The application of ionic liquids (ILs) in Li-ion secondary 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-ion insertion/extraction, had been serious problems of losing their charge-discharge capacity. 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. We have also investigated electrochemical behavior of various anodes in the ILs, e.g., Ni-Si-C composite; the alternative anode also exhibits excellent reversibility in our various FSI-based IL electrolytes.We have also tested the performance of some positive electrodes in FSI-based IL electrolytes: mainly two kinds of active materials, LiNMC and LiNM. 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 (EMIm: 1-ethyl-3-methylimidazolium) with a Li salt 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.Our research concerning charge-discharge behavior of various graphitized negative electrodes in the FSI-based ionic liquid revealed that the reversibility of the graphite electrode is affected by its surface structure. Among them, poor cycleability was observed only for an edge-rich graphite. Thus we clarify mechanism of this poor performance, and improve it with the use of an additive.

Specific effect on LIB electrodes by FSI-based ionic liquid electrolytes

247th National Meeting (2014)

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