Theoretical Exploration of Various Lithium Peroxide Crystal Structures in a Li-Air Battery

Resumen

We describe a series of metastable Li₂O₂ crystal structures involving different orientations and displacements of the O₂^2- peroxy ions based on the known Li₂O₂ crystal structure. Within the vicinity of the chemical potential ?G ~ 0.20 eV/Li from the thermodynamic ground state of the Li₂O₂ crystal structure (i.e., Föppl structure), all of these newly found metastable Li₂O₂ crystal structures are found to be insulating and high-k materials, and they have a common unique signature of an O₂^2- O-O vibration mode (? ~ 799?865 cm^-1), which is in the range of that commonly observed in Li-air battery experiments, regardless of the random O₂^2- orientations and the symmetry in the crystal lattice. From XRD patterns analysis, the commercially available Li₂O₂ powder is confirmed to be the thermodynamic ground state Föppl-like structure. However, for Li₂O₂ compounds that are grown electrochemically under the environment of Li-O₂ cells, we found that the XRD patterns alone are not sufficient for structural identification of these metastable Li₂O₂ crystalline phases due to the poor crystallinity of the sample. In addition, the commonly known Raman signal of O₂^2- vibration mode is also found to be insufficient to validate the possible existence of these newly predicted Li₂O₂ crystal structures, as all of them similarly share the similar O₂^2- vibration mode. However considering that the discharge voltage in most Li-O₂ cells are typically several tenths of an eV below the thermodynamic equilibrium for the formation of ground state Föppl structure, the formation of these metastable Li₂O₂ crystal structures appears to be thermodynamically feasible.