Overpotential from Cosolvent Imbalance in Battery Electrolytes: LiPF₆ in EMC:EC

Most liquid lithium-ion-battery electrolytes incorporate cosolvent blends, but the dominant electrochemical transport models adopt a single-solvent approximation, which assumes in part that nonuniform cosolvent ratios do not affect cell voltage. For the popular electrolyte formulation based on ethyl-methyl carbonate (EMC), ethylene carbonate (EC), and LiPF₆, we perform measurements with fixed-reference concentration cells, finding appreciable liquid-junction potentials when only the cosolvent ratio is polarized. A previously reported junction-potential correlation for EMC:LiPF₆ is extended to cover much of the ternary composition space. We propose a transport model for EMC:EC:LiPF₆ solutions grounded in irreversible thermodynamics. Thermodynamic factors and transference numbers are entwined in liquid-junction potentials, but concentration-cell measurements determine observable material properties we call junction coefficients, which appear in the extended form of Ohm's law that accounts for how composition changes induce voltage drops. Junction coefficients of EC and LiPF₆ are reported and illustrate the extent to which ionic current induces solvent migration.