Electrochemical Investigation of the Solid Electrolyte Interface on Lithium Electrodes Cycled with a Glyme-Litfsi Electrolyte
Authors: Xiong Shizhao , Xie Kai , Jacobsson Per , Matic Aleksandar 
Published: February 2020

Due to their high thermal stability, good compatibility with lithium, and nonflammability, room temperature ionic liquids (RTILs) based electrolytes have attracted considerable attention for application in lithium-sulfur batteries and lithium-air batteries [1, 2]. However, two limiting factors have so far been the low Li+ concentration and low Li+ transference number [3]. A quasi-ionic liquid consisting of glymes and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), in which the lithium cation is the only cation species, has recently been proposed as a way to address these issues [4]. This complex exhibits similar advantages as RTILs but has a higher ionicity and a higher lithium transference number and has shown to contribute to good performance in lithium-sulfur batteries [5, 6].

In this contribution we report on the formation of the solid electrolyte interphase (SEI) on lithium electrodes in glyme (tetra ethylene glycol dimethyl ether, G4) - LiTFSI electrolytes with different molar ratios (G4:LiTFSI, 8:1 to 1:1). As the molar ratio (G4:LiTFSI) decreases the cycling efficiency, in terms of lithium deposition and dissolution on Cu disks, increases from 20% to around 95% for the 1:1 complex. The cycling behavior and impedance measurements in symmetric cells show that the electrolytes with various molar ratio of G4 to LiTFSI result in SEI films with different properties. Furthermore, the SEI film formed in the electrolyte with the equimolar ratio is relatively stable and our results indicate that the equimolar complex of G4 and LiTFSI has an improved compatibility with the lithium electrode. From X-ray photoelectron spectroscopy (XPS), with argon-ion sputtering for depth profiling, we find that the relative amount of fluorinated species, such as LiF and C-F, in the films increases with increasing molar ratio of G4 to LiTFSI. This suggests that the breakdown of TFSI anions is more likely to happen in electrolytes with higher glyme concentration. These results are of importance when considering the implementation of G4-LiTFSI complexes as electrolytes in Li-metal type batteries.

Fig. 1. Cycling efficiency (EFF) of the lithium deposition and dissolution on Cu disk in [Li(G4)x][TFSI] (x = 1, 2, 4, and 8).

Acknowledgements

The support provided by China Scholarship Council (CSC) during a visit of Shizhao Xiong to Chalmers is acknowledged. We are grateful to the support of the Areas of Advance Energy, Materials Science, and Transport at Chalmers.

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