Panasonic just makes the cells, Tesla makes the batteries. Soon Panasonic will be making all the cells under Tesla's roof.

Tesla has hired Li-Ion specialist Jeff Dahn, so at a minimum Tesla is involved is prescribing the additives to their cells to give them the unique properties that Tesla wants.

Tesla has also dictated that the cells have a unique form factor, "2170" instead of the 18650 standard, and most automotive OEM's used prismatic cells instead of round anyway.