Evaluation of the Surface Film Formed on Cu in Li[N(CF3SO2)2]-Tetraglyme Solvate Ionic Liquids
Authors: Katayama Yasushi , Matsumoto Takaaki , Serizawa Nobuyuki 
Published: May 2020

Rechargeable lithium-sulfur batteries have been investigated as a next generation battery due to low cost and abundant resources. Li-S batteries using conventional organic electrolytes have been realized by addition of some additives forming some protecting films on the Li anode in order to avoid self-discharge due to dissolution of lithium polysulfides in the electrolytes. On the other hand, it has been known that the solubility of lithium polysulfides is very low in the ionic liquids composed of [N(CF3SO2)2]– (bis(trifluoromethylsulfonyl)amide, TFSA–) because of its weak coordinating ability. The equimolar mixture of LiTFSA and tetraglyme (G4) has been known to give a liquid composed of [Li(G4)]+ and TFSA– at room-temperature. This liquid is called a solvate ionic liquid (SIL). LiTFSA-G4 SILs have such favorable properties as high lithium ion concentration and low solubility of lithium polysulfides. Therefore, the LiTFSA-G4 SILs have been studied as the promising electrolytes for rechargeable Li-S batteries. In order to realize Li-S batteries using the LiTFSA-G4 SILs, it is necessary to evaluate the performance of the Li anode in the SILs. We have already reported that the coulombic efficiency of deposition and dissolution of lithium on a Cu substrate was improved by the formation of the surface film (solid-electrolyte interphase) on the Cu substrate at the potential of Li(I)/Li[1]. In the present study, the surface film formed in LiTFSA-G4 SILs with and without a diluent, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (HFE), has been characterized using electrochemical impedance spectroscopy (EIS), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS) without exposing the samples to air.
LiTFSA-G4 SILs were prepared by mixing LiTFSA and G4 at the specified molar ratios (50.0 : 50.0 and 54.5 : 45.5). EIS was conducted using an air-tight three electrode cell. A Cu disk electrode (3 mm in diameter) was used as the working electrode. Li foil was used as the counter and reference electrode. The surface film was prepared on the Cu electrode by keeping the potential at 0 V vs. Li/Li(I). The EIS spectra were taken at 0 V vs. Li/Li(I) with an amplitude of 5 mV in the frequency range from 1 Hz to 20 kHz. TEM grids made of Cu and Cu foils in contact with Li foil were immersed in the SILs with different compositions for three days. The samples were washed with monoglyme (G1) and dried under dry Ar atmosphere. The samples were installed into TEM equipped with EDX using a transfer holder. XPS spectra were obtained by focused Al Kα radiation with Ar+ and Ar gas cluster ion beam (GCiB) etching guns.
The impedance spectra assignable to the surface films formed on the Cu electrodes at 0 V vs. Li/Li(I) were observed in the SILs with and without addition of HFE. The resistance of the surface film in 54.5-45.5 mol% LiTFSA-G4 SIL was larger than that in 50.0-50.0 mol% one, suggesting the ionic conductivity of the surface film reflect the concentration of Li(I) in the electrolyte. The resistance of the surface film increased with addition of HFE probably due to a decrease in the concentration of Li(I). The formation of the surface films with the thickness of 20-100 nm was confirmed by TEM. The shrinkage of the films was observed during TEM observation, indicating the decomposition of organic substances and/or elevation of temperature occurred by electron irradiation. Although no spot was observed in the electron diffraction diagram at the beginning of the observation, the spots assignable to LiF and Li2S appeared after the shrinkage of the film by electron irradiation, indicating LiF was not formed electrochemically at 0 V vs. Li/Li(I) in the SILs. In addition, no peak assignable to LiF was not observed in the XPS spectra of the surface films with Ar GCiB etching gun. Therefore, the surface films formed in the SIL were considered to composed of the organic substances derived from reductive decomposition of G4 and the ions existing in the SIL.
Reference
[1] Y. Katayama, N. Tachikawa, and N. Serizawa, 235th ECS meeting, A02-0265, Dallas, (2019).