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\nsolid electrolyte interphase (SEI) is critical for understanding and\ncontrolling interfacial degradation mechanisms in lithium containing battery\nsystems. To study these critical battery materials and interfaces X ray\nphotoelectron spectroscopy (XPS) is a widely used technique that provides\nquantitative chemical insights. However, due to the fact that a majority of\nchemical phases relevant to battery interfaces are poor electronic conductors,\nphase identification that relies primarily on absolute XPS core level\nbinding-energies (BEs) can be problematic. Charging during XPS measurements\nleads to BE shifts that can be difficult to correct. These difficulties are\noften exacerbated by the coexistence of multiple Li containing phases in the\nSEI with overlapping XPS core levels. To facilitate accurate phase\nidentification of battery relevant phases, we propose a straightforward\napproach for removing charging effects from XPS data sets. We apply this\napproach to XPS data sets acquired from six battery relevant inorganic phases\nincluding lithium metal (Li0), lithium oxide (Li2O), lithium peroxide (Li2O2),\nlithium hydroxide (LiOH), lithium carbonate (Li2CO3), and lithium nitride\n(Li3N). Specifically, we demonstrate that BE separations between core levels\npresent in a particular phase (e.g. BE separation between the O 1s and Li 1s\ncore levels in Li2O) provide an additional constraint that can significantly\nimprove reliability of phase identification. Finally, as an exemplary case we\napply the charge-correction methodology to XPS data acquired from a symmetric\ncell based on a Li2S P2S5 solid electrolyte. This analysis demonstrates that\naccurately accounting for XPS BE shifts as a function of current-bias\nconditions can provide a direct probe of ionic conductivities associated with\nbattery materials.\n