| Summary: | 碩士 === 國立成功大學 === 化學系 === 89 === Abstract
Molecular dynamics simulation method has been used to study the conducting behavior of LiClO4 in mixed ethylene carbonate/propylene carbonate. Simulations were performed for both constant temperature (298 K) and constant salt concentration (0.82 M) conditions. The diffusion coefficient of lithium ion was computed firstly from the plot of its mean-squared displacement, utilizations were then made of the Nernst-Einstein equation and the probability of free lithium ion to estimate the conductivity. The average numbers of solvent and anion around the lithium ion, along with the degree of ion association, were also analyzed from the plot of radial distribution function.. Finally, the viscosity of the systems were calculated from the plot of stress autocorrelation function.
At 298 K, the simulated conductivity increases firstly and then decrease, with the increase of the salt concentration. In addition, the number of anions around the lithium ion increases, but that of solvent molecules decreases. It indicates that the aggregation of Li+ and ClO4- is guite severe, especially at high salt concentrations, therefore some of the solvent are excluded from the first solvent shell. It’s also known from the analysis of ion clusters that the probability of finding ion monomers decreases at increasing salt concentration, and that the number of ions for the largest cluster increases from 7 (0.33 M) to 68 (3.3 M). Most of the clusters consist of equal amount of positive and negative ions.
At constant salt concentration (0.82 M), the computed conductivity increases with temperature, but the trend for the variation of the number of anion and solvent around the lithium ion is entirely differently from what obtained upon increasing salt concentration. The reason is that the high mobility of the ions at elevated temperature reduce the probability of ion-ion contact. The structure analysis of the clusters reveals that the number of ion monomer increases considerably, which facilitates the electrical conduction.
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