Asymmetric design of ion-transport channels in a polymeric membrane for lithium-ion sieving

There is an increasing demand for lithium (Li) recovery driven by the expansion of renewable energy storage and electric vehicles. New technologies that provide a more eco-friendly and cost-effective process for Li extraction are urgently needed to complement the existing methods for lithium extraction in the industries (e.g. hard rock mining and precipitation), and membrane-based separation is an emerging alternative. Polymeric ion-selective membranes, offering advantages such as widespread availability and good processability, are considered to have more potential for large-scale application compared to the microporous materials-based membranes; however, the efficient separation of mono- and divalent ions remains challenging for polymeric membranes, primarily due to the difficulties in controlling their pore and channel structures. In this work, we prepared a polymeric membrane using poly (acrylic acid) (PAA) and polyamide (PA) featuring geometrically asymmetric ion-transport channels. The ion-transport channels in the PAA-PA membrane are gradually confined as they progress from the PES layer to the PAA layer, and ultimately to the PA layer. Owing to the synergistic effect of the ion-selective functional groups of PAA and the size-sieving effect of PA, the PAA-PA membrane achieved a Li + /Mg 2+ selectivity of 43.3 (separation factor). This work introduces a new design for asymmetric ion-transport channels, advancing the development of polymeric membranes for high-performance lithium extraction. • Preparation of a poly (acrylic acid) (PAA)-polyamide (PA) membrane featuring asymmetric ion-transport channels. • Successful Li + /Mg 2+ separation achieved with a separation factor of 43.3 using the PAA-PA membrane. • Advancement in the design of polymeric membranes for efficient lithium-ion sieving.