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

Most liquid lithium-ion-batteryelectrolytes incorporatecosolventblends, but the dominant electrochemical transport models adopt asingle-solvent approximation, which assumes in part that nonuniformcosolvent ratios do not affect cell voltage. For the popular electrolyteformulation based on ethyl-methyl carbonate (EMC), ethylene carbonate(EC), and LiPF6, we perform measurements with fixed-referenceconcentration cells, finding appreciable liquid-junction potentialswhen only the cosolvent ratio is polarized. A previously reportedjunction-potential correlation for EMC:LiPF6 is extendedto cover much of the ternary composition space. We propose a transportmodel for EMC:EC:LiPF6 solutions grounded in irreversiblethermodynamics. Thermodynamic factors and transference numbers areentwined in liquid-junction potentials, but concentration-cell measurementsdetermine observable material properties we call junction coefficients,which appear in the extended form of Ohm's law that accountsfor how composition changes induce voltage drops. Junction coefficientsof EC and LiPF6 are reported and illustrate the extentto which ionic current induces solvent migration.

Automated summary

Most liquid lithium-ion battery electrolytes use cosolvent blends but the dominant electrochemical transport models assume a single-solvent approximation which doesn't consider the effect of nonuniform cosolvent ratios on cell voltage. Measurements were performed on electrolytes with fixed-reference concentration cells and liquid-junction potentials were observed when only the cosolvent ratio was polarized. A transport model for EMC:EC:LiPF6 solutions based on irreversible thermodynamics was proposed, and junction coefficients of EC and LiPF6 were reported.