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ARTICLE

Theoretical Study of Solvation Effect on Diffusion Coefficient of Li Ion in Propylene Carbonate

Kentaro Doi^1,2, Yuzuru Chikasako¹, Satoyuki Kawano^1,3

1 Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560–8531, Japan.
2 E-mail: doi@me.es.osaka-u.ac.jp
3 E-mail: kawano@me.es.osaka-u.ac.jp

Fluid Dynamics & Materials Processing 2015, 11(1), 1-26. https://doi.org/10.3970/fdmp.2015.011.001

Abstract

Propylene carbonate (PC) and ethylene carbonate are known as good candidates of organic solvents to be used in Li-ion rechargeable batteries, since Li⁺ ions exhibit preferable charge-discharge characteristics with such solvents. On the other hand, polar solvents usually form solvation shells with solute ions, and cause a drastic reduction of ionic conductivity. Along these lines, there has been a curious question why the diffusion coefficient D_Li of Li⁺ strongly depends on the salt concentration. In the present study, a theoretical model is developed on the basis of the Langevin equation in which the interactions between ions and solvent molecules are explicitly taken into account. Interesting phenomena, which were found in experiments but had not yet been theoretically clarified, are discussed in detail. Molecular dynamics (MD) simulations are also performed to elucidate the relationship between the solvation shell of Li⁺ and D_Li. Analyzing the radial distribution function of PC molecules around Li⁺ ions, the existence of first and second solvation shells (consisting of locally and highly concentrated PC molecules) is numerically clarified. In particular, overlapped regions of the second solvation shells are clearly observed, with the shell volume apparently increasing with an increase in the concentration of Li⁺ . This result indicates that the solvation structures can attractively interact with each other via overlaps of the second shells. The theoretical model and MD simulations are in excellent agreement with experimental data.

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