First-Principles Study of Li-Ion Diffusion Mechanism in Highly Concentrated Li-Salt Electrolye

Li-salt concentration has been recently proposed as an important control parameter of reduction stability of electrolytes and high ion conductivity in Lithium-ion batteries (LIBs)¹. The mechanism of the reduction stability improvement of high concentration (HC) systems has been revealed by our first-principles calculations^1,2, while the rather high Li-ion conductivity under such high-viscosity environment remains an open question. In general, higher viscosity in the HC electrolyte decreases the diffusion coefficient and thus Li-ion conductivity according to the Stokes-Einstein and Nernst-Einstein equations. However, this regime does not seem to hold here. To elucidate this problem, we carried out extensive DFT-MD sampling to examine the Li-ion diffusion mechanism in the HC electrolytes.

The LIBs are often classified by the mechanism of Li-ion diffusion. In conventional LIBs with dilute liquid electrolytes, Li-ions are coordinated only by solvent molecules, and the ions move to an electrode accompanied by the coordinated solvent molecules. In polymer electrolyte LIBs, Li-ions diffuse by “hopping” between oxygen sites of the polymer electrolyte and that fluctuation of the polymer backbone plays a predominant role in ion conduction. In contrast, in all-solid-state LIBs with solid electrolytes, the geometry of the backbone structure is fixed, and Li-ions are conducted through specific pathways in the crystal.

In this study, we investigated the Li-ion diffusion mechanism in acetonitrile (AN) solvent with LiN(SO₂F)₂ (Li-FSA) and LiN(SO₂CF₃)₂ (Li-TFSA) salts systems at some Li-salt concentrations by using density functional theory based molecular dynamics (DFT-MD) calculations. The diffusion coefficients of single-ion in HC systems were also investigated by means of solvation structure analysis.

We calculated the diffusion coefficients of the Li ions, FSA and TFSA anions, and AN solvent molecules in the low-concentration (LC) and HC electrolytes to elucidate how Li-ion diffusion was affected by concentration (Fig. 1). The calculated diffusion coefficients of Li ions were 0.80 × 10^-6 (1.4 × 10^-6) cm² s^-1 and 7.7 × 10^-6 (6.9 × 10^-6) cm² s^-1, respectively, for the HC and LC TFSA (FSA) electrolytes. The calculated values for the Li-FSA/AN electrolyte were on the same order of magnitude as the experimental values. The diffusion coefficients for the HC electrolytes were approximately one order magnitude lower than those for the LC electrolytes. The reported Li-ion diffusion coefficient in a 1 mol dm^-3 solution of LiClO₄ in propylene carbonate³, which is a popular salt-solvent combination for LIBs, is 2.6 × 10^-6 cm² s^-1. In contrast, in 2.75 mol dm^-3 [Li(G4)][TFSA], a HC solvate ionic liquid that can also be used for LIBs⁴, the Li-ion diffusion coefficient is 0.13 × 10^-6 cm² s^-1. These values suggest that the diffusion coefficients of Li ions in the HC electrolytes are large enough for practical use of these electrolytes in LIBs.

In the LC electrolytes, we confirmed that each Li-ion is coordinated only by solvent molecules, and the Li-ions diffuse in the company of the coordinated solvent molecules (vehicle-type diffusion). In the HC electrolytes, the vehicle-type diffusion is difficult because the Li-ions are coordinated both by solvent molecules and by anions arranged in a specific network structure², which results in high viscosity. We analyzed the motions of individual Li ions in the HC electrolytes, and found Li-ion hopping between the oxygen atoms of the anions in both FSA and TFSA anion systems. Additional DFT-MD calculations with different solvents also suggest the Li-ion hopping diffusion mechanism. We concluded that change of the diffusion mechanism can be an origin of the high Li-ion conductivity in the HC electrolytes.

[1] Y. Yamada, K. Furukawa, K. Sodeyama, M. Yaegashi, K. Kikuchi, Y. Tateyama, A. Yamada, J. Am. Chem. Soc. 136, 5039-5046 (2014).

[2] K. Sodeyama, Y. Yamada, K. Aikawa, A. Yamada, Y. Tateyama, J. Phys. Chem. C 118, 14091-14097 (2014).

[3] J. M. Sullivan, D. C. Hanson, R. Keller, J. Electrochem. Soc. 117, 779-780 (1970).

[4] T. Tamura, K. Yoshida, T. Hachida, M. Tsuchiya, M. Nakamura, Y. Kazue, N. Tachikawa, K. Dokko, M. Watanabe, Chem. Lett. 39, 753-755 (2010).