Size-controllable crown ether-embedded 2D nanosheets for the host-guest ion segregation and recovery: Insights from DFT simulations

The recovery of lithium ion from spent lithium sources and from seawater is a strategic way of complementing the existing natural sources to meet the rapid growth in demand. Crown ethers have shown remarkable selectivity in the host-guest extraction of several metal cations. Recent studies have shown that the macrocyclic polyether rings exhibit poor selectivity to lithium when competing with other metals of high abundance and similar cationic size. This study thus presents a strategy to improve the selective extraction of lithium on crown-embedded 2D materials using first principles density functional theory (DFT) simulations. Several ion transmission channels of 9-crown-3, 12-crown-4, 15-crown-5 and 18-crown-6 were created on graphene, hexagonal boron nitride and silicene nanosheets. Compared to the pristine crown ethers, the crown-embedded 2D materials exhibit rapid charge transfer, superior host-guest dipole-dipole interactions and higher Li⁺/Mg²⁺, Li⁺/Na⁺, Li⁺/K⁺ and Li⁺/Ca²⁺ selectivities while maintaining the properties of the 2D materials. Adsorption energies, E_ads of −262.8, −605.1, −566.4 and −231.0 kcal/mol were calculated for the adsorption of Li⁺ ion on 12-crown-4, g-14-crown-4, b-14-crown-4 and s-14-crown-4, respectively. The sp³-hybridized low-buckled crown-4 passivated silicene nanosheet, having a narrower bandgap of 0.588 eV demonstrates the rapid diffusion of Li⁺ ion while slowing down other cations. The present study provides insights on the creation of ion-channels within silicene nanosheet to promote the host-guest selective recovery of lithium in the presence of abundant interfering ions such as in seawater, a prospect which could serve as a feasible strategy for meeting the rapidly thriving energy demand of the metal atoms.