Density Functional Investigation of the Thermodynamic Stability of Lithium Oxide Bulk Crystalline Structures as a Function of Oxygen Pressure

Density functional theory is used together with classical statistical mechanical analyses to investigate the thermodynamic stability of bulk crystalline LiO2, Li2O, and Li2O2 as a function of the oxygen environment. The results indicate that lithium peroxide (Li2O2(s) and superoxide (LiO2(s)) are likely to be stable only under O-2-rich conditions with high oxygen partial pressures (P-O2), whereas Li2O is the most stable at ambient conditions. Additionally, the trends in the density functional calculated equilibrium potential for an ideal reversible Li-O-2 couple can be described by an analytical equation as a function of pressure and temperature. As part of this work, we have also calculated the structure and thermodynamics for lithium superoxide. It is found to be stable with respect to lattice vibrations, with an O-O stretching vibration mode very similar to that of the isolated LiO2 molecule and to the O-2(-) ion radical.