Current-driven solvent segregation in lithium-ion electrolytes

Abstract

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 carbonate:ethyl-methyl carbonate (EC:EMC) as single entities whose constituents travel with identical velocities. We test this “single-solvent approximation” by subjecting LiPF 6 :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-Hittorf 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. • Applied current rearranges neutral cosolvents in lithium-ion electrolytes • A solution-property machine-learning model pinpoints electrolyte composition changes • This polarization effect is expected to be most severe near electrode interfaces While it is understood that electrolyte solvents are consumed throughout the lifetime of battery operation, less is known about intra-cycle variation in cosolvent composition. Wang et al. pair polarization experiments with machine learning to measure solvent segregation occurring under applied current. This effect is especially prominent near electrode surfaces and may illuminate guiding principles for electrolyte design.