Heterogeneous Charge Membranes with 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 positive and negative heterostructure dual-layer NF membrane with sub-10 nanometer PA layer on 2D Zn-TCPP nanosheets through interfacial polymerization method coupled with vacuum assisted filtration was constructed. PA layer with positively charge on the top performed as the ‘shield’ that effectively retains Mg2+, while negatively charged Zn-TCPP coupled with uniform pores performed as the ‘accelerator’ to facilitate Li+ permeate. 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/Mg separation performance. (Water permeance of 6.79 L⋅m−2⋅h−1 bar−1 over 360 hours continuous operation when Mg/Li ratio is 50:1, and Li+/Mg2+ separation factor is 80.79). In the ternary metal ions solution tests (Li⁺, Mg²⁺, X⁺, where X = Na⁺ or K⁺ with doubled concentration), Li+/Mg2+ separation factor remains still above 40. The DFT simulations and molecular dynamics reveal 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, the different ion perforation energy and ion diffusion rate furtherly explain the reason for the membrane with excellent separation performance.