Halogen-free electrolytes enable reversible four-electron iodine conversion

Researchers have demonstrated a new electrolyte framework that allows reversible four-electron iodine conversion in aqueous metal-iodine batteries without the use of halogen-containing additives. Published in Nature, the study presents a kosmotropic anion-polar anion-nucleophilic species (Nuc) electrolyte design that stabilizes reactive hypervalent iodine cations through anionic chemistry rather than conventional interhalogen complexation. This approach addresses a significant limitation in aqueous energy storage, where high-energy iodine redox chemistry has traditionally relied on highly concentrated and corrosive halogen electrolytes to maintain stability.

The central challenge in accessing reversible four-electron iodine redox chemistry is the stabilization of hypervalent iodine cations, which are highly reactive in aqueous environments. Previous solutions typically involved the use of halogen-containing electrolytes that form interhalogen complexes to suppress side reactions. The new framework shifts the mechanistic focus toward coordinating the interfacial microenvironment and molecular interactions using specific anionic components. By harnessing anionic chemistry, the team created a distinct electrolyte composition that supports multi-electron iodine conversion in a halogen-free aqueous setting.

The electrolyte framework consists of three key components: kosmotropic anions (KA), polar anions (PA), and nucleophilic species (Nuc). In the aqueous zinc-iodine battery configurations tested, the kosmotropic anion was provided by 1.5 M zinc sulfate, while the polar anion was supplied by 0.5 M zinc acetate. The nucleophilic species varied across different experiments to demonstrate the broad applicability of the framework. These components were dissolved in deionized water at room temperature to create the working electrolyte solutions.

The mechanism of stabilization involves two distinct roles played by the anions. Polar anions establish a water-deficient, anion-enriched interfacial microenvironment at the electrode surface. Simultaneously, kosmotropic anions regulate the interactions between water and the nucleophilic species. By modulating these interactions, the kosmotropic anions increase the availability of the nucleophilic species to stabilize hypervalent iodine cations through the formation of Nuc-I+-Nuc halogen bonds.

To validate the framework, the researchers tested various nucleophilic species, including urea, acetamide, pyridine, and nicotinamide. The electrolyte compositions were adjusted to maintain specific concentrations, such as 2 M urea, 2 M acetamide, 0.2 M pyridine, or 0.4 M nicotinamide. Comparative analyses were conducted to isolate the roles of the polar and kosmotropic anions. For instance, baseline electrolytes containing only zinc sulfate and nicotinamide were prepared to evaluate the impact of adding different polar anions, such as zinc acetate, zinc trifluoroacetate, or zinc triflate.

The study also explored the applicability of the framework in aqueous aluminum-iodine batteries. In these systems, the electrolyte was prepared by dissolving 1.5 M aluminum sulfate, 0.5 M magnesium acetate, and 0.5 M nicotinamide in deionized water. The negative electrodes in the aluminum-iodine cells were fabricated as aluminum-zinc alloys. These alloys were produced by depositing aluminum ions onto a zinc substrate during the charging process of symmetric cells, creating a Zn-supported Al-Zn alloy structure. This demonstrates the versatility of the electrolyte design across different metal-iodine battery chemistries.

Electrochemical characterization was performed using coin-cell and Swagelok-cell configurations. For the zinc-iodine cells, CR2025 coin cells were used for the halogen-free electrolytes, while Swagelok cells were employed for the halogen-containing control electrolytes. The iodine positive electrodes were prepared by combining iodine-activated carbon composites with polytetrafluoroethylene and Super P conductive additive. The areal iodine loading was controlled by adjusting the thickness of the composite film, with specific loadings such as 2.2 mg cm-2 used in in situ Raman measurements.

In situ analytical techniques were employed to probe the interfacial chemistry and structural changes during battery operation. In situ attenuated total reflectance infrared spectroscopy was used to assess the critical role of polar anions in the interfacial environment. The spectra were recorded during cyclic voltammetry at a scan rate of 0.2 mV s-1. Additionally, in situ Raman spectroscopy was utilized to monitor the iodine positive electrode during galvanostatic charge-discharge cycling. These measurements provided direct evidence of the molecular interactions and phase changes occurring within the battery system.

Theoretical calculations, including classical molecular dynamics simulations and density functional theory, supported the experimental findings. Molecular dynamics simulations were performed using the LAMMPS package to investigate the bulk solvation structure of the aqueous electrolytes.

The research establishes a new electrolyte chemistry framework that moves beyond conventional interhalogen stabilization methods. By leveraging the specific properties of kosmotropic and polar anions, the team achieved reversible four-electron iodine conversion in halogen-free aqueous electrolytes. This represents a mechanistic shift in the understanding of multi-electron iodine chemistry in aqueous environments. The findings offer a promising route towards high-energy aqueous energy storage systems that avoid the use of corrosive and environmentally concerning halogen-containing additives.

The study highlights the importance of tailoring the interfacial microenvironment to stabilize reactive species in aqueous batteries. The distinct roles of the anionic components provide a systematic approach to electrolyte design that can be adapted to other multi-electron redox systems. The validation across diverse nucleophilic species and battery chemistries underscores the robustness of the proposed framework.

Limitations of the study include the specific conditions under which the electrolytes were tested, such as ambient laboratory temperatures and standardized ionic strength conditions.

The publication of this research in Nature underscores the significance of the findings in the field of aqueous energy storage.

Beyond basic cell assembly, the fabrication of the positive electrode involved precise material ratios and thermal treatment. The iodine-activated carbon composite was prepared by manually grinding equal parts of elemental iodine and activated carbon in an agate mortar for ten minutes under ambient laboratory conditions. This mixture was sealed inside a Teflon reactor and heated at 120 degrees Celsius for six hours to achieve adsorption. To fabricate self-standing films for titanium or stainless steel mesh current collectors, researchers blended the composite with Super P carbon and a polytetrafluoroethylene dispersion in an eight-to-one-to-one mass ratio with deionized water before rolling the paste. For electrodes utilizing carbon paper current collectors, the composite was combined in the same ratio with polyvinylidene fluoride binder and Super P in N-methyl-2-pyrrolidone solvent, spread uniformly with a spatula, and dried under vacuum at 60 degrees Celsius for eight hours before being punched into twelve-millimeter disks.

To directly isolate and verify the targeted halogen-bond coordination complexes, the researchers carried out synthetic procedures under an inert nitrogen atmosphere using a standard Schlenk line. Silver salts such as silver sulfate, silver acetate, silver trifluoroacetate, silver triflate, silver nitrate, or silver perchlorate were reacted with pyridine in dry dichloromethane to generate silver-pyridine intermediates. Subsequent addition of elemental iodine induced cation exchange, yielding the halogen-bond complexes after precipitation in dry n-hexane at minus twenty degrees Celsius. Advanced spectroscopic confirmation relied on multiple nuclear magnetic resonance techniques, including proton, carbon-13, and nitrogen-15 spectra, as well as proton-nitrogen heteronuclear multiple-bond correlation experiments conducted on dedicated spectrometers using deuterated solvents. In addition, three-dimensional structural integrity was examined through laboratory micro-computed tomography, using an operating tube voltage of 80 kilovolts and an exposure time of eight seconds to achieve submicron voxel resolution across two-millimeter samples.

Practical validation was extended to a larger cell format through the construction and testing of a 1.1-ampere-hour pouch cell. This assembly incorporated two double-side coated iodine positive electrodes measuring ten by fifteen centimeters, loaded with ten milligrams of iodine per square centimeter, stacked alternately with three zinc negative electrodes. Four glass fibre separators, each wetted with 1.5 milliliters of the ternary electrolyte, were sealed within an aluminium-plastic laminated casing under mechanical constraint. In parallel, electrochemical stability windows were evaluated via linear sweep voltammetry using a titanium mesh working electrode, an activated carbon counter electrode, and a silver/silver chloride reference electrode at a scan rate of two millivolts per second, normalizing measured current to one square centimeter.

To gain deeper insight into interfacial kinetics and impedance behavior, the researchers employed specialized three-electrode configurations. A thin zinc wire inserted between separator layers served as a reference electrode for in situ electrochemical impedance spectroscopy across potential ranges of 0.6 to 1.8 volts. Distribution of relaxation times was evaluated from equivalent circuit models using dedicated computational tools. In addition, differential capacitance measurements were recorded at an alternating current frequency of 1,000 hertz across potentials spanning 1.0 to 1.7 volts versus zinc without applying ohmic-drop corrections, providing quantitative resolution of ion dynamics at the electrode-electrolyte interface.

Subject of Research: Halogen-free aqueous electrolytes for reversible four-electron iodine conversion in metal-iodine batteries

Article Title: Non-halogen aqueous electrolytes for four-electron iodine conversion

Article References: Chen, R., Dong, H., Dai, Y., Wang, J., Chen, J., Liu, J., Feng, J., Zhong, Y., Zong, W., Zhang, W., Yang, H., Cui, Z., & He, G. (2026). Non-halogen aqueous electrolytes for four-electron iodine conversion. Nature, 659(8140), 372-377. https://doi.org/10.1038/s41586-026-11282-0

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11282-0

Keywords: aqueous batteries, iodine redox chemistry, halogen-free electrolytes, four-electron conversion, zinc-iodine batteries, aluminum-iodine batteries, kosmotropic anions, polar anions, nucleophilic species, hypervalent iodine, interhalogen complexation, molecular dynamics, density functional theory, in situ spectroscopy, energy storage