A Divalent Lithium Salt Li2B12F12 Dissolved in Propylene Carbonate Studied by NMR Methods

A divalent lithium salt Li2B12F12 dissolved in propylene carbonate (PC) with four different concentrations was studied by NMR methods and ionic conductivity. The individual diffusion coefficients were measured by H-1, Li-7, and F-19 pulsed-gradient spin-echo NMR for the solvent PC (D-PC), lithium (D-Li), and anion (D-anion). Because two- step salt dissociation processes can be assumed for the divalent Li2B12F12, the existence of LiB12F12- in addition to Li+ and B12F12 (2-) must be taken into account. The comparison was made with the diffusion coefficients of LiPF6 in PC. The plots of ion diffusion coefficients vs solvent diffusion coefficient were linear for both Li2B12F12 and LiPF6 PC in solutions. Although the PF6 diffusion is much faster than lithium diffusion for the LiPF6 in PC, the D-Li and D-anion of the Li2B12F12 have similar values in the temperature range measured for the four different salt concentrations. The experimental parameters R-Li = D-Li/D-PC and R-anion = D-anion/D-PC calculated from the individual diffusion coefficients suggest that the PC solvation around B12F12 can be assumed in addition to the PC solvation around Li. Tentatively, the Nernst-Einstein ionic conductivities sigma(NE) were calculated from the D-Li and D-anion under the assumption of the complete ion dissociation. The temperature- and concentration-dependent behaviors resemble those of the ionic conductivity sigma(ac) observed by the electrochemical ac method. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3160581] All rights reserved.