This study investigates the enhancement of ion selectivity in covalent-organic-framework (COF) membranes by tuning their solvation abilities. The researchers manipulated the lengths of oligoether segments attached to the pore channels to alter the pore solvation abilities, which in turn influenced the transmembrane behavior of Li⁺ and Mg²⁺ ions. Through comparative experiments, they revealed the relationships between pore solvation ability and various ion transport properties, such as partitioning, conduction, and selectivity. The results showed that increasing the length of the oligoether chain facilitated ion transport, but the COF membrane with oligoether chains containing two ethylene oxide units exhibited the most pronounced discrepancy in transmembrane energy barrier between Li⁺ and Mg²⁺, resulting in the highest separation factor. 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 for Li⁺/Mg²⁺ separation. The study provides valuable insights into the molecular-level mechanisms of ion selectivity in confined nanospaces and offers design principles for developing highly selective COF membranes.This study investigates the enhancement of ion selectivity in covalent-organic-framework (COF) membranes by tuning their solvation abilities. The researchers manipulated the lengths of oligoether segments attached to the pore channels to alter the pore solvation abilities, which in turn influenced the transmembrane behavior of Li⁺ and Mg²⁺ ions. Through comparative experiments, they revealed the relationships between pore solvation ability and various ion transport properties, such as partitioning, conduction, and selectivity. The results showed that increasing the length of the oligoether chain facilitated ion transport, but the COF membrane with oligoether chains containing two ethylene oxide units exhibited the most pronounced discrepancy in transmembrane energy barrier between Li⁺ and Mg²⁺, resulting in the highest separation factor. 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 for Li⁺/Mg²⁺ separation. The study provides valuable insights into the molecular-level mechanisms of ion selectivity in confined nanospaces and offers design principles for developing highly selective COF membranes.