Fabrication of Na+-Conducting Ionic Liquids Na[((FSO2)2n)
x
((CF3SO2)(FSO2)N)1-X
], and Its Physical and Electrochemical Properties
Authors: Yokoyama Yoshiki , Masui Rino , Hiraoka Koji , Watanabe Masayoshi , Seki Shiro 
Published: August 2024

To achieve low-carbon society in near future, it’s imperative to research and develop high-performance and low-cost batteries. Therefore, sodium-ion batteries (SIBs) are attracting attention owing to their possibility to be composed of elements with no resource scarcity and low cost.To solve the challenge of low energy density in SIBs, it is essential to focus on developing electrodes of the 4V to 5V class. Simultaneously, electrolytes with wide electrochemical window are crucial from in terms of electrolyte decomposition at high operating voltages. Molten salt electrolytes exhibit relatively high ionic conductivity and electrochemical stability. On the other hands, there is an issue that the electrolyte must be heated to a high temperature to maintain molten state of the electrolytes. In this study, with the goal of expanding the melting temperature range of molten salt electrolytes, we precisely mixed relatively low-melting sodium salts Na(FSO2)2N (NaFSA) and Na(CF3SO2)(FSO2)N (NaFTA) and found a composition with a melting point below 100 ℃. As a result, we successfully fabricated novel single-sodium cationic salt ionic liquid. In this presentation, we will report on its bulk properties and electrochemical characteristics.

All mixed sodium salts were mixed NaFSA and NaFTA in each molar ratio and stirred during gradually cooling after uniformly melting at 170 ℃ in Ar-filled grove box. The thermal properties of the mixed sodium salts were evaluated by differential scanning calorimetry (DSC). The samples were stored at room temperature for 2~3 days to reach equilibrium state, and then sealed in a high-pressure SUS pan. Ionic conductivity (σ) of the mixed sodium salts were also measured from 60 ℃ to 135 ℃ at each temperature. Symmetrical cells were constructed by put the mixed sodium salt between two SUS or sodium metal electrodes. Ionic conductivity of mixed sodium salts was evaluated by electrochemical impedance spectroscopy (EIS) with SUS electrodes symmetric cells. Na+ transference number was measured by potentiostatic polarization and EIS with sodium metal symmetric cells at 90℃.

The binary phase diagram of NaFSA and NaFTA were shown in Fig.1. In the NaFTA-rich composition range (0.1 < x < 0.4), the phase diagram resembles a simple eutectic phase diagram, while, in the NaFTA-rich composition range (0.5 < x < 0.9), the mixed sodium salts exhibited pseudo-single-phase behavior, and the lowest liquid-phase transition temperature (T
L) was observed at 90℃ of Na[(FSA)0.8(FTA)0.2].(1) Additionally, in this composition range, glass transition points (T
g) were also confirmed, indicating the supercooled behavior.

The Arrhenius plot of the ionic conductivity (σ) for Na[(FSA)0.8(FTA)0.2] was shown in Fig.2. At the T
L around 90℃, σ was observed to 0.69 mS cm-1. This is attributed to the strong interactions between Na+ and anion in the absence of solvents, causing high viscosity and decreasing mobility. Below the T
L, there was no remarkable decrease in σ and Nyquist plots remained non-divergent to 55℃, allowing for the attribution of σ. Moreover, below the T
L, the glass transition and crystallization of Na[(FSA)0.8(FTA)0.2] were not observed. The temperature dependence of σ was analyzed by the Vogel-Fulcher-Tamman (VFT) equation.(2) In addition, the σ(T
g) at the observed T
g from DSC were calculated by VFT equation. Also, the decoupling index (R
τ), representing the ratio of ionic conductive relaxation time (τ
σ) to structural relaxation time (τ
s), were calculated.(3) The value of logR
τ value for Na[(FSA)0.8(FTA)0.2] was -0.62 and the low value indicates that τ
s is coupled with τ
σ.

To investigate the Na+ conduction behavior in Na[(FSA)0.8(FTA)0.2], chronoamperograms with an applied potential of 500 mV to the sodium metal symmetric cells were shown in Fig.3(a). Fig.3(b) shows Nyquist plots before and after direct current polarization. The steady current was observed after 8h. Furthermore, there was slightly changes of Nyquist plots before and after direct current (DC) polarization, stable interface between sodium metal and Na[(FSA)0.8(FTA)0.2] was also confirmed. The Na+ transference Number (t
Na+) in the Na[(FSA)0.8(FTA)0.2] was estimated to be 0.86. Tatara et al. reported the t
Na+ of 0.18 in the 1M NaPF6/PC+0.5vol% FEC.(4) Since molten salts consisting of only cations and anions should not be occurred concentration polarization, it exhibited a considerably higher t
Na+ in Na[(FSA)0.8(FTA)0.2] compared to the t
Na+ in dilute non-aqueous sodium electrolyte. In the presentation, we will discuss the cycle characteristics obtained by constant current charge-discharge tests of both half-cell and full-cell configurations using generic positive and negative electrode materials.

(1) K. Kubota et al., J. Chem. Eng. Data, 55, No. 9, 2010, 3142–3146.

(2) H. Vogel., Phys. Z. 1921, 22, 645.

(3) C. A. Angell, Annu. Rev. Phys. Chem. 1992, 43, 693.

(4) R. Tatara et al., Electrochemistry, 89(6), 2021, 590–596.

Figure 1