Intrinsic roles of nanosheet characteristics in two-dimensional montmorillonite membranes for efficient Li+/Mg2+ separation

Abstract

Stacking two-dimensional (2D) nanosheets into lamellar membranes holds great promise in the selective separation of Li⁺ and Mg²⁺ from salt-lake brines, but revealing the intrinsic effect of nanosheet properties on the ion transport remains a great challenge. The primary reasons are inevitable emerging defects and changes in surface functional groups during nanosheet preparation. Here, we successfully demonstrated the intrinsic dependence of ion separation on the size and layer charge density of 2D building blocks using defect-free and inherently permanent charged clay nanosheets. The smaller-sized nanosheets readily assembled into lamellar membranes with narrower nanochannel dimension, which facilitated the steric hindrance effect to improve the Li⁺/Mg²⁺ selectivity. Experiments and calculations demonstrated the layer charge density-dependent ion separation as well, for which a novel mechanism of intrinsic selective separation driven from the energy barrier difference of ions transport was proposed. Based on the “internal” regulation of the intrinsic nanosheet properties, MMT membranes realized stable and efficient Li⁺/Mg²⁺ separation under extreme conditions, multi-cycle and long-term experiments, with an optimal S_Li/Mg of 38.9, superior to most of the reported state-of-the-art membranes. This work reveals the intrinsic interplay of nanosheet properties tuning the ion transport and separation, which will inspire the design and development of advanced 2D lamellar membranes, particularly for sustainable and environmental energy exploitation.