Researchers at Kobe University and their collaborators have developed a polymer membrane that allows singly charged ions to pass through rapidly while effectively blocking multiply charged ions, whether positively or negatively charged. By engineering interconnected water-filled channels at the angstrom scale, the researchers demonstrated efficient lithium enrichment from magnesium-rich brines and high-purity salt recovery from simulated seawater, opening new possibilities for sustainable resource recovery.
Lithium is essential for rechargeable batteries used in electric vehicles and renewable energy storage systems. However, recovering lithium from natural salt-lake brines remains challenging because these solutions often contain large amounts of magnesium and other dissolved salts. Similar difficulties arise in seawater treatment, where valuable salts must be separated from complex mixtures of dissolved ions.
Membrane-based separation offers a promising alternative to conventional chemical treatment, but existing membranes often face a fundamental trade-off between ion transport efficiency and selectivity. Enlarging membrane channels facilitates ion movement but allows unwanted ions to pass through, while narrowing these channels improves separation at the expense of transport efficiency.
Moreover, conventional ion-exchange membranes generally favor either positively or negatively charged ions. Developing a membrane capable of efficiently distinguishing singly and multiply charged ions across both cationic and anionic systems remains a major challenge.
To address these limitations, Kobe University membrane engineering researchers LIN Yuqing, MATSUYAMA Hideto and their colleagues developed a new class of ion-exchange membranes based on Tröger's base-derived porous organic polymers.
By combining specially designed molecular building blocks, the researchers constructed a rigid, three-dimensional polymer network containing interconnected pores approximately 0.5–0.7 nanometers wide. They then introduced charged functional groups to promote the formation of continuous water-filled pathways, known as hydration channels, while maintaining the structural stability of the membrane. Unlike conventional approaches that primarily rely on adjusting pore size or membrane charge, this strategy simultaneously controls the pore structure, connectivity and local chemical environment through which ions move.
"The key is not simply to make membrane pores smaller, but to control how water molecules organize and connect within them," explains Lin. "By engineering these hydrated pathways, we can facilitate the movement of singly charged ions while creating strong barriers against multiply charged ions."
Experiments and molecular simulations revealed that the membrane's selectivity arises from several mechanisms working together. Singly charged ions can partially reorganize their surrounding water molecules and move through the narrow channels with relatively low resistance. Multiply charged ions, however, face substantially higher transport barriers because of stronger hydration, spatial confinement and electrostatic interactions. In particular, positively charged magnesium ions are hindered by confinement and electrostatic repulsion, while negatively charged sulfate ions can become strongly associated with charged sites inside the membrane, restricting their movement. This ability to discriminate between singly and multiply charged ions in both cationic and anionic systems represents a distinctive feature of the developed membrane.
To demonstrate the practical potential of the membrane, the researchers applied it to a three-stage electrodialysis process for lithium recovery from magnesium-rich salt-lake brine. Electrodialysis uses an electric field to drive dissolved ions through selective membranes, allowing valuable ions to be separated and concentrated.
The magnesium-to-lithium ratio decreased progressively from approximately 40 in the initial brine to just 0.08 after three separation stages, representing an almost 500-fold reduction. The lithium-enriched solution was subsequently converted into crystalline lithium carbonate with a purity of 99.5%. The membrane also maintained stable separation performance during more than 400 hours of continuous electrodialysis operation, demonstrating its potential for sustained use under demanding conditions. Beyond lithium recovery, the researchers demonstrated that the same membrane could selectively enrich sodium chloride from simulated seawater. After treatment, sodium and chloride accounted for 99.4% of the measured ion mass in the enriched solution. These findings highlight the versatility of the membrane design, which enables selective separation of different ion mixtures rather than being limited to a single application.
"The ability to combine rapid ion transport with precise selectivity is an important step forward in membrane separation technology," says Matsuyama. "This approach offers new opportunities for recovering valuable resources from complex aqueous solutions."
The findings demonstrate how molecular-level control of water-filled channels can help overcome the longstanding trade-off between ion transport efficiency and selectivity. By combining rapid transport, strong ion discrimination and structural stability, the developed membrane provides a promising platform for more resource-efficient separation processes. Further development will focus on evaluating the membrane with actual industrial and natural water sources, improving large-scale membrane fabrication and integrating the technology into practical separation systems. Beyond lithium extraction and salt purification, the approach could contribute to future membrane technologies for resource recovery, wastewater treatment and other electrochemical separation processes.
This research was conducted by researchers from Kobe University in collaboration with East China University of Science and Technology, Nanjing Tech University and Hanyang University. The study was supported by the Japan Society for the Promotion of Science (JSPS KAKENHI JP25K23530), Intergovernmental Cooperation of Science and Technology Program of Shanghai (22520710800), China Postdoctoral Science Foundation (2025M781178), Science and Technology Program of XPCC (2025AA005), National Research Council of Science & Technology of Korea (25081-300) and National Research Foundation of Korea (RS-2025-02309471).
Kobe University is a national university with roots dating back to the Kobe Higher Commercial School founded in 1902. It is now one of Japan's leading comprehensive research universities with over 16,000 students and over 1,700 faculty in 11 faculties and schools and 14 graduate schools. Combining the social and natural sciences to cultivate leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society's challenges.
