Lithium extraction from brine or seawater using membrane technology has attracted extensive attention in recent years. Graphene oxide (GO), as one of the two-dimensional materials, has been proven as a competitive candidate for membranes. However, the GO membranes still suffer challenges for ion sieving due to the swelling in the aqueous solution. In this work, a GO–PEI membrane with positively charged channels was constructed by polyelectrolyte polyethyleneimine (PEI) molecular chain-grafted GO nanosheets. The GO–PEI membrane showed a high selectivity of 22.2 for Li+/Mg2+, together with a competitive Li+ permeation rate of 0.09 mol m–2 h–1 in a binary permeation test. In addition, the membrane showed excellent stability during the separation process. The enhanced Li+/Mg2+ separation performance of the GO–PEI membrane could be mainly attributed to the synergistic effect of size sieving and electrostatic repulsion. We further systematically studied the influence of various variables, such as PEI molecular weight, PEI content, membrane thickness, and ion concentration, on separation performance. This work deepens the understanding of membrane ion selectivity from multiple perspectives and puts forward a strategy to design a two-dimensional membrane structure from microscopic materials.

Graphene-oxide (GO) membrane with notable ions sieving properties has attracted significant attention for many applications. However, because of the water swelling of GO membrane, the rejection of monovalent metal cations is generally low. In this work, we developed a fast and facile method to fabricate a kind of reduced GO membranes using the thermal treatment method at 160 °C for only one minute, which denoted as fast reduced GO membrane (FRGO). Surprising, the FRGO membrane represents high ion sieving ability and ultrahigh water/ions selectivity, compared with other reduced GO membranes with similar average interlayer spacings, and even superior to most of GO-based membranes reported in literature. Building on these findings, we provide a new light on fabricating of energy- and environment-related high desalination performance of GO-based membranes as well as a new insight into the transport mechanism within 2D laminar nanochannels.