X-Ray Photoelectron Spectroscopy of Lithium Salts in Batteries

By Chinmay ChariReviewed by Susha Cheriyedath, M.Sc.Sep 9 2022

In a paper recently published in the open-access journal ACS Energy Letters, researchers presented a comprehensive X-ray photoelectron spectroscopy (XPS) study of three Li salts, namely lithium hexafluorophosphate (LiPF₆), lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI), and lithiumbis(fluorosulfonyl)imide (LiFSI).

Study: Degradation and Speciation of Li Salts during XPS Analysis for Battery Research. Image Credit: Veleri/Shutterstock.com

Background

Li metal anodes (LMAs) have attracted significant attention in industry as well as academia over the last decade, with the potential to double the Li-ion batteries' energy densities by substituting graphite anodes. However, the development of rechargeable LMAs in practical uses is hampered by (1) the formation of Li dendrite causing short circuits, (2) the constant loss of Li inventory, and (3) the high Li metal reactivity, leading to unmanageable side reactions.

Designing novel solid and liquid-state electrolytes is a viable avenue for enhancing LMA safety and cyclability. The creation of the solid-electrolyte interphase (SEI), which determines the cycle life of a battery, is directly influenced by electrolyte compositions.

XPS is among the most extensively utilized methods in the battery research field for the chemical identification of SEI. Li metal along with other Li compounds, however, are commonly sought to be sensitive to electron beams, air, and water. Although XPS can offer useful information about the SEI, the influence of these confounding variables must be carefully evaluated throughout the XPS analysis. The existence of LiF inside the SEI layer is demonstrated solely by XPS. To avoid potential problems in XPS analysis, proper control experiments must be designed and executed, and the actual SEI species created by battery operation must be assigned robustly.

About the Study

In this study, the team provided a comprehensive XPS examination of three Li salts typically utilized for battery research. After the bulk of organic solvents was extracted by the XPS vacuum, surface layers of Li salts were produced directly using liquid electrolytes.

When compared to direct evaluation of solid Li salt powder, this preparative method has three significant advantages: (1) close simulation of the residual salt situations because of incomplete washing before XPS analysis, (2) investigating possible Li salt speciation behavior within the liquid electrolyte, and (3) quantification of the remnant organic solvent on the bulk and surface areas of precipitated salt layers.

Prior to Ar+ sputtering, the three Li salts' surface films were studied by XPS to benchmark their constituent BE values. Furthermore, the density functional theory (DFT) was used to compute the charge density of various atoms on LiTFSI and LiFSI. XPS measurements were also performed on solid powders of LiPF₆, LiFSI, and LiTFSI.

Observations

The spectroscopic and BE properties of the three Li salts varied, indicating their diverse speciation processes and chemical environments. Primarily, it was observed that all three Li salts readily produced LiF, particularly during Ar+ sputtering. Regardless of battery performance, any salt remaining in the electrode sample would likely yield LiF during XPS testing. Washing the electrode samples thoroughly is thus necessary to achieve total salt removal, particularly for electrodes having porous architectures.

Furthermore, the different BE values for each of the three Li salts benchmarked give excellent references for identifying any remnant salt or other species that may create LiF artifacts following Ar+ sputtering. Interestingly, Ar+ sputtering eliminated the trace H₂O impurities that were previously located at the surface. Post-sputtering data acquired from different sections of the LiPF6 specimen revealed that higher sputtering power resulted in improved LiF production. However, the lack of any PF5 molecules in these areas revealed that its concentration varied spatially throughout the sample surface.

Despite significant salt speciation and degradation, inorganic species apart from LiF, including Li₂S, Li₃N, and Li₂O, may act as SEI chemical fingerprints throughout the battery operation. Despite the fact that the three Li salts exhibit comparable LiF enrichment phenomena, their production routes most likely differ in XPS conditions. The degradation of LiFSI into LiF includes the breaking of the S-F bond, whereas LiTFSI degradation to LiF begins with the breaking of the C-F bond. This result implies that the CS bonds of LiTFSI play an important role in the production of detectable byproducts by XPS.

This finding implies that LiTFSI's C-S bonds play an important role in the production of detectable byproducts by XPS. The quantity of residual organic solvent in liquid electrolytes is highly dependent on the particular solvent-salt combination. Furthermore, organic residues like carbonates may decompose during Ar+ sputtering based on their chemical structure, thereby complicating analysis of the depth profiling data. Moreover, the LiTFSI decomposition following Ar+ sputtering results in additional peaks in the C 1s spectra at BE of about 288 eV that must exclude assigning other organic SEI components.

Conclusions

To summarize, the team demonstrated that XPS might identify the LiPF₆ salt's speciation products during dissolution and capture minor H₂O impurities within a commercial electrolyte, triggering further side reactions. When examining the real SEI chemistry, the reaction products from these extra pathways showing in the O 1s and P 2p spectra must be carefully eliminated. According to the authors, these benchmark studies provide a useful reference for peak assignment and even underline the need for control trials to avoid possible errors and detect true SEI components.

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