Lithium ion batteries don't actually contain that much lithium by mass. Lots of water is indeed the recommended approach to dealing with Li-ion battery fires.

Lithium ion batteries don't actually contain that much lithium by mass. Lots of water is indeed the recommended approach to dealing with Li-ion battery fires. The lithium in the batteries is far less of a concern than the magnesium used elsewhere in the car. If that gets hot enough to ignite, then you're going to have a very hard time putting it out (though the approach to burning magnesium in car fires (which is actually pretty common) is still water, water, and more water).