Reactions and SEI Formation during Charging of Li-O₂ Cells

In this letter we combine detailed electrochemical impedance measurements with quantitative measurements of O2 evolution and Li2O2 oxidation to describe the charge mechanisms during charge of Li-O2 batteries with porous carbon electrodes. We identify Li2O2 oxidation at 3.05 V and an apparent chemical formation of a solid electrolyte interface (SEI) layer as the first monolayer of Li2O2 is oxidized, leading to a voltage increase. The first electrochemical degradation reaction is identified between 3.3 V and 3.5 V, and the chemical degradation is limited above 3.5 V, suggesting that a chemically stable SEI layer has been formed.