Critical Factors Controlling Superoxide Reactions in Lithium-Oxygen Batteries

Superoxide (LiO2) formation on charging is one of the major causes of poor cycle life of Li-O-2 batteries. The underlying mechanism controlling LiO2 formation remains elusive. Here, we reveal that the formation of LiO2 is mainly controlled by oxidation potential and solvent donicity, which regulates the rates of LiO2 formation and consumption via electrochemical oxidation and disproportionation. We quantify LiO2 in the electrolyte under a wide range of charging potentials via rotating-ring disk chronoamperometry and reveal that the amount of LiO2 decreases with increasing charging potential and decreasing solvent donicity. X-ray absorption near-edge structure spectroscopy results support the formation of LiO2 at low charge potentials. Li2CO3 is observed along with LiO2 formation, indicating severe side reactions resulting from LiO2. Our study reveals the underlying mechanism controlling LiO2 formation on charging Li-O-2 batteries and highlights that strategies which simultaneously reduce charge potential and bypass LiO2 (e.g., redox mediator) should be developed to enable efficient and reversible Li-O-2 batteries.