Characterization of Solid Electrolyte Interphase on Some Electrodes in the Bis(fluorosulfonyl)Amide Anion-Based Ionic Liquids with Different Li Salt Concentrations

Aprotic ionic liquids are investigated as the next generation electrolytes for lithium ion battery (LIB) because non-flammability of ionic liquids is expected to improve the safety of the LIB. Bis(fluorosulfonyl)amide (FSA) anion-based ionic liquids have been attracting attention because of the formation of stable solid-electrolyte interphase (SEI) on the electrode surface during charge-discharge reactions. The SEI is known to form as a result of reductive decomposition of electrolytes and considered as a lithium ion conductor, though its nature like composition has not been clarified yet especially in ionic liquid electrolytes. In the present study, the SEIs on some different electrodes in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (MPPFSA) containing LiFSA at different concentrations were characterized by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The formation and growth of the SEI were confirmed by the EIS spectra of a Cu electrode kept at 0 V vs. Li|Li(I) and Li deposited on a Cu electrode in MPPFSA containing different concentrations of LiFSA with the elapse of time. The SEIs prepared on the Cu electrode and the deposited Li in the electrolytes for 1 week were characterized by XPS washing with monoglyme (G1) without exposure to air. The XPS peaks corresponding to C, N, O, F, S atoms were observed on the Cu electrode, suggesting the ionic liquids are adsorbed on the electrode surfaces even after washing by G1. Such decomposition products of the electrolytes as LiF and LiOH were detected on the Cu electrode and the deposited Li. In case of the deposited Li, the peak assignable to alkyl lithium (R-Li) in the C1s spectra was observed probably due to the reaction of Li with the organic cation of the ionic liquids. The ratio of the peak intensity for LiF to that for SO 2 -F on the Cu electrode and the deposited Li increased with an increase in the LiFSA concentration in the electrolyte. On the other hand, the relative intensity for R-Li on the deposited Li decreased with an increase in the LiFSA concentration, suggesting an increase in the LiFSA concentration suppressed the decomposition of MPP + and promoted the decomposition of FSA - .