Protection method and device for secondary battery, secondary battery, battery module, battery pack, and electric apparatus

A protection method for a secondary battery comprising an injection opening, the method comprising:

• step (1):
• evacuating the secondary battery through the injection opening to within 100 kPa, optionally to within 100 Pa;
• step (2):
• introducing a first reactant into the secondary battery through the injection opening until a pressure in the secondary battery rises by more than 1 Pa, optionally more than 1 kPa, and maintaining the pressure for more than 1 ms, optionally for 0.1s-600s;
• and optionally
• step (3):
• evacuating the secondary battery through the injection opening to within 100 kPa again, optionally to within 100 Pa;
• and
• step (4):
• introducing a second reactant into the secondary battery through the injection opening until the pressure rises by more than 1 Pa, optionally more than 1 kPa, and maintaining the pressure for more than 1 ms, optionally for 0.1s-600s;
• wherein the second reactant is capable of reacting with the first reactant to form a protection layer on an internal surface of the secondary batter battery.