A Study on Rate Capacity and Safety of Ionic Liquid-Based Lithium Ion Battery

Lithium ion battery (LIB) is expected to be applied to large-format energy storage system and the cell performance with higher safety along with high energy density and high power density is demanded in industry applications. In order to improve the safety performance of LIB, much attention has been paid to an ionic liquid, instead of an organic solvent, as an electrolyte. LIB with the ionic liquid-based electrolyte, however, shows relatively poor rate capability compared to LIB with the organic solvent. Recently, several papers have revealed that LIB with bis(fluorosulfonyl)imide (FSI) anion-based ionic electrolyte shows good rate capability^1,2). The reason is because of the reduction of charge transfer resistance between FSI anion-based ionic liquid electrolyte and electrode. The point of further development of the LIB using the ionic liquid-based electrolyte is to achieve both the improvement in the rate capability and safety by optimizing cell design including electrode, separator and others.

 In this study, influence of the electrode design and the nonwoven fabric separator on rate capability of a LIB with N-methyl-N-propylpyrrolidinium (Py13)/FSI-based ionic liquid electrolyte was investigated. The safety behavior of the LIB using Py13/FSI-based ionic liquid electrolyte was also evaluated by the overcharging test and nail penetrating test, comparing with the LIB using organic electrolyte. The rate capability of the LIB using Py13/FSI-based ionic liquid electrolyte was improved by optimizing the electrode design with higher porosity and thinner thickness and by using the nonwoven separator with more than 80 % of porosity. It is suggested that the enhancement of lithium ion mobility in both the porous electrode and the porous separator filled with the ionic liquid electrolyte is more effective to improve the rate capability, comparing the organic electrolyte. As a result of evaluation of the safety behavior, LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂cell using Py13/FSI-based ionic liquid electrolyte, as compared with the organic electrolyte, exhibited a superior safety performance in the nail penetrating test. The heat generation was also drastically suppressed by the high thermal stability ionic liquid electrolyte. The heat generation was drastically suppressed in comparison with the organic electrolyte due to high thermal stability of the ionic liquid electrolyte.

 References

1) H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono , J. Power Sources., 160, 1308 (2006)

2) T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, M. Ishikawa, J. Power Sources., 183, 436 (2008)