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Rechargeable lithium batteries represent one of the most important developments in energy storage for 100 years, with the potential to address the key problem of global warming. However, their ability to store energy is limited by the quantity of lithium that may be removed from and reinserted into the positive intercalation electrode, Li_xCoO₂, 0.5 <x <1 (corresponding to 140 mA·h g^-1 of charge storage). Abandoning the intercalation electrode and allowing Li to react directly with O₂ from the air at a porous electrode increases the theoretical charge storage by a remarkable 5−10 times! Here we demonstrate two essential prerequisites for the successful operation of a rechargeable Li/O₂ battery; that the Li₂O₂ formed on discharging such an O₂ electrode is decomposed to Li and O₂ on charging (shown here by in situ mass spectrometry), with or without a catalyst, and that charge/discharge cycling is sustainable for …