Origins of Permselectivity in Lithium/Sodium Reverse-Selective 12-crown-4 Ether Functionalized Polymer Membranes*

Despite the technological innovations spurred by the advent of the lithium-ion battery, lithium production still utilizes antiquated solar evaporation processes that are inefficient, environmentally deleterious, and lack the scalability required to meet projected demand. Direct lithium extraction via membrane separations could revolutionize lithium salt production, but progress in this area is limited by the lack of monovalent ion selectivity exhibited by conventional polymeric membranes.

Recently, a 12-crown-4 functionalized polymer membrane was shown to have one of the largest lithium/sodium permselectivities observed to date. Using atomistic molecular dynamics simulations, we reveal that this selectivity is due to strong interactions between sodium ions and 12-crown-4. These interactions decrease the diffusivity of sodium ions while leaving lithium ions relatively unaffected. We show that diffusive selectivity for lithium over sodium in the membrane persists across a range of water volume fractions and identify several parameters to guide the design of future lithium/sodium reverse-selective membranes.