Batteries supply electrons by undergoing reversible chemical reactions. That has meant that all the reactants have to be inside the battery, which adds to its weight and volume. Lithium-air batteries could potentially change that situation. At one electrode, they have pure lithium metal rather than a lithium-containing chemical. At the other, the lithium reacts with oxygen in the air. When the battery is charged, this reaction is reversed, and the oxygen is returned to our atmosphere.