The High Temperature Operation of Lithium Secondary Batteries with Using Ionic Liquids

Ionic liquids (ILs) composed of perfluoroanion such as BF₄⁻ and bis(trifluoromethylsulfonyl)amide (([(CF₃SO₂)₂N]⁻, Tf₂N⁻) have attracted attention as less flammable and less volatile electrolyte for a lithium secondary battery for past decade. Especially, Tf₂N^- forms ILs not only 1-ethyl-3-methylimidazolium (C₂mim⁺), but also various aliphatic quaternary ammonium (AQA⁺). The AQA-Tf₂N exhibited large apparent electrochemical windows enough wide to use a solvent free electrolyte for a 4 V-class battery such as a LiCoO₂ half-cell. Though the rate properties of the cell at room temperature were much lower than that for a conventional organic solvents, another promising ILs composed of bis(fluoromethylsulfonyl)amide (f₂N⁻) are dramatically improved the rate property of the cells using ILs. However, it should be focused on not only the rate performance but also the actual thermal stability of the cells using ILs, which might not depend on only the thermal stability of ILs.[1]

  In this presentation, we would like to show the effect of cationic species of Tf₂N^--ILs on the cycle stability of a cathode half-cell at high temperature over 100 °C. For this purpose, a conventional LiFePO₄ cathode composed of polyvinylidene fluoride (PVDF) (KUREHA, #1120) as binder and acetylene black (AB) as the conductive additive. We found that the rate property of the LiFePO₄half-cells were much improved over 85 °C and also the cycle performance of the cells over 100 °C were much depend on the kind of cation in ILs.

Acknowledgements

A part of this work was supported by the R&D project for Li batteries by NEDO of Japan (Li-EAD) and Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST).

Reference

[1] H. Matsumoto, in Electrolytes for Lithium-ion Batteries, eds. T. R. Jow, K. Xu, O. Borodin and M. Ue, Springer, 2014, ch.4 and references therein.