(Invited) Fsi-Based Ionic Liquid Electrolyte and Lifsi-Based Solvent Electrolyte for Excellent Lib Performance
Authors: Ishikawa Masashi , Yamagata Masaki , Uchida Satoshi , Matsui Yukiko 
Published: February 2020

The application of ionic liquids (ILs) in Li-ion batteries (LIBs) would ensure safety taking advantage of their lower flammability than conventional organic electrolytes. However, decomposition of ILs on a negative electrode and intercalation of an organic cation, which lead to irreversible Li+ insertion/extraction, had been a serious problem of losing their charge-discharge capacity. Nevertheless, we successfully applied bis(fluorosulfonyl)imide anion (FSI)-based ILs to electrolytes for LIBs, which provide reversible Li-ion insertion/extraction and high efficiency for a graphitized anode without any additives [1-6].
We also tested the performance of some positive electrodes in FSI-based IL electrolytes: two kinds of active material, LiNi1/3Mn1/3Co1/3O2 (NMC) and LiNi0.5Mn1.5O4 (NM). According to 1.0/1.0 C-rate charge/discharge measurements in a potential range of 3.0–4.5 V vs. Li, the NMC cathode in EMImFSI with a Li salt (LiTFSI) maintains 95% of the initial discharge capacity even after 100 cycles, suggesting that the FSI-based IL provides relatively stable operation of the NMC cathode without any additives. Another remarkable point is that the NMC cathode in our FSI-based ionic liquids represents an excellent rate capability of ca. 70% even at 10 C-rate. Surprisingly, the value is rather better than that obtained in a conventional electrolyte of LiPF6/EC+DMC. We also tested Li-ion batteries containing FSI ILs as a “full cell”. A Li-ion battery with NMC as cathode material maintained over 80% of its initial capacity in 2000 cycles at 100% DOD [7].
As our IL battery technology have been developed very well, we applied our FSI-based IL-LIBs to a rechargeable power source on a small satellite in space. The Hodoyoshi-3 satellite with our IL-LIBs was launched into space on June 2014. Our space batteries can be still operated on the Hodoyoshi-3 in an earth orbital [8].
Moreover, we are also applying LiFSI-based solvent-type electrolytes to LIBs. We achieved significant reduction of a reaction resistance at the de-solvation process of EC molecule from Li ion by a novel electrolyte design focusing on the solvation state of Li ion by using the LiFSI salt.

References

[1] M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, E. M. Kono, J. Power Sources, 162, 658 (2006).
[2] T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, M. Ishikawa, J. Power Sources,
183, 436 (2008).
[3] T. Sugimoto, Y. Atsumi, M. Kono, M. Kikuta, E. Ishiko, M. Yamagata, M. Ishikawa, J. Power Sources, 195, 6153 (2010).
[4] M. Yamagata, Y. Matsui, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono, M. Ishikawa, J. Power Sources, 227, 60 (2013).
[5] M. Yamagata, N. Nishigaki, S. Nishishita, Y. Matsui, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono, M. Ishikawa, Electrochim. Acta,
110, 181 (2013).
[6] Y. Matsui, M. Yamagata, S. Murakami, Y. Saito, T. Higashizaki, E. Ishiko, M. Kono, M. Ishikawa, J. Power Sources, 279, 766 (2015).
[7] Y. Matsui, S. Kawaguchi, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono, M. Yamagata, M. Ishikawa, Electrochemistry, 80, 808 (2012).
[8] M. Yamagata, K. Tanaka, Y. Tsuruda, Y. Sone, S. Fukuda, S. Nakasuka, M. Kono, M. Ishikawa, Electrochemistry,
83, 918 (2015).