Secondary battery

A secondary battery comprising:

• an electrode assembly to which an electrode lead is bonded;
• a pouch case including a case body portion and a cover portion covering the case body portion to form a cavity accommodating the electrode assembly in such a manner that a portion of the electrode lead protrudes externally of the pouch case;
• and
• a sealing portion in which outer circumferential portions of the case body portion and the cover portion of the pouch case contact each other and are sealed by thermal fusion, the sealing portion is positioned between the cavity and an exterior of the pouch case in the outer circumferential portions,
• wherein a polymer compound layer is attached on at least one of an upper surface and a lower surface of the electrode lead,
• the polymer compound layer includes a thermally expandable polymer compound of a composite of a hydroxy group-containing compound and silica;
• or at least one heat-shrinkable polymer compound selected from the group consisting of polyphenylene ether (PPE), polycarbonate (PC), polyoxymethylene (POM) and polyamide (PA),
• the polymer compound layer has a first end located inside of the cavity, and a second end located inside or outside of the sealing portion, and
• the polymer compound layer is configured to expand or contract in response to increased temperature and break the sealing portion to form a passage for gas discharge from the cavity.