Al writes "A company called PolyPlus has developed lithium metal-air batteries that have 10 times the energy density of regular lithium-ion batteries. The anode is made up entirely of lithium metal, and the surrounding air acts as the cathode, making the batteries incredibly energy dense. Previous efforts to make lithium metal batteries have been stymied by the sensitivity of lithium to water in the air. The new batteries use a sophisticated membrane to protect the lithium anode and PolyPlus has even created a version that works underwater, by drawing oxygen through the membrane. Lithium metal-air batteries could be light-weight power sources for demand for plug-in hybrid vehicles and consumer electronics; IBM also recently announced that it would develop lithium metal-air batteries for the energy grid and for transportation."