A research team led by Prof. GAO Jun from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of the Chinese Academy of Sciences, in collaboration with researchers from Qingdao University, has developed an innovative membrane that mimics biological ion channels to achieve highly selective lithium ion separation from complex brines. This study was published in Nature Communications.
Lithium, which is essential for batteries and clean energy technologies, is often found in low concentrations alongside high levels of sodium, potassium, magnesium, and calcium ions. Traditional extraction methods can be inefficient, costly, or harmful to the environment.
Inspired by biological ion channels, the team designed a sulfonic acid-functionalized covalent organic framework (COF)-r-TpPa-SO3H. The membrane's randomly oriented nanocrystalline structure creates ultra-narrow, winding channels that can differentiate ions based on size and hydration energy.
This unique structure enables an unconventional reverse-sieving mechanism that allows the selective passage of Na+, K+, and even divalent ions like Mg2+ and Ca2+ under an electric field while effectively blocking hydrated Li+ ions.
"Our approach doesn't extract lithium by pulling it through the membrane. Instead, we selectively remove all other ions and retain lithium," said co-first author BAO Shiwen from QIBEBT. "This enables efficient, single-step purification."
In laboratory tests, the membrane demonstrated remarkable Na+/Li+ and K+/Li+ selectivity, comparable to that of biological ion channels. Its performance remained stable in complex solutions, including real salt-lake brines. Under electrodialysis conditions, the membrane consistently removed major interfering ions, resulting in a lithium-enriched solution ready for downstream processing.
"Combining high selectivity with practical ion flux is a significant challenge, but this membrane achieves both," said co-corresponding author Prof. GAO. "Its compatibility with scalable electrodialysis systems makes it well-suited for sustainable lithium extraction."
To understand the mechanism, the researchers utilized computational modeling. Simulations revealed that the sulfonic acid groups in the COF structure strongly attract partially dehydrated Na+ and K+ ions, facilitating their transport. In contrast, Li+ ions retain their hydration shells and are excluded due to size and energy barriers.
This membrane design eliminates the need for complex multi-step separation processes, offering a promising route for recovering lithium from low-grade or magnesium-rich brines, which are becoming increasingly important in the global supply chain.
According to the authors, the membrane's design is not only effective but also versatile. Although it was initially fabricated on anodic aluminum oxide, it can be adapted for use on ceramic substrates for industrial-scale production, making it a practical option for future deployment.
