A cover assembly includes a cover plate including a gas vent, a gas exhaust valve configured to seal the gas vent and deform in response to an increase in temperature to get out of a state of sealing the gas vent, and a sealing part. The gas exhaust valve includes a metal sheet and a fixing part. The metal sheet seals the gas vent. The sealing part is arranged between the metal sheet and the cover plate for sealing a gap between the metal sheet and the cover plate. The fixing part is connected to the metal sheet. The metal sheet is fixed to the cover plate by the fixing part. The fixing part is configured to deform in response to an increase in temperature in such a manner that the metal sheet gets out of the state of sealing the gas vent.