Shifting-reference concentration cells to refine composition-dependent transport characterization of binary lithium-ion electrolytes

Abstract

We present a novel ‘shifting-reference concentration-cell’ method, altering the traditional protocol for measuring liquid-junction potentials by using a sequence of reference concentrations in regularly spaced intervals, rather than a fixed reference. The method, applied to solutions of lithium hexafluorophosphate (LiPF6) in propylene carbonate (PC) and ethyl methyl carbonate (EMC) at 25 °C, helps to determine thermodynamic factors more accurately, and is useful across a wider concentration range. For LiPF6:PC, good agreement with prior fixed-reference measurements is shown, and new data at low concentrations is consistent with Debye–Hückel theory. Original composition-dependent property correlations are produced for LiPF6:EMC up to 2 M, including the density and thermodynamic factor, as well as isothermal-transport properties such as transference number, conductivity, diffusivity, and viscosity. Polarization-relaxation simulations validate these correlations. For LiPF6:EMC, the low thermodynamic factor and cation/anion Stefan–Maxwell diffusivity, as well as Walden analysis, suggest that ion association dominates, even at high dilution.