Separator for lithium ion secondary battery

A separator, used for a lithium ion secondary battery that produces electromotive force by lithium doping/dedoping, said separator

• being composed of a porous film comprising an organic polymer (component A) which can swell in an electrolyte solution to retain the electrolyte solution, surrounding a nonwoven fabric which cannot swell in the electrolyte solution, said porous film including an electrolyte solution non-swelling organic polymer with a melting point of 210° C. or above (component B) and an electrolyte solution non-swelling organic polymer with a melting point of 180° C. or below (component C),
• wherein component B is incorporated in the fiber composing the nonwoven fabric,
• wherein said nonwoven fabric is a nonwoven fabric formed of component B and component C, and
• wherein the fiber composing said nonwoven fabric is a core-sheath fiber with component B as the core and component C as the sheath.