But there is probably a hard ceiling to the efficiency of safe and cost effective Li-ion batteries. We can't be entirely sure because we don't fully understand how Li-ion batteries work, but my best layman's educated guess is that those are close to tapped out as well.

The strength of the battery+motor electric drivetrain is that it's modular. Come up with another energy storage method, build it into a power pack, then plug it right into existing Tesla S cars, get the over the air software update, and you're back in business. There's also all of that frunk space.