Mechanism of lithium ion selectivity through membranes: a brief review

The recycling and reuse of lithium resources from spent lithium-ion batteries have become a major research area to address the contradiction between limited resources and increasing market demand. Membrane separation, as a highly efficient and easy-to-operate process, has attracted more attention among various lithium recycling technologies. However, the complicated ion migration process results in an insufficient understanding of the lithium ion diffusion mechanism in membrane separation. In this review, recent research efforts on membrane separation technology for lithium recovery are summarized, with the mechanism of ion selectivity through membranes being emphasized. To illustrate the ion diffusion mechanism, the ion transportation process through a membrane is divided into 3 sequential stages: the entering of ions into the channel openings of the membrane, the diffusion of ions within the channels, and the returning of ions to the solution on the other side of the membrane. In each stage, the main factors that affect lithium ion selectivity through membranes are discussed, including the ion charge, hydration energy, channel size and functional groups. It is found that the selectivity of the membrane separation process is highly correlated to the ion hydration energy and the interaction between the ion and the functional groups. In particular, the ion hydration energy is the key factor affecting the entry stage, while the anchoring strength between the ion and the functional groups of the channel wall dominates the diffusion stage within the channel. We conclude with a discussion of the challenges and future development trends of membrane separation technology for lithium recovery. The ion transportation process through a membrane was divided into 3 sequential stages, where the ion selectivity could be determined by the ion charge, hydration energy, channel size and surface chemistry.