Degradation and Speciation of Li Salts during XPS Analysis for Battery Research

X-ray photoelectron spectroscopy (XPS) is one of the most common techniques to characterize the solid–electrolyte interphase (SEI) in battery research. However, residual salt or solvent can produce spectroscopic artifacts that complicate the evaluation of actual SEI chemistry. Herein, we present a systematic XPS study of three different Li salts, namely lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium hexafluorophosphate (LiPF6). A side-by-side comparison reveals that the binding energies of constituent elements sensitively shift in response to their distinct chemical environments. Strikingly, all three Li salts consistently transformed into LiF under Ar+ sputtering, pointing out the importance of complete salt removal before XPS analysis. Residue from organic solvent or impurities from sample surface to bulk were found specific to the solvent–salt combination, which should be distinguished from the real organic SEI. Overall, this set of benchmark studies not only offers a valuable reference for peak assignment but also emphasizes the significance of control experiments to avoid potential pitfalls while identifying actual SEI components.