Preparation of Low Melting Temperature Lithium-Based Binary Salts and Their Physicochemical Properties
Authors: Masui Rino , Yokoyama Yoshiki , Watanabe Masayoshi , Seki Shiro 
Published: December 2024

Achieving carbon neutrality demands the development of high-performance Li-ion secondary batteries (LIB), driven by a pressing need for electric vehicles and renewable energy storage. Molten salt electrolytes offer promise for enhancing safety and performance due to their exceptional thermal stability and high Li+ transport numbers(1). However, the high operating temperature range of molten salts, stemming from high melting temperatures, presents a significant challenge. In this study, we propose Li salts with low melting points achieved through increased entropy and eutectic effects by mixing multi-Li salts. Our focus lies on the anion structure of fluorosulfonyl compounds, aiming to create salts with low melting points. Through mixing of an asymmetric anion salt (Li[N(SO2 CF3)(SO2F)], LiFTA) and versatile Li salts (Li[N(SO2F)2], LiFSA and Li[N(SO2CF3)2)], LiTFSA) we identified compositions with melting points lower than those of individual salts. We investigated the preparation scheme of the salt mixtures, their bulk properties, and electrochemical characteristics.

Preparation of mixed salts was conducted in an Ar-filled glove box. Two salts from LiFTA, LiFSA, and LiTFSA were mixed in prescribed molar ratios, dissolved in acetonitrile (AN), and removed AN by heating at 373 K~413 K. The absence of residual AN in the Li mixed salt was confirmed through thermogravimetry (TG-DTA) and differential scanning calorimetry (DSC). The thermal properties of mixed salts were evaluated using TG-DTA and DSC. Ionic conductivity (σ) of the mixed Li salts was measured from 403 K to 263 K, with an alternating current (AC) amplitude of 500 mV over a frequency range of 500 kHz to 50 mHz.

For low melting point LiFTA (T
m = 376 K), pulsed-field gradient NMR (PFG-NMR) was obtained to measure self-diffusion coefficients (D) of each nucleus (Li+, FTA-) above T
m (413 K~378 K). Compared to Li+, FTA- exhibited higher values of D, with activation energies of 74 kJ mol-1 for Li+ and 71 kJ mol-1 for FTA- (Figure 1). The difference in D between Li+ and FTA- suggests individual diffusion of the cation and anion.

Figure 2 illustrates DSC profiles of each Li salt (LiFSA, LiTFSA) and mixed salts (LiFSA/LiTFSA). Mixing LiFSA and LiTFSA revealed a eutectic effect in the salt mixtures, particularly evident in the mixed salt Li([FSA]0.6[TFSA]0.4), which exhibited the lowest T
m at 391 K. This melting point was lower than that of single salts, highlighting the effectiveness of mixing two versatile Li salts. Furthermore, highly amorphous nature was observed in Li([FSA]0.2[TFSA]0.8) through DSC analysis. The incorporation of LiFTA into the mixed salts (Li([FSA]0.6[TFSA]0.4)) holds promise for fabricating mixed salts with even lower T
ms.

Figure 3(a) displays DSC profiles of each Li salt (LiFTA, LiFSA) and mixed salts (LiFTA/LiFSA), while Figure 3(b) also depicts those of each Li salt (LiFTA, LiTFSA) and mixed salts (LiFTA/LiTFSA). The prepared mixed salts resulted in either transparent, highly viscous liquids or transparent to white solids. Mixed salts using LiFTA and LiTFSA became complete amorphous, indicating the potential for further reduction in melting points or the creation of supercooled Li ionic liquids.

The Arrhenius plot of the ionic conductivity (σ) for Li([FSA]0.6[FTA]0.4) is presented in Figure 4. At 403 K, σ was rather low and 0.18 mS cm-1, which can be attributed to strong interactions between Li+ and the anion in the absence of solvents, resulting in high viscosity and reduced mobility. The conductivities exhibited close values between Li([FSA]0.6[FTA]0.4) and LiFTA. LiFTA showed a change from VFT to Arrhenius-type temperature dependence at 308 K, whereas Li([FSA]0.6[FTA]0.4) revealed stronger supercooling behavior compared to LiFTA.

(1)Y. Yamada, A. Yamada, J. Electrochem. Soc.,
162, A2406-A2423 (2015).

Figure 1