Binder for secondary battery, negative electrode for secondary battery including the same, and lithium secondary battery including the same

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
• R1 and R3 are independently substituted or unsubstituted hydrocarbyl having 1 to 10 carbon atoms;
• R2 and R4 are independently hydrogen or substituted or unsubstituted hydrocarbyl having 1 to 10 carbon atoms;
• Mn+ is a cation having an oxidation number of n except for a hydrogen ion;
• and
• n is an integer of 1 to 3.