  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, 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. 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.
