Solvation and Ion Association Studies of LiBF₄−Propylenecarbonate and LiBF₄−Propylenecarbonate−Trimethyl Phosphate Solutions

Concentrated LiBF 4 solutions in propylenecarbonate (PC) or PC−trimethyl phosphate (TMP) were studied regarding the ion-association and solvation of Li + ions in each solution from the measurements of conductivities, viscosities, Raman spectra, and diffusion coefficients by field gradient NMR spectroscopies. From the Walden product evaluated, the dissociation degree of LiBF 4 in the LiBF 4 −PC solution was roughly deduced to be about 30% in concentrations over 1 mol dm -3 . About 1.8 PC molecules were found bound strongly to a Li + ion in the LiBF 4 −PC solution, and this number was smaller than 3 for LiPF 6 in PC solutions. The small solvation number could be evidence of the formation of contact ion-pairs of Li + −BF 4 - . The diffusion coefficients of BF 4 - came closer to those of Li + ions as concentrations increased, and this is also in accordance with the assumption that the contact ion-pairs are formed. In LiBF 4 −PC−TMP solutions, about 50% of LiBF 4 dissociates at 1 mol dm -3 . TMP as a Lewis base is expected to suppress the contact ion-pairs of Li + −BF 4 - and to improve the conductivity.