Characterization of Solid Electrolyte Interphase Formed in Li[N(CF3SO2)2]-Sulfolane-Based Electrolytes
Authors: Uchiya Tomohiro , Serizawa Nobuyuki , Katayama Yasushi 
Published: December 2020

A highly concentrated electrolyte composed of sulfolane (SL) and lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) attracts attention as an electrolyte for the lithium-sulfur battery because of the low solubility of lithium polysulfides. 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) is often added to the electrolyte in order to decrease the viscosity[1]. It is important to improve the cyclability of the Li anode to develop the Li-S battery having a high energy density. The deposition and dissolution of Li have been known to be affected by the formation and growth of a solid electrolyte interphase on the anode surface. In the present study, formation of the SEI on Cu and Li electrodes in LiTFSA-SL-based electrolytes was investigated by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM).
The mixtures of LiTFSA, SL and HFE at various ratios were used as the electrolytes. The SEI was formed on a Cu substrate by holding its potential at 0 V vs. Li|Li(I) in the electrolytes. The impedance measurements were conducted at the open circuit potential using a coin-type or three-electrode cell using a Li foil as a reference and counter electrode. The samples for TEM and XPS analysis were prepared by immersing a Cu foil or grid in the electrolytes in contact with Li. After washing the sample with monoglyme, TEM observation and XPS measurement were conducted without exposure to air. The SEI was formed on Li deposited galvanostatically on a Cu grid placed in a coin-type cell.
Formation of the SEI was not confirmed on a Cu electrode after keeping the potential at 0 V for 24 hours in LiTFSA-SL (1 : 2) by EIS, suggesting sluggish formation and/or poor adhesion of the reduced products. On the other hand, a semicircle attributable to the SEI appeared in LiTFSA-SL-HFE (1 : 2 : 2), indicated the decomposition of HFE played an important role in the SEI formation.
The XPS spectra of a Cu electrode after the SEI formation at 0 V in LiTFSA-SL-HFE indicated the existence of TFSA– and/or its decomposition products in the SEI. On the other hand, LiF was found to be absent in the SEI, suggesting LiF was not formed by the cathodic decomposition of the electrolyte on a Cu electrode.
The SEI with a thickness of 10-100 nm was observed by TEM on a Cu grid after the SEI formation in LiTFSA-SL-HFE. The shrinkage of the film was observed during the observation, suggesting that the SEI was composed of organic materials. No spot was observed in the electron diffraction diagrams of the SEI. On the other hand, the ring asignable to Li2O was observed in the electron diffraction diagram of the SEI formed on Li deposits. Thus, the composition of the SEI formed on Li was considered different from that on Cu.
Acknowledgment
This study was supported by Advanced Low Carbon Technology Research and Development of Program (ALCA) of the Japan Science and Technology Agency (JST).
Reference
[1] A. Nakanishi et al., J. Phys. Chem. C, 123, 14229 (2019).