This study investigates the performance of different membranes for selective Li/Mg separation in electrodialysis (ED). A unified mass transport model, the solution-friction model, is used to compare the performance of monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes at the coupon scale. The results show that monovalent selective CEMs with a dense surface thin film, such as a polyamide (PA) film, are more effective in enhancing Li/Mg separation than those with a loose but highly charged thin film. PA film-coated CEMs in ED perform similarly to PA-based NF membranes in NF. NF membranes, when used in ED, can replace monovalent selective CEMs if they have a thin support layer with low tortuosity and high porosity to reduce internal concentration polarization. The study also highlights the importance of membrane structure and properties in achieving high Li/Mg selectivity. The performance of different membranes is influenced by factors such as solution composition, operating conditions, and membrane charge density. The results indicate that PA-based NFMs have a higher Li/Mg selectivity than PC-CEMs, especially when the PA film has a positive charge. The performance of NFMs in ED is strongly dependent on the porous support structure, and thin support layers with high porosity and low tortuosity are necessary for effective Li/Mg separation. The study also compares the performance of composite membranes in ED and NF. It shows that PA-based NFMs have a similar Li/Mg selectivity to PA-CEMs in ED, but the current efficiency is limited due to the unavoidable counterion flux in the reverse direction. The performance of NFMs in NF is similar to their performance in ED, but the driving forces are different. The study concludes that selective ED is a promising membrane-based process for Li/Mg separation, and further research is needed to optimize membrane design for high selectivity and high Li flux.This study investigates the performance of different membranes for selective Li/Mg separation in electrodialysis (ED). A unified mass transport model, the solution-friction model, is used to compare the performance of monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes at the coupon scale. The results show that monovalent selective CEMs with a dense surface thin film, such as a polyamide (PA) film, are more effective in enhancing Li/Mg separation than those with a loose but highly charged thin film. PA film-coated CEMs in ED perform similarly to PA-based NF membranes in NF. NF membranes, when used in ED, can replace monovalent selective CEMs if they have a thin support layer with low tortuosity and high porosity to reduce internal concentration polarization. The study also highlights the importance of membrane structure and properties in achieving high Li/Mg selectivity. The performance of different membranes is influenced by factors such as solution composition, operating conditions, and membrane charge density. The results indicate that PA-based NFMs have a higher Li/Mg selectivity than PC-CEMs, especially when the PA film has a positive charge. The performance of NFMs in ED is strongly dependent on the porous support structure, and thin support layers with high porosity and low tortuosity are necessary for effective Li/Mg separation. The study also compares the performance of composite membranes in ED and NF. It shows that PA-based NFMs have a similar Li/Mg selectivity to PA-CEMs in ED, but the current efficiency is limited due to the unavoidable counterion flux in the reverse direction. The performance of NFMs in NF is similar to their performance in ED, but the driving forces are different. The study concludes that selective ED is a promising membrane-based process for Li/Mg separation, and further research is needed to optimize membrane design for high selectivity and high Li flux.