To enhance capacity of a battery, and to improve a charge/discharge characteristic by cooling a binding agent polymer at a specific speed after being melted in N-methyl-2-pyrolidone at a specific temperature for a specific time by heating.

A binding agent polymer uses a dicarboxylic acid expressed by formula I, diol expressed by formula II and polyamide having reduced viscosity of 0.5 to 7 dL/g as constitutive components, and is composed of 100 pts.wt. of polyester amide oligomer including two or more of functional groups selected from a carboxyl group, an amino group and a hydroxyl group in a molecular chain and 5 to 50 pts.wt. of diisocyanate expressed by formula III or IV. In the formula I to IV, R1 represents an alkylene group having the carbon number 2 to 8, R2 represents an alkylene group having the carbon number 2 to 6, and R3 represents an alkylene group or a phenylene group having the carbon number 2 to 15. After melting this binding agent polymer in N-methyl-2-pyrolidone at 140 to 180°C for 1 to 3 hours by heating, the binding agent polymer is cooled at a speed of 2 to 10°C/hour while agitating the binding agent polymer, and a temperature is lowered.