Li-air batteries are the subject of intense investigations due to their potentially high gravimetric energy density. While it is generally accepted that lithium peroxide is formed during discharge, the exact mechanism of formation of the discharged product is not completely understood. Understanding the mechanism is important as it controls the shape and size of lithium peroxide particle formation, and thus, influences the overall cell performance. While the exact morphology of the lithium peroxide formed during discharge of the cell has not been determined, the growth likely involves nanoparticles and possibly agglomerates of nanoparticles. In this presentation, we report on density functional theory calculations of stoichiometric lithium superoxide clusters that provide evidence for the influence of LiO2 on the shape of the discharged product. In particular, our calculations suggest that (LiO2)n clusters are stabilized in planar ring configurations. While current investigations are restricted for cluster sizes up to n = 12, initial trends indicate that the larger clusters may have stacked-ring configurations. The clusters exhibit high spins and superoxide-like O�O and Li�O bond distances. Typical energy barriers for the removal of oxygen molecule from these clusters is ~1 eV, suggesting that (LiO2)n rings may not be just transient intermediates, but exist long enough to agglomerate into large lithium superoxide nanoparticles that influence the morphology of the discharge product in Li-air batteries. These results have been confirmed by higher level G4MP2 calculations that also indicate superior thermodynamic stability of the ring-shaped structures and high activation barriers for dissociation of oxygen from the clusters.

Electronic structure of lithium superoxide clusters and their role in growth of discharge product in Li-air batteries

247th National Meeting (2014)

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