Oxide supersedes peroxide

Science 361, 777–781 (2018)

Under ambient conditions, the mechanism of operation for the oxygen electrode of Li–O₂ batteries is dominated by the reduction of O₂ to lithium peroxide (Li₂O₂) during discharge, and by the oxidation of Li₂O₂ during charge. Unfortunately, Li₂O₂ is insoluble and it can clog the oxygen electrode, which is typically made of porous carbon. Additionally, the ionic and electronic conductivities of Li₂O₂ are low and it is highly reactive towards the organic electrolytes that are typically used in Li–O₂ batteries. These factors all have deleterious effects on the activity of the carbon electrode and the organic electrolyte, and thus the battery cyclability. Now, Linda Nazar and colleagues from the University of Waterloo report a Li–O₂ battery with highly reversible formation of a different discharge product, lithium oxide (Li₂O), avoiding the problems associated with Li₂O₂.

In their battery, the researchers use a Ni-based composite catalyst consisting of lithiated nickel oxides on the surface of Ni nanoparticles (Ni/Li_xNiO₂) at the oxygen electrode, a molten nitrate electrolyte (LiNO₃ and KNO₃) that has been previously shown to be chemically stable, and a Li anode protected by a solid-electrolyte membrane. Because oxygen reduction to Li₂O₂ is thermodynamically more favourable than oxygen reduction to Li₂O at ambient conditions, an elevated operating temperature (150 °C) is applied. The researchers show that during discharge Li₂O₂ forms first, but that Ni/Li_xNiO₂, with the aid of the inorganic electrolyte and the elevated temperature, catalyses the breaking of the O–O bond in Li₂O₂, quickly converting it to Li₂O. During charge, direct oxidation of Li₂O takes place without involving the Li₂O₂ intermediate. Stable discharge–charge up to 150 cycles is demonstrated.