just to point out, ultra capacitors, tech isn't quite there yet, comercially at 30Wh/kg and in the lab at 155Wh/kg, vs 100-250Wh/kg for lithium ion. however they have the potential to store much more (wiki has a figure of 6KWh/kg @95% efficiency)

but they have several advantages over batteries. they can dump a lot of charge quickly without getting hot - If I remember correctly tesla have a special water cooling setup on the lithium-ion set up. adding weight to the overall setup (450kg for a 63KWh battery), on top of that they have a limited rate of charging (again related to heat). as already mentioned lithium-ions wear out, whereas ultracapacitors can to thousands of charge/discharge cycles with little degradation. and more importantly, you can dump juice into them quickly. so make refilling them possible quite easy.

did a post on this before here

just for reference, theres the wikipage (not accessible in a few hours)
the 155Wh/kg figure is from this article