Fabrication of enhanced Mg2+/Li+-selective membranes via aminoacetonitrile-modulated PEI/TMC interfacial polymerization

Abstract

High-efficiency lithium extraction from salt lakes is pivotal to meet escalating lithium demand driven by energy storage and electric vehicle industries. In this study, the small-molecule aminoacetonitrile (AAN) was introduced as an aqueous-phase co-monomer to modulate the interfacial polymerization process between polyethyleneimine (PEI) and trimesoyl chloride (TMC). Specifically, the primary amine in AAN molecules reacted with acyl chloride moieties in TMC, occupying reaction sites and thereby preserving a larger fraction of unreacted primary amine groups on the PEI chains within the separation layer, which significantly enhanced the membrane's surface positive charge. Simultaneously, the nitrile groups of AAN preferentially coordinated with Li⁺ ions to effectively counteract the repulsive effect of the strongly positively charged membrane surface on Li⁺. This “charge-coordination” synergistic mechanism enabled precise control over the separation process. When treating mixed salt solution with a Mg²⁺/Li⁺ mass ratio of 100, the optimized membrane achieved a separation factor of 74.7, along with enhanced MgCl₂ rejection of 99.3 % compared to virgin membrane's 93.9 %, and reduced LiCl rejection from 56.3 % to 49.3 %. Notably, upon treatment of simulated salt lake brine, the membrane manifested a pronounced Li⁺ enrichment effect, with a LiCl rejection rate of −8.8 % while retaining a high MgCl₂ rejection capacity. This study employed a simple preparation process with industrialization potential, providing a guiding strategy for constructing high-selectivity membranes for Mg²⁺/Li⁺ separation.