Current-driven solvent segregation in lithium-ion electrolytes

Liquid lithium-battery electrolytes universally incorporate at least two solvents to balance conductivity and viscosity. Almost all continuum models treat cosolvent systems such as ethylene carbo-nate:ethyl-methyl carbonate (EC:EMC) as single entities whose con-stituents travel with identical velocities. We test this single-solvent approximation by subjecting LiPF6:EC:EMC blends to constant -current polarization in Hittorf experiments. A Gaussian process regression model trained on physicochemical properties quantifies changes in composition across the Hittorf cell. EC and EMC are found to migrate at noticeably different rates under applied current, demonstrating conclusively that the single-solvent approximation is violated and that polarization of salt concentration is anticorrelated with that of EC. Simulations show extreme solvent segregation near electrode/liquid interfaces: a 5% change in EC:EMC ratio, post-Hit-torf polarization, implies more than a 50% change adjacent to the interface during the current pulse. Understanding how lithium-ion flux induces local cosolvent or additive imbalances suggests new approaches to electrolyte design.

Automated summary

Liquid lithium-battery electrolytes commonly use multiple solvents to balance conductivity and viscosity. Current continuum models that treat cosolvent systems as single entities have been found to be inaccurate. Simulations have shown extreme solvent segregation near electrode/liquid interfaces, providing new insights for electrolyte design.