Intrinsically Lithium-Selective Nanofiltration Membranes Enabled by 1,3,5-Tris(aminomethyl)benzene

Rising demand for lithium, driven by its essential role in energy storage technologies, necessitates efficient, selective recovery from saline brines. However, Li + /Mg 2+ separation remains challenging due to their similar hydrated sizes and the high Li + /Mg 2+ ratios in natural brines. In this study, we proposed a method to fabricate intrinsically lithium-selective nanofiltration (NF) membranes by incorporating 1,3,5-tris(aminomethyl)benzene (TAB) as an aqueous-phase monomer in interfacial polymerization. Thin-film composite (TFC) membranes were prepared using varying ratios of piperazine (PIP) and TAB, forming polyamide (PA) selective layers with tunable structure and charge properties. The incorporation of TAB, a rigid aromatic triamine, promotes the formation of a densely cross-linked network via stable covalent amide bonds while increasing the density of protonatable amine groups. This results in enhanced positive surface charge and improved Mg 2+ rejection via Donnan exclusion, without requiring postfabrication modification. The membrane prepared with a combined PIP–TAB system exhibited a significantly enhanced Li + /Mg 2+ selectivity of 118.7, achieving 97.9% Mg 2+ rejection and −149.2% Li + rejection, compared to a selectivity of 25.4 for the conventional PIP-only membrane. A higher selectivity of 172.3 was achieved using TAB as the sole aqueous-phase monomer; however, this came at the expense of reduced water permeability (1.6 L m –2 h –1 bar –1 ), compared to 5.4 and 4.6 L m –2 h –1 bar –1 for the PIP and PIP–TAB membranes, respectively. This work introduces a design strategy for high-performance NF membranes, offering a scalable and efficient approach for lithium recovery from complex brine systems.