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

We describe a series of metastable Li2O2 crystal structures involving different orientations and displacements of the O-2(2-) peroxy ions based on the known Li2O2 crystal structure. Within the vicinity of the chemical potential Delta G similar to 0.20 eV/Li from the thermodynamic ground state of the Li2O2 crystal structure (i.e., Foppl 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 O-2(2-) O-O vibration mode (omega similar to 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 Li2O2 powder is confirmed to be the thermodynamic ground state Foppl-like structure. However, for Li2O2 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 Li2O2 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 Li2O2 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 Foppl structure, the formation of these metastable Li2O2 crystal structures appears to be thermodynamically feasible.