First Principles Study of Hybrid Li-Ion/Li-O2 Battery Materials

Using first principles density functional theory (DFT) calculations, we explore a class of materials which facilitate both Li-ion and Li-O₂ reactions. These "hybrid" materials contain high lithium and oxygen contents together with a transition-metal oxide component. The prototype hybrid Li-ion/Li-O₂ material is Li₅FeO₄ which, when used as an electrocatalyst in Li-O₂ cells, reduces overpotentials and demonstrates a high capacity of ~500 mAh/g unrelated to Li₂O₂ formation. While hybrid materials exhibit promising capacities of up to 1000 mAh/g, their reaction mechanisms remain unknown. We performed DFT investigations of the reaction mechanisms in several of these materials, including Li₅FeO₄, Li₆MnO₄, Li₆CoO₄, and Li₈IrO₆. DFT calculations are performed to investigate the thermodynamics of Li and Li-O removal from these materials, in order to establish the details of the charge reaction. Atomistic and electronic structural changes in the materials are tracked during the reactions. Results on the energetics indicate a cross-over between pure lithium removal and concomitant oxygen loss, which is electronically shown to be correlated with development of the peroxide-like character of oxygen.