Diffusional Priority Enables Kinetic 6 Li Selectivity in a Covalent Organic Framework

Lithium isotope (6Li/7Li) separation is hindered by nearly identical electronic structures and strong solvation. Adsorption strategies based on ligand–ion thermodynamic affinity often struggle to explain the experimentally observed preference for 6Li over 7Li. A crown ether-functionalized covalent organic framework is employed as a representative adsorption system to distinguish the contributions of equilibrium binding and transport processes. Density functional theory calculations show that amino-benzo-15-crown-5 exhibits a slight thermodynamic preference for 7Li, with a lower Gibbs free energy of complexation. Batch experiments reveal an unexpected preference for 6Li adsorption. This behavior arises from its higher zero-point energy and lower mass, which facilitate desolvation and accelerate diffusion, leading to kinetically favored coordination over thermodynamically preferred 7Li. Once coordinated, the small Gibbs free energy difference between isotopes provides limited driving force for ligand exchange, hindering replacement of 6Li by 7Li. These findings support the important role of nonequilibrium transport processes in adsorption-based lithium isotope separation.