UCL Water-Based Zinc-Iodine Battery Keeps Capacity Over Thousands of Cycles

How the new electrolyte works

The UCL-led team designed the electrolyte so that iodine transfers four electrons during charging and discharging. Conventional iodine battery chemistry manages two. That gives the cell access to more of iodine's storage capacity.

The hard part is stability. Highly reactive forms of iodine appear along the way and tend to react with water.

Earlier experiments used highly concentrated electrolytes containing halogens. Those mixtures helped, but they were corrosive and costly, which made them difficult to manufacture at scale.

The UCL team instead changed the chemistry of the liquid immediately surrounding the iodine. Some molecules in the new electrolyte keep water away. Others bind to the reactive forms of iodine and help prevent unwanted reactions.

Pouch and coin cells tested

The researchers demonstrated the electrolyte in pouch cells and coin cells. Pouch cells are the flat packages commonly used in phones and laptops. Coin cells are small, disk-shaped batteries used in devices with lower power demands. The chemistry worked in both formats.

In separate durability tests, the zinc-iodine cells kept nearly all their initial capacity after thousands of cycles. That matters for grid storage, where batteries must repeatedly absorb surplus electricity from wind and solar farms and release it when generation drops.

The team said the design could also be adapted for aluminum-iodine batteries, giving researchers another chemistry to explore for stationary storage.

What comes next

Large battery farms currently rely on lithium-ion technology. Water-based designs could give grid operators another option, but the team still has to test its electrolyte in larger systems and under conditions closer to those of a battery farm.

Guanjie He, a professor at UCL Chemistry and the study's corresponding author, said he is excited that the chemistry "can help aqueous batteries move one step closer to commercialization and wide deployment." He added that next steps include testing at a larger scale and improving performance under realistic operating conditions.

First author Ruwei Chen pointed to a broader challenge in designing rechargeable iodine batteries. "We also need to understand how they interact with the water-based electrolyte and how those interactions change as the battery charges and discharges," he said.