Stability of Lithium Superoxide LiO2 in the Gas Phase: Computational Study of Dimerization and Disproportionation Reactions

Knowledge of the precise molecular mechanisms during the discharge and recharge processes in the lithium-air battery is critical for achieving desired improvements in specific capacity, current density, and cyclability. The initial oxygen reduction product formed in the presence of Li+ ions is lithium superoxide LiO2. In this study, we report the computed structures and thermodynamic parameters of LiO2 dimerization in the gas phase, which enables us to provide a baseline for the reaction free energy profile of the subsequent disproportionation of (LiO2)(2) to lithium peroxide Li2O2 and O-2. Our calculations identified several low-lying (LiO2)(2) dimers, with the singlet bipyramidal structure giving IR bands that are consistent with the characteristic IR vibration frequencies of (LiO2)(2) in the oxygen matrix at T = 15-40 K. The activation barrier for (LiO2)(2) = Li2O2+O-2 is 10.9 kcal/mol at the UCCSD(T)/CBS level (T = 298 K), suggesting that in the gas phase LiO2 and its aggregates could only be observed at low temperatures.