Secondary battery

A secondary battery comprising:

• an electrode assembly including a first electrode sheet and a second electrode sheet, the first electrode sheet and the second electrode sheet having different polarities;
• and
• a first electrode current collector electrically connected to the first electrode sheet,
• wherein the first electrode sheet includes a first electrode core and a first electrode active material layer on the first electrode core,
• the first electrode core is made of copper or a copper alloy,
• the first electrode current collector is made of copper or a copper alloy,
• the electrode assembly includes a first electrode core laminate of the first electrode core,
• the first electrode core laminate is bonded to the first electrode current collector by ultrasonic bonding,
• a core recess is formed in a bonding region of the first electrode core laminate bonded to the first electrode current collector by ultrasonic bonding,
• a region of the first electrode core laminate in which the core recess is formed includes a solid-state bonding layer and a central layer, the solid-state bonding layer being formed by solid-state bonding of an interface between layers of the first electrode core, the central layer being disposed between the solid-state bonding layers formed on both faces of the first electrode core, and
• metal crystal grains constituting the solid-state bonding layer have a first average grain size smaller than a second average grain size of metal crystal grains constituting the central layer.