lithium-air

Research is underway on a new class of rechargeable powerpack known as a lithium-air or lithium-oxygen battery. These devices could store more than 10 times the energy found in current lithium-ion batteries, which would greatly extent the time required between recharging cycles. Because lithium-air uses freely available oxygen as a cathode reactant, the weight of the battery significantly decreased.

These power cells could even beat the energy density of hydrogen fuel cells, making one efficient lithium-air car battery able to store the equivalent energy of a tank of gasoline. The technology is not yet commercially available, although it's actively being developed in several places.