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 Mg 2+ 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 (Δ H ) for Mg 2+ than for Li +, due to the energy required to break Mg 2+ –COOH coordination bonds. Cu 2+ cross-linking generated a denser and more ordered channel structure, increasing the spatial confinement for Mg 2+ while maintaining a more favorable entropy (Δ S ) profile for Li + transport. This design achieved a Li + permeation rate of 1.02 mol m –2 h –1 and a Li + /Mg 2+ 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.