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

Understanding the thermal stability of major reaction products, Li 2 O 2 (space group P6 3 /mmc) and Li 2 O (space group ) is critical to improve the safety characteristics of Li-air batteries. The changes in the crystal structure and surface chemistry of Li 2 O 2 and Li 2 O 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 Li 2 O 2 were found at 280°C and higher. These structural changes can be attributed to the transformation of Li 2 O 2 to Li 2 O 2-δ , which is supported by density functional theory calculations. Upon further heating to 700°C, a lithium-deficient Li 2-δ O phase appeared at 300°C and gradually became stoichiometric upon further heating to ∼550°C. XPS measurements of Li 2 O 2 revealed that Li 2 O 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 Li 2 CO 3 on the surface was found at 250°C, which can be attributed to chemical reactions between Li 2 O 2 /Li 2 O 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.