LiF Artifacts in XPS Analysis of the SEI for Lithium Metal Batteries

The solid electrolyte interphase (SEI) is considered to be the key to the performance of lithium metal batteries (LMBs). The analysis of the SEI and cathode electrolyte interphase (CEI) composition (especially F 1s spectra) by X-ray photoelectron spectroscopy (XPS) has become a consensus among researchers. However, the surface-sensitive XPS characterization is susceptible to LiF artifacts due to several factors, leading to the overexaggerated role of LiF in the analysis of the SEI and CEI. In this paper, we conduct a systematic study on the reasons for the LiF artifacts in the XPS characterization of LMBs. The decomposition of the SEI and CEI components under argon ion sputtering, the reaction between Li2CO3 and LiPF6 in the electrolyte, influence of different sample pretreatments, the selection of the XPS measurement region, and the measurement time on the resulting spectra are investigated. The results indicate that the high content of LiF in the SEI and CEI may be attributed to the LiF artifacts, and the role of LiF in the SEI may be overexaggerated as a consequence. This work sounds an alarm about the potential misuse of argon ion sputtering and the lack of rigorous XPS characterization in SEI studies. This work also helps to set up standardized XPS characterization to provide a more accurate understanding of the role of SEI components.