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.