A novel polyelectrolyte-modified membrane for selective lithium extraction from water in an electrified process

The selective extraction of lithium ions (Li⁺) from salt lake brines holds substantial significance for environmental and energy applications. However, the effective extraction of Li⁺ is hindered by the low concentration of Li⁺ and the high Mg²⁺ to Li⁺ ratio in salt lakes. In this study, polyelectrolyte modified cation exchange membranes (CEMs) were prepared by coating poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) (PAH/PSS) bilayers using a layer-by-layer method, and their effects on the selective separation of Li⁺ and Mg²⁺ ions were investigated. By optimizing the bilayer number and current density, an average Li⁺/Mg²⁺ selectivity exceeding 10 was achieved. Increasing the bilayer number from 0.5 to 2.5 led to a rise in peak Li⁺/Mg²⁺ selectivity from 12.8 to 31.6, which was significantly higher than that of the unmodified CEM. An optimal Li⁺/Mg²⁺ selectivity of 97.2 was further achieved by increasing the current density to 2.0 mA/cm². The mechanism of Li⁺/Mg²⁺ separation in PAH and PSS monolayers were also elucidated. PSS demonstrated a higher Li⁺ adsorption ability compared to Mg²⁺, while PAH enhanced Li⁺/Mg²⁺ selectivity through stronger electrostatic repulsion against Mg²⁺. This study highlights the potential of polyelectrolyte-modified membranes for Li⁺ extraction, inspiring a novel electrified process for the highly selective separation of valuable species using tailor-designed membranes with polyelectrolyte bilayers.