How Does the Solvent Composition Influence the Transport Properties of Electrolyte Solutions?
Authors: Uchida Satoshi , Kiyobayashi Tetsu 
Published: December 2020

The electrolyte solutions used in lithium ion batteries have been optimized based on the empirical knowledge and concept. Yet the ion transport phenomena in the electrolyte solution, which composed of mixed solvent with salt concentration of about 1 mol kg−1, are complicated and significantly different from those in dilute solutions. In the present study, the characteristics of the electrolyte solutions are reanalyzed by using physical and spectroscopic data for the electrolyte solutions, which were obtained from our experiment, and by referring to recent theoretical studies 1,2) and the experimentally determined diffusion coefficient in previous studies 3,4).

We experimentally measured the viscosity η and ionic conductivity σ of the electrolyte solutions of 1 mol kg−1 of LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSA) dissolved in the binary mixture solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) in the temperature range of 288 ≤ T/K ≤ 328 with varying the EC content from 0 to 60 vol.%, which translates into the molar fraction of EC in 0 ≤ x
EC ≤ 0.6 and the solute to solvent ratio constant (total ca. 11 solvent molecules are present per solute in the system). The diffusion coefficients D of each species, Li+, PF6, FSA−, EC and DMC, were determined by pulse gradient spin echo NMR. The state of molecules around Li+ was examined by the Raman spectra of the solvents and anions.

With the increase in x
EC, DMC solvating Li+ is replaced by EC, resulting in the increase in the total number of solvents around Li+. Reanalysis of Raman spectra based on Borodin's calculations 2) reassessed the solvation numbers and confirmed that the preferential coordination of EC to Li+, but the extent of the preference was not as strong as previously conceived. The addition of EC, more viscous fluid than DMC, into the system leads to the increase in η, which should be responsible for the monotonous decrease in D of all the species with x
EC. On the other hand, σ initially increases until marking the maximum at x
EC = ~0.3. The opposite trend between D and σ, i.e., dD/dx
EC < 0 while dσ/dx
EC > 0, at low x
EC suggests that the Nernst-Einstein relation does not hold as it ignores the inter-ionic correlations. The analysis of the Stokes radii r
St based on η and D obtained in the present study implies that the increase in x
EC results in the decrease in r
St for all the species, among which the anions are most influenced. Although only an equivocal picture on the cation-anion correlation, such as the ion pair formation, was obtained from the Raman spectroscopy, the present study is consistent with the view that the highly Li+-solvating EC, with its higher dielectric shielding effect than DMC, liberates the anions from the attraction from Li+, enhancing the cation-anion anti-correlation, which positively contributes to the ionic conductivity until the viscosity prevails at high x
EC. The Einstein-Stokes formula and van der Waals radius are certainly "classical" concepts; nonetheless, if properly formulated, they can be consistent with the solvation number obtained from Raman spectra not only qualitatively but also semi-quantitatively.

References

Borodin, G. V. Zhuang, P. N. Ross and K. Xu, Phys. Chem. C, 117, 7433 – 7444 (2013).

Borodin, M. Olguin, P. Ganesh, P. R. C. Kent, J. L. Allen and W. A. Henderson, Chem. Chem. Phys., 18, 164 – 175 (2016).

Hayamizu, Y. Aihara, S. Arai, C. Garcia-Martinez, J. Phys. Chem. B, 103, 519 – 524 (1999).

Hayamizu, J. Chem. Eng. Data, 57, 2012 – 2017 (2012).

Figure 1