Transport Properties of Highly Concentrated Li Salt Solutions
Authors: Watanabe Kenta , Tatara Ryoichi , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

1. Introduction
Highly concentrated Li salt solutions (>3 mol dm-3) have attracted considerable attention due to their unique properties, such as low volatility, thermal stability, electrochemical stability, and high lithium transference number [1]. However, the origin of unique nature of the concentrated electrolyte is still unclear. In this work, we evaluated the dissociativity of electrolyte solutions with various Li salt concentrations and solvents. The ratio of the molar conductivity (Λ
imp) of solution measured using AC impedance and the calculated molar conductivity (Λ
NMR) using Nernst-Einstein equation from the self-diffusion coefficients of ions reflects the degree of dissociation (i.e. ionicity) of salt in the solution [2]. We previously reported ionicity value (Λ
imp/Λ
NMR) for the Li[TFSA]/glyme mixutres increases as increasing the Li salt concentration [3]. On the contrary, in propylene carbonate solution of LiPF6, Λ
imp/Λ
NMR value was found to decrease with increasing salt concentrations [4]. Although the physicochemical implication of the ionicity value of electrolyte solutions, especially for concentrated electrolytes, remains a matter of controversy, it is of great interest to evaluate the electrolyte solutions with various solvents as a function of Li salt concentration. Transport properties such as viscosity of the solution and self-diffusion coefficients of Li+, anion and solvent in the solution will also be discussed in detail.

2. Experimental
The solutions were prepared by mixing Li[TFSA] ([TFSA]:bis(trifluoromethanesulfonyl)amide) and solvents in different molar ratios in an Ar-filled glovebox. Diethyl carbonate (DEC), acetonitrile (AN), dimethyl sulfoxide (DMSO) and propylene carbonate (PC) were used as solvents. Ionic conductivities of the solutions were measured by AC impedance method. Diffusion coefficients of the solvents, Li+ and [TFSA]- ions were measured by pulsed-field gradient spin-echo NMR (PGSE-NMR) method.

3. Result and discussion

Fig. 1 shows concentration dependence of Λ
imp/Λ
NMR value in Li[TFSA] solution with various solvents. Permittivity (ε

r) and donor number (DN) of the solvents used here is also shown in Table 1. In the region of lower Li[TFSA] concentration, Λ
imp/Λ
NMR values of PC solution and DMSO solution, which have high permittivity, are higher than those of other solutions. It indicates that polar solvent molecules can reduce interaction between solvate cation and anion in the solution by means of strong electrostatic shielding, resulting in higher dissociativity.
As for highly concentrated region, ionicity of DMSO solution, which has high DN, showed higher value than other solvents systems. In the case of highly concentrated DMSO solution (>3 mol dm-3
, molar ratio of [DMSO]/[Li+] is less than 3), almost all the DMSO molecules in the electrolyte solution must coordinate to Li cation. In general, diffusion coefficient of solvent (D
sol) is larger than that of Li ion (D
Li) even for concentrated region, because solvent exchange of solvate cation easily occurs. As well as Li+-solvent interaction, there also exists Li+-anion interaction, which is an important factor for the contact ion pair (CIP) formation. In the solution with large DN solvent such as DMSO, interaction between Li+-DMSO is stronger than that of Li+-[TFSA]-. As shown in Fig. 2, D
sol/D
Li in highly concentrated Li[TFSA]/DMSO solution was smaller than that in other electrolyte solutions and close to one, suggesting that stable solvate cation can be formed in highly concentrated DMSO solutions. Therefore, in such case, it is considered that ionicity of concentrated DMSO solution is higher because ion exchange between ion pairs would be faster than solvent exchange. On the other hand, ionicity of the solution based on lower permittivity and DN solvents decreases because relatively stronger cation-anion interaction makes the lifetime of ion pair in the solution longer.

4. Acknowledgements
This study was supported in part by the Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of the Japan Science and Technology Agency (JST).

5. References
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, 13121 (2011); L. Suo et al., Nat. Commun., 4, 1481 (2013); Y. Yamada et al., J. Electrochem, Soc., 162, A2409 (2015).
[2] H. Tokuda et al., J. Phys. Chem. B, 109, 6103 (2005); C. Zhang et al., J. Phys. Chem. B, 118, 5144 (2014).
[3] K. Yoshida et al.,J. Phys. Chem. C, 115, 18384 (2011); C. Zhang et al., J. Phys. Chem. B,
118, 5144 (2014).
[4] M. Takeuchi et al., J. Mol. Liq., 148, 99 (2009).

Figure 1