Exploring the stability of lithium metal surface by X-ray photoelectron spectroscopy

The energy density of solid-state lithium metal battery has been greatly improved if it employs lithium metal as the anode because it has high theoretical capacity and most negative electrode potential. However, the highly active surface of lithium metal is usually covered by a passivation layer, which can not be completely removed by conventional methods and greatly affects the performance and safety of the corresponding battery. The existence of the passivation layer is a common sense, but still lacks detailed characterizations, and the researchers rarely focus on lithium metal under the passivation layer. In this work, we use X-ray photoelectron spectroscopy (XPS) to analyze the composition and thickness of the passivation layer to account for the stability of Li metal surface, the results of different pretreatment methods are systematically compared. The parameters used for fitting the XPS results of Li surface are summarized, and the relevant results can be used to compare the general proportion of each component on the surface conveniently and quickly. With argon ion sputtering, we can successfully remove the passivation layer and obtain a clean and fresh surface. With this fresh Li, a symmetric cell is built for electrochemical tests, where it is found significant decrease on the interfacial resistance between the clean lithium metal electrode and the electrolyte with regard to that in the un-treated lithium metal based one. This work opens a rewarding avenue towards the accurate application of XPS to elucidate the specific role of Li metal in stabilizing the interface of solid-state batteries.