Complexation mechanism of 12-crown-4-ether-calix[4]arene with Li⁺: toward efficient recycling of end-of-life lithium-ion batteries

In this study, a novel extractant, 12-crown-4-ether-calix[4]arene (12C4@C[4]A), was designed for the selective recovery of lithium from the leach solution of end-of-life lithium-ion batteries. Density functional theory (DFT) results demonstrate that 12C4@C[4]A exhibits superior Li⁺ complexation affinity compared with three reference host molecules, namely 12-crown-4 (12C4), benzo-12-crown-4 (B12C4), and calix[4]arene (C[4]A). This exceptional binding is predominantly governed by the synergistic host–guest interaction between the crown ether oxygen donors and the aromatic π system of the C[4]A unit. In contrast, strong steric hindrance suppresses 12C4@C[4]A complexation with common competing metal ions (Ni²⁺, Co²⁺, and Mn²⁺), thereby rendering its highly selective for lithium-ion recovery. Overall, these findings identify 12C4@C[4]A as a promising supramolecular extractant for efficient and selective lithium recovery from end-of-life lithium-ion batteries.

DFT calculations were performed using Gaussian 16, Multiwfn 3.6, and molecular visualizations were generated with VMD. Geometry optimizations were carried out at the M06-2X/def2-SVP level, followed by single-point energy calculations at the M06-2X/def2-TZVP level. The Boys-Bernardi counterpoise method was applied to correct for basis set superposition error (BSSE), and thermodynamic parameters including internal energy and Gibbs free energy changes were evaluated for complex formation. Structural optimization, electrostatic potential (ESP), frontier molecular orbitals (FMOs), Hirshfeld charge transfer, natural bond orbital (NBO), reduced density gradient (RDG), independent gradient model (IGM), and atoms in molecules (AIM) analyses were performed to elucidate the host–guest recognition mechanism.