Conductivity and Solvation of Li⁺ Ions of LiPF₆ in Propylene Carbonate Solutions

Solutions of lithium hexafluorophosphate, LiPF₆, in propylene carbonate (4-methyl-1,3-dioxolan-2-one; denoted by PC hereafter) in the concentration range from 0.0 to 3.29 M (M = mol dm^-3) have been studied regarding their conductivities, viscosities, and self-diffusion coefficients of PC by the NMR field gradient technique, Raman spectra, and NMR spectra. Walden's products are almost constant in the range up to and over 3.0 M. Therefore, Li ions are considered to be quite free from the firm interaction with anions even in such concentrated solutions. The appearance of the maximum conductivity at about 0.8 M is explained by associating with the concentration dependence of the solution viscosity. A remarkable increase in the solution viscosity was observed in a concentration beyond 2.0 M, and it can be ascribed to the cluster formation of lithium ions with PC molecules of the solvent. Such an idea of clusters can reasonably interpret some of the characteristic changes of the viscosities, the diffusion coefficients, the Raman spectra, and the NMR spectra at concentrations over 2 M.