Amine-Modified SPEEK Membranes via Interfacial Polymerization for Li+/Mg2+ Separation in Electrically Driven Systems

Lithium is a critical resource essential for energy storage, yet its selective extraction from salt-lake brines remains challenging due to the low Li⁺ concentration and the high Mg²⁺/Li⁺ ratio. This study investigates the potential of sulfonated poly(ether ether ketone) (SPEEK) as a substrate material for lithium ion (Li⁺) separation in electrically driven systems. SPEEK membranes were prepared with varying sulfonation reaction times (12, 24, and 36 h) to identify the optimal conditions by evaluating their mechanical properties, ionic flux, and Li⁺ selectivity. The SPEEK-12 membrane exhibited superior performance and was utilized as a substrate for interfacial polymerization with amine monomers, including polyethylenimine (PEI), piperazine (PIP), and m-phenylenediamine (MPD), which were cross-linked with 1,3,5-benzenetricarbonyl trichloride (TMC). This process formed positively charged thin-film composite layers, enhancing Mg²⁺ rejection and Li⁺/Mg²⁺ selectivity. The modified membranes were extensively characterized using ATR FT-IR, XPS, SEM, and AFM to confirm chemical and morphological changes. PIP-TMC-SPEEK exhibited the highest Li⁺/Mg²⁺ selectivity (434.01) and Li⁺ flux (48.75 mmol m^–2 h^–1), exceeding the performance of other membranes under simulated natural brine conditions. These findings demonstrate that SPEEK membranes modified via interfacial polymerization are promising candidates for efficient and selective Li⁺ extraction in electrically driven systems.