Lithium-Selective Membranes Face a Hard Trade-Off Between Selectivity and Productivity, Review Finds

Lithium is the metal that powers the modern world, and getting it out of salty brines, seawater, and dead batteries is one of the hardest separation problems in chemistry. A new mini-review published in Frontiers in Membrane Science and Technology by a team from Kuban State University, Platov South-Russian State Polytechnic University, and Université Paris-Est Créteil CNRS argues that the field of lithium-selective membranes has been measuring its own success the wrong way, and that a single stubborn trade-off governs everything researchers have built so far.

The review, led by Vasiliy Troitskiy and Dmitrii Butylskii, takes aim at a problem that has quietly plagued the literature for years: researchers report the efficiency of new lithium-selective membranes using wildly different parameters, making it nearly impossible to compare one material against another. The team’s solution is to re-evaluate recent studies using identical metrics, specifically the selective ion separation coefficient and the specific ion flux, expressed in moles per square meter per hour. This standardization reveals a pattern that the authors call the trade-off effect: as selectivity rises, productivity falls, and vice versa.

Direct lithium extraction, or DLE, is the umbrella term for technologies that pull lithium ions straight from liquid sources without the crushing, leaching, washing, heat treatment, filtration, and evaporation steps that define traditional hydrometallurgical processes. In the Lithium Triangle countries of Chile, Bolivia, and Argentina, evaporation ponds remain the dominant method, concentrating brines over months while simultaneously enriching the solution. But this approach depends entirely on climate and topography, and it fails in regions where conditions preclude pond evaporation, including parts of Russia, China, the United States, Portugal, and Serbia.

Lithium-selective membranes are designed to permit the passage of lithium ions while rejecting sodium, potassium, magnesium, and calcium, the ions that coexist in virtually every natural brine. The review classifies the most widely used types into three groups: ion-imprinted membranes, ion sieve membranes, and liquid membranes. Each group achieves selectivity through a different physical mechanism, and each carries its own set of structural constraints that determine where the trade-off lands.

Ion-imprinted membranes trace their lineage to molecular imprinting polymers, a concept pioneered by G. Wulff in 1972 and extended to ion templates by Nishide and colleagues in 1976. The synthesis involves functional monomers, template ions, and cross-linkers that form specific recognition sites for lithium. The template is then leached out, leaving a cavity sized to the target ion. The most common chelating agent is 12-crown-4 ether, whose cavity closely matches the ionic radius of lithium. But the review notes a critical flaw: the structure remains permeable to smaller sodium and potassium ions, and 12-crown-4 ether can also interact with magnesium and calcium to form sandwich complexes. As a result, these membranes are actually selective toward coexisting cations rather than lithium itself, with separation coefficients often significantly lower than one.

Ion sieve membranes represent the fastest-growing category, with Scopus recording nearly four times more publications on ion sieve membranes than on ion-imprinted membranes since 2000. These composite materials feature microporous frameworks where the fundamental separation mechanism is a pore-size sieving effect, requiring monovalent ions to partially or completely dehydrate before transport. The review highlights metal-organic frameworks, covalent organic frameworks, porous organic cages, and MXenes as the leading crystalline materials. Advanced strategies include MOF-on-MOF and COF-on-MOF architectures grown layer by layer on porous substrates, oriented MOF membranes with preferential growth directions, and crown ethers perfectly confined within MOF structures. A 3D UiO-66-COOH membrane achieved an exceptional selective ion separation coefficient for lithium over magnesium of 136.5 under an applied electric field, according to the review’s calculations.

Liquid membranes, first patented by N. Li in 1968, essentially advance conventional liquid-liquid extraction by immobilizing the organic extractant phase within a membrane support. The extractant forms lipophilic organometallic complexes with lithium ions, which diffuse across the selective liquid layer before back-extraction into an aqueous receiving solution. The review classifies these into supported liquid membranes and non-supported variants including emulsion and bulk liquid membranes. A PVDF-based supported liquid membrane employing fluorinated ligands achieved a selectivity toward sodium of 400 with a lithium flux of 0.052 moles per square meter per hour, while a sandwiched bulk liquid membrane configuration using ionic liquids achieved simultaneous back-extraction and extractant regeneration within a single system.

The standardized comparison reveals stark differences across the three membrane families. Composite ion-imprinted membranes typically demonstrate low specific lithium fluxes, often below 0.1 moles per square meter per hour, but the review notes that for this membrane type, low fluxes imply low lithium loss because the target product remains in the feed compartment rather than passing through the membrane. Ion sieve membranes achieve higher selectivity coefficients but suffer from complex fabrication procedures and high production costs, particularly for MOF-based variants. Liquid membranes show the highest specific permselectivity values but face surface area limitations that constrain their performance and hinder broader adoption.

The review also emphasizes that laboratory testing conditions vary enormously across studies, from simple H-type diffusion cells to multi-compartment electrodialysis stacks, and from ternary electrolytes containing lithium and magnesium salts to multicomponent solutions with six or more competing ions. The authors note that membranes can exhibit specific permselectivity only at current densities below the limiting value, due to coupled effects of concentration polarization and overlimiting phenomena. This means that performance measured under simplified conditions using artificial brines may not translate to real-world operation, and the technology readiness level of most laboratory prototypes remains too low for commercialization.

Only a small fraction of laboratory prototypes has undergone detailed, long-term testing in multicomponent solutions resembling natural brines, and the review concludes that moving to high levels of availability requires significant experimental development. The complexity of fabrication methods, the need for continuous monitoring of membrane stability, the scarcity of comprehensive performance evaluations, and the inconsistent reporting of productivity, selectivity, and efficiency metrics collectively hinder the implementation of lithium-selective membrane technologies for direct lithium extraction. The authors expect that continued research aimed at improving membrane design and optimizing extraction processes will enhance the overall feasibility and efficiency of DLE systems, but the trade-off between selectivity and productivity remains the fundamental constraint that every new material must navigate.

Subject of Research: Materials Science

Article Title: Advanced strategies for designing the structure of lithium-selective membranes and its performance: a mini-review

Article References: Troitskiy, V., Ponomar, M., Ruleva, V., Butylskaia, T., Gorobchenko, A., Dammak, L., Mareev, S., Sharafan, M., & Butylskii, D. (2026). Advanced strategies for designing the structure of lithium-selective membranes and its performance: a mini-review. Frontiers in Membrane Science and Technology, 5, Article 1753282. https://doi.org/10.3389/frmst.2026.1753282

Image Credits: AI Generated

DOI: 10.3389/frmst.2026.1753282

Keywords: lithium extraction, lithium-selective membranes, direct lithium extraction, ion-imprinted membranes, ion sieve membranes, liquid membranes, metal-organic frameworks, MXenes, membrane technology, trade-off effect, Advanced, strategies