Three-dimensional cationic covalent organic framework membranes for rapid and selective lithium extraction from saline water

Abstract

The development of high-efficiency ion transport membranes is of great importance in the fields of energy, water purification and resource recovery. In the application of lithium extraction from salt lakes, membranes dominated by size sieving and Donnan exclusion typically enhance Li+/Mg2+ selectivity by sacrificing Li+ flux, which inevitably increases the energy consumption of the separation dramatically. In this work, we manipulate the pore charge density to demonstrate the important role of counterion-mediated positively charged channels in efficient Li+ transport. The potential relationship between the transport behaviour of cations and the membrane charge density was revealed after decoupling the transport of anions and cations using the electric field. On the basis of Manning’s counterion condensation theory and density functional theory calculations, the transport mode in monovalent cations by interacting with the anchored counterions in the positively charged pores to form a high-velocity transport pathway is revealed. The cationic covalent organic framework membranes displayed high Li+/Mg2+ selectivity of 321 in electrodialysis tests while possessing superior lithium permeation rates (0.53 mol m−2 h−1). Therefore, our results suggest that counterion-mediated covalent organic framework membranes have great potential in the field of lithium resource extraction. The counterion-mediated positively charged channels in covalent organic framework membranes enable the fast transport of lithium ions, realizing high lithium/magnesium selectivity without compromising lithium ion flux.