FTA-Based Ionic Liquids: Promising Electrolytes for Alkali Metal-Ion Secondary Batteries
Authors: Nishijima Shu , Yadav Alisha , Yamamoto Takayuki , Nohira Toshiyuki 
Published: December 2020

1. Introduction

Lithium secondary batteries have been widely used as power sources for small electronic devices. However, there are still many challenges for the widespread use as large storage batteries, such as the volatility and flammability of the organic-solvent-based electrolytes, and the uneven distribution and scarcity of lithium and cobalt resources.
Therefore, we have been developing sodium and potassium-ion secondary batteries using FSA-based ionic liquid (IL) electrolytes (FSA = bis(fluorosulfonyl)amide) because the FSA-based ILs have excellent electrochemical stability and ionic conductivities, and sodium and potassium resources are abundant in the Earth’s crust [1,2].
In this study, we focus on FTA-based ILs (FTA = (fluorosulfonyl)(trifluoromethylsulfonyl)amide) as promising electrolytes for alkali metal-ion batteries. The FTA-based ILs have lower melting points and higher thermal stability than the FSA-based ones, which is advantageous for broadening the operating temperature range of batteries [3,4]. The physicochemical properties of M[FTA]–[C4C1pyrr][FTA] ILs (molar fraction x(M[FTA]) = 0.20; M = Li, Na, K, Rb, Cs; C4C1pyrr = N-butyl-N-methylpyrrolidinium) were investigated to discuss the effect of alkali metal-ion species.

2. Experimental

All reagents were handled under argon atmosphere. The M[FTA]–[C4C1pyrr][FTA] (x(M[FTA]) = 0.20; M = Li, Na, K, Rb, Cs) ionic liquids were prepared, and their ionic conductivities, viscosities, densities, and electrochemical windows were measured. The electrochemical windows were determined by cyclic voltammetry (CV) measurements using a three-electrode cell. The components of the three-electrode cell were as follows; a copper disk electrode and a glassy carbon disk electrode were used in the negative and positive potential region as the working electrodes, respectively, a platinum mesh electrode was used as the counter electrode, and an Ag+/Ag electrode was used as the reference electrode.

3. Result and discussion

Fig. 1 shows the Arrhenius plots of the ionic conductivities for M[FTA]–[C4C1pyrr][FTA] ILs. The order of the ionic conductivities (mS cm−1) is Na(1.7) < Li(2.0) < K(2.2) < Rb(2.3) < Cs(2.6) at 298 K. In terms of ion size, the smallest Li+ is considered to be the most favorable for the ionic conduction. However, its ionic conductivity is lower than those of K+, Rb+, and Cs+-based systems. One possible explanation is that the higher charge densities of Li+ and Na+ lead to the strong ion interaction with FTA−, resulting in the lower ion mobilities of alkali-metal ion complexes. We also measured their viscosities and densities of the FTA-based ILs at 273–368 K and constructed a Walden plot, which suggested that all the FTA-based ILs do not have a special ion conduction mechanism such as the Grotthuss mechanism.
Fig. 2 summarizes the results of the CV measurements for M[FTA]–[C4C1pyrr][FTA] ILs. The order of their electrochemical windows (V) is Na(5.33) < Li(5.45) < K(5.58) < Rb(5.64) < Cs(5.74) at 298 K. The redox peaks were observed in the negative potential regions for all the electrolytes, which corresponds to the deposition and dissolution of alkali metals. On the other hand, irreversible oxidation currents were observed in the positive potential region for all the electrolytes. Since there was little difference in the anode limits for all the electrolytes, these oxidation currents are considered to be the decomposition of FTA− anions [4]. The trends of the physicochemical properties obtained in this study are very similar to those for FSA-based ionic liquids reported by our group [1].

R
eferences

[1] T. Yamamoto, K. Matsumoto, R. Hagiwara, T. Nohira, J. Phys. Chem. C, 121, 18450 (2017).
[2] R. Hagiwara, K. Matsumoto, J. Hwang, T. Nohira, Chem. Rec., 18, 1 (2018).
[3] K. Kubota, T. Nohira, R. Hagiwara, H. Matsumoto, Chem. Lett., 39, 1303 (2010).
[4] K. Kubota, H. Matsumoto, J. Phys. Chem. C, 117, 18829 (2013).

Figure 1