• • NF270 membrane, despite looser pore structure, exhibits Cl−/SO4^2− selectivity nearly tenfold higher than NF90, directly challenging the pore-size-centric paradigm and highlighting charge homogeneity as a superior design lever.
• • Multimodal AFM (KPFM/EFM) and AFM-IR mapping reveal that NF270 has a uniform –COOH distribution, whereas NF90 shows patchy, structure-dependent distribution, with defects acting as low-energy pathways for SO4^2− leakage.
• • Molecular dynamics simulations confirm that SO4^2− ions preferentially localize in regions devoid of –COOH on heterogeneous surfaces, providing mechanistic evidence for charge heterogeneity-induced selectivity loss.
• • The PEI-MTV strategy, particularly via EDC/NHS-catalyzed amidation, achieves the most homogeneous positive charge distribution and highest charge density, yielding exceptional Li+/Mg2+ separation performance, demonstrating a scalable route to high-selectivity membranes.