Time-Resolved Evolution of Lithium Metal Surfaces in Ambient Air through X-ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry

Understanding the dynamic oxidation behavior of lithium metal is critical for fundamental surface chemistry. However, the reliable characterization of lithium-based systems involving air transfer obstructs our understanding of the true state of the lithium metal anode. Here, we map the time-resolved oxidation dynamics of lithium metal upon exposure to ambient air using multimodal characterization techniques, such as X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. We show that lithium oxidation preferentially initiates at mechanically damaged regions and subsequently extends in both lateral and depth directions. While an ultrathin layer of Li₂O, LiOH, and Li₂CO₃ forms within minutes, it fails to effectively passivate the surface against further reaction with ambient species due to its hygroscopic nature and continuous diffusion of reactive gaseous species. With prolonged exposure, the surface evolves into a continuously thickening Li₂CO₃-dominated phase that progressively fragments the underlying metallic lithium framework. Notably, we identify a 2 min window as a practical guideline during which chemical and morphological degradation remains negligible. Restricting ambient air exposure to this limited timeframe mitigates measurement artifacts, establishing a robust protocol for the characterization of highly reactive battery materials.