AINEWS
A Flinders University team built a zinc iodine rechargeable battery whose cyclodextrin (a sugar derived polymer) cage traps iodine in place.
A Flinders University team has reported an aqueous zinc-iodine battery that held its capacity across more than 60,000 charge cycles in published lab tests, using a low-cost polymer "cage" borrowed from the food and pharmaceutical industry to keep the cell's active iodine from drifting where it shouldn't.
The result, published this week in Angewandte Chemie International Edition, attacks the central problem with zinc-iodine cells: keeping them alive past a few thousand charge cycles. Aqueous zinc-iodine batteries store energy by shuttling iodine back and forth in a water-based electrolyte, which makes them attractive: iodine is cheap, abundant, and packs a lot of charge per gram.
The catch has always been the polyiodide shuttle. Once iodine reacts in a cell, intermediate forms dissolve in the electrolyte and leak between the electrodes, killing capacity over time. The Flinders answer is a cyclodextrin-based polymer, the Flinders University release explains. Cyclodextrins are ring-shaped sugar molecules already used in food, drugs, and cosmetics. The team built them into a porous polymer whose cavities trap iodine and polyiodide species in place, suppressing the shuttle that otherwise bleeds the cell dry.
Performance is a deliberate trade-off between energy density and cycle life. At a slower charge, the cell stores about 200 mAh/g (milliamp-hours per gram, the standard measure of how much charge a battery can hold for its weight) and survives 8,000 cycles. Crank the charge rate to a three-minute top-up, and the cell still holds roughly 150 mAh/g and runs past 60,000 cycles, Tech Xplore reports. The operating voltage sits between 1.3 and 1.4 V, comparable to a single alkaline cell and well below the 3.6 to 3.7 V of a typical lithium-ion phone battery. Several zinc-iodine cells would need to be stacked to replace one Li-ion unit.
That trade-off matters more in some applications than others. Grid storage, where weight is irrelevant and longevity is everything, would value the 60,000-cycle figure over a slightly lower energy density. A phone or laptop, which needs a single high-voltage cell and replaces the device every few thousand cycles, would value neither.
The paper arrives at a useful moment for the country that built it. Australia's Department of Climate Change, Energy, the Environment and Water projects that domestic lithium-ion battery waste will climb from about 3,300 tonnes a year today to more than 136,000 tonnes by 2036. Australia also holds roughly a fifth of the world's known zinc reserves, giving a domestic supply chain a reason to watch zinc-based chemistries closely. Aqueous zinc-iodine cells use different materials than lithium-ion batteries, though they trade one supply question for another: whether iodine production can scale to grid-storage volumes.
The 60,000-cycle figure is from small-format lab tests under controlled conditions, not from a production-format pouch or prismatic cell. The energy density, the trade-off the cage makes possible, is lower than a comparable lithium-ion cell, and the team has not published a calendar-life figure, only a cycle count. Jia said the group is working with industry to set up a prototyping platform, but no commercial partner has been named.
Cyclodextrins are produced by the ton for other industries. That existing supply is the one piece of the Flinders result that already lives outside a research lab. The next paper, and the prototyping platform Jia described, will have to show whether the polymer survives industrial synthesis, keeps its cavities under real-world cycling, and can be made in the volumes a battery factory needs.