Sulfonium-polyamide membranes for high flux Mg2+/Li+ separation

High performance Mg2+/Li+ separation membranes are needed to meet the rising demands for lithium supply, yet state-of-the-art polyamide nanofiltration membranes show limited permeance due to relatively dense structures. This work exploited sulfonium salts for the first time to modify nanofiltration membranes, leading to sulfonium-polyamide membranes with high Mg2+/Li+ separation performance. The occurrence and competition of three types of reactions during the surface modification process were studied and correlated to properties of resultant membranes. Both the positive charge and loose structures of polyamide membranes were enhanced as a result of sulfonium modification, leading to ∼4.3 times improved permeance. The modified membrane exhibited water permeance (∼26.4 L m-2 h-1 bar-1) that is one of the top tier values among recent membranes, coupled with good Mg2+/Li+ selectivity (∼11) in separating high Mg2+/Li+ ratio feed (∼103). Moreover, the modified membrane showed good stability during one-month continuous operation. This work shows the promising potential of sulfoniums for engineering advanced lithium extraction membranes.