Thermodynamics of Li+–Crown Ether Interactions in Aqueous Solvent

Lithium ion-based batteries are ubiquitous in modern technology due to applications in personal electronics and high-capacity storage for electric vehicles. Concerns about lithium supply and battery waste have prompted interest in lithium recycling methods. The crown ether, 12-crown-4, has been studied for its abilities to form stable complexes with lithium ions (\ce{Li+}). In this paper, molecular dynamics simulations are applied to examine the binding properties of a 12-crown-4—\ce{Li+} system in aqueous solution. It was found that 12-crown-4 did not form stable complexes with \ce{Li+} in aqueous solution due to the binding geometry which was prone to interference by surrounding water molecules. In addition, the binding properties of sodium ions (\ce{Na+}) to 12-crown-4 are examined for comparison. Subsequently, calculations were performed with the crown ethers 15-crown-5 and 18-crown-6 to study their complexation with \ce{Li+} as well. It was determined that binding was unfavorable for both types of ions for all three crown ethers tested, though 15-crown-5 and 18-crown-6 showed a marginally greater affinity for \ce{Li+} than 12-crown-4. Metastable minima present in the potential of mean force for \ce{Na+} render binding marginally more likely there. We discuss these results in the context of membrane based applications of crown ethers for \ce{Li+} separations.