Cap assembly for lithium ion secondary battery

A cap assembly (100, 200) for a lithium ion secondary battery (1000, 2000). The cap assembly includes a cap plate (30, 97), a leakproof film (20, 98), and a cap cover (10, 99). The cap plate (30, 97) includes a supporting portion (31, 971) and a safety film (32, 972). The safety film is positioned in the supporting portion (31, 971). The safety film (32, 972) defines safety grooves (33, 973). The safety film (32, 972) is capable of break down along the safety grooves (33, 973). The leakproof film (20, 98) is attached on the supporting portion (31, 971). The leakproof film (20, 98) is made of a material with high temperature resistance and elasticity properties. The cap cover (10, 99) is attached on the leakproof film (20, 98).