Existing battery's energy density is already a long obstacle to prevent us from making lighter smaller EVs. And super capacitors energy densities are even lower[1]! Now with new materials like graphene or carbon nanotubes are promising, but they are definitely not ready for primetime in 2017 [2]!

[1] http://my.teslamotors.com/fr_CA/forum/forums/use-battery-pac... [2] https://en.wikipedia.org/wiki/Supercapacitor