Characterization of the Solid-Electrolyte Interphase between a Cu Electrode and LiN(CF 3 SO 2 ) 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, CH 3 (OCH 2 CH 2 ) 3 OCH 3 ) 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.