Formation of Li3O4 nano particles in the discharge products of non-aqueous lithium–oxygen batteries leads to lower charge overvoltage

Abstract

Density functional theory calculations are made for bulk thermodynamic properties and surface energies of Li₂O₂, a primary discharge product, and Li₃O₄, a possible byproduct in the discharge products, of the non-aqueous lithium–oxygen batteries. Results show that the standard formation Gibbs free energy of bulk Li₃O₄ is marginally higher than that of Li₂O₂, but the surface energy of Li₃O₄ is much lower. Low surface energy results in both lowered nucleation energy and formation Gibbs free energy in the nanometer regime, allowing the Li₃O₄ nano particles to nucleate ahead of Li₂O₂ during the discharge process and to exist stably when particle sizes are smaller than about 40 nm. The scanning transmission electron microscopy (STEM) image of Li₃O₄ crystals is simulated and compared with the measured STEM image of the discharge product particles. The consistency between the simulated and measured STEM images suggests that the Li₃O₄ phase can exist stably as a discharge product. The energy profile of the oxygen evolution reaction (OER) occurring on the most abundant surfaces of Li₃O₄ is also calculated. The predicted overpotential for the OER on the {0001} surface (0.30 V) shows a good agreement with experimental data. The presence of more electronically conductive Li₃O₄ nano particles in the primary discharge product Li₂O₂ tends to decrease the charge overvoltage of the batteries, explaining why the lower voltage area (<3.5 V) was widely observed during the charging of the batteries. An increase in the oxygen pressure or a decrease in temperature enhances the stability of the Li₃O₄ phase and increase the proportion of the Li₃O₄ phase in the discharge products, consequently leading to a lower overall charge overvoltage.