Halogen-Free Electrolyte Kept Four-Electron Iodine Chemistry Reversible

A new aqueous electrolyte design lets iodine store charge through a reversible four-electron reaction without relying on highly concentrated chloride or bromide salts. In laboratory zinc–iodine cells, the chemistry retained 96% of capacity after 1,000 cycles at 1 ampere per gram and 94% after 2,000 cycles at twice that rate.

Why iodine usually stops at two electrons

Water-based metal–iodine batteries are attractive for stationary storage because their electrolyte is less flammable than common organic battery liquids and their active materials can be abundant. Most designs cycle iodine between iodide, I⁻, and neutral iodine, I⁰. Reaching the higher I⁺ state transfers two additional electrons and raises both capacity and voltage.

The difficulty is that I⁺ is highly reactive in water. Earlier four-electron designs stabilized it using concentrated halogen-containing electrolytes, where chloride or bromide helps form interhalogen complexes. Those formulations can be corrosive, salt-heavy and expensive to manufacture.

Three chemical roles replace concentrated halogens

The team combined a kosmotropic anion, a polar anion and a nucleophilic molecule. The polar anion creates a water-poor, anion-rich environment near the electrode. The kosmotropic anion changes how water interacts with the nucleophile, leaving more nucleophile available to bind and stabilize I⁺. The researchers tested four nucleophiles, including urea and nicotinamide.

Nuclear magnetic resonance, Raman spectroscopy, electrochemical measurements and simulations supported this mechanism. Reversible conversion appeared when the three roles were present together, and the framework worked in both zinc–iodine and aluminium–iodine cells.

Key Number: 422 mAh g⁻¹. That is iodine’s theoretical capacity when the reaction transfers four electrons rather than the conventional two.

What the cell tests established

Zinc–iodine coin cells retained 96% capacity after 1,000 cycles at 1 A g⁻¹ and 94% after 2,000 cycles at 2 A g⁻¹. Four-electron behavior persisted at iodine loadings up to 12.2 milligrams per square centimetre. The researchers also tested different current collectors and pouch-cell configurations, showing that the chemistry was not limited to one tiny laboratory arrangement.

NewTqnia previously covered a zinc–iodine cell that restrained mobile iodine compounds with a polymer cage. That work targeted the polyiodide shuttle and long cycle life. The new study tackles a different problem: stabilizing iodine’s higher oxidation state without concentrated halogen salts. It also complements research in which machine learning helped select a sodium-battery electrolyte solvent.

Before we overstate the result

What comes next

The next persuasive step is repeated testing in larger cells with realistic electrode thickness, limited excess electrolyte and balanced negative-electrode capacity. Researchers also need performance data across temperature, long idle periods and variable charging schedules.

If the chemistry remains reversible under those constraints, removing concentrated halogen salts could make four-electron iodine storage easier to scale. For now, the strongest result is mechanistic: the study shows that controlling the local water environment and molecular binding can stabilize reactive I⁺ without the conventional corrosive formulation.