Mechanisms of lithium selection from mixed LiCl-NaCl solution by nanopores: The synergistic effects of nanoconfinement and electric fields

Abstract

In this work, molecular dynamics simulations were performed to investigate the separation mechanism of Na+/Li+ by graphene nanopores under different electric fields and concentrations. The results showed that the Li+ selectivity of nanopores can mainly be attributed to the association-dissociation mechanism near nanopores and not the dehydration of Na+/Li+. Ions will pass through three typical regions near nanopores: the low ion association region, high ion association region, and high water density region. Abnormal enhancement of ion association occurs on cation side near nanopores, which can be attributed to the difference of ion transports under confinement. The association-dissociation dynamics of Na+/Li+ and Cl− near nanopores can be significantly affected by adjusting the electric field intensity and pore size. In contrast, the effect of ion concentration on ion selectivity is less obvious. Due to the confinement of nanopores, the association of cations and anions near nanopores will be enhanced. Meanwhile, ions need to dissociate from ion-associated species before being transported through nanopores. Thus, ion permeation will be affected by the ion association degree as well as the stability of ion-associated species.