Effect of the Lithium Salt Concentration on Deposition and Dissolution of Lithium in a Bis(fluorosulfonyl)Amide-Based Ionic Liquid Electrolyte
Authors: Yamashita Ryo , Serizawa Nobuyuki , Katayama Yasushi 
Published: December 2020

Lithium metal is considered to be an ideal anode material for high-power lithium secondary batteries because of the high theoretical specific capacity of 3860 mAh g-1 and the negative redox potential. However, dendritic, needle-like or whisker-like growth of Li deposits may cause the internal short circuit and thermal runaway of the battery. The detachment of lithium from the current collector also leads to the capacity loss. The formation of homogeneous and stable solid electrolyte interphase (SEI) on the anode is considered to suppress the concentration of the current and dendritic growth of Li deposits.

The addition of LiFSA (FSA- = bis(fluorosulfonyl)amide) to organic electrolytes has been reported to improve the reversibility of the Li anode probably due to the formation of FSA--derived SEI. Moreover, the reversibility of the Li anode was known to be improved in ionic liquids containing a high concentration of LiFSA[1, 2]. However, the formation mechanism and the chemical composition of the SEI in the electrolytes composed of FSA- are still unclear. In the present study, the charge-discharge behavior of the Li anode and the characterization of the SEI were investigated in FSA--based ionic liquids containing different concentrations of LiFSA.

BMPFSA (BMP = 1-butyl-1-methylpyrrolidinium) and LiFSA were mixed at the molar ratios of 1 : 0.3 (1.1 M) and 1 : 1 (2.9 M). The charge-discharge measurement was conducted using a coin-type cell at 0.1 mA cm-2. Cu was used as a working electrode and Li was used as a counter electrode. Celgard 3501 was used as a separator. SEI was prepared on a Cu electrode by keeping the potential at 0 V prior to the measurement. The morphology of Li deposits on the Cu electrode was observed by scanning electron microscopy (SEM) after the charge-discharge measurement. The electrochemical impedance spectroscopy (EIS) was carried out at the open circuit potential before and during the SEI formation. The SEI formed on Cu by immersing a Cu substrate in the electrolytes in contact with Li metal was characterized using X-ray photoelectron spectroscopy (XPS) and transmittance electron microscopy (TEM) without exposure to air.

The coulombic efficiency for deposition and dissolution of Li on a Cu electrode was ca. 97 % after cycling in both 1.1 and 2.9 M LiFSA/BMPFSA. Although needle-like Li deposits were observed in the 1st cycle, nodule-like deposits were obtained after 100 cycling regardless of the compositions. A semicircle corresponding to the SEI appeared after keeping the potential of a Cu electrode at 0 V for 6 hours in the Nyquist plots. The diameter of the semicircle in 1.1 M LiFSA/BMPFSA was larger than that in 2.9 M LiFSA/BMPFSA probably because the Li+ conductivity of the SEI reflected the concentration of Li+ in the electrolyte. XPS spectra of the SEI formed on the Cu substrate indicated the existence BMP+, FSA-, and/or their decomposition products. The SEI with a thickness of 20 nm was observed by TEM regardless of the composition. Furthermore, the spots corresponding to LiF and Li2S were found in the electron diffraction diagrams of the SEI.

Reference

[1] G. M. A. Girard, M. Hilder, N. Dupre, D. Guyomard, D. Nucciarone, K. Whitbread, S. Zavorine, M. Moser, M. Forsyth, D. R. MacFarlane, and P. C. Howlett, ACS Appl. Mater.
Interfaces, 10, 6719 (2018).

[2] T. Iwahashi, Y. Miwa, W. Zhou, Y. Sakai, M. Yamagata, M. Ishikawa, D. Kim, and Y. Ouchi, Electrochem. Commun.,72, 54 (2016).