Lithium Fsi-Based Low Ethylene Carbonate Content Electrolyte for High-Power Lib
Authors: Ishikawa Masashi , Uchida Satoshi 
Published: February 2020

Among the numerous factors that may influence the charge-discharge rate performance of Li-ion batteries, a desolvation process may be a crucial factor; T. Abe et al. have reported that the Li ion intercalation process into active materials has large activation energies 50 – 60 kJ mol−1in EC-containing conventional organic electrolyte systems and the large activation energy is attributed to the desolvation process of EC molecules from Li ions at an electrode/electrolyte interface [1]. This suggests that the rate-determining process of a charge-discharge reaction is considered the desolvation process of EC molecules from Li ions.

Based on the idea described above, the solvation number of EC to a Li ion should be reduced as much as possible to improve charge-discharge rate performance of Li-ion batteries. However, decreasing EC content should decrease the dissociation degree of Li salts and thus the ionic conductivity should decrease. Such a decrease in ionic conductivity would be fatal when we attempt to enhance rate performance of an electrode in a Li-ion battery because the ionic conduction of an electrolyte in a porous structure like an inside structure of positive or negative composite electrodes was found to be much lower than that of the corresponding bulk electrolyte [2]. This finding suggests that decreased ionic conductivity by limited dissociation should strongly impede charge-discharge reaction kineticsinside an electrode, even if the activation energy of a desolvation process is decreased. It is obvious that, therefore, the rate performance of Li ion batteries cannot be improved by just reducing EC content in the usual electrolytes.

To avoid this trade-off, we have focused on the interaction between a Li ion and a counter anion in this study. In our previous study on the ionic liquid electrolyte composed of 1-ethyl-3-methylimidazorium bis(fluorosulfonyl)imide (EMImFSI) and Li bis(fluorosulfonyl)imide (LiFSI), we already had known that the interaction between a Li ion and an FSI anion is quite weak [3, 4]. Based on this knowledge, we had expected that LiFSI disassociates sufficiently without usually required high-permittivity solvent conditions. Thus, we prepared LiFSI-based organic electrolytes without EC or with low EC-content and measured their ionic conductivities. As we had expected, these electrolytes showed higher ionic conductivities compared with the case of using conventional Li salts. Furthermore, we observed different electrode/electrolyte interfacial behavior from the conventional electrolytes containing a large amount of EC.

This paper mainly reports the electrochemical properties and unusual interfacial behavior of LiFSI-based low-EC-content organic electrolytes and the corresponding superior Li-ion battery performances [5]. We prepare the LiFSI-based low EC content electrolyte (LiFSI / EC:DMC = 1:9) with a low viscosity and high ionic conductivity comparable to conventional LiPF6/ EC:DMC = 1:1 on the basis of our knowledge that the interaction between a Li ion and FSI anion is quite weak. The Li ions in LiFSI / EC:DMC = 1:9 have an unusual solvation state when compared to conventional high EC content electrolyte systems and we considered that the solvation number of EC molecules to a Li ion in LiFSI / EC:DMC = 1:9 is significantly decreased. The graphite half-cell assembled with LiFSI / EC:DMC = 1:9 shows an overwhelmingly small Li ion transfer resistance mainly attributed to a desolvation process. In addition, both the graphite half-cell and the graphite / LiNi1/3Mn1/3Co1/3O2full-cell assembled with LiFSI / EC:DMC = 1:9 show outstanding charge and discharge rate performance when compared to conventional EC:DMC = 1:1 systems. We consider that these results are related to a facile desolvation process attributed to an unusual solvation state of Li ion in LiFSI / EC:DMC = 1:9. Furthermore, the graphite / LiNi1/3Mn1/3Co1/3O2full-cell assembled with LiFSI / EC:DMC = 1:9 shows superior cycle stability compared to the conventional LiPF6/ EC:DMC = 1:1 system, which seems to be attributed to a quite small amount of HF in our LiFSI / EC:DMC = 1:9 system. We will also report electrolyte additives inhibiting corrosion caused by FSI at Al current collector of a cathode.

References

[1] T. Abe, H. Fukuda, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc. 151 (2004) A1120.

[2] T. Fukutsuka, K. Koyamada, S. Maruyama, K. Miyazaki, T. Abe, Electrochim. Acta 199 (2016) 380.

[3] T. Sugimoto, Y. Atsumi, M. Kikuta, E. Ishiko, M. Kono, M. Ishikawa, J. Power Sources 189 (2009) 802.

[4] M. Yamagata, N. Nishigaki, S. Nishishita, Y. Matsui, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono, M. Ishikawa, Electrochim. Acta 110 (2013) 181.

[5] S. Uchida, M. Ishikawa, J. Power Sources, 359 (2017) 480.