Highly Selective Lithium-Ion Separation by Regulating Ion Transport Energy Barriers of Vermiculite Membranes.

Selective extraction of lithium from brines faces significant challenges due to the difficulty of separating Li+ from competing Mg2+ by using conventional membranes. Taking inspiration from natural mineral ion-recognition mechanisms, we developed a vermiculite-based membrane functionalized with copper-coordinated sodium alginate (Cu-SA) to regulate ion transport energy barriers and overcome limitations in selectivity. The incorporation of Cu-SA serves a dual function by reinforcing interlayer covalent networks to stabilize the membrane structure with minimal spacing fluctuation (0.55 Å) and by enhancing ion selectivity through unreacted carboxyl groups. Thermodynamic analysis revealed a higher enthalpic barrier (ΔH) for Mg2+ than for Li+, due to the energy required to break Mg2+-COOH coordination bonds. Cu2+ cross-linking generated a denser and more ordered channel structure, increasing the spatial confinement for Mg2+ while maintaining a more favorable entropy (ΔS) profile for Li+ transport. This design achieved a Li+ permeation rate of 1.02 mol m-2 h-1 and a Li+/Mg2+ selectivity of 34 in single-ion systems. When applied in an electrodialysis system at an external field of 0.8 V cm-1, the Li+ flux increased to 1.8 mol m-2 h-1, six times higher than diffusion-driven transport. This work not only offers a practical route for lithium recovery from complex brines but also provides a general strategy for designing next-generation ion-selective membranes through thermodynamic and structural tuning.