The article Homer linked to states that the super capacitors are not intended as primary storage but just to handle the big current surges during accellerating and braking. It makes sense to use super capacitors there in order to prolong the life of the Li-ion batteries. I don't see super capacitors as a primary energy storage any time soon because their energy density is way to low.
Plus in all the work I've done with them (solar race cars), they are invaluable for heavily dampening the HUGE switchmode / ripple that exists.
I've measured up to 30V across a very wide bandwidth. Most battery chemistries only go to ~100Hz, maybe 200Hz before they start going HiZ.
2 other issues with Supercap only -
1/ Voltage output is linear with capacity - so when they are 50% depleted, you are 50% down on Vop. A disaster !!
With Lithium, we usually take Vref as 3.6-3.7V, which is ~85% of capacity. If you have a 200-300Km range, then you can push that to ~90%
3.8V-4.2V is only ~2-3%, depending on C. The rest is below 3.6V, and can be used safely, if you derate C load (I create a load profile for each pack).
2/ Given that the max voltage of a supercap is ~3.0V, the more you put in series the more "equalization current" you need across each cap !!
Unlike Li, which can hold equalization for months (again depending on C rates), these MUST be connected all the time.
By my calculations, the current volumetric ratio (total energy) is ~20X, based on what is actually available now.
They definitely have a place, but not as a complete replacement, not for a long time.