Hydroxyl-Modified Carbon Nanotube Cathode Helps Aqueous Zinc-Iodine Battery Last Over 10,000 Cycles

Aqueous zinc batteries are attracting attention as a next-generation energy storage technology that can more easily reduce fire risk, because they use a non-flammable water-based electrolyte. Compared with conventional lithium-ion batteries, which use flammable organic solvents, they offer safety advantages, and researchers are studying their use in large-scale storage systems and home batteries. Among them, aqueous zinc-iodine batteries, which use iodine (${\mathrm{I}}_2$) as the cathode active material, have drawn interest for their favorable theoretical capacity and redox potential.

However, a major problem accompanying the cathode reaction stands in the way of practical use: the "shuttle effect." During charging and discharging, polyiodide ions dissolve into the electrolyte and migrate between the cathode and anode. When the dissolved iodine species react with the zinc anode, they cause self-discharge and loss of active material, reducing the battery's capacity and lifespan.

A study (DOI: 10.1007/s11426-026-3487-0) published in the journal Science China Chemistry proposes a way to address this by controlling the surface chemistry of carbon nanotubes (CNTs). Using density functional theory (DFT) calculations, the research team compared how several functional groups adsorb iodine species, and found that the hydroxyl group ($-\mathrm{OH}$) is well suited to capturing polyiodide ions. In small cells built with the fabricated cathode, the team reports that the battery retained 89.34% of its capacity after more than 10,000 charge-discharge cycles at 1 A/g. Rather than modifying the zinc anode, the work aims to extend the life of aqueous batteries by improving the surface structure of the cathode.

The "Shuttle Effect" That Shortens Aqueous Zinc-Iodine Battery Life

In an aqueous zinc-iodine battery, electrical energy is stored through the redox reaction between iodine and iodide ions (${\mathrm{I}}^{-}$) at the cathode. During discharge, iodine accepts electrons and is reduced to iodide ions via intermediates such as triiodide ions (${\mathrm{I}}_3^{-}$). During charging, the reaction runs in reverse, producing iodine from iodide ions.

This mechanism faces two main challenges. One is iodine's low electrical conductivity: for the electrode reaction to proceed efficiently, iodine must be dispersed in a highly conductive material such as carbon. In addition, phase and volume changes of iodine species during cycling can damage the electrode structure, making long-term stable operation difficult.

The other, more serious problem is the dissolution of polyiodide ions, the reaction intermediates, into the electrolyte. Triiodide ions (${\mathrm{I}}_3^{-}$) in particular dissolve readily in aqueous electrolytes, and once they leave the cathode they diffuse toward the anode along the concentration gradient. Iodine species that reach the zinc anode react spontaneously with metallic zinc, causing self-discharge and anode corrosion. This repeated migration and side reaction is called the shuttle effect, and it leads to lower Coulombic efficiency, loss of active material, and shorter cycle life.

Previous approaches have included confining iodine species in the micropores of porous carbon and adding heteroatoms to carbon materials to create adsorption sites. However, a systematic design principle had not been well established for which surface functional groups capture polyiodide ions most effectively and through what kind of interactions they hold them.

DFT Calculations Select the Hydroxyl Group, With an Adsorption Energy of -7.42 eV

The team investigated how the functional groups introduced onto the carbon nanotube surface change the adsorption of iodine species. They compared four surfaces: unmodified carbon nanotubes (pristine CNT) and CNTs functionalized with hydroxyl ($-\mathrm{OH}$), carboxyl ($-\mathrm{COOH}$), or amino ($-{\mathrm{NH}}_2$) groups.

First, they used DFT, which computes electron distributions and interatomic interactions quantum-mechanically, to compare the adsorption of triiodide ions (${\mathrm{I}}_3^{-}$). Among the four, the hydroxyl-modified carbon nanotube showed the strongest adsorption affinity.

On the hydroxyl-modified surface, the adsorption energy of ${\mathrm{I}}_3^{-}$ was calculated as $-7.42\mathrm{eV}$ and the interaction energy as $-5.69\mathrm{eV}$. These are indicators of adsorption stability and interaction in the computational model, not directly measured values of actual battery performance or bond strength. Even so, they suggest that introducing hydroxyl groups changes the charge distribution on the carbon surface and may strengthen electrostatic and chemical interactions with iodine species.

Based on these calculations, the team fabricated a hydroxyl-functionalized free-standing carbon nanotube sheet electrode (SSCC-OH). Microscopy and various spectroscopic measurements reportedly confirmed that the functional groups had been introduced and that iodine was uniformly loaded. The resulting cathode has a specific surface area of about 1,005 ${\mathrm{m}}^2/\mathrm{g}$, and the team explains that this large surface area and the three-dimensional structure of the nanotubes contribute to both the physical confinement and the chemical adsorption of iodine species.

Over 10,000 Cycles at 1 A/g, and 93,400 Cycles at 10 A/g

Using a cathode in which iodine was loaded onto the hydroxyl-modified carbon nanotubes (SSCC-OH/${\mathrm{I}}_2$), the team evaluated charge-discharge performance and durability in small cells.

At 1 A/g, the cell delivered an initial reversible capacity of 175.5 mAh/g. It then retained 89.34% of its capacity after more than 10,000 cycles. The test lasted more than 3,500 hours (about 145 days), confirming stable long-term operation in an aqueous zinc-iodine battery, where dissolution of iodine species is a known problem.

In a high-rate test at a higher current density of 10 A/g, the cell showed an initial capacity of 120 mAh/g and reportedly reached 93,400 cycles. Sustaining so many cycles at a high current density is an important result that points to the durability of the cathode.

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However, the two results cannot be compared simply, because the test conditions differ. The 89.34% capacity retention after more than 10,000 cycles at 1 A/g and the 93,400 cycles reached at 10 A/g are independent measurements. It therefore does not mean that 89.34% of capacity was retained after 93,400 cycles. Because higher current densities shorten each cycle, a larger cycle count does not necessarily mean a longer lifespan in actual operating time.

Electrodes modified with unmodified CNTs, carboxyl groups, and amino groups were also used as controls, but the comparative data available in public materials is limited. Detailed measurement results across the full cycling of each control group, as well as statistical information such as sample sizes, confidence intervals, and error ranges, are not sufficiently presented. Rigorously assessing the size and reproducibility of the improvement from hydroxyl modification will therefore require additional data and independent replication.

Tracking Iodine Reactions With Spectroscopy: How Far Was the Shuttle Effect Suppressed?

To investigate the mechanism behind the longer life, the team analyzed how iodine species change inside the electrode during charging and discharging. They used operando Raman spectroscopy, which observes reactions while the battery is running, and X-ray photoelectron spectroscopy (XPS), which examines the chemical state of the electrode surface after cycling.

Operando Raman measurements showed iodine species inside the electrode changing reversibly with charge and discharge. In the SSCC-OH/${\mathrm{I}}_2$ cathode in particular, the Raman signal from the triiodide ion (${\mathrm{I}}_3^{-}$) intermediate disappeared near the end points of both charging and discharging. The team interprets this as showing that the intermediate is not lost by diffusing into the electrolyte but instead proceeds efficiently to the next reaction at the electrode surface. They explain that this promotion of the reaction leads to suppression of the shuttle effect.

That said, the disappearance of the Raman signal alone cannot establish that the shuttle effect has been eliminated. Even if no ${\mathrm{I}}_3^{-}$ signal is detected at the end points of charge and discharge, trace amounts of polyiodide ions may still dissolve into the electrolyte during intermediate stages. Raman spectroscopy also has limits in the region it can observe and its detection sensitivity, and it does not directly measure diffusion throughout the electrolyte. The results support suppression of the shuttle effect, but they do not prove that dissolution is completely prevented under all conditions.

The reaction mechanism the team proposes combines chemical adsorption by hydroxyl groups, redistribution of surface charge, physical confinement by the nanotube structure, and promotion of iodine redox reactions. Acting together, these are thought to help the cathode retain reaction intermediates and reduce losses from side reactions. However, the adsorption energies from DFT calculations, the changes in chemical state seen in spectroscopic measurements, and the durability from long-term cycling tests each evaluate a different aspect. How much each factor contributed to the improved lifespan requires further investigation.

Pouch Cell With 150 mg of Iodine Also Works, but Challenges Remain for Practical Use

Beyond the small cells, the study also built a pouch cell loaded with 150 mg of iodine. This test was designed to check whether electrode performance obtained in the lab can be carried over to a more practical battery format.

The pouch cell operated at a current density of 3.612 ${mA/cm}^2$, with an operating voltage of 1.1 V and a measured capacity of 23.8 mAh. It reportedly retained 91.7% of its capacity after 350 charge-discharge cycles. That the performance seen in small electrodes was maintained to a certain extent in a cell with a larger iodine loading is an early-stage result on the path to practical use.

Still, the pouch cell test was limited to 350 cycles, a large gap from the more than 10,000 and 93,400 cycles recorded in the small cells described above. As electrode area and active material mass increase, electrolyte distribution, ion transport, and the uniformity of reactions within the electrode change, so the long-life characteristics of small cells are not necessarily reproduced as they are. Whether pouch cells can achieve comparable durability needs to be confirmed in future long-term tests.

In addition, capacity per mass of active material (mAh/g) alone is not enough to judge performance as a commercial battery. It is necessary to evaluate the gravimetric energy density (Wh/kg) and volumetric energy density (Wh/L) of the whole cell, including the current collector, electrolyte, separator, and packaging. Manufacturing cost and mass-production yield for functionalizing carbon nanotubes, calendar life (capacity fade during long-term storage), and stability in low- and high-temperature environments will also matter. Many of these practical requirements have not yet been sufficiently tested in this study.

The significance of the work lies not only in recording a long cycle life. By selecting a promising functional group through DFT calculations, then fabricating real electrodes and evaluating their performance, the study demonstrates a rational design approach for aqueous zinc-iodine battery cathodes. This approach could potentially be applied to other halogen-based cathodes and to battery materials that exploit multi-electron reactions. Whether similar effects can be obtained in different material systems, and whether the non-uniformity of reactions and mass transport that comes with scaling up batteries can be overcome, will be key to judging the technology's practicality.