A binder for battery comprising composite polymer particles in which two or more polymers having different chemical structures form a heterogeneous phase structure. Preferably, said two or more polymers include two polymers having a glass transition temperature (Tg) different from each other by 5.degree. C. or more. When a slurry prepared by mixing the binder with a liquid material and an active material is used for the production of an electrode for a secondary battery, especially, a lithium secondary battery, the resulting battery exhibits an improved performance because the slurry imparts a well-balanced binding power and a binding durability.