Engineering Li/Na selectivity in 12-crown-4–functionalized polymer membranes*

Isolation of lithium from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could circumvent these challenges by efficiently extracting lithium from aqueous solutions, they exhibit poor ion-specific selectivity. Toward this end, we have incorporated host–guest interactions into a tunable polynorbornene network by copolymerizing: (1) 12-crown-4 ligands to impart ion selectivity, (2) poly(ethylene oxide) side-chains to control water content, and (3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (~2.3) for LiCl over NaCl—the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na⁺ ions more strongly than Li⁺ in an aqueous environment, which reduces Na⁺ mobility and offsets the increase in Na⁺ solubility due to binding with crown ethers.