(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

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