Bioinspired Polypeptide Dendrimer‐Modified Thin‐Film Composite Membranes for Selective Lithium‐Magnesium Separation with DFT Insights

Selective ion transport in nanofiltration (NF) enables sustainable lithium (Li + ) recovery. While many membranes rely on strong positive charge for Li⁺/Mg 2 ⁺ separation, we show that negatively charged membranes can also excel using a biomimetic approach. Inspired by biological ion channels that achieve cation selectivity via specific binding sites despite their negative charge, we designed a nitrogen‐rich polypeptide dendrimer (amino acid–based) bearing carboxylate coordination sites with higher affinity for Mg 2 ⁺ than Li⁺, while moderating the membrane's net negative charge. This biomimetic design enhanced Li + recovery by inhibiting Mg 2+ transport through stronger interactions, thereby allowing for preferential Li + permeation. This process occurred through a combination of electrostatic modulation and ligand‐assisted coordination. Density functional theory (DFT) calculations indicated strong oxygen‐donor coordination: lysine motifs bind hydrated Mg 2+ (E ≈ −170 kcal.mol −1 ) far more strongly than Li + (E ≈ −50.2 kcal.mol −1 ). The optimized membrane achieved Li + /Mg 2+ selectivity of 15.6 at neutral pH with 23 LMH flux, and 136 at pH 4, highlighting strong performance in acidic feeds. Long‐term tests showed ∼0.4% leaching over 10 days with stable rejection and enrichment of Li⁺ (feed Li⁺/Mg 2 ⁺ increased from 0.05 to 0.20). Antifouling tests showed a twofold lower flux‐decline ratio and higher flux‐recovery than the unmodified TFC.