−) for lithium-ion batteries since 2006. [1] This liquid with a lithium salt offers incredible charge-discharge performances; despite the relatively high viscosity of the electrolyte, it affords not only reversible and stable cycling but also rapid charge-discharge performance for both negative and positive electrodes. [2] In particular, we are interested in the mechanism of the high rate capability and high stability of the negative electrode of a LIB in FSI-based ionic liquids. By applying several electrochemical techniques, we also revealed the specific electrode–electrolyte interface formed in the presence of both Li+ and FSI− in the ionic liquid electrolyte [3]. On the negatively polarized carbon electrode, minimal Li+ can exist dominantly at the primary layer because of the moderately strong interaction between Li+ and FSI−, leading to the exclusion of EMIm+ away from the electrode. As a result, the presence of FSI− apparently improves the cathodic stability of the electrolyte.Herein, we report the results from charge-discharge tests, focusing on the effect of the electrolyte composition; we investigated how the high concentrated ionic liquid electrolyte including the lithium salts promotes the more stable and rapid charging-discharging of lithium-ion cell.
To clarify the effect of concentration of LiFSI on the dependencies of the discharge capacity retention of a graphite electrode in EMImFSI and EMImTFSI (EMIm+ = 1-ethyl-3-methylimidazolium) on the charge-discharge C-rate, which were evaluated by determining the 5-cycle durability for each C-rate operation after primary charge-discharge cycling at the 0.1/0.1 C-rate, are summarized in Figure 1. Here, the discharge capacity retention was calculated as Cx/C2, where Cx and C2 are the specific discharge capacities for each cycle (x) and for the second cycle (1.0 C-rate), respectively. The remarkable aspect in this figure is that the high concentration (1.46 mol dm−3) of LiFSI improves the output characteristics in comparison with 0.43 mol dm−3 LiFSI/EMImFSI despite the electrolyte possessing a lower ionic conductivity, while 1.46 mol dm−3 LiTFSI/EMImFSI decreases the rate capability of the graphite compared with 0.43 mol dm−3 LiTFSI/EMImFSI.
References 1. M. Ishikawa et al, J. Power Sources, 2006, 162, 658-662.2. Y. Matsui et al., Electrochemistry, 2012, 80, 808-811; M. Yamagata et al., J. Power Sources, 2013, 227, 60-64. 3. M. Yamagata, N. Nishigaki, S. Nishishita, Y. Matsui, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono, and M. Ishikawa, Electrochim. Acta, 110, 181 (2013).
