Modeling viscosity and conductivity of lithium salts in γ-butyrolactone

Viscosity and conductivity properties of Li-salts (lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium bis-(trifluoromethylsulfone)-imide (LiTFSI)) dissolved in γ-butyrolactone (BL) have been investigated. The B- and D-coefficients of the Jones–Dole (JD) equation for the relative viscosity of concentrated electrolyte solutions (concentration: C=0.1–1.5 M): ηr=1+AC1/2+BC+DC2, have been determined as a function of the temperature. The B-coefficient is linked to the hydrodynamic volume of the solute and remains constant within the temperature range investigated (25–55 °C). The D-coefficient, which originates mainly from long-range coulombic ion–ion interactions, is a reciprocal function of the temperature. The variations of the molar conductivity (Λ) with C follow the cube root law Λ=Λ0′−S′C1/3 issued from quasi-lattice theory of electrolyte solutions. From the Walden product W=Λη which does not vary with C and the JD equation, the bell shape of the conductivity–concentration relationship is explained and it is shown that the concentration in salt at the maximum of conductivity is linked to the D-coefficient. Raman spectroscopy has been used as an additional tool to investigate ion pairing in BL. Ions pairs have been evidenced for LiClO4 solutions in BL but not for LiPF6. As little variations occur for the ions pairs dissociation coefficient when the salt concentration is increased, the cube root law remains valid, at least in the concentration range investigated.