To provide a secondary battery with high reliability in which a stress in a wound body is alleviated as well as securing a sufficient capacity.
The wound body 20 of a secondary battery includes a positive electrode 21 and a negative electrode 22 laminated and wound with a separator 23 in-between and is housed inside a battery can 11 of a hollow parallelepiped shape. Further, it is wound along the winding direction R from the winding center side toward the winding outer circumference side and includes a flat shape being conformed to the shape of the battery can 11 so as to include a pair of flat portions 20S and a pair of curved portions 20R facing each other. The positive electrode current collector 21A is an aluminum alloy having a tensile strength of 90 N/mm2 or more. When the surface density of a positive electrode active material layer 21B located on a wound inner circumferential face side of the positive electrode current collector 21A is denoted by A and the surface density of the positive electrode active material layer 21B located on the wound outer circumferential face side of the positive electrode current collector 21A is denoted by B, the following formula (1): 0.35{A/(A+B)}0.45 is satisfied at the curved portions 20R.
Yasushi Santanzaki
Masao Hasebe
Takaaki Tanaami