Study of the Electronic Structure of Alkali Peroxides and Their Role in the Chemistry of Metal–Oxygen Batteries

[Image: see text] We use a multiconfigurational and correlated ab initio method to investigate the fundamental electronic properties of the peroxide MO(2)(–) (M = Li and Na) trimer to provide new insights into the rather complex chemistry of aprotic metal–O(2) batteries. These electrochemical systems are largely based on the electronic properties of superoxide and peroxide of alkali metals. The two compounds differ by stoichiometry: the superoxide is characterized by a M(+)O(2)(–) formula, while the peroxide is characterized by [M(+)](2)O(2)(2–). We show here that both the peroxide and superoxide states necessarily coexist in the MO(2)(–) trimer and that they correspond to their different electronic states. The energetic prevalence of either one or the other and the range of their coexistence over a subset of the MO(2)(–) nuclear configurations is calculated and described via a high-level multiconfigurational approach.