Evaluation of the Solid Electrolyte Interphase Formed in Lithium Bis(trifluoromethylsulfonyl)Amide-Tetraglyme Solvate Ionic Liquids with Different Compositions
Authors: Katayama Yasushi , Tachikawa Naoki , Serizawa Nobuyuki 
Published: February 2020

Rechargeable lithium-sulfur (Li-S) battery has attracted much attention because of abundant sulfur resources and low cost. Lithium polysulfides (Li2S
x
) produced during discharge have been known to dissolve in conventional organic electrolytes, resulting in self-discharge due to shuttle effect. It has been found that the solubility of Li2S
x
is very low in the solvate ionic liquids (SILs) composed of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4, CH3(OCH2CH2)4OCH3).1 Therefore, LiTFSA-G4 SILs are considered a promising electrolyte for rechargeable Li-S battery. We have reported deposition and dissolution of Li can be performed with the coulombic efficiency higher than 90% in LiTFSA-G4 SILs.2 The coulombic efficiency increased with an increase in the molar fraction of LiTFSA, suggesting the dissolved Li species might affect the morphology of Li deposits. On the other hand, formation of the solid electrolyte interphase (SEI) is considered unavoidable even in the LiTFSA-G4 SILs. In the present study, the SEI formed in the LiTFSA-G4 SILs with different compositions was evaluated with electrochemical impedance spectroscopy.

The SILs were prepared by mixing LiTFSA and G4 at 50.0-50.0 and 54.5-45.5 mol%. The water content in the SILs was less than 50 ppm, which was determined by Karl Fischer titration. Cu was used as a substrate for the SEI formation and deposition of Li. The surface of the Cu substrate was electrolytically degreased before use. A silver wire immersed in 50.0-50.0 LiTFSA-G4 SIL containing 0.1 M AgCF3SO3 was used as a reference electrode for a three-electrode cell. Li foil was used as a counter electrode. Celgard 3501 was used as a separator for a 2032 coin-type cell. Electrochemical impedance spectroscopy was performed in the frequency range from 20 kHz to 1 Hz with an amplitude of 5 mVpp.

The SEI was prepared on a Cu electrode by applying 0 V vs. Li|Li(I) in LiTFSA-G4 SILs. Semi-circles appeared in the Nyquist plots of the Cu electrode at 0 V vs. Li|Li(I) after formation of the SEI. The resistance of the SEI increased continuously during application of the potential within 2 hours, suggesting decomposition of the SILs was sluggish at room-temperature. The resistance of the SEI prepared by applying the potential for 10 minutes in 54.5-45.5 mol% LiTFSA-G4 SIL was smaller than that in 50.0-50.0 mol% one, indicating the ionic conductance and/or thickness of the SEI depends on the composition of the SIL.

The coulombic efficiency for deposition (with the charge density of 0.2 C cm–2) and dissolution (with the cut-off potential of 2.0 V vs. Li|Li(I)) of Li using a coin-type cell at the current density of ±0.1 mA cm–2 with the open-circuit intervals of 10 minutes reached to the steady-state value close to 90% after four or five cycles. The coulombic efficiency for 54.5-45.5 mol% LiTFSA-G4 SIL was slightly higher than that for 50.0-50.0 mol% one, probably reflecting the difference in the SEI formed on the Cu electrode. On the other hand, the coulombic efficiency was found to be improved when a sufficient open-circuit interval was taken after deposition of Li in the first cycle, indicating it is important to take an enough reaction time for formation of the better SEI on the Cu electrode.

Acknowledgment

This study was supported by the Advanced Low Carbon Technology Research and Development of Program (ALCA) of the Japan Science and Technology Agency (JST).

Reference

K. Dokko, N. Tachikawa, K. Yamauchi, M. Tsuchiya, A. Yamazaki, E. Takashima, J.-W. Park, K. Ueno, S. Seki, N. Serizawa, and M. Watanabe, J. Electrochem. Soc.,160, A1304 (2013).

N. Tachikawa, S. Hosoda, T. Ishida, K. Yoshii, M. Watanabe, and Y. Katayama, PRiME2016, Abst#558, Hawaii, (2016).