Characterization of the Solid-Electrolyte Interphase between a Cu Electrode and LiN(CF₃SO₂)₂-triglyme Solvate Ionic Liquid

Abstract

The formation of the solid electrolyte interphase (SEI) on a Cu electrode was investigated in 50.0–50.0 mol% lithium bis(trifluoromethylsulfonyl)amide (LiTFSA)-triglyme (G3, CH3(OCH2CH2)3OCH3) solvate ionic liquid, [Li(G3)]TFSA, using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR). The Li+ conductive SEI was formed on a Cu electrode by keeping the potential at 0 V vs Li∣Li(I). The resistance and capacitance attributable to the SEI changed over a period of 24 h in EIS spectra, suggesting the sluggish SEI formation on the Cu electrode in [Li(G3)]TFSA. XPS and FT-IR spectra indicated the existence of G3, TFSA–, [Li(G3)]+, and/or their decomposition products in the SEI. In contrast, LiF was not found in the SEI, suggesting LiF was not formed by the cathodic decomposition of [Li(G3)]TFSA on the Cu electrode at 0 V. The whisker-like growth of Li on a Cu electrode was suppressed after the SEI formation at 0 V probably due to the formation of homogeneous and low-conductive SEI layer. The coulombic efficiency for deposition and dissolution of Li was improved with the SEI formation at 0 V prior to the measurements, suggesting the SEI on a Cu electrode affected the reversibility of the Li anode.

Dissecting the Solid Polymer Electrolyte–Electrode Interface in the Vicinity of Electrochemical Stability Limits

“…A similar trend was observed at the interface between Cu and a glyme-based solvate ionic liquid with LiTFSI at 0 V, attributed to the formation of a low-conductivity SEI layer. 34 In comparison, the resistance of the SPE is only ∼2.5 kΩ, despite having a thickness of ∼125 μm. Below 0 V, the interphase resistance (R SEI E<0 ) drops rapidly and stabilizes at 7 kΩ from −0.2 to −1 V. In conjunction, the charge transfer resistance (R ct ) also decreases, suggesting an initial kinetic barrier for lithium deposition.…”

“…Possible candidates for this peak include Li 2 CO 3 , LiOH, RO−Li, LiF, Li 3 N, and Li 2 S. 21,41,47,48 However, the absence of matching peaks in the O 1s, F 1s, N 1s, and S 2p spectra at 0 V rules out LiOH, LiF, Li 3 N, and Li 2 S. In agreement with this work, no LiF was observed at the interface between triglyme:LiTFSI solvate ionic liquid and Cu at 0 V, indicating minimal TFSI − degradation. 34 According to the relative atomic composition, the concentration of lithium species is approximately 5 times larger than the CO 3 2− concentration; see Figure S12. Hence, it can be concluded that the Li 1s peak does not exclusively belong to Li 2 CO 3 .…”

Understanding the Reductive Decomposition of Highly Concentrated Li Salt/Sulfolane Electrolytes during Li Deposition and Dissolution

“…To investigate the SEI on the deposited Li, XPS measurements were performed on the electrodes after Li deposition in the [LiTFSA]/[SL] = 1/2 and [LiFSA]/[SL] = 1/3 electrolytes. Ar + etching was not performed on the samples to prevent the decomposition of the anion species by Ar + irradiation. , The surface atomic ratios of Li, C, N, O, F, and S on the deposited Li in different electrolytes were estimated from the XPS spectra (Figure S7). We could not find remarkable differences in the atomic ratios in the SEIs formed in the two electrolytes.…”

Effect of EtOMgCl Salt to Suppress Reductive Decomposition of TFSI⁻ Anion in Electrolyte for Magnesium Rechargeable Battery

“…S4) and the previous studies. 16,[26][27][28][29][30][31] Although any peaks were not observed in the spectrum at 0 V before the deposition (red line, 0 V vs. OCP), the downward peaks assigned to either TFSIanion or G3 were observed in the spectrum at -0.3 V (blue line, -0.3 V vs. 0 V). The result indicated that both TFSIanions and the G3 approached to the interface between electrolyte and…”

“…5). Based on the previous studies, [29][30][31] it was estimated that the G3 solvated Mg 2+ ion (Table 3). It indicates that magnesium is deposited in EtOMgCl/Mg(TFSI)2/G3 by the reduction of Mg 2+ ion solvated by G3.…”