| Summary: | 碩士 === 國立成功大學 === 化學系碩博士班 === 98 === Molecular dynamics simulations have been used to study the diffusion, conductivity, coordination and ion association properties for LiCF3SO3 in organic solvents propylene carbonate (PC)、dimethoxyethane (DME)、tetrahydrofuran (THF) and in N,N-dimethylformamide (DMF) / polyvinylidene fluoride (PVDF) gel. Simulations were conducted at 298、308、328 and 348 K. The salt concentration in binary system is 0.5 and 1.0 M. In gel system, DMF：LiCF3SO3 ratio is 6：1 and 15：1, respectively, and PVDF：DMF ratio is 3：7 .
The simulated diffusion coefficients are close to that obtained from the NMR measurement and specific conductivities agree well with the experiment. The simulations show that the changes in temperature and concentration, and the addition of PVDF polymer have little effect on coordination distances. The number of solvent molecules around the first coordination sphere of Li+ ion is related to the dielectric constant of solvent. The coordination number in high dielectric PC and DMF (about 3.8 ) is greater than that in low dielectric in DME and THF (about 1.7 ). It is known from the radial distribution function that in binary systems Li+ is closest to oxygen atom of DME, followed by THF, and then PC. In PC system, the fraction of small ion cluster is higher than that in DME and THF, indicating the low degree of ion association. Ion aggregation occurs more easily in DME and THF containing systems, which leads to the relatively low conductivity. For example, the fractional free ions in 1.0 M DME and THF systems is 0.155 and 0.130 , respectively, but whose value is remarkably large in PC system ( 0.54 ).
In the gel system, the high salt concentration facilitates the ion pair formation, but depletes solvent and PVDF coordination around Li+ ion. At high salt concentration, ion association between cations and anions is more pronounced, resulting an increasing cluster size, and a decreasing fractional free ions. Take temperature 308 K for example, as DMF：LiCF3SO3 ratio varies from 15：1 to 6：1, the fraction of free ions decreases from 0.435 down to 0.255. With the addition of PVDF, the system is gel-like, and the fractional free ions descends. For example, after the addition of PVDF at temperature 308 K and DMF：LiCF3SO3＝6：1 , the fractional free ions decreases from 0.350 to 0.255.
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