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 carbonate:ethyl-methyl carbonate (EC:EMC) single entities whose constituents travel with identical velocities. We test this “single-solvent approximation” by subjecting LiPF6:EC:EMC blends constant-current polarization in Hittorf experiments. A Gaussian process regression model trained on physicochemical properties quantifies changes composition across the cell. EC EMC are found migrate noticeably different rates under applied current, demonstrating conclusively that single-solvent approximation is violated of salt concentration anticorrelated EC. Simulations show extreme solvent segregation near electrode/liquid interfaces: a 5% change EC:EMC ratio, post-Hittorf polarization, implies more than 50% adjacent interface during current pulse. Understanding how lithium-ion flux induces local or additive imbalances suggests new approaches electrolyte design.