<p>Efficient separation of lithium (Li⁺) and magnesium (Mg²⁺) is critical for enhancing sustainable lithium extraction from natural brines, which is vital for battery production and renewable energy technologies. Here we present a method for highly selective Li⁺/Mg²⁺ separation driven by concentration gradients across negatively charged membranes with high charge densities. In contrast to typical electric field-driven transport in negatively charged membranes, where divalent cations generally permeate faster than monovalent cations, Li⁺ ions in our system permeate the membrane at substantially higher rates than Mg²⁺ ions. This unexpected selectivity stems from the selective ion partitioning properties of the membrane and the uphill transport of Mg²⁺ ions against their external concentration gradient. We demonstrate the efficacy of this separation approach through bench-scale dialysis experiments using a model Atacama brine solution, achieving efficient separation of monovalent and divalent cations. The high separation efficiency observed in this study suggests a promising approach for monovalent/divalent ion separations, offering higher selectivity compared to current technologies.</p><p></p>