Engineering transport highways in microporous membranes for lithium extraction: The double role of covalent organic frameworks

Microporous polyamide membranes that allow lithium ions to pass through easily but exclude magnesium ions hold vast potential in the lithium extraction from brine lakes. The efficacy, nevertheless, has long been limited by a trade-off that the magnesium/lithium selectivity is received at the expense of the permeability, arising from the highly cross-linked polyamide. This work describes that rationally lowering the internal density of polyamides by embedding functionalized covalent organic framework (COF) nanosheets can boost the selectivity and permeability simultaneously. A cationic COF, TpTGCl, is used to build transport highways in membranes, which shows the double role that not only lowers the polyamide density in a controllable way, but also strengthens the positive charge property. The resulting membrane exhibits a large water permeance of 19.6 L m−2 h−1 bar−1, along with an excellent separation factor of up to 21.3 under a high magnesium/lithium ratio of 30. This engineering strategy offers a means by which charged and nanoporous two-dimensional frameworks can be leveraged as a booster for microporous membranes enabling fast and precise ion transports.