Power storage device packaging material and power storage device using the packaging material

A power storage device packaging material, comprising

• a structure including at least a substrate layer, an adhesive layer, a metal foil layer, a sealant adhesive layer, and a sealant layer laminated in this order, wherein
• the substrate layer is formed of a polyester film exhibiting ΔA, as expressed by the following Formula (1), of 10% or more and a 50% elongation stress of 75 MPa or more after heat treatment at 160° C.:
• ΔA=(break elongation after 160° C. heat treatment)−(break elongation before 160° C. heat treatment) (1)
• wherein the break elongation and the 50% elongation stress are each an average of tensile tests at 23° C. (test piece shape:
• dumbbell No. 5 according to JIS K7127, length between chucks:
• 75 mm, gauge length:
• 25 mm, test speed:
• 50 mm/min) in four directions (0° (MD), 45°, 90° (TD), 135°) of the substrate layer.