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

Abstract

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 (Δ<i>H</i>) 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 (Δ<i>S</i>) 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.