The research, led by K. Li and J. E. Dykstra of Wageningen University & Research together with A. N. Shocron of Ben-Gurion University of the Negev, P. Śledzik and S. Porada of Wroclaw University of Science and Technology, and H. H. M. Rijnaarts, tackles a deceptively simple question: when two competing technologies claim to be selective, how do you know which one actually performs better? The authors’ answer is that, until now, you often could not tell, because conventional selectivity metrics provide limited practical value when applied across different systems.

Selectivity matters far beyond the laboratory. In agriculture, irrigation water must be upgraded so that sodium and chloride are stripped away while nutrients remain. In industry, recovering valuable metals such as silver from streams contaminated with copper requires membranes and electrodes that discriminate between ions differing by only a single charge or a fraction of a nanometre in size. In environmental protection, the targeted removal of nitrate, fluoride or heavy metals from drinking water depends on the same underlying physics. The new paper situates itself squarely in this landscape of water treatment and resource circularity, where the difference between a good separator and a mediocre one is measured in energy, cost and sustainability.

To make their case, the researchers performed a rigorous comparative analysis of two leading electrochemical technologies: electrodialysis, or ED, and membrane capacitive deionization, known as MCDI. Electrodialysis drives ions through a stack of ion-exchange membranes using an electric field, with charged membranes acting as gates that admit some ions and block others. Membrane capacitive deionization works differently: ions are stored electrically in porous carbon electrodes, with ion-exchange membranes shielding each electrode from counter-charged species. Both technologies can be made selective, but they achieve selectivity through fundamentally different mechanisms, and the study reveals that their selective separation behaviour is not directly comparable under the traditional metrics.

The heart of the problem is representation. A typical selectivity number, such as a separation factor between two ions, compresses an entire separation process into a single value, discarding information about purity, water recovery and the degree of removal. A technology might achieve impressive selectivity while recovering only a trickle of usable water, or deliver high recovery while quietly failing to remove the target ion. Because each subfield reports its results in its own format, cross-technology comparison becomes an exercise in apples-to-oranges reasoning. The authors demonstrate this by mapping electrodialysis and MCDI results side by side, showing how the same underlying performance looks misleadingly different depending on the metric chosen.

Their proposed solution is an integrative framework in which selectivity, purity and water recovery can be visualized together on a single plot. Instead of collapsing performance into one number, the framework preserves the trade-offs that engineers actually face: how much of the target ion is removed, how much of the competing ion is left untouched, and how much water comes out the other end. By plotting separation results in this unified space, the researchers show that the advantages of different removal regimes become visible at a glance, allowing a flexible and concise analysis that single metrics cannot provide.

Perhaps the most striking findings concern two advantages that the analysis identifies as unique to membrane capacitive deionization. The first is electrode-enhanced selectivity: in MCDI, the carbon electrodes themselves contribute to discrimination between ions, beyond what the ion-exchange membranes provide. The second is more remarkable still. Because MCDI operates cyclically, adsorbing ions during charging and releasing them during discharge, the timing of the cycle can be tuned to exploit differences in how quickly different ions are absorbed and desorbed. The study shows that this cyclic operation enables what the authors call infinite selectivity, a regime in which target ions are completely removed while competing ions achieve zero removal, based purely on the distinct absorption and desorption dynamics of each ion species.

This time-dependent selectivity builds on a body of theory showing that in porous electrodes, ions do not all arrive at the same rate. Smaller, more mobile ions race ahead during the early stages of charging, while others catch up later. By cutting the adsorption phase at precisely the right moment, an operator can harvest a stream enriched almost exclusively in the target ion. The paper’s simulations, developed with COMSOL for the electrodialysis model and Maple for the MCDI model, trace these dynamics in detail, and the authors have made both codebases openly available through Code Ocean, a move that supports reproducibility and invites the community to build on their framework.

The implications reach across the water technology sector. For designers of electrodialysis stacks, the framework offers a way to quantify honestly how monovalent-selective membranes perform under realistic multi-ionic conditions. For the capacitive deionization community, it provides a common language for reporting the gains from modified electrodes, coating chemistries and cycle-timing strategies. And for decision-makers choosing between technologies for a specific application, whether softening hard groundwater, recovering nutrients from greenhouse drainage water or removing micropollutants, the single-plot visualization promises a more transparent basis for comparison than the fragmented metrics of the past.

The study also joins a growing conversation about measurement itself in separation science. Recent work in the literature has questioned how selectivity is reported in membrane-based separations generally, and this paper extends that critique into electrochemistry with a concrete alternative. By demonstrating that electrodialysis and MCDI behave in fundamentally different ways under equivalent conditions, the authors make a compelling argument that the field needs not just better materials but better ways of seeing what those materials do. As water scarcity intensifies and resource recovery becomes an industrial imperative, the ability to compare separation technologies on shared, meaningful terms may prove as important as the technologies themselves.

Subject of Research: Selective ion removal and selectivity metrics in electrochemical water separation technologies

Article Title: Comparative analysis of selective ion removal in electrochemical technologies reveals the need for a new data representation framework

Article References: Li, K., Shocron, A. N., Śledzik, P., Rijnaarts, H. H. M., Porada, S., & Dykstra, J. E. (2026). Comparative analysis of selective ion removal in electrochemical technologies reveals the need for a new data representation framework. Nature Water. https://doi.org/10.1038/s44221-026-00715-9

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00715-9

Keywords: ion selectivity, electrodialysis, membrane capacitive deionization, water treatment, desalination, resource recovery, ion-exchange membranes, porous carbon electrodes, separation metrics, water recovery, electrochemistry, Nature Water

The Rice team, drawn from the departments of Chemical and Biomolecular Engineering, Chemistry, and Materials Science and Nanoengineering, along with the Rice Advanced Materials Institute, has developed a relatively soft and flexible porous material that selectively transports lithium ions. The work, published in the peer-reviewed journal Chemical Science, introduces a new design philosophy for porous solid electrolytes, one that treats mechanical adaptability not as an afterthought but as a core performance criterion, sitting alongside ionic conductivity and chemical stability in the designer’s toolkit.

The material at the heart of the study is called ZnBTCA, and it belongs to a family of compounds known as metal-organic frameworks, or MOFs. These are porous crystalline materials built from metal atoms connected by organic molecules into repeating, lattice-like structures. What makes MOFs so compelling for battery applications is their architecture: they contain tiny, ordered channels whose size and chemistry can be tailored to move or hold specific molecules and ions. In principle, a MOF can act as a molecular sieve for charge, allowing lithium ions to stream through its pores while blocking everything else. Many MOFs previously explored as battery electrolytes, however, have been constructed from relatively rigid aromatic linkers — the organic struts that connect the metal nodes — which produce frameworks that are stiff and mechanically unyielding.

ZnBTCA takes a fundamentally different route. Instead of an aromatic linker, it employs a flexible aliphatic linker, an organic molecule with a pliable carbon-chain backbone. That single design choice ripples through the entire material: the framework itself becomes softer and more mechanically adaptable, capable of deforming and conforming rather than resisting. The researchers also emphasized practicality in their selection of building blocks. The material is assembled from relatively abundant, low-cost components, including zinc — one of the most accessible metals in the battery chemist’s repertoire — and an inexpensive aliphatic linker, an important consideration if such materials are ever to move from the laboratory bench toward real-world manufacturing.

First author Zina Deriche, a graduate student in chemical and biomolecular engineering at Rice, framed the shift in thinking that underpins the work. MOF electrolytes, she noted, are often designed primarily around how effectively they transport ions. The Rice study demonstrates that the mechanical properties of the framework can also be an important part of the design equation. It is a deceptively simple reframing, but one with significant consequences: a material that can flex with the battery as it charges and discharges may hold up better over thousands of cycles than one that cracks or delaminates under mechanical stress.

The dual functionality of ZnBTCA is what excites the team most. The material’s flexible molecular building blocks make it relatively soft, while its negatively charged framework structure favors the movement of positively charged lithium ions through its channels. Stavroula Alina Kampouri, a corresponding author of the study and assistant professor across chemical and biomolecular engineering, chemistry, and materials science and nanoengineering, described the convergence of these properties as the most thrilling aspect of the result. The same material, she explained, brings together flexible building blocks that make the framework softer and more adaptable with a negatively charged structure that promotes selective lithium-ion transport. Bringing those properties together, she said, offers a new way to design MOF electrolytes by considering how ions move and how the material behaves mechanically at the same time.

The experimental results lend strong support to the design concept. The researchers first exchanged the sodium ions residing within the framework for lithium ions, a process known as ion exchange that is common in MOF chemistry. After the exchange, they found that roughly 95 percent of the mobile charge-balancing ions in the material were lithium, while the framework itself remained structurally intact — a critical demonstration that the delicate crystalline lattice could survive the swapping of its guest ions without collapsing. Selectivity matters enormously in an electrolyte: a material that carries only lithium ions, rather than a mixed crowd of mobile species, delivers cleaner, more efficient battery operation and helps prevent parasitic reactions that degrade performance over time.

Further electrochemical testing quantified just how selective the material is. In measurements of the current flowing through the material, lithium ions accounted for approximately 79 percent of the total ionic current — a high lithium-ion transference number that indicates the framework’s channels and charged surfaces are indeed preferentially shepherding lithium rather than the counterions left behind by the exchange process. For electrolyte designers, that figure represents a meaningful benchmark, because a high transference number reduces the concentration gradients that build up inside a cell during fast charging and discharging, which in turn improves power delivery and reduces energy losses.

Perhaps the most demanding test the team ran addressed one of the most feared failure modes in next-generation batteries: short circuiting. In lithium-metal batteries, where a strip of pure lithium metal serves as one electrode, repeated charging and discharging can cause lithium to grow unevenly, forming needle-like structures that eventually pierce the electrolyte and create an unwanted electrical pathway — a short circuit that can disable or even ignite a cell. To probe how ZnBTCA would cope with this challenge, the researchers incorporated the material into a solid electrolyte membrane and sandwiched it between two lithium-metal electrodes. The cells then operated stably for nearly 300 hours as the researchers progressively increased the current, with no evidence of short-circuiting under the conditions tested. While laboratory cycling tests are far from the final word on commercial viability, the result suggests that a softer, more conformable electrolyte membrane may help suppress the mechanical pathways through which dendrites typically propagate.

The broader significance of the work lies in the design principle it establishes. To the team’s knowledge, ZnBTCA is the first MOF electrolyte built from an aliphatic linker with a flexible carbon-chain backbone, a distinction that opens an essentially unexplored region of chemical space for electrolyte development. Sibani Lisa Biswal, chair of chemical and biomolecular engineering and the William M. McCardell Professor in Chemical Engineering at Rice, as well as a professor of materials science and nanoengineering, summarized the lesson: researchers can think beyond just the chemistry and pore structure of these materials, because mechanical flexibility can also become a design tool for developing MOF electrolytes for solid-state batteries. The results suggest that mechanical flexibility could take its place alongside chemical composition, pore structure, and framework charge as a tunable parameter in the rational design of battery electrolytes.

The research was supported by a GEM Fellowship as well as the National Science Foundation under grant No. 2404376, and portions of the work used resources of the Shared Equipment Authority at Rice. As solid-state batteries edge closer to commercial reality in electric vehicles, consumer electronics, and grid storage, the challenge is no longer simply finding materials that conduct lithium ions well — it is finding materials that can survive the mechanical realities of a battery’s life. By proving that a MOF can be simultaneously soft, selective, and structurally robust, the Rice team has added a promising new entry to the electrolyte design playbook, and perhaps a hint that the future of the solid-state battery will be built not on rigid crystals, but on frameworks that know how to flex.

Subject of Research: Flexible metal-organic framework electrolytes for selective lithium-ion conduction in solid-state batteries

Article Title: A softer porous material could help improve solid-state batteries

Article References: A softer porous material could help improve solid-state batteries. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: solid-state batteries, metal-organic frameworks, lithium-ion transport, ZnBTCA, electrolytes, mechanical flexibility, Rice University, Chemical Science, aliphatic linker, lithium-metal anodes, ion selectivity, energy storage