Atomistic Modeling of the Charge Process in Lithium/Air Batteries

We present a combined classical and density functional theory (DFT) based Molecular Dynamics (MD) study of the mechanisms underlying the oxygen evolution reactions during the charging of lithium/air batteries. As models for the Li₂O₂ material at the cathode we employ small amorphous clusters with a 2:2 Li:O stoichiometry, whose energetically most stable atomic configurations comprise both O atoms and O–O pairs with mixed peroxide/superoxide character, as revealed by their bond lengths, charges, spin moments, and densities of states. The oxidation of Li₈O₈ clusters is studied in unbiased DFT-based MD simulations upon removal of either one or two electrons, either in vacuo or immersed in dimethyl sulfoxide solvent molecules with a structure previously optimized by means of classical MD. Whereas removal of one electron leads only to an enhancement of the superoxide character of O–O bonds, removal of two electrons leads to the spontaneous dissolution of either an O₂ or a LiO₂⁺ molecule. These results are interpreted in terms of a two-stage process in which a peroxide-to-superoxide transition can take place in amorphous Li₂O₂ phases at low oxidation potentials, later followed by the dissolution of dioxygen molecules and Li⁺ ions at higher potentials