| abstract |
The present invention provides a polyester film that can be suitably used for battery exterior packaging, particularly a deep-drawn battery exterior that can be suitably used for a lithium ion secondary battery having an electrolyte (solid, liquid). For packaging purposes, the polyester film can also be suitably used for medical packaging applications that can correspond to various shapes. The polyester film is characterized in that the stress (F5 value) and the stress (F10 value) when the film is stretched by 5% in the longitudinal direction and the width direction of the film at 25° C. and the stress (F10 value) when stretched by 10% satisfy the following formula (I ), (II). 1.5≥F10 MD /F5 MD≥1 ...(I) 1.5≥F10 TD /F5 TD≥1 ...(II). |