Electrochemical Performance of Ionic Liquids with Bis(trifluorosulfonyl)Imide Anion for Lithium-Ion Battery Application

Room temperature ionic liquids (RTILs) formed from bulky cations and anions have attracted significant attention as potential electrolyte components for the next generation of lithium-ion batteries. Unfortunately, the lithium–ion batteries fabricated with neat ILs as electrolytes possess low ionic conductivity and lithium corrosion problems. Both surface-active and ionic IL additives have been used overtime to improve the electrochemical performance.^a

 Bis(trifluoromethylsulfonyl)imide (TFSI^-) - based ionic liquids (Fig.1) are known to have low cathodic reduction potentials at graphite electrodes that affect formation of protective layer on solid electrolyte interface (SEI)^b. On the contrary, at metal electrodes including lithium, TFSI^- anions favor electrolyte reduction which is due to stabilization of LiTFSI ion pair formed under reducing conditions in presence of lithium cations and TFSI anions.

Compared to other lithium salts as IL ionic additives, such as LiBF₄ and LiTFSI, lithium hexafluorophosphate (LiPF₆) demonstrated an improved electrochemical cathodic behavior. Furthermore, the presence and concentration of a lithium salt have an influence on the cathodic potential electrochemical window. For example, addition of LiPF₆ to a neat solution of 1-butyl-2,3-dimethyl-imidazolium bis(fluoromethyl-sulfonyl)imide (FSI) has been shown to change the cathodic electrochemical window from  -2.8 to -4.10V.^c

Present study is focused on electrochemical performance evaluation of ionic liquids, specifically 1-ethyl-3 methylimidazolium (EMIM TFSI) and 1-butyl-1-methylpyrrolidonium (PYR14 TFSI) (Fig.1) in symmetrical cells with two lithium metal electrodes and Celgard^TM separator.

The results for symmetrical coin cells (CR2025) constructed with lithium disks deposited onto 0.5mm spacers and separated by Celgard^TM 20mm polyethylene membrane with 12 mL of ionic liquid on each side of the separation membrane will be presented.  The electrochemical performance of the symmetrical cells will be compared with the results obtained for the half-cells with the earlier developed silicon-based working electrode. 

EMIM TFSI and PYR14 TFSI ionic liquids in presence and absence of LiPF₆ for symmetrical cells and half-cells will be discussed in terms of coin cell performance, impedance, chemical transformations, and changes to the morphology that take place during cell storage, solid electrolyte interface (SEI) formation, and charge-discharge cycling.

 NMR study of the ionic liquids will be addressed to study the dynamics of the cations, anions, and ion pairs in presence and absence of ionic additive LiPF₆. Pulsed field gradient NMR is used to determine self-diffusion coefficients of ¹H and ¹⁹F.

REFERENCES:

^a J. Xiang, F. Wu, R. Chen, L.Li, H. Yu, J. of Power Sources, 233, 115 (2013)

^b Y. Katayama, T. Morita, M. Yamagata, and T. Miura, Electrochemistry (Tokyo, Jpn.), 71, 1033 (2003)

^c A. S. Best, A. I. Bhatt, and A.F. Hollendamp, J. of  the electrochemical Society, 157 (8) A903-A911 (2010)