Molecular Dynamics Study of Mg2+/Li+ Separation via Biomimetic Graphene-Based Nanopores: The Role of Dehydration in Second Shell.

Residual Mg2+ reduces the performance of lithium-ion batteries. However, separating Mg2+ and Li+ is difficult because of their similar ionic properties. Inspired by the high selectivity of biological Mg2+ channels, this work utilizes atomistic simulations to investigate the ability of graphene-based nanopores with diameters of 0.789, 1.024, and 1.501 nm to separate Mg2+ and Li+ under a series of transmembrane voltages. We analyzed the spatial distribution of molecules in the nanopores' vicinity, structure properties of ionic hydration, and potential of mean force of ions traveling through the nanopores. Separation was mainly caused by the difference in dehydration between the second hydration shells of Mg2+ and Li+. When ions traveled through nanopores, Li+ had to overcome a greater energy barrier than Mg2+ because it had to shed more water molecules and break more hydrogen bonds in the second hydration shell compared with Mg2+. Moreover, the ionic Coulomb blockade of Mg2+ occurred near the pore mouth, impeding Li+ transport and increasing selectivity when the pore diameter decreased to subnanometer.