Although Li-O-2 batteries can potentially achieve greater than two-fold higher energy densities than Li-ion batteries, the basic performance of Li-O-2 batteries remains poor. In particular, the large over potential of positive electrode reactions during the charging process results in low round-trip energy efficiency and limited cycle life, and is therefore the main barrier to the practical use of rechargeable Li-O-2 batteries. In the present study, we demonstrate that the charging performance of Li-O-2 batteries can be significantly improved by simply adding barium (Ba) ions into the electrolyte. Elemental analysis revealed that Ba-incorporated Li-O-2 was obtained as the main discharge product of a Li-O-2 cell operated in the presence of Ba2+. Notably, the improvement of charging performance was confirmed to originate from the Ba-incorporated Li-O-2 deposits, rather than the Ba2+ present in the electrolyte. The present results suggest that the incorporation of heteroatoms into the discharge product is an effective approach for improving the charging performance of Li-O-2 batteries. (C) 2017 Elsevier B.V. All rights reserved.