Thermal Stability of Li2O2and Li2O for Li-Air Batteries: In Situ XRD and XPS Studies

Understanding the thermal stability of major reaction products, Li2O2 (space group P63/mmc) and Li2O (space group ) is critical to improve the safety characteristics of Li-air batteries. The changes in the crystal structure and surface chemistry of Li2O2 and Li2O were examined as a function of temperature via in situ X-ray diffraction (XRD) and in situ X-ray photoelectron spectroscopy (XPS). Significant decreases in the lattice parameters and the c/a ratio of Li2O2 were found at 280°C and higher. These structural changes can be attributed to the transformation of Li2O2 to Li2O2-δ, which is supported by density functional theory calculations. Upon further heating to 700°C, a lithium-deficient Li2-δO phase appeared at 300°C and gradually became stoichiometric upon further heating to ∼550°C. XPS measurements of Li2O2 revealed that Li2O appeared on the surface starting at 250°C, which is in agreement with the onset temperature of phase transformation as detected by XRD. In addition, the growth of Li2CO3 on the surface was found at 250°C, which can be attributed to chemical reactions between Li2O2/Li2O and carbon-containing species (e.g. hydrocarbons) present in the XPS chamber. This finding highlights the challenges of developing stable carbon-based oxygen electrode for Li-air batteries.