Atomistic Modeling of the Charge Process in Lithium/Air Batteries
T Dabrowski and LC Ciacchi, JOURNAL OF PHYSICAL CHEMISTRY C, 119, 25807-25817 (2015).
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 O-2 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.
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