Transport properties and solvation state of magnesium salts dissolved in propylene carbonate and [formula omitted]-butyrolactone: A comparison with monovalent cation counterparts

Abstract

We experimentally measured the conductivity and viscosity of non-aqueous solutions of magnesium electrolytes; Mg(TFSA)2 (TFSA = bis(trifluoromethanesulfonyl) amide) and Mg(ClO4)2 dissolved in propylene carbonate (PC) and γ-butyrolactone (GBL) up to the concentration of m/molkg-1=1.4-1.7 in the temperature range 288⩽T/K⩽328. With the previously obtained results, the matrix of experimental data, comprising three cations (Mg2+, Li+, Na+), two anions (ClO4-, TFSA−) and two solvents (PC, GBL), enables a systematic comparison of their transport properties. Raman spectra of Mg(TFSA)2, LiTFSA, and NaTFSA dissolved in GBL yield the solvation state of each cation, suggesting that the GBL molecules solvating Mg2+ are classified into tightly and less tightly bound states. The lattice gas Monte-Carlo simulation of conductivity quantitatively explains well the reason why the divalent cation systems are less conductive than the monovalent ones; namely, the negative effect of the strong inter-ionic interaction of Mg2+ overwhelms the positive effect of the high valence, because the former effect exponentially attenuates the conductivity while the latter is merely proportional to the harmonic mean valence of the salt. The simulation revealed that, in a hypothetical scenario wherein the long-range interactions are absent, the conductivity is dominantly determined by the solvent. Consequently, the solvent-specific upper limit of the conductivity is predicted for PC and GBL.