Cap assembly, secondary battery including cap assembly, and method of manufacturing cap assembly

A secondary battery comprising:

• a case having an opening formed therein;
• an electrode assembly accommodated in the case, the electrode assembly comprising a first electrode, a separator, and a second electrode;
• and
• a cap assembly sealing the opening of the case,
• wherein the cap assembly comprises:
• an upper cap;
• a vent plate disposed below the upper cap, the vent plate being deformable in response to pressure change inside the case, the vent plate comprising a vent protruding downward;
• a lower cap disposed below the vent plate, with a through hole being formed in the lower cap, and with the vent being inserted into the through hole;
• a sub-plate disposed below the lower cap and joined to the vent and the lower cap to electrically connect the vent plate and the lower cap;
• and
• an insulator disposed between the vent plate and the lower cap,
• wherein at least a portion of a surface of the lower cap in contact with the insulator is surface treated to have a greater roughness than another surface of the lower cap.

A method of manufacturing a cap assembly, the method comprising:

• surface treating at least a portion of a surface of a lower cap that is configured to contact with an insulator such that the portion of the surface has a greater roughness than another surface of the lower cap;
• disposing the insulator above the lower cap;
• disposing a vent plate above the insulator so that a vent of a vent plate is inserted into a through hole formed in the lower cap;
• connecting a sub-plate to a lower portion of the lower cap and the vent so that the lower cap and the vent are electrically connected by the sub-plate;
• and
• disposing a upper cap above the vent plate and joining the upper cap and the vent plate.