Reversibility of anodic lithium in rechargeable lithium–oxygen batteries

Non-aqueous lithium–air batteries represent the next-generation energy storage devices with very high theoretical capacity. The benefit of lithium–air batteries is based on the assumption that the anodic lithium is completely reversible during the discharge–charge process. Here we report our investigation on the reversibility of the anodic lithium inside of an operating lithium–air battery using spatially and temporally resolved synchrotron X-ray diffraction and three-dimensional micro-tomography technique. A combined electrochemical process is found, consisting of a partial recovery of lithium metal during the charging cycle and a constant accumulation of lithium hydroxide under both charging and discharging conditions. A lithium hydroxide layer forms on the anode separating the lithium metal from the separator. However, numerous microscopic ‘tunnels’ are also found within the hydroxide layer that provide a pathway to connect the metallic lithium with the electrolyte, enabling sustained ion-transport and battery operation until the total consumption of lithium.