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

Concentrated LiBF4 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 LiBF4 in the LiBF4−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 LiBF4−PC solution, and this number was smaller than 3 for LiPF6 in PC solutions. The small solvation number could be evidence of the formation of contact ion-pairs of Li+−BF4 -. The diffusion coefficients of BF4 - 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 LiBF4−PC−TMP solutions, about 50% of LiBF4 dissociates at 1 mol dm-3. TMP as a Lewis base is expected to suppress the contact ion-pairs of Li+−BF4 - and to improve the conductivity.