Layer-by-layer assembly of polycationic electrolytes onto cation exchange membranes for enhanced Li+/Mg2+ selectivity: A comparative study of polyethyleneimine and chitosan

The selective separation of Li + from Mg 2+ in salt lake brines is critical for sustainable lithium supply but remains challenging due to their similar hydrated radii. Unlike conventional approaches that rely on a single polyelectrolyte system, this study introduces a comparative layer-by-layer (LbL) assembly strategy to systematically screen optimal polycationic building blocks. We employ two distinct polycations, polyethyleneimine (PEI) and chitosan (Cs), paired with poly(sodium 4-styrenesulfonate) (PSS) on a sulfonated poly(ether ether ketone) (SPEEK) substrate, and systematically elucidate how polycation chemistry governs membrane structure, surface charge, and ion-transport behavior across the entire LbL assembly series. The optimized PEI7.5 membrane delivers a Li + /Mg 2+ permselectivity of 9.7, a 13-fold improvement over pristine SPEEK (0.73) and a 31% enhancement over Cs7.5 (7.4), while maintaining a high Li + flux of 0.22 mol·m −2 ·h −1 . This performance represents one of the highest selectivity values reported for LbL-assembled monovalent cation exchange membranes. Combined experimental characterization and molecular dynamics simulations reveal that the superior performance of PEI-based membranes originates from synergistically enhanced denser positive charge sites and narrower pore size distribution, which collectively impose stronger electrostatic repulsion and steric hindrance against Mg 2+ . Overall, this work not only establishes a robust structure–property relationship for LbL-assembled polyelectrolyte membranes, but also provides a polycation-screening paradigm and a mechanistic framework for rationally designing next-generation cation exchange membranes toward high-efficiency lithium extraction from brines.