Chris Lee, writing for Ars Technica:
The performance of the current generation of lithium ion batteries is about to hit a wall, and if we want batteries with higher energy densities, an entirely new system will have to be developed. Among the many possible candidates, lithium-air batteries look very promising. When lithium oxidizes, it releases a lot of energy—so much so that, like sodium, it catches on fire. Lithium is also very light and reasonably abundant, making it the perfect element.
Keep an eye on this technology. Lithium-air batteries could potentially have ten times the energy density per kilogram compared to Lithium-ion batteries, the current star battery technology.