Lithium extraction from brine typically uses electrodialysis — electric current drives ions across a charged membrane. Under current, magnesium crosses first. It has twice the charge of lithium and responds more strongly to the electric field. The process works, but magnesium-rich brines jam the system: magnesium forms solids with lithium in evaporation ponds, requiring extra chemicals to separate them. Many brines are too magnesium-heavy to process economically.
A team at the University of Michigan removed the electricity. They placed a negatively charged membrane between brine and pure water, applied no current, no pressure. Just diffusion. Lithium crossed first.
The inversion is physical. Without an external field, the driving force is chloride diffusion — chloride ions move down their concentration gradient into the pure water. Charge balance requires a positive ion to follow. Lithium pairs with chloride more readily than magnesium does. So when chloride goes, lithium goes with it. The preference is strong enough that magnesium ions that leak through are recaptured by the membrane's negative charges.
The electric field was selecting the wrong ion. Under current, the stronger charge wins because the field couples to charge directly. Under diffusion, the relevant property is the ion's pairing preference with the co-diffusing anion — a completely different selectivity criterion. The two mechanisms don't just rank ions differently; they select by different properties entirely.
The method works at high salinities and uses less water than evaporation ponds. Brines that were uneconomical under electrodialysis become viable when you stop trying to force the separation. The force was the problem. Remove it, and the chemistry does the work.