Secondary battery

A secondary battery, comprising:

• an electrode assembly;
• an electrode lead attached to the electrode assembly;
• a case configured to receive the electrode assembly therein;
• a lead film configured to surround a part of an outer surface of the electrode lead, the lead film being interposed between the electrode lead and the case;
• a vent region formed in at least a part of the case;
• and
• a vent member inserted into the vent region, wherein the vent member contains linear low-density polyethylene having a poly dispersity index (PDI) of from 2.6 to 4,
• wherein the vent member has a maximum sealing strength of less than 6 kgf/15 mm at from 100° C. to 120° C. and a maximum sealing strength of 6 kgf/15 mm or more at room temperature to 60° C.,
• wherein the case includes a sealing portion formed to seal the electrode assembly, the sealing portion contains a sealant resin, and the linear low-density polyethylene has a lower melting point than the sealant resin,
• wherein a difference between a crystallization temperature of the sealant resin and a crystallization temperature of linear low-density polyethylene is 10° C. or less,
• wherein the linear low-density polyethylene is made by polymerizing in the presence of a metallocene catalyst, and wherein the linear low-density polyethylene has a crystallization temperature of 90° C. to 115° C.