This study investigates how the solvation ability of covalent-organic-framework (COF) membranes can be tuned to enhance ion selectivity, particularly for lithium (Li⁺) and magnesium (Mg²⁺) ions. By adjusting the lengths of oligoether segments attached to the pore channels, the researchers manipulated the solvation abilities of the COF membranes. Through comparative experiments and molecular dynamics simulations, they revealed the relationships between pore solvation ability and ion transport properties such as partitioning, conduction, and selectivity. The study emphasizes the competition between Li⁺ and Mg²⁺ with the solvating segments in modulating selectivity. The COF membrane with oligoether chains containing two ethylene oxide units exhibited the most pronounced discrepancy in transmembrane energy barriers between Li⁺ and Mg²⁺, resulting in the highest separation factor among all evaluated membranes. Under electro-driven binary-salt conditions, this specific COF membrane achieved an exceptional Li⁺/Mg²⁺ selectivity of up to 1352, making it one of the most effective membranes available for Li⁺/Mg²⁺ separation. The insights gained from this study significantly contribute to advancing our understanding of selective ion transport within confined nanospaces and provide valuable design principles for developing highly selective COF membranes. The study also highlights the importance of ion-membrane binding energies in enhancing ion selectivity while maintaining high ion conduction. The results demonstrate that the length of the oligoether chains significantly influences ion transport, with longer chains providing greater facilitation. The COF-EO₂/PAN membrane, with a moderate length of oligoether chains, exhibited the largest discrepancy in the relative transport rates of Li⁺ and Mg²⁺ ions. This finding aligns well with the results obtained from MD simulations. The study also shows that the COF-EO₂/PAN membrane exhibits excellent selectivity of Li⁺ over Na, Ca, or B, elements commonly found in salt lakes. The results emphasize that the observed selectivities are not characteristic of COF membranes but can be rationalized through coordination chemistry. The study also demonstrates the practical potential of COF-EO₂/PAN in Li-extraction applications, with excellent long-term stability and performance under various conditions. The findings suggest that tailoring the ion-membrane binding energies holds promise for enhancing ion selectivity while maintaining high ion conduction. This understanding opens up avenues for optimizing COF membranes and designing materials with improved ion separation properties for various practical applications.This study investigates how the solvation ability of covalent-organic-framework (COF) membranes can be tuned to enhance ion selectivity, particularly for lithium (Li⁺) and magnesium (Mg²⁺) ions. By adjusting the lengths of oligoether segments attached to the pore channels, the researchers manipulated the solvation abilities of the COF membranes. Through comparative experiments and molecular dynamics simulations, they revealed the relationships between pore solvation ability and ion transport properties such as partitioning, conduction, and selectivity. The study emphasizes the competition between Li⁺ and Mg²⁺ with the solvating segments in modulating selectivity. The COF membrane with oligoether chains containing two ethylene oxide units exhibited the most pronounced discrepancy in transmembrane energy barriers between Li⁺ and Mg²⁺, resulting in the highest separation factor among all evaluated membranes. Under electro-driven binary-salt conditions, this specific COF membrane achieved an exceptional Li⁺/Mg²⁺ selectivity of up to 1352, making it one of the most effective membranes available for Li⁺/Mg²⁺ separation. The insights gained from this study significantly contribute to advancing our understanding of selective ion transport within confined nanospaces and provide valuable design principles for developing highly selective COF membranes. The study also highlights the importance of ion-membrane binding energies in enhancing ion selectivity while maintaining high ion conduction. The results demonstrate that the length of the oligoether chains significantly influences ion transport, with longer chains providing greater facilitation. The COF-EO₂/PAN membrane, with a moderate length of oligoether chains, exhibited the largest discrepancy in the relative transport rates of Li⁺ and Mg²⁺ ions. This finding aligns well with the results obtained from MD simulations. The study also shows that the COF-EO₂/PAN membrane exhibits excellent selectivity of Li⁺ over Na, Ca, or B, elements commonly found in salt lakes. The results emphasize that the observed selectivities are not characteristic of COF membranes but can be rationalized through coordination chemistry. The study also demonstrates the practical potential of COF-EO₂/PAN in Li-extraction applications, with excellent long-term stability and performance under various conditions. The findings suggest that tailoring the ion-membrane binding energies holds promise for enhancing ion selectivity while maintaining high ion conduction. This understanding opens up avenues for optimizing COF membranes and designing materials with improved ion separation properties for various practical applications.