Apparatus and method for welding single cells for the production of electrical energy storage devices

A welding apparatus for welding single cells stacked together to define a conductive cell for the production of electrical energy storage devices, each single cell including at least one first electrode, a second electrode and at least one separator film interposed between the first and second electrode, the welding apparatus having at least one first ultrasonic welding member and a second ultrasonic welding member, each configured to emit ultrasound in counterphase relative to the other.