Method for producing a lithium ion secondary battery

A method for producing a lithium ion secondary battery in which a positive electrode, a heat-resistant insulating layer provided separator having a heat-resistant insulating layer formed of oxide particles on one surface of a resin porous substrate, and a negative electrode are laminated on one another, and a nonaqueous electrolyte is impregnated in the heat-resistant insulating layer provided separator, comprising:

• drying the heat-resistant insulating layer provided separator before laminating so that a water content in the heat-resistant insulating layer provided separator remains in a predetermined range,
• wherein, in the drying, the water content in the heat-resistant insulating layer provided separator is decreased to the predetermined range by controlling a dew point and maintaining the predetermined range after reaching the predetermined range of the water content, and
• wherein, in the drying, the heat-resistant insulating layer provided separator is dried by setting the dew point so as to attain a water content decrease rate such that a shape of the heat-resistant insulating layer provided separator after the drying defined by a length ratio Y that is a ratio of a length of the resin porous substrate to a length of the heat-resistant insulating layer satisfies an equation:
• 1−4πX≤Y≤1+4πX,
• wherein D represents a thickness of the separator, L represents a length of the separator, and X represents a ratio (D/L) of the thickness D to the length L of the separator,
• wherein, in the drying, the water content is decreased such that a water decrease rate ((1−W)/T) satisfies an equation:
• ((1−W)/T)≤1.2, and
• wherein W is a water content ratio W=w2/w1, T is a drying time of 0.5 hour, w1 is a water content before starting the drying, and w2 is a water content that is measured when T elapses after starting the drying.