Water-Based Iodine Battery Keeps 96% of Capacity After 1,000 Cycles, Nature Study Reports

As renewable energy spreads, demand is growing for large-scale batteries that can absorb fluctuations in power generation. Lithium-ion batteries, now widely used, carry fire risks and the environmental burden of mining resources such as cobalt. Water-based (aqueous) batteries have been studied as a safer alternative, but they have relatively low energy density and face many hurdles to practical use. Against this backdrop, a research team including University College London (UCL) published in the journal Nature (DOI: 10.1038/s41586-026-11282-0) on November 11, 2026, a new electrolyte design approach that drives iodine's four-electron reaction reversibly without relying on highly corrosive halide electrolytes. How far does this result improve iodine battery performance, and what challenges does it leave for commercialization?

What Does Doubling Theoretical Capacity Mean, and How Does This Differ from Earlier Work?

Aqueous metal-iodine batteries, which use iodine as the cathode, store charge by changing iodine's oxidation state rather than by moving ions in and out of a crystal lattice, as lithium-ion batteries do. Most earlier studies exploited the two-electron reaction between iodide (${\mathrm{I}}^{-}$) and elemental iodine (${\mathrm{I}}^0$). In this reaction, iodine's theoretical capacity is 211 mAh g${}^{-1}$, with a redox potential of 0.54 V relative to the standard hydrogen electrode (SHE).

If iodine can instead be oxidized further to the monovalent iodine cation (${\mathrm{I}}^{+}$), using the sequential ${\mathrm{I}}^{-}/{\mathrm{I}}^0/{\mathrm{I}}^{+}$ reaction, that is, the four-electron reaction, iodine's theoretical capacity doubles to 422 mAh g${}^{-1}$. In addition, the ${\mathrm{I}}^0/{\mathrm{I}}^{+}$ redox reaction has a high potential of 1.07 V vs. SHE, which could also raise the battery's operating voltage. However, the increase from 211 to 422 mAh g${}^{-1}$ is only a comparison of theoretical capacity based on the mass of iodine, the cathode active material. It does not mean that the energy density (Wh kg${}^{-1}$) of the whole battery, including the electrolyte, anode, and casing, has doubled.

This is not the first attempt to use iodine's four-electron reaction in an aqueous battery. A study published in Nature Communications in 2021 achieved the four-electron reaction in a zinc-iodine ($\mathrm{Zn}-{\mathrm{I}}_2$) battery using a chloride-containing electrolyte and reported a specific capacity of 550 mAh g${}^{-1}$. That value exceeds the theoretical 422 mAh g${}^{-1}$ because it also includes the electrical double-layer capacitance contributed by the carbon support. A 2024 study in Inorganic Chemistry Frontiers achieved the four-electron reaction without chloride, using an electrolyte combining betaine, nicotinamide, and zinc sulfate, and reported a specific capacity of 355.5 mAh g${}^{-1}$.

What distinguishes the new UCL-led study is not simply that it developed an electrolyte free of halide salts. The team established a design approach called the KA–PA–Nuc framework, which combines control of the environment at the electrode interface with molecular-level stabilization of the iodine cation to achieve a reversible four-electron reaction. The researchers confirmed that the mechanism works with four types of nucleophiles (Nuc) and demonstrated operation not only in $\mathrm{Zn}-{\mathrm{I}}_2$ batteries but also in aluminum-iodine ($\mathrm{Al}-{\mathrm{I}}_2$) batteries.

Method / benchmark	Electrolyte and iodine reaction	Reported figures	Scope and limits of the evidence
Conventional two-electron reaction	${\mathrm{I}}^{-}/{\mathrm{I}}^0$ conversion in aqueous electrolyte	211 mAh g${}^{-1}$, 0.54 V (vs. SHE)	Theoretical capacity per mass of iodine, not whole-battery energy density
2021 prior study (Nature Communications)	Four-electron reaction in a chloride-containing electrolyte	550 mAh g${}^{-1}$ (measured)	Depends on high-concentration chloride salts; includes capacity contribution from the carbon support
2024 prior study (Inorganic Chemistry Frontiers)	Chloride-free electrolyte using betaine, nicotinamide, and zinc sulfate	355.5 mAh g${}^{-1}$ (measured)	Lab-scale result from a specific combination of additives
This study (Nature, 2026)	${\mathrm{I}}^{-}/{\mathrm{I}}^0/{\mathrm{I}}^{+}$ conversion in KA–PA–Nuc electrolyte	422 mAh g${}^{-1}$ (theoretical); high-potential reaction at 1.07 V (vs. SHE)	Four-electron reaction demonstrated in lab cells; whole-battery energy density and commercial lifetime not yet verified

When comparing these figures, theoretical and measured capacities must be distinguished. The significance of this study lies not in exceeding earlier theoretical capacity but in showing a new mechanism for designing electrolytes that allow the four-electron reaction to be used repeatedly.

How Do Three Components Protect the Unstable Iodine Cation in Water?

The ${\mathrm{I}}^{+}$ needed for the four-electron reaction is electron-deficient and highly electrophilic. When it forms in an aqueous electrolyte, it is easily attacked by water molecules and lost through hydrolysis. This has been a major cause of poor reversibility and a barrier to stable charge-discharge cycling.

To suppress this hydrolysis, earlier four-electron iodine batteries used electrolytes containing high concentrations of chloride or bromide. With abundant chloride (${\mathrm{Cl}}^{-}$) or bromide (${\mathrm{Br}}^{-}$) ions, ${\mathrm{I}}^{+}$ is bound into interhalogen complexes such as $\mathrm{IC}{\mathrm{l}}_2$ or $\mathrm{IB}{\mathrm{r}}_2$, preventing reaction with water. But such electrolytes require large amounts of salt, and their corrosiveness and material cost are obstacles to practical use. This study investigated a way to stabilize ${\mathrm{I}}^{+}$ without adding halide salts such as chloride or bromide. "Halogen-free" here does not mean that the iodine cathode active material is absent; it means the system does not depend on the halide salts formerly needed for stabilization.

The KA–PA–Nuc framework developed by the team assigns different roles to three components. The first, the polar anion (PA), gathers at the electrode-electrolyte interface and creates a local environment with a high anion concentration and few water molecules. This reduces the chance that water molecules take part in reactions at the electrode surface and suppresses hydrolysis of ${\mathrm{I}}^{+}$. The distinguishing feature is that it does not remove water from the whole electrolyte but reduces water molecules near the interface where the reaction occurs.

The second component is the kosmotropic anion (KA), typified by the sulfate ion ($\mathrm{S}{\mathrm{O}}_4^{2-}$). KA affects the hydration structure of water and weakens the hydrogen bonds formed between nucleophiles and water molecules. This makes it easier for nucleophiles, previously tied up in interactions with water, to bind to ${\mathrm{I}}^{+}$. In other words, KA tunes the relationship between water and nucleophiles in the electrolyte and helps stabilize the iodine cation.

The third component, the nucleophile (Nuc), binds to ${\mathrm{I}}^{+}$ and forms a linear $\mathrm{Nuc}-{\mathrm{I}}^{+}-\mathrm{Nuc}$ structure through halogen bonding. This bond stabilizes ${\mathrm{I}}^{+}$ in place of conventional interhalogen complexes. In the $\mathrm{Zn}-{\mathrm{I}}_2$ battery experiments, 1.5 M zinc sulfate ($\mathrm{ZnS}{\mathrm{O}}_4$) was used as the KA source and 0.5 M zinc acetate ($\mathrm{Zn}(\mathrm{C}{\mathrm{H}}_3\mathrm{COO}{)}_2$) as the PA source. Four nucleophiles were then added individually, urea (2 M), acetamide (2 M), nicotinamide (0.4 M), and pyridine (0.2 M), and the reaction was tested under each condition.

Control Experiments, Spectroscopy, and Computation Support the Reaction Mechanism

To confirm how the three components act, the team combined control experiments, spectroscopic measurements, and molecular simulations. First, in experiments with urea as the nucleophile, combining KA and urea alone produced only the two-electron ${\mathrm{I}}^{-}/{\mathrm{I}}^0$ reaction. When PA and urea were combined, the ${\mathrm{I}}^0/{\mathrm{I}}^{+}$ oxidation also occurred at high potential, but with poor reversibility. The team interprets the excess capacity that appeared on the second charge plateau under this condition as the result of irreversible iodine oxidation occurring simultaneously with undesirable water oxidation.

By contrast, in an electrolyte containing KA, PA, and urea together, two pairs of reversible redox peaks and corresponding clear charge-discharge plateaus were observed. Similar results were obtained when urea was replaced by acetamide, nicotinamide, or pyridine, and reversibility of the four-electron reaction was also confirmed in comparisons with matched ionic strength. In addition, control experiments without nucleophiles, nuclear magnetic resonance (NMR) spectroscopy, and theoretical calculations supported a stabilization mechanism through formation of $\mathrm{Nuc}-{\mathrm{I}}^{+}-\mathrm{Nuc}$.

Differences among PA types were also examined in detail. With the trifluoromethanesulfonate ion ($\mathrm{C}{\mathrm{F}}_3\mathrm{S}{\mathrm{O}}_3^{-}$), almost no ${\mathrm{I}}^0/{\mathrm{I}}^{+}$ conversion occurred, and with the trifluoroacetate ion ($\mathrm{C}{\mathrm{F}}_3\mathrm{CO}{\mathrm{O}}^{-}$), oxidation was observed but reversibility was limited. The acetate ion ($\mathrm{C}{\mathrm{H}}_3\mathrm{CO}{\mathrm{O}}^{-}$), in contrast, showed the highest reversibility and a stable reaction. Molecular dynamics simulations and in situ attenuated total reflection infrared spectroscopy (in situ ATR-IR) also support the explanation that acetate changes the distribution of water molecules and anions at the electrode interface.

Furthermore, in situ ultraviolet-visible (UV-vis) spectroscopy and micro-CT (computed tomography) observations showed that interface control by acetate keeps iodine inside the electrode and suppresses its leakage into the electrolyte. The effect of sulfate, acting as KA, in weakening hydrogen bonds between nucleophiles and water was likewise supported by NMR measurements and molecular simulations. The reversible formation and transformation of $\mathrm{Nuc}-{\mathrm{I}}^{+}-\mathrm{Nuc}$ during charging and discharging was tracked by in situ Raman spectroscopy. Together, these measurements support the explanation that the three components interact to stabilize the four-electron reaction. However, molecular dynamics simulations and theoretical calculations mainly support interpretation of the reaction mechanism; they alone do not prove practical battery performance.

Lifetime Confirmed in Lab Cells, and Pouch-Cell Conditions Left Unquantified

The performance of the new electrolyte was evaluated in laboratory-scale batteries such as coin cells and pouch cells. According to the team, the $\mathrm{Zn}-{\mathrm{I}}_2$ battery retained 96% of its capacity after 1,000 charge-discharge cycles at a current density of 1 A g${}^{-1}$, and in a separate test recorded 94% capacity retention after 2,000 cycles at 2 A g${}^{-1}$. The team also reports that the four-electron reaction was maintained even when the iodine loading per cathode area was raised to 12.2 mg cm${}^{-2}$. However, the absolute capacity values serving as the baseline for these retention rates are not stated in the paper's main text, so the battery's overall storage performance cannot be judged from retention rates alone.

The team also examined applicability to different current collectors and cell configurations. The four-electron reaction was confirmed in an $\mathrm{Al}-{\mathrm{I}}_2$ battery, but the anode used in that experiment was not pure aluminum metal but an aluminum-zinc alloy (Al–Zn). It has therefore not been confirmed that equivalent performance would be obtained in batteries using aluminum metal anodes.

To check operation in a larger cell, a 1.1 Ah pouch cell was also built. It incorporates two double-side-coated iodine cathodes measuring 10 × 15 cm with an iodine loading of 10 mg cm${}^{-2}$, and three zinc anodes 70 µm thick. During measurement, the cell was mechanically constrained with a dedicated fixture to suppress swelling and similar effects, but the paper's Methods section gives no specific value for the applied pressure. Because constraint pressure can affect internal contact conditions and charge-discharge behavior, it is one of the conditions that should be clarified for future replication tests and scale-up.

In addition, the capacity retention rates of 96% and 94% were obtained in laboratory cells, and the main text gives no statistical variation such as confidence intervals. Although the study is valuable for confirming the electrolyte's effectiveness under various conditions, these figures cannot be directly tied to lifetime guarantees for commercial batteries.

Degradation Factors, Potential Cost Savings, and Challenges for Commercialization

The reversibility of the four-electron reaction has improved greatly, but not all degradation arising from long-term cycling has been eliminated. When the team analyzed the reaction mechanism and the cells' interior after testing, they found that the apparent decline in Coulombic efficiency and capacity fade were linked to reversible or irreversible loss of the total amount of iodine usable as active material. Irreversible loss was especially pronounced under higher iodine loading. Therefore, in addition to stabilizing ${\mathrm{I}}^{+}$, it is important to keep iodine inside the battery through charging and discharging so that it continues to serve as effective active material.

The paper also considers the potential for cost reduction from this electrolyte design. However, this is not a measurement of the cost of actually manufactured batteries but a techno-economic analysis (TEA) calculated with standardized models, premised on, for example, eliminating the need for high-concentration halide electrolytes and expensive current collectors. What the team demonstrated is mainly electrochemical performance and reaction mechanisms in a laboratory setting. Grid integration into large-scale storage facilities, long-term lifetime as a commercial product, and comparative safety tests including fire risk are outside the scope of this study.

This result was first released as a preprint on Research Square (DOI: 10.21203/rs.3.rs-6372434/v1) on May 28, 2025, under a different title, and was published in Nature on November 11, 2026, after peer review. According to UCL's official announcement of November 13, 2026, the research was supported by a European Research Council (ERC) Starting Grant, guaranteed by UK Research and Innovation (UKRI).

Ruwei Chen of the UCL Department of Chemistry, the paper's first author, explained that to make iodine batteries rechargeable, it is necessary not only to stabilize the chemical species involved in the reaction but also to understand their interactions with the aqueous electrolyte and how those change during charging and discharging. Professor Guanjie He, the corresponding author, named testing at larger scales and improving performance under real operating conditions as the next challenges. The study can be seen as presenting a new design guideline for stabilizing the four-electron reaction in aqueous iodine batteries without relying on corrosive halide salts. Going forward, a key factor in judging practicality will be whether the same reaction reversibility and lifetime can be maintained in larger cells, while suppressing iodine loss at high loading and clarifying cell design conditions such as constraint pressure.