This study investigates the performance of different monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes in selective electrodialysis (ED) for separating lithium (Li) from magnesium (Mg) ions, a critical step in lithium extraction from brine lakes. Using a unified mass transport model, the authors compare the Li/Mg separation performance of various CEMs and NF membranes at the coupon scale. They find that CEMs with a dense surface thin film, such as polyamide (PA) films, are more effective in enhancing Li/Mg separation compared to those with a loose but highly charged thin film. PA film-coated CEMs perform similarly to PA-based NF membranes in ED, indicating that PA-based NF membranes can replace CEMs in ED for Li/Mg separation. However, to achieve this, a thin support layer with low tortuosity and high porosity is necessary to reduce internal concentration polarization. The study provides insights into the design of composite membranes for ED-based selective ion separation, highlighting the importance of membrane structure and properties in achieving high selectivity and flux.This study investigates the performance of different monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes in selective electrodialysis (ED) for separating lithium (Li) from magnesium (Mg) ions, a critical step in lithium extraction from brine lakes. Using a unified mass transport model, the authors compare the Li/Mg separation performance of various CEMs and NF membranes at the coupon scale. They find that CEMs with a dense surface thin film, such as polyamide (PA) films, are more effective in enhancing Li/Mg separation compared to those with a loose but highly charged thin film. PA film-coated CEMs perform similarly to PA-based NF membranes in ED, indicating that PA-based NF membranes can replace CEMs in ED for Li/Mg separation. However, to achieve this, a thin support layer with low tortuosity and high porosity is necessary to reduce internal concentration polarization. The study provides insights into the design of composite membranes for ED-based selective ion separation, highlighting the importance of membrane structure and properties in achieving high selectivity and flux.