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

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