We describe a series of metastable Li
2O
2 crystal structures involving different orientations and displacements of the O
22− peroxy ions based on the known Li
2O
2 crystal structure. Within the vicinity of the chemical potential ΔG ~
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We describe a series of metastable Li
2O
2 crystal structures involving different orientations and displacements of the O
22− peroxy ions based on the known Li
2O
2 crystal structure. Within the vicinity of the chemical potential ΔG ~ 0.20 eV/Li from the thermodynamic ground state of the Li
2O
2 crystal structure (
i.e., Föppl structure), all of these newly found metastable Li
2O
2 crystal structures are found to be insulating and high-
k materials, and they have a common unique signature of an O
22− 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
22− orientations and the symmetry in the crystal lattice. From XRD patterns analysis, the commercially available Li
2O
2 powder is confirmed to be the thermodynamic ground state Föppl-like structure. However, for Li
2O
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
2O
2 crystalline phases due to the poor crystallinity of the sample. In addition, the commonly known Raman signal of O
22− vibration mode is also found to be insufficient to validate the possible existence of these newly predicted Li
2O
2 crystal structures, as all of them similarly share the similar O
22− 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
2O
2 crystal structures appears to be thermodynamically feasible.
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