XPS on Li-Battery-Related Compounds: Analysis of Inorganic SEI Phases and a Methodology for Charge Correction

Accurate identification of chemical phases associated with the electrode and solid-electrolyte interphase (SEI) is critical for understanding and controlling interfacial degradation mechanisms in lithium-containing battery systems. To study these critical battery materials and interfaces X-ray photoelectron spectroscopy (XPS) is a widely used technique that provides quantitative chemical insights. However, due to the fact that a majority of chemical phases relevant to battery interfaces are poor electronic conductors, phase identification that relies primarily on absolute XPS core-level binding-energies (BEs) can be problematic. Charging during XPS measurements leads to BE shifts that can be difficult to correct. These difficulties are often exacerbated by the coexistence of multiple Li-containing phases in the SEI with overlapping XPS core levels. To facilitate accurate phase identification of battery-relevant phases (and electronically insulating phases in general), we propose a straightforward approach for removing charging effects from XPS data sets. We apply this approach to XPS data sets acquired from six battery-relevant inorganic phases including lithium metal (Li⁰), lithium oxide (Li₂O), lithium peroxide (Li₂O₂), lithium hydroxide (LiOH), lithium carbonate (Li₂CO₃), and lithium nitride (Li₃N). Specifically, we demonstrate that BE separations between core levels present in a particular phase (e.g., BE separation between the O 1s and Li 1s core levels in Li₂O) provide an additional constraint that can significantly improve reliability of phase identification. For phases like Li₂O₂ and LiOH where the Li-to-O ratios and BE separations are nearly identical, X-ray excited valence-band spectra can provide additional clues that facilitate accurate phase identification. To accurately identity the phases in this study, we show that in situ growth of Li₂O on Li⁰ provides a means for determining absolute core-level positions, where all charging effects can be accurately removed. Finally, as an exemplary case we apply the charge-correction methodology to XPS data acquired from a symmetric cell based on a Li₂S-P₂S₅ solid electrolyte. This analysis demonstrates that accurately accounting for XPS BE shifts as a function of current-bias conditions can provide a direct probe of ionic conductivities associated with battery materials.