Lithium cells are prone to outgassing, bursting and catching fire when the CHARGER malfunctions. (They do not emit steam, however.) This bursting hazard is a serious problem for Li-Ion and Li-Polymer cells. We model airplane builders use Li-Polymer cells a lot, because they are very light and have a high energy density. But we have to use great care - many cars and garages have been set alight by the little widgets.
They are NOT suitable for a car, and not because of the fire hazard. The fact is, their performance is drastically reduced by low temperatures, so unless you live in the South, you would not like them. They also require a very slow charge cycle - fast charging is a fiery no-no.
Prius uses Nickel-Hydride cells. Heavier, but a very reliable technology. Disposal is a problem - they are considered Haz-Mat when spent, so must be collected and recycled.
Latest tech is Lithium Iron (A123) and Lithium Manganese, (Saphion) cells, both of which are proving VERY capable, have little or no fire hazard, and can be recharged very fast. You can already buy them in battery packs for portable power tools. A little heavier per unit of power than Li-Polymer, but safe. THESE you may find in cars pretty soon.
Interesting to know. What is their half-life for energy output assuming a daily recharge?