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…

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