Another problem with Li-ion cars is that, apart from being very heavy, the batteries spectacularly explode in the event of a fire (such as a car-crash).

Keep in mind that gasoline cars have a well-documented history of explosion risk. It's not all that clear why batteries would be more dangerous than gallons of gasoline. The amount of energy involved is far lower (a fully-charged EV battery has 50kWh, which is the energy equivalent of just 1.5 gallons of gasoline), and it should be much simpler to engineer safety systems around non-moving solid objects than around a constantly-circulating fluid.

You're making a claim that you don't have nearly enough evidence to back up. You're stating as fact something that has never actually happened. Moreover, you're thinking of thermal runaway, which has caused some laptop batteries to burst into flames. That's got nothing to do with external shock (like a car crash), and is generally a problem with faulty manufacturing rather than external heat.

EDIT: it seems like the battery chemistries which are being considered for the Volt are intrinsically safer, and material properties (such as cells that tear rather than exploding under pressure) are being used to lower risk.