LiTFSA (Lithium Bis(trifluoromethanesulfonyl)imide) and LiFSA (Lithium bis(fluorosulfonyl)imide) are well-known as a salt of an electrolyte of a Li-based secondary battery, and also used as a salt of an electrolyte of a secondary battery using metal Li as a negative electrode. In many cases, when using these electrolytes to electrodeposit metal Li, side reactions occur at about 1.8 V vs Li⁺/Li.^1,2 Recently, we have discovered that this side reaction is reversible and appears prominently at temperatures as high as 150 ° C.

The electrolyte was prepared by dissolving 1 M of Li [TFSA] in Pyr13 [TFSA] (N-Propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide). A Ni (Nickel) substrate was immersed in this electrolytic solution to form a working electrode. A metal Li was used as a counter electrode to form a two-electrode cell. A current was flowed to the reduction side at a current density of 0.2 and 1 mA cm^-2, and a current was flowed to the oxidation side when it reached 0.1 V. The voltage-time profiles are shown in Figure (a)(b). At the time of reduction, it has a plateau at a voltage of 1.5 V, indicating that some electrochemical reaction has occurred. The oxidation has a plateau near 1.9 V, suggesting that this reaction is electrochemically reversible.

The result of performing cyclic voltammetry measurement for the cells of the same configuration is shown in Figure (c). Reversible reduction and oxidation waves can be confirmed at the potential (ca. 1.5 V and 1.9 V) corresponding to the plateau potential in constant current measurement. Also, it was seen that the peak increases with each cycle.

At the meeting site, we will also describe the results of using LiFTA (lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide), which is a molten Li single salt, and discuss the origin of side reactions.

Acknowledgments

This work was performed as a cross-appointment project between Osaka University and the National Institute of Advanced Industrial Science and Technology (AIST).

References

1. D.Aurbach, M. Daroux, P. Faguy, E. Yeager, J. Electroanal. Chem. Interfacial Electrochem., 297, 225 (1991).
2. T. Shiga, Y. Kato, M. Inoue, N. Takahashi, Y. Hase, J. Phys. Chem. C, 119, 3488 (2015).