The Importance of Electrolyte Composition to the Charge-Discharge Performance of Lithium-Ion Batteries Using Ionic Liquids
Authors: Yamagata Masaki , Matsui Yukiko , Ishikawa Masashi 
Published: February 2020

Ionic liquids such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) are still attractive candidates for use as electrolytes in electrochemical energy storage devices, because of their relatively low flammability and reactivity, and because of their electrochemical stability, wide liquidus temperature range, and acceptable ionic conductivity. We have reported a promising ionic liquid based on bis(fluorosulfonyl)imide (FSI-) 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
/C
2, where Cx
and C
2 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-3LiTFSI/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).

Figure 1