Beyond sieving: A review on active ion recognition via interfacial nanoarchitecture for selective lithium recovery

To address the impending lithium supply crisis, membrane-based extraction from salt-lake brines has emerged as a pivotal area of research in the field of critical metals. Traditional polymeric membranes demonstrate limited Li⁺/Mg²⁺ selectivity and suffer from rapid degradation in highly saline, alkaline brines where the Mg/Li mass ratio exceeds 40. This review provides a systematic analysis of how precision membrane surface engineering strategies have advanced separation capabilities of lithium over the past decade. Specifically, (i) the angstrom-scale narrowing of interlayer nanochannels in laminated MXene/graphene oxide and the contraction of pore sizes in covalent-organic frameworks effectively distinguish hydrated Li⁺ ions (dia. 0.38 nm) from the larger hydrated Mg²⁺ ions. (ii) By adjusting surface charge to boost the Donnan effect, pH/voltage-responsive polyelectrolytes or self-assembled monolayers are applied to nanoscale channels in membranes, achieving a Li⁺ transference number over 0.9 and a Li⁺/Mg²⁺ selectivity coefficient above 1000. (iii) The immobilization of crown ether creates specific recognition sites that increase Li⁺/Na⁺ selectivity to 25 in supported films and to over 40 in ideal benchmark scenarios. (iv) Hierarchical integration of steric, electrostatic and chemical-recognition effects pushes the upper bound of the permeability-selectivity trade-off. Future development requires an integrated design of 'identification-transmission-anti-contamination': membrane surface modification to construct ultra-thin stable selective layers, machine learning to optimize multi-mechanism coupling parameters, and the integration of in-situ characterization and process simulation to achieve efficient and long-lasting lithium extraction from high magnesium-to-lithium ratio brines.