In-Depth Characterization of Lithium-Metal Surfaces with XPS and ToF-SIMS: Toward Better Understanding of the Passivation Layer

To significantly increase the energy density of lithium-based batteries, the use of lithium metal as an anode is an option despite all of the associated challenges. Due to its high reactivity, lithium is covered with a passivation layer that may affect cell performance and reproducibility of electrochemical characterization. In most studies, this is ignored and lithium metal is used without considering the passivation layer and carrying out a proper characterization of the surface. Against this background, we systematically characterized various lithium samples with X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), and complementary energy-dispersive X-ray spectroscopy (EDX), resulting in a complete three-dimensional chemical picture of the surface passivation layer. On all analyzed lithium samples, our measurements indicate a nanometer-thick inorganic passivation layer consisting of an outer lithium hydroxide and carbonate layer and an inner lithium oxide-rich region. The specific thickness and composition of the passivation layer depend on the treatment before use and the storage and transport conditions. Besides, we offer guidelines for experimental design and data interpretation to ensure reliable and comparable experimental conditions and results. Lithium plating through electron beam exposure on electrically contacted samples, the reactivity of freshly formed lithium metal even under ultrahigh-vacuum (UHV) conditions, and the decomposition of lithium compounds by argon sputtering are identified as serious pitfalls for reliable lithium surface characterization.