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A new quasi-solid electrolyte combines zeolite with isolated iron atoms to control both iodine reactions and zinc deposition. Laboratory cells retained capacity after 6,800 cycles, while pouch cells demonstrated the approach at a larger format.
Zinc–iodine batteries are attractive because they can be built around aqueous electrolytes and materials that differ from those used in today's lithium-ion batteries. But the water that makes ion transport easy can also contribute to side reactions at the zinc electrode. At the same time, dissolved polyiodides can migrate between the electrodes and undermine battery stability.
One possible solution is a quasi-solid electrolyte containing less free water. That can limit several of these problems, but it creates another trade-off: reducing the liquid content often slows charge transfer at the interfaces.
In a peer-reviewed study in Nature Communications, researchers now report an electrolyte in which zeolite is modified with isolated metal atoms and wetted with an aqueous zinc sulfate solution. The aim is for the quasi-solid structure to do more than simply hold the electrolyte. The material itself is designed to assist the electrochemical reactions.
The researchers prepared zeolites containing individual atoms of iron, cobalt, nickel and copper. Calculations and experiments showed that the metal atoms affected both the kinetics of iodine conversion and the way polyiodides were bound within the material.
The iron version produced the strongest overall performance. According to the study, the isolated iron sites both catalyze iodine conversion and act as adsorption sites for polyiodides. This can reduce the so-called shuttle effect, in which soluble iodine species move between the electrodes and contribute to losses and unwanted reactions.
The material also affected the other side of the battery. The researchers found that the iron-doped zeolite stabilized water molecules, suppressed side reactions and promoted more uniform zinc deposition at the negative electrode.
That combination is the central idea of the work. Rather than optimizing only one reaction, the electrolyte uses atomically dispersed active sites to address several bottlenecks in zinc–iodine chemistry at once.
In laboratory cells, the batteries retained a discharge capacity of 81.2 mAh per gram of iodine after 6,800 cycles at a high current of 4 A per gram.
The figure is notable, but it needs to be read together with the test conditions. This was not a finished pouch cell completing 6,800 realistic use cycles. The long-term test was performed on laboratory cells at a high specific current, and the capacity is reported relative to the mass of iodine in the positive electrode.
The researchers also built pouch cells as a proof of concept. After 200 cycles at 0.5 A per gram, these delivered 147.6 mAh per gram when capacity was calculated from iodine mass. When calculated from the total mass of the positive electrode, the capacity was 52 mAh per gram.
The difference between these figures illustrates why the mass basis matters when battery results are compared. A value calculated only from the active iodine says something different about a practical battery than a value that includes the entire electrode.
The study also shows that the iron-doped zeolite is compatible with other positive-electrode materials for rechargeable zinc batteries. That suggests the principle may extend beyond the particular zinc–iodine cell tested most extensively here.
Even so, there is still a gap between a promising laboratory material and a commercial energy-storage system. The researchers demonstrate pouch cells, but the study does not establish cost, manufacturing yield, full-system energy density or lifetime across a broad range of realistic temperatures and charging profiles.
The result is therefore primarily an advance in electrolyte design. The zeolite does not act merely as a passive porous host. By placing metal atoms individually within its structure, the researchers create active sites that can influence the chemistry around both iodine and the zinc electrode.
If that control can be maintained as cells are scaled up, it could contribute to zinc-based batteries that combine aqueous chemistry with improved stability. The present study shows that the mechanism works in laboratory cells and can be transferred to a pouch-cell prototype. The path to a practical storage system still needs to be demonstrated.