Binder composition for lithium ion secondary battery positive electrode, slurry composition for lithium ion secondary battery positive electrode, positive electrode for lithium ion secondary battery and lithium ion secondary battery

A binder composition for a lithium ion secondary battery positive electrode including a copolymer containing a nitrile group-containing monomer unit and a conjugated diene monomer unit and a solvent, wherein,

• a degree of swelling in electrolysis solution of the copolymer is at least 200 mass % and no greater than 700 mass %,
• a storage elastic modulus of a binder coating obtained through a film formation of the binder composition is at least 1×104 Pa and no greater than 1×109 Pa, and
• the copolymer includes an alkylthio group of a carbon number from 12 to 16 having at least three tertiary or higher carbon atoms and a sulfur atom directly bonded to at least one of the tertiary or higher carbon atoms.