Lithium-Ion-Sieve-Embedded Hybrid Membranes for Anion-Exchange- and Cation-Concentration-Driven Li/Mg Separation

Abstract

There is an urgent need to develop efficient and environmentally friendly technologies for separating Li⁺ from brines containing abundant Mg²⁺ to meet the growing demand for lithium resources. In this work, we prepared hybrid membranes by integrating hydrogen manganese oxide (HMO), a lithium-ion sieve, as a filler into anion-exchange membranes (AEMs), the quaternary ammonium-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO) and poly(m-terphenyl piperidinium) (m-PTP). Cations are transported by electrostatic attraction originating from anions and the concentration difference across membranes. Because of the effects of electrostatic repulsion of the fixed cationic groups and steric resistance in AEMs, Li⁺ with less charge and smaller radius will preferentially pass through the membrane. In addition, the presence of HMO provides an additional fast transport channel for Li⁺, resulting in an enhanced Li⁺/Mg²⁺ separation performance. The results show that 20%HMO@m-PTP exhibits high Li⁺ flux (0.48 mol/m²·h) and Li⁺/Mg²⁺ selectivity (β_{Li⁺/Mg²⁺} = 14.1). Molecular dynamics simulations show that m-PTP has more free volume than QPPO, which is beneficial for rapid cation transport. Spectral analysis confirms the insertion and sieving of Li⁺ in HMO. This work illustrates the great potential of anion-exchange- and cation-concentration-driven hybrid membranes based on lithium-ion sieves for low-energy and efficient Li⁺ extraction processes.