Binder, method of preparing the same, electrode for secondary battery including the binder, and secondary battery including the electrode

1. A binder comprising a metal salt of a graft copolymer, which is a polymerization product of polyvinyl alcohol, an ethylenically unsaturated carboxylic acid, and a polymerizable monomer having a long-chain alkyl group,

wherein the binder is a copolymer comprising a first repeating unit represented by Formula 1, a second repeating unit represented by Formula 2 or Formula 2a, and a third repeating unit represented by Formula 3:

wherein, in Formula 2 or Formula 2a,

R 1 and R 2 are each independently hydrogen or a substituted or unsubstituted C1-C30 alkyl group,

R 3 is hydrogen, a monovalent metal, a divalent metal, a trivalent metal, or a combination thereof,

L 1 as a linker is a simple chemical bond, a C1-C30 alkylene group, or a C6-C30 arylene group, and

k is 0 or a number from 1 to 10,

wherein, in Formula 3,

m is a number from 3 to 20, and

wherein in Formulae 1 to 3, a, b, and c are mole fractions of the first, second, and third repeating units, respectively, and are each independently a number from 0.01 to 0.99; provided that the sum of a, b, and c is 1.

2. A binder comprising a metal salt of a graft copolymer, which is a polymerization product of polyvinyl alcohol, an ethylenically unsaturated carboxylic acid, and a polymerizable monomer having a long-chain alkyl group, wherein the binder further comprises another graft copolymer of the polyvinyl alcohol, the ethylenically unsaturated carboxylic acid, and the polymerizable monomer having a long-chain alkyl group.

3. The binder of claim 1 , wherein the ethylenically unsaturated carboxylic acid is a compound represented by Formula 1a:

CHR 1 ═C(R 2 )(R 3 COOH)  Formula 1a

wherein, in Formula 1a,

R 1 is hydrogen, a substituted or unsubstituted C1-C30 alkyl group, a substituted or unsubstituted C6-C30 aryl group, or —COOH,

R 2 is hydrogen, a substituted or unsubstituted C1-C30 alkyl group, a substituted or unsubstituted C6-C30 aryl group, or —(CH 2 ) a COOH wherein a is a number from 1 to 10, and

R 3 is a simple chemical bond, a C1-C30 alkylene group, or a C6-C30 arylene group.

4. The binder of claim 1 , wherein the ethylenically unsaturated carboxylic acid is at least one selected from methacrylic acid, acrylic acid, itaconic acid, fumaric acid, crotonic acid, 2-ethyl acrylic acid, 2-pentenoic acid, trans-2-methyl-2-butenoic acid, and trans-2,3-dimethylacrylic acid.

5. The binder of claim 1 , wherein the polymerizable monomer having a long-chain alkyl group is pentanoic acid, octanoic acid, hexanoic acid, heptanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, neodecanoic acid, 2,2-dimethyloctanoic acid, or a mixture thereof.

6. The binder of claim 1 , wherein the binder comprises at least one additional monomer selected from a polymerizable monomer having a thiol group, a polymerizable monomer having a silane group, and a polymerizable monomer having an alkylene oxide group.

7. The binder of claim 6 , wherein

the polymerizable monomer having a thiol group is HS—(CH 2 ) n —COOH wherein n is a number from 1 to 20,

the polymerizable monomer having a silane group is (R 1 )(R 2 )(R 3 )Si—(CH 2 ) n —COOH wherein n is a number from 1 to 20, and R 1 to R 3 are each independently hydrogen, a C1-C30 alkyl group, or a C6-C30 aryl group, and

the polymerizable monomer having an alkylene oxide group is glycidyl methacrylate, glycidyl acrylate, allyl glycidyl ether, or a combination thereof.

8. The binder of claim 1 , wherein a metal of the metal salt is a monovalent metal, a divalent metal, a trivalent metal, or a combination thereof.

9. The binder of claim 1 , wherein a metal of the metal salt is lithium (Li), magnesium (Mg), calcium (Ca), sodium (Na), potassium (K), aluminum (Al), cobalt (Co), or a combination thereof.

10. The binder of claim 1 , wherein the binder further comprises at least one selected from a fourth repeating unit represented by Formula 4 and a fifth repeating unit represented by Formula 5:

wherein, in Formula 4,

m is a number from 1 to 20, and

d is a number from 0.01 to 0.99, and

wherein, in Formula 5,

m is a number from 1 to 20, R 1 to R 3 are each independently hydrogen, a C1-C30 alkyl group, or a C6-C30 aryl group, and

e is a number from 0.01 to 0.99.

11. The binder of claim 1 , wherein the binder is a graft copolymer comprising a repeating unit represented by Formula 1, a repeating unit represented by Formula 2-1, and a repeating unit represented by Formula 3; or

a graft copolymer comprising a repeating unit represented by Formula 1, a repeating unit represented by Formula 2-1, a repeating unit represented by Formula 2-2, and a repeating unit represented by Formula 3:

wherein, in Formula 2-1,

R 1 is hydrogen, or a substituted or unsubstituted C1-C30 alkyl group,

wherein, in Formula 2-2,

R 1 is hydrogen, or a substituted or unsubstituted C1-C30 alkyl group,

wherein, in Formula 3,

m is a number from 4 to 20, and

in Formulae 1, 2-1, 2-2, and 3,

a, b, and c are mole fractions of the corresponding repeating units, respectively, and are each independently a number from 0.01 to 0.99, provided that the sum of a, b, and c is 1.

12. The binder of claim 1 , wherein the binder is a graft copolymer represented by Formula 6 or a graft copolymer represented by Formula 7:

wherein, in Formula 6,

a, b, and c are each independently a number from 0.01 to 0.99, provided that the sum of a, b, and c is 1, and

m is a number from 3 to 20,

R 1 is hydrogen or a substituted or unsubstituted C1-C30 alkyl group,

wherein, in Formula 7,

a, b1, b2 and c are each independently a number from 0.01 to 0.99, provided that the sum of a, b1, b2 and c is 1,

m is a number from 3 to 20,

R 1 is hydrogen or a substituted or unsubstituted C1-C30 alkyl group, and

the graft copolymer of Formula 7 is charge balanced.

13. The binder of claim 1 , wherein a mixed weight ratio of the polyvinyl alcohol to the ethylenically unsaturated carboxylic acid is about 9:1 to about 1:9.

14. The binder of claim 1 , wherein a mixed weight ratio of the polyvinyl alcohol to the ethylenically unsaturated carboxylic acid is about 9:1 to about 5:5.

15. A binder comprising a metal salt of a graft copolymer, which is a polymerization product of polyvinyl alcohol, an ethylenically unsaturated carboxylic acid, and a polymerizable monomer having a long-chain alkyl group, wherein

the metal salt comprises a lithium salt and a calcium salt, and

a mixed molar ratio of repeating units comprising the lithium salt to repeating units comprising the calcium salt is about 1:1 to about 5:1.

16. The binder of claim 1 , wherein an amount of the polymerizable monomer having a long-chain alkyl group is about 0.1 parts to about 10 parts by weight with respect to 100 parts by weight of a total weight of the polyvinyl alcohol and the ethylenically unsaturated carboxylic acid.

17. An electrode for a secondary battery, wherein the electrode comprises the binder of claim 1 and an electrode active material.

18. The electrode of claim 17 , wherein the electrode active material is at least one anode active material selected from a silicon-based active material, a tin-based active material, a silicon-tin alloy-based active material, and a silicon-carbon-based active material.

19. The electrode of claim 17 , wherein

the electrode is obtained by coating and drying an electrode active material composition comprising the binder, the electrode active material, and a solvent; and

an amount of the binder is about 0.5 parts to about 15 parts by weight with respect to 100 parts by weight of the electrode active material composition.

20. The electrode of claim 19 , wherein the electrode active material composition has a viscosity of less than 5,000 centipoise.

21. The electrode of claim 19 , wherein the electrode active material composition further comprises at least one anti-foaming agent selected from ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, and decanol.

22. The electrode of claim 17 , wherein the electrode has a cohesive strength of about 7 gram force per millimeter or greater, and an adhesive strength of about 1 gram force per millimeter or greater.

23. A secondary battery comprising a first electrode, a second electrode, and an electrolyte disposed between the first electrode and the second electrode, wherein the first electrode is the electrode of claim 17 .

24. A method of preparing the binder according to claim 1 , the method comprising:

performing a polymerization reaction of the polyvinyl alcohol, the ethylenically unsaturated carboxylic acid, and the polymerizable monomer having a long-chain alkyl group to obtain a graft polymerization reaction product; and

reacting the graft polymerization reaction product with a monovalent metal compound to prepare a monovalent metal salt of a graft copolymer.

25. The method of claim 24 , further comprising reacting the monovalent metal salt of the graft copolymer with a divalent metal compound, a trivalent metal compound, or a combination thereof to thereby prepare a composite of the monovalent metal salt of the graft copolymer and a divalent metal salt, a composite of the monovalent metal salt of the graft copolymer and a trivalent metal salt, or a composite of the monovalent metal salt of the graft copolymer, a divalent metal salt, and a trivalent metal salt.

26. The method of claim 24 , wherein

the monovalent metal compound is lithium hydroxide, sodium hydroxide, potassium hydroxide, or a combination thereof, and

an amount of the monovalent metal compound is about 0.5 moles to about 3.0 moles with respect to 1 mole of the ethylenically unsaturated carboxylic acid.

27. The method of claim 25 , wherein

the divalent metal compound or the trivalent metal compound is calcium hydroxide, calcium acetate, magnesium acetate, cobalt nitrate hexahydrate, or aluminum acetylacetonate, and

an amount of the divalent metal compound or the trivalent metal compound is about 0.5 moles or less with respect to 1 mole of the ethylenically unsaturated carboxylic acid.

28. The method of claim 25 , wherein a total amount of the monovalent metal compound and one selected from the divalent metal compound and the trivalent metal compound is about 1 mole to 3 moles with respect to 1 mole of the ethylenically unsaturated carboxylic acid.