PolyPlus is working on the development of lithium metal-air based batteries, these batteries promise an energy density of more than 10 times that of today's lithium-ion batteries, while being a lot lighter as well. The Berkely, CA company is currently working on single-use lithium metal-air batteries for the government, these batteries may be available within a few years. Rechargeable versions are still in early stages of development. More details about these batteries can be found at TechnologyReview.
In lithium-ion batteries, the electrodes are made of materials such as graphite, while in a lithium-metal battery, the anode is made up entirely of lithium metal, and the surrounding air can act as the cathode.
Lithium-metal batteries approach the energy density of fuel cells without the plumbing needed for these devices; in theory, the maximum energy density is more than 5,000 watt-hours per kilogram, or more than 10 times that of today's lithium-ion batteries. Lithium metal-air batteries are also very lightweight because it's not necessary to carry a second reactant. Lithium metal is "the holy-grail battery material," says Steven Visco, chief technical officer and founder of PolyPlus.