LiO₂: Cryosynthesis and Chemical/Electrochemical Reactivities

The reduction of O₂ to solid Li₂O₂, via the intermediates O₂^- and LiO₂, is the desired discharge reaction at the positive electrode of the aprotic Li-O₂ batteries. In practice, a plethora of byproducts are identified together with Li₂O₂ and have been assigned to the side reactions between the reduced oxygen species (O₂^-, LiO₂, and Li₂O₂) and the battery components (the cathode and electrolyte). Understanding the reactivity of these reduced oxygen species is critical for the development of stable battery components and thus high cycle life. O₂^- and Li₂O₂ are readily available, and their reactivities have been studied in depth both experimentally and theoretically. However, little is known about LiO₂, which readily decomposes to Li₂O₂ and is thus unavailable under usual laboratory conditions. Here we report the synthesis and reactivity of LiO₂ in liquid NH₃ at cryogenic temperatures and conclude that LiO₂ is the most reactive oxygen species in Li-O₂ batteries.