We use a multiconfigurational and correlated ab initio method to investigate the fundamental electronic properties of the peroxide MO2- (M = Li and Na) trimer to provide new insights into the rather complex chemistry of aprotic metal-O2 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+O2- formula, while the peroxide is characterized by [M+]2O22-. We show here that both the peroxide and superoxide states necessarily coexist in the MO2- 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 MO2- nuclear configurations is calculated and described via a high-level multiconfigurational approach.