Preparation and application of MOF-based ion exchange membrane for lithium selective separation

Lithium, as a power source material in rechargeable lithium batteries, has aroused increasing demand. Salt-lakes with 62% of the total worldwide lithium resources are attracting people’s attention to reinforce the exploitation.

Compared to traditional methods such as salting out, solvent extraction, and precipitation, membrane methods are emerging with great promise for lithium extraction from aqueous environments due to their simple operation, environmental sustainability, low manufacturing cost, high energy efficiency, and excellent ionic selectivity. Depending on the driving force, membranes can be classified as pressure-driven membranes (e.g., nanofiltration membranes), electro-driven membranes (e.g., ion exchange membranes), temperature-driven membranes and concentration-driven membranes. Among them, the nanofiltration membranes and ion exchange membranes with the suitable membrane pore size and controllable membrane surface charge are mostly used for selective separation of target ions. Especially, in electrodialysis, ion exchange membranes can avoid issues of a high concentration gradient and osmotic pressure partial to nanofiltration membranes, and is widely used to separate ions in various aqueous environments.

Porous metal-organic frameworks (MOFs) are an increasingly popular class of porous materials due to their tailorable structure comprised of ordered porous cavities, high specific surface areas, and versatile functional organic ligands. Implementation of these porous materials into membrane separation technologies can create an energy efficient alternative for ion-selective separation.

Ideal MOF-based membranes for target ion-selective separation should be prepared with a uniform dispersion of MOF particles, and meanwhile exhibits high ionic conductivity, excellent stability, anti-scaling and antifouling performance. However, most MOFs have low ionic conductivity, hence the resulting MOF-based membranes may not be conducive to ion selection and transport, which inspires us to develop MOF-based cation exchange membranes with high ionic conductivity for high lithium selectivity.