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Buzz around zinc ion batteries may be overstated, researchers say

A zinc anode brings high safety and low cost for aqueous zinc-ion batteries. But challenges remain to be solved, researchers say.

A recent study by Canadian and U.S.-based researchers suggests that zinc-ion battery chemistry for grid-scale energy storage may face more challenges than is widely believed.

Aqueous zinc ion battery (AZIB) chemistry has been identified as a promising technology for grid storage based on its use of zinc, a key sustainable metal, as the anode material in the cell.

Compared to lithium in commercialized lithium-ion batteries, zinc is more naturally abundant and compatible with water, thus allowing for the direct use of cost-effective, nonflammable, aqueous-based electrolytes.

However, researchers at the University of Waterloo working as part of the Joint Center for Energy Storage Research based at the U.S. Department of Energy’s Argonne National Laboratory, said that numerous claims in the open literature have been “mistakenly overestimated,” and that zinc-ion batteries still face many challenges.

“A zinc anode brings high safety and low cost for aqueous zinc-ion batteries because it is stable with most water-based electrolytes,” said University of Waterloo Professor Linda Nazar, who along with graduate student Chang Li are lead authors of the study. ​However, during charge-discharge cycling, zinc tends to grow as random and spiky crystals — called dendrites. The researchers said these dendrites can easily cause a short-circuit when charging batteries. 

“Although many strategies have been reported to solve the zinc dendrite issue, only few of them can address the requirements of practical applications,” Nazar said.

Challenges also remain on the cathode side. According to Nazar, the water molecules in the aqueous-based electrolyte can spontaneously dissociate into hydroxide ions and protons. While the protons compete with the zinc ions in the process of shuttling back and forth into the battery’s cathode materials — a process known as intercalation — the remaining hydroxide ions can combine with the zinc.

That reaction results in compounds called layered zinc double hydroxides. Nazar said these precipitate on the cathode surface and ​“take zinc out of the equation, insulating the surface in a very deleterious side reaction.”

The main question facing zinc batteries, in Nazar’s view, is how to suppress the water activity. The answer could lie in a non-aqueous/aqueous hybrid electrolyte that can help to sequester the water, preventing its dissociation. 

​“These electrolytes have been shown to be quite effective at ensuring that zinc-ion insertion dominates the chemistry,” she said.

Nazar identified an additional drawback to some research involving zinc batteries: Studies that had run the batteries at exceptionally high cycling rates tended to be based on proton insertion much more than zinc intercalation. ​

“Superfast cycling of zinc batteries won’t help in large-scale grid storage,” she said. ​She said a better approach is to “cycle only at moderate rates” and then perform electrolyte engineering to inhibit the water dissociation. 

A paper based on the study, ​“Toward practical aqueous zinc-ion batteries for electrochemical energy storage,” appeared in the online edition of Joule this past summer.

The work was supported by the Joint Center for Energy Storage Research, DOE Office of Science Energy Innovation Hub and by the Center for Mesoscale Transport Properties, a DOE Office of Science Energy Frontier Research Center.

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