Aqueous zinc–iodine batteries have attracted intense interest because they combine the inherent safety of water-based electrolytes with the low cost and abundance of zinc and iodine. Unlike many lithium-based systems that rely on flammable organic electrolytes, aqueous batteries are far less vulnerable to combustion. Iodine is also capable of storing a large amount of charge through reversible redox reactions. Yet converting that theoretical potential into a durable commercial battery has proved difficult, particularly when the cell is designed to use the full four-electron iodine reaction.

The key chemistry involves the transformation of iodide through multiple oxidation states, represented broadly as I⁻/I⁰/I⁺. Accessing this four-electron pathway can substantially increase the amount of energy stored per unit of iodine. However, the high-valence iodine species generated during charging are chemically reactive and difficult to control. They can form soluble polyiodides, migrate through the electrolyte, and reach the zinc electrode, where they trigger unwanted reactions. These processes reduce the battery’s efficiency, accelerate self-discharge, and cause active material to escape from the cathode reaction zone.

The zinc anode presents a second major obstacle. During charging, zinc ions must be reduced and deposited back onto the metal surface. If the deposition is uneven, needle-like dendrites or rough, porous structures can develop. Such irregular growth may increase electrical resistance, consume electrolyte, and in severe cases create conditions for internal short circuits. At the same time, water can participate in parasitic reactions at the zinc surface, producing hydrogen and changing the local chemical environment. A successful battery therefore needs to control iodine at the cathode while also regulating zinc deposition at the anode.

To solve both problems, the research team, led by Prof. Chen Zhongwei and Prof. Wang Dongdong, systematically screened nitrogen-containing cationic ligands and identified N-methylimidazolium chloride, or MImCl, as a multifunctional electrolyte additive. Its positively charged MIm⁺ component does not remain confined to one location in the cell. Instead, the cations dynamically migrate between the cathode and anode interfaces as the battery operates, acting as mobile chemical regulators in the two regions where instability is most severe.

At the iodine cathode, MIm⁺ coordinates with reactive iodine intermediates. This coordination changes the local chemical environment around the iodine species and helps stabilize the high-valence forms required for the four-electron reaction. By binding or interacting with these intermediates, the additive can also reduce the tendency of iodine to assemble into soluble polyiodides. Suppressing this shuttle effect is crucial because polyiodides can diffuse away from the cathode, cross the electrolyte, and react directly with zinc instead of contributing to useful charge storage.

The same additive performs a different function at the zinc electrode. MIm⁺ modifies the interfacial environment through which Zn²⁺ ions approach and are reduced onto the metal surface. This encourages more uniform nucleation and growth, helping zinc plate into a smoother and more compact layer. During discharge, the regulated interface also supports more even stripping of zinc. The researchers describe this simultaneous control of iodine chemistry and zinc electrochemistry as “dual-interface coordination orchestration,” because one mobile electrolyte component coordinates the behavior of both electrodes rather than treating them as separate problems.

The results are particularly notable because the team moved beyond small laboratory cells and tested high-loading configurations designed to better reflect practical battery operation. In the reported pouch cell, the strategy sustained the demanding zinc–iodine chemistry over more than 800 cycles while delivering 1.4 Ah of capacity. The stated energy density reached 455 Wh kg⁻¹ when calculated using the mass of the active cathode material. That measurement basis is important: it highlights the performance of the iodine-based cathode chemistry, but it does not represent the full packaged-cell energy density, which would also include the electrolyte, current collectors, separator, casing, and other components.

The study offers a broader lesson for aqueous metal batteries. Rather than relying solely on new electrode materials, the researchers show that a carefully selected electrolyte additive can act as an active participant in battery chemistry, moving to the interface where it is needed and coordinating unstable reaction intermediates. If the approach can be validated under larger-scale conditions, with leaner electrolyte quantities, thicker electrodes, and full-cell mass accounting, it could help close the gap between the impressive theoretical capacity of zinc–iodine systems and the durability required for grid storage. The researchers say the work provides new insights into electrolyte-mediated interface regulation and creates a promising route toward safer, higher-energy aqueous zinc batteries.

Subject of Research: Not applicable

Article Title: Unveiling Dual-Interface Coordination Orchestration for Durable Aqueous Zinc–Iodine Pouch Cells with Four-Electron Chemistry

Web References: https://doi.org/10.1021/jacs.6c11308; https://english.dicp.cas.cn/

References: Journal of the American Chemical Society, DOI: 10.1021/jacs.6c11308

Keywords

Aqueous batteries, zinc–iodine batteries, zinc metal anodes, iodine redox chemistry, electrolyte additives, MImCl, N-methylimidazolium chloride, polyiodide shuttle, pouch cells, energy storage, electrochemistry, zinc deposition, four-electron chemistry

Zinc-iodine batteries have long been hailed for their theoretical promise, boasting a high specific capacity of 211 mAh g⁻¹ and leveraging iodine’s abundant availability. These attributes position them as a front-runner for safe, cost-effective, and scalable energy storage solutions crucial for integrating renewable energy sources. However, their practical deployment has been hampered by intrinsic material and electrochemical limitations. The iodine cathode, despite its high capacity, suffers from inherently poor electronic conductivity and sluggish redox kinetics. This leads to the formation of soluble polyiodides—intermediates like I₃⁻ and I₅⁻—which dissolve into the electrolyte, migrate between electrodes, and degrade cell performance through a phenomenon known as the shuttle effect. Concurrently, the zinc anode is vulnerable to dendritic growth and parasitic hydrogen evolution reactions, which compromise cycle life and safety. The complex interplay between these factors has rendered traditional single-faceted improvements insufficient.

The ingenious breakthrough in this work lies in the deliberate exploitation of the dual ionic components of TMAI—tetramethylammonium cations (TMA⁺) and iodide anions (I⁻)—to enact complementary and mutually reinforcing functions at both the cathode and anode interfaces. Rather than treating each challenge in isolation, the team embraced a holistic “collaboration” strategy that transforms these ions into multifunctional agents, unlocking synergistic mechanisms that simultaneously enhance reaction kinetics, suppress deleterious shuttle processes, and stabilize zinc plating behavior. This conceptually transformative approach encapsulates a shift from piecemeal additive use to integrated interface engineering through ionic synergy.

At the cathode, this additive orchestrates a novel solid-liquid-solid iodine conversion arc. Traditionally, iodine reduction suffers from slow solid-state I₂ to I₃⁻ conversion kinetics. The iodide anion acts as a catalytic species, accelerating the dissolution of solid iodine into soluble triiodide ions, effectively bypassing kinetic bottlenecks. Meanwhile, the cation TMA⁺ promptly complexes with I₃⁻, precipitating as an insoluble TMA-I₃ solid that remains anchored in the cathode region. This engineered immobilization not only halts polyiodide dissolution and diffusion across the electrolyte—thereby quashing the shuttle effect—but also maintains the electrochemical activity of iodine species, balancing capacity retention with coulombic efficiency. This refined “solid-liquid-solid” reaction pathway represents a meticulous orchestration of phase transformations underpinning superior cathode performance.

On the opposing anode side, the TMAI additive extends its protection via a sophisticated dual-layer passivation schema. Positively charged TMA⁺ cations preferentially adsorb on nascent zinc protrusions, forming an electrostatic shield that modulates local electric fields. This shields active sites from uneven Zn²⁺ deposition, steering ion flux toward more uniform plating in micro-scale valleys, thereby impeding the initiation and propagation of zinc dendrites—primary causes of cell failure and short-circuiting. Complementing this, iodide anions adsorb distinctly onto zinc surfaces, effectively lowering the nucleation barrier for zinc deposition. This facilitates the growth of a dense, flat, and compact zinc layer. Together, these mechanisms synergistically construct a robust, self-regulating double protective interphase that enhances the reversibility and safety of zinc plating and stripping processes.

The culmination of these multi-faceted advancements manifests in remarkable electrochemical performance metrics. Cells incorporating the TMAI additive exhibit remarkably low polarization voltages—approximately 90 millivolts—reflecting rapid reaction kinetics and minimized overpotentials. Energy efficiency peaks near 92.8%, indicative of lower intrinsic losses during charge-discharge cycling. Most strikingly, the symmetric Zn||Zn cells demonstrate unprecedented cycling lifespans surpassing 5500 hours, vastly exceeding conventional electrolytes which typically sustain only around 120 hours under similar testing conditions. In full Zn||I₂ configurations, capacity retention nears 100% after an extraordinary 50,000 cycles at a formidable rate of 5 A g⁻¹. The average coulombic efficiency stabilizes at an astounding 99.95%, underscoring the efficacy of shuttle suppression and surface stabilization schemes. The self-discharge behavior is also markedly attenuated, reflecting the electrolyte’s ability to preserve stored charge over extended periods.

Adding another dimension to the practical viability of the approach, the team tested their system in simplified configurations devoid of traditional electrodes—so-called “electrode-less” cells—still achieving stable and efficient cycling. This suggests the strategy’s broader applicability and robustness, with potential for diverse architectural adaptations in next-generation battery systems. The facile, one-step additive introduction to the aqueous electrolyte further enhances scalability prospects, while avoiding the complexity and cost of extensive electrode material modifications commonly employed in prior efforts.

The conceptual innovation in this research transcends the immediate zinc-iodine system, offering a blueprint for exploiting multifunctional ion pairs to fine-tune and harmonize interface chemistry in complex electrochemical energy devices. By harnessing intrinsic ionic complementarity and their distinct adsorption behaviors, it becomes feasible to orchestrate multi-target advances—including kinetic acceleration, shuttle suppression, and dendrite inhibition—simultaneously. This integrative strategy may well be broadly translatable to other metal-halogen couples, such as zinc-bromine or metal-sulfur chemistries, catalyzing a new generation of scalable, durable, and high-performance aqueous batteries.

From a broader energy landscape perspective, the importance of safe, affordable, and environmentally benign energy storage solutions is escalating as renewable energy sources proliferate globally. Zinc-based aqueous batteries, exemplified by the newly optimized zinc-iodine system, are emerging as front-runners in fulfilling grid-scale and decentralized energy buffering roles. Their use of earth-abundant, non-toxic materials mitigates supply risks and environmental concerns inherent to lithium-ion technologies, positioning them as sustainable alternatives. The present electrolyte design paradigm—centered on ionic synergy—addresses fundamental electrochemical constraints that have historically limited aqueous zinc battery commercialization, ushering in renewed optimism for their wide-scale deployment.

This work also reinforces the critical, sometimes underestimated role of electrolyte chemistry in governing battery performance. While electrode materials often receive primary attention, the electrolyte and its additives wield profound influence on interfacial reactions, ion transport pathways, and degradation mechanisms. Strategic molecular engineering of electrolyte constituents, especially through multifunctional ion pairs, stands as a powerful tool for tuning battery interface properties and kinetics. Moving forward, continued exploration of synergistic ion interactions may unlock further transformative breakthroughs, potentially enabling aqueous batteries with unmatched energy densities, cycle lives, and safety profiles.

Published as an open access article in CCS Chemistry—the flagship journal of the Chinese Chemical Society—this study exemplifies international scientific collaboration and dissemination aimed at addressing global energy challenges. The reported findings are poised to stimulate extensive interdisciplinary research spanning electrochemistry, materials science, and chemical engineering, accelerating innovation toward safer and longer-lasting energy storage solutions. As renewable energy integration intensifies and electrification expands, such advancements could play a pivotal role in achieving low-carbon, sustainable energy futures worldwide.

In summary, the research led by Professor Huang Zhang deftly solves the notorious “performance triangle” problem of zinc-iodine batteries through a simple yet elegant synergistic anion-cation additive approach. By fundamentally reengineering electrode interfaces via multifunctional ion complementarity, they have achieved a rare trifecta: dramatically improved reaction kinetics, effective shuttle suppression, and robust anode protection. This milestone inspires a fresh vision for electrolyte design and battery architecture, signaling a new era where complex electrochemical interfaces are precisely controlled through cooperative ionic chemistry. The ripple effects of this work will undoubtedly resonate deeply across the evolving energy storage landscape, catalyzing safer, more efficient, and more sustainable battery technologies worldwide.

Subject of Research: Not applicable

Article Title: Synergistic Anion-Cation Pair Additive Unites Shuttle-Suppressed and Kinetics-Accelerated I2 Chemistry for Aqueous Zn Batteries

News Publication Date: 7-Jan-2026

Web References:
https://www.chinesechemsoc.org/journal/ccschem
http://dx.doi.org/10.31635/ccschem.025.202506944

Image Credits: CCS Chemistry

Keywords

Batteries, Electrochemistry, Zinc-Iodine Batteries, Electrolyte Additives, Ion Synergy, Energy Storage, Aqueous Zinc Batteries, Polyiodide Shuttle Suppression, Zinc Dendrite Inhibition, Electrochemical Interfaces, Reaction Kinetics, Sustainable Batteries

Central to this pioneering advance is the innovative incorporation of ferrocene, a stable organometallic compound known for its reversible redox properties, into the cathode architecture. Unlike conventional hosts that remain electrochemically inert, ferrocene actively participates in redox reactions. This redox activity enables a dynamic conversion process between ferrocene and its oxidized counterpart, ferrocenium, which in turn forms insoluble complexes with polyiodides. Such coupling effectively immobilizes the polyiodides, dramatically reducing their dissolution and migration within the electrolyte—a primary cause of shuttle effects that compromise battery efficiency and longevity.

The significance of this electroactive redox coupling strategy extends beyond merely curbing shuttle phenomena. By engaging ferrocene in reversible redox cycling, the researchers achieved a notable elevation in discharge capacity. Coin-cell configurations revealed an impressive discharge capacity of 160.5 mAh per gram of cathode material, alongside a Coulombic efficiency surpassing 99.5% at a current density of 1 C. These figures underscore a potent blend of high energy output and exceptional efficiency, laying the groundwork for practical application scenarios.

Zooming out to a more application-driven scale, the team successfully fabricated a pouch cell delivering a capacity of 1.2 ampere-hours. Featuring cathodes with a remarkable areal capacity of 8.4 mAh per square centimeter, the pouch cells sustained over 600 stable charge-discharge cycles at a moderately high rate of 0.5 C. Throughout this extensive cycling, the average Coulombic efficiency remained steadfast at 99.8%, a testament to the robustness of the electroactive redox coupling concept in mitigating capacity fade and enhancing overall battery durability.

Diving deeper into the mechanistic intricacies, the study elucidates how ferrocene’s redox behavior offers a dual advantage: it transforms the inert host from a passive spectator to an active participant in energy storage, and concurrently it represses the dissolution and migration of soluble polyiodide species. These soluble polyiodides, when free to shuttle between anode and cathode, notoriously cause self-discharge, lowering capacity and efficiency. The formation of insoluble ferrocenium–polyiodide complexes effectively sequesters these species, strongly suppressing shuttle-induced degradation pathways.

This breakthrough also addresses a long-standing trade-off in zinc–iodine battery technology—the balance between cathode host activity and energy density. Typically, electrochemically inactive hosts add parasitic weight without contributing to capacity, reducing the practical energy density of the battery. Here, ferrocene’s inherent electroactivity adds value directly to the energy storage process, contributing to the overall discharge capacity rather than merely serving as a vessel for active iodine species. This paradigm shift opens the door to designing future cathodes that integrate redox-active components to maximize energy density.

Moreover, this approach exemplifies the strategic advantage of leveraging organometallic chemistry in aqueous battery systems. Ferrocene and its derivatives have long been studied in organic and non-aqueous electrochemical contexts, but their application in aqueous zinc–iodine batteries represents a novel trajectory. The compatibility of ferrocene with the aqueous environment, alongside its stable and reversible redox behavior, makes it an ideal candidate for advancing next-generation batteries that combine safety, efficiency, and environmental benignity.

The high-rate capability observed in these batteries can be attributed to the rapid electron transfer kinetics enabled by ferrocene’s redox cycling. Unlike sluggish processes typically seen in inactive hosts, the ferrocene/ferrocenium couple facilitates fast charge transfer reactions, thus supporting high current densities without substantial capacity loss. This characteristic is particularly valuable for grid-scale energy storage, where batteries often face fluctuating power demands requiring quick response times coupled with enduring stability.

From a practical perspective, the pouch cell demonstration is especially compelling. It showcases the feasibility of scaling lab-scale advances to practical device architectures without sacrificing performance integrity. The retention of high Coulombic efficiency and capacity over hundreds of cycles positions this technology as a robust candidate for real-world applications, including renewable energy integration and load-leveling in power grids.

Furthermore, the electroactive cathode material design confers intrinsic safety benefits. Aqueous zinc–iodine batteries inherently reduce fire risk relative to flammable organic electrolytes common in lithium-ion systems. By enhancing energy density and cycle life without resorting to hazardous materials or compromising structural integrity, the ferrocene incorporation strategy aligns well with the increasing demand for safer, sustainable battery technologies.

Looking ahead, this work propels a broader research agenda aiming to exploit redox-active molecules and complexes within battery electrodes. The approach could be extended beyond zinc–iodine chemistries to other aqueous systems facing shuttle and solubility challenges. It invites interdisciplinary collaboration, merging electrochemistry, organometallic synthesis, and materials engineering to innovate electrodes that transcend traditional passive hosts.

Moreover, the precise tuning of ferrocene derivatives or exploration of alternative redox-active moieties could yield tailored electrochemical profiles, optimizing batteries for specific applications, whether they require ultra-high capacity, rapid charging, or extreme cycling longevity. The modularity of the redox coupling concept presents wide applicability, fostering versatile energy storage platforms.

In summary, the integration of ferrocene into zinc–iodine battery cathodes not only mitigates shuttle effects through the formation of insoluble ferrocenium–polyiodide complexes but also enriches the battery’s energy storage capability via its intrinsic redox activity. This dual-functionality realigns the design philosophy of aqueous iodine cathodes, moving from passive containment to active involvement in the electrochemical process. The resulting improvement in capacity, efficiency, and cycling durability signifies a critical stride toward practical, scalable aqueous battery technologies suitable for grid-scale applications.

This research, published in Nature Chemistry, illuminates the path toward safer, greener, and more effective battery systems. By marrying organometallic chemistry and battery engineering, Zhang, Hao, Wu, and their collaborators deliver a transformative strategy poised to influence both academic inquiry and industrial development. As energy storage demands continue to escalate worldwide, innovations like this ferrocene-mediated redox coupling mechanism stand as beacons guiding the future of sustainable power technologies.

Subject of Research: Electroactive redox coupling in aqueous zinc–iodine batteries to suppress shuttle effects and enhance energy density.

Article Title: Electroactive ferrocene/ferrocenium redox coupling for shuttle-free aqueous zinc–iodine pouch cells.

Article References:
Zhang, SJ., Hao, J., Wu, H. et al. Electroactive ferrocene/ferrocenium redox coupling for shuttle-free aqueous zinc–iodine pouch cells. Nat. Chem. (2025). https://doi.org/10.1038/s41557-025-01986-7

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41557-025-01986-7