Lithium ion batteries are the current state of the art in battery technology, but they have two problems:

1) They have a nasty habit of exploding

2) They are relatively expensive

Now there may be a better technology on the horizon:

Rechargeable zinc-air batteries can store three times the energy of a lithium-ion battery

A Swiss company says it has developed rechargeable zinc-air batteries that can store three times the energy of lithium ion batteries, by volume, while costing only half as much. ReVolt, of Staefa, Switzerland, plans to sell small “button cell” batteries for hearing aids starting next year and to incorporate its technology into ever larger batteries, introducing cell-phone and electric bicycle batteries in the next few years. It is also starting to develop large-format batteries for electric vehicles.

More info from RevoltTechnologies.com: Rechargeable Zn-air Batteries

See also these zinc-air batteries used in a bus:

More info: Electric Vehicle Division

[[[Jump to previous invention - new muscle/computer interface will let you type without a keyboard and play a real air guitar]]]