Structure and transport properties of the LiPF6 doped 1-ethyl-2,3-dimethyl-imidazolium hexafluorophosphate ionic liquids: a molecular dynamics study.
Molecular dynamics simulations have been performed on 1-ethyl-2,3-dimethyl-imidazolium hexafluorophosphate (EMMIPF(6)) ionic liquids (ILs) doped with different molar ratios of LiPF(6) at 523.15 K and 1 bar. Ionic conductivity, self-diffusion coefficients, density, and viscosity predicted by MD simulations were found to be in good agreement with previous studies. Structural analysis shows that the Li(+) cation is strongly coordinated by the F atom of the PF(6)(-) anion, and the number of F atoms coordinated with a Li(+) cation in the first solvation shell is about six for all molar ratios of LiPF(6)/EMMIPF(6) 0.05, 0.15, 0.30, and 0.50. The coordination number of the PF(6)(-) anion within the first solvation shell of Li(+) cation is about four, which tends to increase slightly when the salt concentration is increased. The two-dimensional radial-angular distribution study shows that the Li(+)-PF(6)(-) complex tends to form the C(2v) conformation at low salt concentration, whereas C(4v) conformation becomes important at higher salt concentration. It is found that the aggregation of Li(+)-PF(6)(-) complexes occurs in all four molar ratios, whereas ionic conductivity decreases and viscosity increases at higher salt concentration. The residence time correlation of PF(6)(-) within the first solvation shell of Li(+) shows a strong memory effect. The Li(+)-hopping function further shows that the hopping of Li(+) is strongly affected by its environment with different exchange rates of the PF(6)(-) anions for the structure diffusion, and the system of 0.5 LiPF(6)/EMMIPF(6) molar ratio has the slowest hopping rate.