Threshold-Voltage Ion Sieving in Ultrathin Polymeric Membranes for Energy-Efficient Lithium Extraction

Biological ion channels achieve multi-ion discrimination through parallel chemically distinct channel types, each optimized for a single ion species. Replicating this within a single synthetic membrane has remained beyond reach, for conventional membrane resistance blocks the bonding energy difference between different ions. Here we show that an ultrathin polymeric membrane (ca. 8.56 nm) bearing aligned angstrom-scale porphyrin pores, fabricated by interfacial superspreading, reduces internal resistance sufficiently and achieves simultaneous multi-ion discrimination. Under a microvolt-scale applied voltage, the membrane separates six ion species across three valence groups. In particular, the membrane exhibits anomalous selectivity for Li⁺, with a threshold voltage V_t ≈ 400 mV which is 370 mV above the nearest competitor. Applied to lithium extraction from low-quality brines, a two-stage device achieves >98% recovery at 1.1 × 10^-4 Wh g^-1 _(Li), three orders of magnitude below conventional electrodialysis. This membrane architecture offers a broadly applicable design principle for energy-intensive separations in which resolving ions of similar charge and size is the critical challenge.