prepare a copolymer, wherein based on the total weight (100 wt %) of the monomer mixture, a) 40 to 55 wt % of aliphatic conjugated diene-based first monomers, b) 40 to 55 wt % of aromatic vinyl-based second monomers, c) 0.1 to 3 wt % of unsaturated carboxylic acid-based third monomers, and d) 1 to 11 wt % of polyethylene glycol mono (meth)acrylate monomers are included.

17. An anode mixture for a secondary battery, comprising the binder for an anode of a secondary battery according to claim 1, and anode active material.

18. The anode mixture for a secondary battery according to claim 17, further comprising a conductive material.

19. An anode of a secondary battery comprising: an anode mixture layer comprising the anode mixture for a secondary battery according to claim 17, and an anode current collector.

20. A secondary battery comprising the anode of a secondary battery according to claim 19.

Description

EXAMPLE 1

(1) Preparation of a Binder for an Anode

[0119] A monomer mixture comprising (a) 1,3-butadiene (47.5 parts by weight), (b) styrene (47.5 parts by weight), (c) acrylic acid (2 parts by weight), and (d1) polyethylene glycol monoacrylate (3 parts by weight); sodium lauryl sulfate emulsifier (0.3 parts by weight); and potassium persulfate polymerization initiator (0.1 parts by weight); was added to water solvent (150 parts by weight).

[0120] The temperature of the mixture was raised to 75° C., and then, it was subjected to a polymerization reaction for about 5 hours while maintaining 75° C., thus obtaining a binder comprising a copolymer in the form of latex particles and water.

[0121] After polymerization, the pH of the binder was adjusted to a range of pH 7 to 8 using sodium hydroxide, and the binder of Example 1 was obtained.

[0122] The obtained binder of Example 1 had a total solid content of 40%, and the average particle diameter of latex particles measured using particle size analyzer (NICOMP AW380, manufactured by PSS Inc.) was 155 nm.

(2) Preparation of an Anode Mixture

[0123] Artificial graphite (95 parts by weight) was used as anode active material, acetylene black (1.5 parts by weight) as conductive agent, the binder of Example 1 (2.0 parts by weight) as a binder, and carboxy methyl cellulose (1.5 parts by weight) as a thickener, and they were stirred in a dispersion medium, water, for 1 hour and mixed. Wherein, the slurry phase was controlled such that the total solid content became 45 wt %, thus obtaining the anode mixture of Example 1.

(3) Preparation of an Anode

[0124] A copper foil with a thickness of 10 μm was prepared and used as an anode current collector. Using a comma coater, the anode mixture of Example 1 was coated on both sides of the anode current collector in the loading amount of 8.0 mg/cm.sup.2 per one side, hot air dried in an oven of 80° C. for 10 minutes, and then, roll-pressed such that the total thickness became 190 μm. Thereby, the anode of Example 1 was obtained.

(4) Preparation of a Secondary Battery

[0125] 90 parts by weight of Li.sub.1.03Ni.sub.0.6Co.sub.0.6Mn.sub.0.2O.sub.2 was used as cathode active material, 5.0 parts by weight of acetylene black as a conductive agent, and 50 parts by weight (10% solid content) of polyvinylidene fluoride (PVdF) as a binder, and they were stirred in NMP solvent for 1 hour and mixed. Wherein, the slurry phase was controlled such that the total solid content became 70 wt %, thus obtaining the cathode mixture of Example 1.

[0126] An aluminum foil with a thickness of 20 μm was prepared and used as a cathode current collector. Using a comma coater, the cathode mixture of Example 1 was coated on both sides of the cathode current collector in the loading amount of 15.6 mg/cm.sup.2 per one side, hot air dried in an oven of 80° C. for 10 minutes, and then, roll-pressed such that the total thickness became 190 μm. Thereby, the cathode of Example 1 was obtained.

[0127] A separator was inserted between the anode and cathode of Example 1 and assembled, and then, electrolyte was injected, and a lithium ion battery was completed according to a method commonly known in the art.

[0128] As the electrolyte, LiPF6 was dissolved in a mixed solvent(weight ratio of EC:PC:DEC=3:2:5) of ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) such that the concentration became 1.3 M, and fluoroethylene carbonate (FEC) was added such that it constituted 10 wt % of the total weight of the electrolyte.

Examples 2 to 7 and Comparative Example 1 to 9

(1) Preparation of a Binder for an Anode

[0129] The binders of Examples 2 to 7 and Comparative Examples 1 o 9 were respectively prepared by polymerization by the same method as Example 1, except that the monomer compositions were changed according to the following Table 1.

[0130] The binders of Examples 2 to 7 and Comparative Example 1 to 9 commonly comprise a copolymer in the form of latex particles, and had a total solid content of 40%.

(2) Preparation of an Anode Mixture, an Anode, and a Secondary Battery

[0131] The anode mixtures, anodes, and lithium secondary batteries of Examples 2 to 7 and Comparative Examples 1 to 9 were respectively prepared by the same method as Example 1, except that the binders of Examples 2 to 7 and Comparative Examples 1 to 9 were respectively used instead of the binder of Example 1.

[0132] In the following Table 1, the first monomers indicate (a) 1,3-butadiene (BD), the second monomers indicate (b) styrene (SM), the third monomers indicate (c) acrylic acid (AA), the fourth monomers indicate (d1) polyethylene glycol monoacrylate(PEGA) or (d2) polyethylene glycol monomethacrylate (PEGMA).

[0133] And, the monomer content is based on the content (wt %) in the total weight (100 wt %) of the monomers, and the content ratio of monomers is based on weight ratio. In case there is a number of 3 or more decimal places, it was rounded to two decimal places.

TABLE-US-00001 TABLE 1 Monomer content (wt %) Monomer content ratio M1 M2 M3 M4 M1/M4 M2/M4 M3/M4 M1/M2 Example 1 47.5 47.5 2 3 15.83 15.83 0.67 1 Example 2 47.5 47.5 2 3 15.83 15.83 0.67 1 Example 3 47.5 47.5 2 3 15.83 15.83 0.67 1 Example 4 48.5 48.5 2 1 48.5 48.5 2 1 Example 5 44 44 2 10 4.4 4.4 0.2 1 Example 6 48.5 48.5 2 1 48.5 48.5 2 1 Example 7 44 44 2 10 4.4 4.4 0.2 1 Comparative 49 49 2 0 — — — 1 Example 1 Comparative 48.6 48.6 2 0.8 60.75 60.75 2.5 1 Example 2 Comparative 43 43 2 12 3.58 3.58 0.17 1 Example 3 Comparative 48.6 48.6 2 0.8 60.75 60.75 2.5 1 Example 4 Comparative 43 43 2 12 3.58 3.58 0.17 1 Example 5 Comparative 39 56 2 3 13 18.67 0.67 0.70 Example 6 Comparative 56 39 2 3 18.67 13 0.67 1.44 Example 7 Comparative 48.5 48.5 0 3 16.17 16.17 0 1 Example 8 Comparative 46.5 46.5 4 3 15.5 15.5 1.33 1 Example 9 *M1: first monomers *M2: second monomers *M3: third monomers *M4: fourth monomers

[0134] In the Table 1, the monomer mixtures of Examples 1 to 7 respectively meet the kinds and contents of monomers limited in one embodiment.

[0135] Specifically, the monomer mixture comprising, based on the total weight (100 wt %) of the monomer mixture, a) 40 to 55 wt % of aliphatic conjugated diene-based first monomers, b) 40 to 55 wt % of aromatic vinyl-based second monomers, c) 0.1 to 3 wt % of unsaturated carboxylic acid-based third monomers, and d) 1 to 11 wt % of polyethylene glycol mono(meth)acrylate was subjected to emulsion polymerization to prepare binder.

[0136] On the other hand, Comparative Example 1 does not comprise fourth monomers, and Comparative Examples 2 to 5 comprise fourth monomers but the content range is under (Comparative Examples 2 and 4) or exceeds (Comparative Examples 3 and 5) the content range limited in one embodiment. Comparative Example 6 and 7 comprise fourth monomers, but the content ranges of first and second monomers do not fall within preferable ranges. Comparative Example 8 and 9 comprise fourth monomers, but do not comprise third monomers or the content range does not fall within preferable range.

[0137] Furthermore, in each monomer mixture of Examples 1 to 7, the weight ratio of the first monomers/fourth monomers (i.e., weight ratio of first repeat units/fourth repeat units) is within a range of 3.64 to 55, the weight ratio of the second monomers/fourth monomer (i.e., weight ratio of second repeat units/fourth repeat units) is within a range of 3.64 to 55, the weight ratio of the third monomers/fourth monomers (i.e., weight ratio of third repeat units/fourth repeat units) is within a range of 0.01 to 3, and the weight ratio of the first monomers/second monomers(i.e., weight ratio of first repeat units/second repeat units) is within a range of 0.73 to 1.38.

[0138] On the other hand, in each monomer mixture of Comparative Examples 1 to 5, the weight ratio of the third monomers/fourth monomers is within a range of 0.01 to 3, and the weight ratio of the first monomers/second monomers is within a range of 0.73 to 1.38, but the weight ratio of the first monomers/fourth monomers and the weight ratio of the second monomers/fourth monomers do not meet the above explained ranges. In each monomer mixture of Comparative Examples 6 and 7, the weight ratio of the first monomers/second monomers does not fall within a preferable range. In the monomer mixture of Comparative Example 8, the weight ratio of the third monomers/fourth monomers does not fall within a preferable range.

Comparative Example 10

(1) Preparation of a Binder for an Anode

[0139] The binder of Comparative Example 10 was prepared by polymerization by the same method as Example 1, except that a monomer mixture comprising methyl methacrylate (45 parts by weight), polyethylene glycol monoacrylate (45 parts by weight), acrylic acid (1.3 parts by weight), methacrylic acid (3.7 parts by weight), and trimethylol propane triacrylate (5 parts by weight), and water solvent (500 parts by weight) were used.

[0140] The binder of Comparative Example 10 comprises a copolymer in the form of latex particles, and has a total solid content of 17%.

(2) Preparation of an Anode Mixture, an Anode, and a Secondary Battery

[0141] The anode mixture, anode, and lithium secondary battery of Comparative Example 10 were prepared by the same method as Example 1, except that the binder of Comparative Example 10 was used instead of the binder of Example 1.

Comparative Example 11

(1) Preparation of a Binder for an Anode

[0142] The binder of Comparative Example 11 was prepared by polymerization by the same method as Example 1, except that a monomer mixture comprising polyethylene glycol monoacrylate (86.4 parts by weight), and trimethylol propane triacrylate (13.6 parts by weight), and water solvent(500 parts by weight) were used.

[0143] The binder of Comparative Example 11 comprises a copolymer in the form of latex particles, and has a total solid content of 17%.

(2) Preparation of an Anode Mixture, an Anode, and a Secondary Battery

[0144] The anode mixture, anode, and lithium secondary battery of Comparative Example 11 were prepared by the same method as Example 1, except that the binder of Comparative Example 11 was used instead of the binder of Example 1.

Comparative Example 12

(1) Preparation of a Binder for an Anode

[0145] The binder of Comparative Example 12 was prepared by polymerization by the same method as Example 1, except that a monomer mixture comprising polyethylene glycol monoacrylate (30 parts by weight), vinyl acetate (15 parts by weight), acrylic acid (1.3 parts by weight), methacrylic acid (3.7 parts by weight), methyl methacrylate (45 parts by weight), and trimethylol propane triacrylate (5 parts by weight), and water solvent (500 parts by weight) were used.

[0146] The binder of Comparative Example 12 comprises a copolymer in the form of latex particles, and has a total solid content of 17%.

(2) Preparation of an Anode Mixture, an Anode, and a Secondary Battery

[0147] The anode mixture, anode, and lithium secondary battery of Comparative Example 12 were prepared by the same method as Example 1, except that the binder of Comparative Example 12 was used instead of the binder of Example 1.

Comparative Example 13

(1) Preparation of a Binder for an Anode

[0148] The binder of Comparative Example 13 was prepared by polymerization by the same method as Example 1, except that a monomer mixture comprising polyethylene glycol monoacrylate (30 parts by weight), vinyl acetate (55 parts by weight), acrylic acid (2 parts by weight), and trimethylol propane triacrylate (13 parts by weight), and water solvent (500 parts by weight) were used.

[0149] The binder of Comparative Example 13 comprises a copolymer in the form of latex particles, and has a total solid content of 17%.

(2) Preparation of an Anode Mixture, an Anode, and a Secondary Battery

[0150] The anode mixture, anode, and lithium secondary battery of Comparative Example 13 were prepared by the same method as Example 1, except that the binder of Comparative Example 13 was used instead of the binder of Example 1.

Comparative Example 14

(1) Preparation of a Binder for an Anode

[0151] The binder of Comparative Example 14 was prepared by polymerization by the same method as Example 1, except that a monomer mixture comprising polyethylene glycol dimethacrylate (5 parts by weight), cyclohexyl methacrylate (30 parts by weight), methacrylic acid (1 part by weight), acrylic acid (1 part by weight), acrylamide (0.1 parts by weight), 2-hydroxyethyl methacrylate (5 parts by weight), 2-ethylhexylacrylate (55 parts by weight), methyl methacrylate (1 part by weight), butyl methacrylate (0.4 parts by weight), butyl acrylate (1 part by weight), and trimethylol propane triacrylate (0.5 parts by weight) was used.

[0152] The binder of Comparative Example 14 comprises a copolymer in the form of latex particles, and has a total solid content of 40%.

(2) Preparation of an Anode Mixture, an Anode, and a Secondary Battery

[0153] The anode mixture, anode, and lithium secondary battery of Comparative Example 14 were prepared by the same method as Example 1, except that the binder of Comparative Example 14 was used instead of the binder of Example 1.

Experimental Example 1: Evaluation of a Binder for an Anode

[0154] Each binder of Examples 1 to 7 and Comparative Examples 1 to 14 was evaluated under the following conditions, and the results were shown in the following Table 2.

[0155] (1) Average particle diameter of latex particles: The arithmetic mean particle diameter of latex particles in the binder, specifically intensity distribution mean particle diameter was obtained using a particle size analyzer (NICOMP AW380, manufactured by PSS Inc.).

[0156] (2) Gel content in the binder: calculated using Mathematical Formula 1. Specifically, a designated binder was dried at 80° C. for 24 hours, and then, about 0.5g was taken and the accurate weight was measured, which was put into Ma of Mathematical Formula 1.

[0157] And then, the binder of which weight had been measured was dipped in 50 g of tetrahydrofuran (THF) for 24 hours. And then, the binder contained in THF was filtered through 200 Mesh of which weight was known, and then, the Mesh and the copolymer remaining on the mesh were dried together at 80° C. for 24 hours, and then, the weight of the mesh and copolymer remaining on the mesh was measured, and a value calculated by subtracting the weight of 200 Mesh therefrom was used as the weight of copolymer, Mb.

Gel content (%)=M.sub.b/M.sub.a*100   [Mathematical Formula 1]

[0158] Per each binder, the average value of 3 or more samples was calculated, and the results were shown in the following Table 2.

[0159] (3) Stability of a binder: Using a homogenizer, 100 g of a binder was put in a container and fixed such that the head was immersed in latex, and then, shear was applied at 300 rpm for 10 minutes, followed by filtering through 200 mesh, thus measuring coagulum.

TABLE-US-00002 TABLE 2 Average particle Binder stability diameter of polymer Gel content in test (latex particles) (μm) binder (%) (coagulum, ppm) Example 1 155 98.9 150 Example 2 151 98.8 210 Example 3 150 98.7 170 Example 4 152 98.7 180 Example 5 153 98.2 220 Example 6 150 98.8 190 Example 7 155 98.1 220 Comparative 153 98.5 180 Example 1 Comparative 155 98.7 180 Example 2 Comparative 160 97.8 230 Example 3 Comparative 153 98.8 190 Example 4 Comparative 161 97.7 250 Example 5 Comparative 153 98.8 170 Example 6 Comparative 155 98.7 210 Example 7 Comparative 237 99.1 7,980 Example 8 Comparative 150 99.0 190 Example 9 Comparative 109 93.7 350 Example 10 Comparative 121 99.2 1,250 Example 11 Comparative 102 95.2 550 Example 12 Comparative 210 98.2 980 Example 13 Comparative 165 90.3 390 Example 14

[0160] From the Table 2, it can be confirmed that the binders of Examples 1 to 7 respectively comprise a copolymer (latex particles) having an average particle diameter of 50 to 500 nm, and exhibit a gel content of 95% or more, and coagulum of 250 ppm or less in the stability test.

[0161] On the other hand, it is confirmed that the binders of Comparative Examples 10 to 14 exhibit inferior gel content or stability, because polyethylene glycol mono(meth)acrylate-based fourth monomers were used but aliphatic conjugated diene-based first monomers and aromatic vinyl-based second monomers were not used and replaced with other monomers.

[0162] Meanwhile, in the case of the binders of Comparative Examples 1 to 5, although polyethylene glycol mono(meth)acrylate-based fourth monomers were not used or the amount used did not fall within the range limited in one embodiment, a gel content and stability equivalent to those of Examples 1 to 7 were secured because aliphatic conjugated diene-based first monomers and aromatic vinyl-based second monomers were used. In the case of the binders of Comparative Examples 6 and 7, although the content ranges of aliphatic conjugated diene-based first monomers and aromatic vinyl-based second monomers did not meet the limited ranges, a gel content and stability equivalent to those of Examples 1 to 7 were secured because the first and second monomers were used. In the case of the binder of Comparative Example 8, stability was deteriorated because the third monomers were not used. In the case of the binder of Comparative Example 9, although the content range of the third monomers do not meet the limited range, a gel content and stability equivalent to those of Examples 1 to 7 were secured because aliphatic conjugated diene-based first monomers and aromatic vinyl-based second monomers were used.

[0163] However, with regard to the binders of Comparative Examples 1 to 9, it is necessary to confirm the effects when applied in a battery.

Experimental Example 2: Evaluation of an Anode and a Secondary Battery

[0164] The anodes and lithium secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 14 were evaluated under the following conditions, and the results were reported in the following Table 3.

[0165] (1) Anode adhesion: For each anode of Examples and Comparative examples, peel strength was measured 5 times or more, and the average value was calculated and shown in the following Table 3. Wherein, peel strength is a measurement of a force (N) required to peel off a tape from an anode at the peel angle of 180°, after attaching the anode to an adhesive tape having a width of 10 mm, using a tension meter (Stable Micro System, TA-XT)

[0166] (2) Initial discharge resistance of a secondary battery: In a 25° C. constant temperature chamber, voltage drop generated when progressing discharge at 150 A for 10 seconds, from the state when the lithium ion battery is 50% SOC (state of charge) was reported, and DC-resistance value was calculated using R=V/I (Ohm's law).

[0167] (3) Capacity retention rate of a secondary battery after 100 cycles: In a 25° C. constant temperature chamber, charge of the lithium secondary battery at CC/CV mode at 36 A to 4.15 V, followed by discharge at CC mode to 3.0 V was set as one cycle, with 20 minutes pause between the charge and discharge, and total 100 cycles were progressed. The rate of discharge capacity measured at 100th cycle to the discharge capacity measured at the first cycle was calculated.

TABLE-US-00003 TABLE 3 Initial discharge Capacity retention Anode resistance of rate of secondary adhesion secondary battery after (gf/cm) battery (mΩ) 100 cycles (%) Example 1 29.1 1.15 92.5 Example 2 28.9 1.14 92.7 Example 3 29.3 1.15 92.5 Example 4 27.2 1.25 92.1 Example 5 25.8 1.05 91.5 Example 6 27.1 1.25 92.0 Example 7 25.7 1.07 91.4 Comparative 21.1 1.29 88.3 Example 1 Comparative 25.5 1.27 89.1 Example 2 Comparative 22.7 1.05 88.5 Example 3 Comparative 25.7 1.25 89.0 Example 4 Comparative 22.5 1.07 88.7 Example 5 Comparative 18.2 1.15 89.3 Example 6 Comparative 23.7 1.20 89.0 Example 7 Comparative 19.1 1.18 87.5 Example 8 Comparative 21.1 1.15 88.9 Example 9 Comparative 19.2 1.11 89.5 Example 10 Comparative 15.8 1.10 81.5 Example 11 Comparative 17.3 1.15 85.7 Example 12 Comparative 14.1 1.19 85.5 Example 13 Comparative 22.3 1.23 87.5 Example 14

[0168] From the Table 3, it can be confirmed that Examples 1 to 7 exhibit excellent anode adhesion, initial discharge resistance of a secondary battery, and life.

[0169] It means that as the result of emulsion polymerization of a monomer mixture meeting the kinds and contents of monomers (derived repeat units) limited in one embodiment, anode adhesion of a binder comprising the polymer is excellent, resistance of a secondary battery is minimized, and ultimately, life of a secondary battery is improved.

[0170] However, in case a binder that does not comprise the fourth monomers was used (Comparative Example 1), anode adhesion was just 21.1 gf/cm, initial discharge resistance of a secondary battery was as high as 1.29 mΩ, and above all, capacity retention rate of a secondary battery after 100 cycles was just 88.3%.

[0171] In case the fourth monomers were included but the content range was under the range limited in one embodiment (Comparative Examples 2 and 4), due to the existence of the polyethylene glycol mono(meth)acrylate-based fourth monomers, anode adhesion increased compared to Comparative Example 1, and initial discharge resistance of a secondary battery decreased to a level of 1.25˜1.27 mΩ, but capacity retention rate of a secondary battery after 100 cycles was just 89.0˜89.1%.

[0172] In the Comparative Examples 2 and 4, the weight ratios of the aliphatic conjugated diene-based first monomers and aromatic vinyl-based second monomers to polyethylene glycol mono(meth)acrylate-based fourth monomers (i.e., the weight ratio of the first monomers/fourth monomers, and the weight ratio of the second monomers/fourth monomers) also exceed the above explained ranges, respectively.

[0173] To the contrary, in case the fourth monomers were included but the content range exceeded the range limited in one embodiment (Comparative Examples 3 and 5), due to the existence of the polyethylene glycol mono(meth)acrylate-based fourth monomers, anode adhesion increased and initial discharge resistance of a secondary battery partially decreased compared to Comparative Example 1. However, since the contents of the first and second monomers relatively decreased, anode adhesion decreased and initial discharge resistance of a secondary battery partially increased compared to Comparative Examples 2 and 4. Above all, capacity retention rate of a secondary battery after 100 cycles was just 88.5˜88.7%.

[0174] In the Comparative Examples 3 and 5, the weight ratios of the aliphatic conjugated diene-based first monomers and aromatic vinyl-based second monomers to polyethylene glycol mono (meth)acrylate-based fourth monomers (i.e., the weight ratio of the first monomers/fourth monomers, and the weight ratio of the second monomers/fourth monomers) are also under the above explained ranges, respectively.

[0175] In case the fourth monomers were included but the content ranges of the aliphatic conjugated diene-based first monomers and aromatic vinyl-based second monomers did not fall within the limited ranges (Comparative Examples 6 and 7), anode adhesion was low or resistance increased, and capacity retention rate of a secondary battery after 100 cycles was just 89.0˜89.3%.

[0176] In case the fourth monomers were included but the content range of the unsaturated carboxylic acid-based third monomers did not fall within the limited range (Comparative Examples 8 and 9), anode adhesion was low and capacity retention rate of a secondary battery after 100 cycles was just 87.5˜88.9%.

[0177] And, in case the polyethylene glycol mono(meth)acrylate-based fourth monomers were included, but the aliphatic conjugated diene-based first monomers and the aromatic vinyl-based second monomers were not included and replaced with other monomers (Comparative Examples 10 to 14), adhesion and life were inferior to Examples 1 to 7.

[0178] Meanwhile, in Examples 1 to 7, it is confirmed that by adjusting the content of each monomer and the content ratio of two kinds of monomers, anode adhesion, initial discharge resistance and life of a secondary battery may be controlled.

[0179] It means that by adjusting the content of each monomer and the content ratio of two kinds of monomers referring to Examples 1 to 7 in conjunction with the explanations of one embodiment, anode adhesion, and initial resistance and life of a secondary battery can be controlled to aimed ranges.