The material and thickness of your battery tab may mean that slits are required in order to optimise the welding result.
When batttery tabs are too thick, having no slit can result in the welding current bypassing the desired weld area and travelling through the tab and back up into the other electrode. This can be observed in the left-hand image.
The inclusion of a slit in your battery can improve your welding results by directing the current through the welding area. The slit in effect, makes the path of least resistance through the desired weld area. This can be observed on the image on the right-hand side.