Heterogeneous charge membranes with a sub-10-nanometer polyamide/Zn-TCPP dual-layer for lithium extraction

Abstract

High ion selectivity and robust long-term stability are critical performance metrics for nanofiltration (NF) membranes targeting lithium extraction from high-salinity brines with impurities and high magnesium/lithium ratios. Herein, a positively and negatively charged heterostructure dual-layer NF membrane with a sub-10 nanometer PA layer on 2D Zn-TCPP nanosheets was constructed through an interfacial polymerization method coupled with vacuum assisted filtration. The PA layer with positive charge on the top acted as the ‘shield’ to effectively retain Mg²⁺, while negatively charged Zn-TCPP coupled with uniform pores acted as the ‘accelerator’ to facilitate Li⁺ permeation. The addition of Zn-TCPP can increase the membrane free volume fraction and construct effective screening channels. An appropriate amount of Zn-TCPP nanosheets can construct PA layers as thin as 5 nm and lead to the best Li⁺/Mg²⁺ separation performance (water permeance of 6.79 L m⁻² h⁻¹ bar⁻¹ over 360 hours of continuous operation when the Mg²⁺/Li⁺ ratio is 50 : 1 and the Li⁺/Mg²⁺ separation factor is 80.79). In the ternary metal ion solution tests (Li⁺, Mg²⁺, and X⁺, where X = Na⁺ or K⁺ with doubled concentration), the Li⁺/Mg²⁺ separation factor remains still above 40. DFT simulations and molecular dynamics reveal that the transmembrane energy barrier for Mg²⁺ significantly exceeds that for Li⁺ across the membrane. The adsorption heights of ions on Zn-TCPP in a stable state, different ion perforation energies and ion diffusion rates further explain the reason for the membrane with excellent separation performance.