Article

XPS Insights of Fluorinated Ether Electrolyte for High Voltage Li Metal Batteries

Surface Analysis Spotlight: XPS

by Sarah Zaccarine XPS Scientist by Malli Komarneni XPS Scientist

The increasing demand for high-density energy storage systems has driven the research on lithium metal batteries (LMBs) with high-voltage (>4.0 V vs. Li⁺/Li) operation. However, the thermodynamic instability of metallic Li and the formation of an unstable solid electrolyte interphase (SEI) at the interface with the Li metal anode have hindered the practical application of LMBs. The electrolyte chemistry can play a crucial role in stabilizing the SEI layer and extending the cycle life. Conventional carbonate-based electrolytes face limitations due to their narrow electrochemical windows, necessitating the development of new solvents with improved compatibility and voltage stability. In a study by Coskun  et al., published in Nature Communications, an innovative solvent was introduced, 2,2-dimethoxy-4-(trifluoromethyl)-1,3-dioxolane (DTDL), which combines the desirable  oxidative stability of a cyclic fluorinated segment and the Li⁺ solvation ability of a linear ether segment.

Understanding the composition of the interfacial layers formed after cycling is crucial for designing 0stabilized battery systems. X-ray photoelectron spectroscopy (XPS) can provide detailed chemical state information of surfaces as well as bulk features when combined with ion sputtering to profile into a sample. In this work, XPS analysis was performed using the PHI Quantes instrument to investigate the chemical composition at interfaces with both the anode (SEI) and cathode (cathode electrolyte interface, CEI) for cells using different electrolytes.

First, Li|Cu half cells were prepared using lithium bis(fluorosulfonyl)imide (LiFSI) with varying solvents. Analysis of the SEI in the half cells (Figure 1) reveals two chemical states of F in the SEI, corresponding to Li-F and S-F bonds from FSI anion decomposition, regardless of which solvent was used. But the SEI layer formed in the 1 M LiFSI-DTDL electrolyte exhibited higher LiF and S-F compositions compared to the 1 M LiFSI-DME (1,2-dimethoxyethane) electrolyte, indicating a more favorable SEI composition. Additionally, the SEI layer in the DTDL-based electrolyte was rich in N and S contents, indicative of an anion-derived SEI layer.

Figure 1: F 1s XPS spectra obtained on the plated Li metal surface in Li|Cu half cells using (a) 1 M LiFSI-DTDL and (b) 1 M LiFSI-DME electrolytes after 5 cycles at 1 mA cm⁻² with a cutoff capacity of 1 mAh cm⁻².

Comparative XPS analysis was also performed on the SEI and CEI compositions in Li|NCM811 (LiNi_0.8Co_0.1 Mn_0.1O₂) full cells using three different electrolytes. Here, depth profiles were performed to analyze both surface chemistry as well as composition further into the layers. The XPS concentration depth profiles (Figure 2) of the SEI layers showed that the SEI formed in the 1 M LiPF₆-EC-DEC (EC -Ethyelene carbonate and DEC – Diethyl carbonate) electrolyte had a higher carbon content at the outer surface, while the inorganic species increased with depth. In contrast, the SEI layers formed in the 1 M and 2 M LiFSI-DTDL electrolytes exhibited a more uniform composition with increased depth, with the 2 M LiFSI-DTDL electrolyte showing higher S and F atomic contents, indicating greater FSI anion participation in SEI formation. The F 1s spectra confirmed a constant and high LiF content in the SEI layers of DTDL-based electrolytes. The SEI layer formed in the 1 M LiPF₆-EC-DEC electrolyte was relatively inhomogeneous, while the DTDL-based electrolytes showed a more consistent concentration of species, especially the inorganic ones.

Figure 2: Quantified atomic percent of SEI components at different sputter times in Li|NCM811 full cells after 30 cycles at 0.5 C with (a) 1 M LiPF₆-EC-DEC, (b) 1 M LiFSI-DTDL, and (c) 2 M LiFSI-DTDL electrolytes.

The XPS spectra of the CEI that formed on the NCM811 cathode (Figure 3) also revealed differences between the electrolytes. The C 1s spectrum showed an additional peak at 293.3 eV in the 1 M and 2 M LiFSI-DTDL electrolytes, attributed to the -CF3 functional group, indicating the participation of DTDL in CEI formation through its decomposition. The F 1s spectra demonstrated a LiF-rich character of the CEI in both 1 M and 2 M LiFSI-DTDL electrolytes, which differed significantly from the 1 M LiPF6-EC-DEC electrolyte.

Figure 3: XPS spectra for (a) C1s and (b) F1s transitions of NCM811 cathode surface in Li|NCM811 full cells after 30 cycles at 0.5 C.

In conclusion, this study presents a new class of fluorinated ether solvent, DTDL, which exhibits controlled solvation ability and improved stability at high voltages. The unique solvation structure enabled the formation of strong FSI anion-coordinated Li⁺ clusters at low concentrations, as demonstrated by PHI Quantes XPS results, which led to excellent cycling performance in both Li|Cu half-cell and Li|NCM 811 full cell configurations. The molecular design of DTDL highlights its potential for future lithium metal batteries.

For more information about how PHI XPS systems can be used to analyze interfacial chemistry and degradation in battery devices, please visit Dr. Sarah Zaccarine’s talk at the Florida AVS Symposium March 11-12 and poster at the International Battery Summit March 13-15.