Binder for secondary battery, negative electrode for secondary battery including the same, and lithium secondary battery including the same
Provided are a binder for a secondary battery, a negative electrode including the same, and a secondary battery including the same. More particularly, the binder for a secondary battery prepared by reacting a copolymer including specific repeating units and a crosslinking agent including two or more aldehyde groups has excellent mechanical properties and effectively improves a binding force. The negative electrode and the secondary battery including the binder for a secondary battery effectively suppress expansion of a negative electrode to manufacture a secondary battery having excellent charge/discharge cycle characteristics and battery performance.
1 . A binder for a secondary battery, the binder comprising a copolymer which is crosslinked by a crosslinking agent comprising an aldehyde group,
wherein the copolymer comprising a repeating unit (a) of the following Chemical Formula 1, a repeating unit (b) of the following Chemical Formula 2, a repeating unit (c) of the following Chemical Formula 3, and a repeating unit (d) of the following Chemical Formula 4,
wherein the copolymer satisfies the following Equation 2:
wherein
R 1 and R 3 are independently substituted or unsubstituted hydrocarbyl having 1 to 10 carbon atoms;
R 2 and R 4 are independently hydrogen or substituted or unsubstituted hydrocarbyl having 1 to 10 carbon atoms;
M n+ is a cation having an oxidation number of n except for a hydrogen ion; and
n is an integer of 1 to 3
0.55<( b+d )/( a+b+c+d )<0.9 [Equation 2]
wherein
a, b, c, and d are mole fractions of the repeating units (a), (b), (c), and (d), respectively in the copolymer.
2 . The binder for a secondary battery of claim 1 , wherein the crosslinking agent comprises two or more aldehyde groups.
3 . The binder for a secondary battery of claim 1 , wherein the crosslinking agent is represented by the following Chemical Formula 5:
wherein
R 5 is a single bond, or substituted or unsubstituted hydrocarbylene having 1 to 20 carbon atoms.
4 . The binder for a secondary battery of claim 1 , wherein the crosslinking agent is one or a combination of two or more selected from the group consisting of glyoxal, malondialdehyde, succinaldehyde, fumaraldehyde, glutaraldehyde, 2-methylglutaraldehyde, 3-methylglutaraldehyde, 2-hydroxyglutaraldehyde, adipaldehyde, pimelaldehyde, suberaldehyde, malealdehyde, sebacaldehyde, phthalaldehyde, isophthalaldehyde, and terephthalaldehyde.
5 . The binder for a secondary battery of claim 1 , wherein the crosslinking agent is glutaraldehyde.
6 . The binder for a secondary battery of claim 1 , wherein the copolymer and the crosslinking agent satisfy a weight ratio of 80 to 99.99:20 to 0.1.
7 . The binder for a secondary battery of claim 1 , wherein the copolymer has (a+b):(c+d) of 0.05 to 0.95:0.95 to 0.05, in which a, b, c, and d are mole fractions of the repeating units (a), (b), (c), and (d), respectively in the copolymer.
8 . The binder for a secondary battery of claim 1 , wherein c of the copolymer is 0.02 or more, in which c is a mole fraction of the repeating unit (c) in the copolymer.
9 . The binder for a secondary battery of claim 1 , wherein the copolymer is a linear polymer.
10 . The binder for a secondary battery of claim 1 , wherein the copolymer has a weight average molecular weight of 20,000 to 2,0000,000 Da.
11 . The binder for a secondary battery of claim 1 , wherein the binder for a secondary battery is a binder for a lithium secondary battery negative electrode.
12 . A secondary battery comprising: a positive electrode and a negative electrode for a secondary battery,
wherein the negative electrode for a secondary battery comprises a current collector; and a negative electrode active material layer disposed on the current collector, and the negative electrode active material layer comprises the binder for a secondary battery of claim 1 and a negative electrode active material.
13 . The secondary battery of claim 12 , wherein the negative electrode active material comprises a silicon-based active material.
14 . The secondary battery of claim 13 , wherein the negative electrode active material further comprises a graphite-based active material.
15 . The secondary battery of claim 14 , wherein a mass ratio between the silicon-based active material and the graphite-based active material is 3 to 97:97 to 3.
16 . The secondary battery of claim 12 , wherein the binder for a secondary battery is comprised at 0.5 to 30 wt % with respect to the negative electrode active material layer.
17 . A method of preparing a binder for a secondary battery, the method comprising the steps of:
step (A) saponifying a copolymer comprising a repeating unit (a) of the following Chemical Formula 1 and a repeating unit (c) of the following Chemical Formula 3 to prepare a saponified copolymer; and
step (B) mixing the saponified copolymer and a crosslinking agent comprising two or more aldehyde groups,
wherein the saponified copolymer comprises the repeating unit of the following Chemical Formula 1, a repeating unit (b) of the following Chemical Formula 2, the repeating unit (c) of the following Chemical Formula 3, and a repeating unit (d) of the following Chemical Formula 4,
wherein a degree of saponification in step (A) is more than 0.55 and less than 0.9, and the degree of saponification is calculated by (b+d)/(a+b+c+d), in which a, b, c, and d are mole fractions of the repeating units (a), (b), (c), and (d) in the saponified copolymer:
wherein
R 1 and R 3 are independently substituted or unsubstituted hydrocarbyl having 1 to 10 carbon atoms;
R 2 and R 4 are independently hydrogen or substituted or unsubstituted hydrocarbyl having 1 to 10 carbon atoms;
M n+ is a cation having an oxidation number of n except for a hydrogen ion; and
n is an integer of 1 to 3.
18 . The method of preparing a binder for a secondary battery of claim 17 , wherein the crosslinking agent is comprised at 0.1 to 10 wt % with respect to the binder for a secondary battery.