Lithium is a valuable element used in contemporary energy applications, but its isolation from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could in principle circumvent these challenges by efficiently performing aqueous separations, they currently exhibit poor ion-specific selectivity. Here, we introduce a tunable poly(norbornene) membrane platform that can separate lithium ions from aqueous solutions by copolymerizing: (1) 12-crown-4 ligand that impart a mechanism for selectivity, (2) poly(ethylene oxide) side-chains to control water content, and (3) a crosslinker to form robust solids at room temperature. Salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (~2.3) of Li+ over Na+—the highest documented to date for a polymer membrane under aqueous conditions to date. As demonstrated by molecular dynamics simulations, this effect originates from the ability of 12-crown-4 to bind Na+ ions more strongly than Li+ in an aqueous environment, which acts to reduce the mobility of Na+ relative to Li+ and offsets increases in Na+ solubility. These results reveal new insights into the design of advanced membranes for solute-specific separations and establish a promising strategy for lithium recovery.

Polymers for recovery of lithium from aqueous streams

ACS Spring 2022

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