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Two commercial NF membranes and their surface-modified membranes were used to investigate the factors influencing Li+/Mx+ selectivity in model and spent LIB leachates. The EDA-modified NF membranes exhibited higher Li+ passage rates and superior Li+/Mx+ selectivity across varying pH and operating pressures, indicating that surface charge enhancement can improve Li+/Mx+ selectivity. The results for NF1000-based membranes and NF270-based membranes suggested that a smaller pore radius and narrower pore radius distributions also contribute to improved Li+/Mx+ selectivity. The NF1000-EDA membrane exhibited an exceptionally high Li+/Mx+ selectivity for the spent LIB leachates, achieving selectivity values of 645.9 for Li+/Ni2+, 508.8 for Li+/Co2+, and 307.4 for Li+/Mnx+, respectively. Furthermore, battery-grade Li2CO3 product with a purity of 99.76% was also successfully produced without the use of hazardous chemicals. By employing the Donnan-Steric Pore Model with Dielectric Exclusion (DSPM-DE), ion flux and selectivity for spent LIB leachates were accurately predicted, confirming that ion transport in LIB leachates during NF separation follows this model. Overall, this work demonstrates that commercially available membranes can achieve efficient Li recovery from spent LIBs through simple surface modification, and advances highly efficiency, short process, and environmentally friendly LIB recycling technologies. Moreover, the study of ion transport in LIB leachates provides a mechanistic foundation for developing NF membranes capable of producing battery-grade Li2CO3.