Next-generation aqueous zinc battery retains 96% capacity after 5,000 charge cycles

South Korean researchers have developed a new design technology for zinc batteries that sustains performance even after repeated charging cycles.

The Gwangju Institute of Science and Technology (GIST) announced Tuesday that a research team led by Kim Sang-ryun, a chemistry professor at the institute, working jointly with teams from the Korea Institute of Energy Research and Ewha Womans University, has identified a new electrolyte design principle. The principle uses borohydride complexes to suppress electrode degradation and extend the lifespan of aqueous zinc metal batteries.

The research demonstrates that a stable interface can be formed through electrolyte composition design alone, without separate surface coatings or complex electrode treatment processes.

As large-scale energy storage systems (ESS) have grown in importance, aqueous zinc metal batteries have drawn attention as a next-generation rechargeable battery option valued for their safety and cost-effectiveness.

Aqueous zinc metal batteries use water-based electrolytes instead of volatile and toxic organic solvents, improving safety. They also use zinc as an electrode material — an element that is inexpensive and offers high storage capacity.

However, repeated charging and discharging causes water and zinc to react, generating hydrogen gas and producing byproducts such as zinc oxide (ZnO). Zinc also accumulates unevenly on the electrode surface, gradually degrading the electrode and limiting long-term stable use.

To address these problems, the research team added borohydride complexes to the electrolyte of an aqueous zinc metal battery and analyzed changes at the zinc metal surface and in the electrolyte during charging and discharging.

When dissolved in water, the borohydride complex separates into zinc ions and complex anions. These complex anions interact only weakly with water and surrounding ions, leaving the key properties of the existing electrolyte — including its structure, ionic conductivity and viscosity — largely unchanged.

Once charging and discharging begins, however, the complex anions react at the zinc electrode surface to form a hydride-based protective layer.

This protective layer, or interfacial layer, forms only on the zinc surface where charging and discharging occurs, while the electrolyte itself remains intact. The layer reduces direct contact between water and zinc, suppressing the hydrogen evolution reaction and zinc oxidation that cause electrode degradation, and promoting more uniform zinc deposition on the electrode surface.

In electrochemical experiments, the electrolyte containing the borohydride complex suppressed hydrogen evolution and uneven zinc growth. In symmetric cell tests used to evaluate zinc electrode stability, the battery operated stably for up to 3,000 hours.

In a zinc-vanadium oxide battery, the electrolyte retained 96.55 percent of initial capacity after 5,000 charge-discharge cycles, recording a Coulombic efficiency of 99.8 percent. A battery using a conventional electrolyte under the same conditions retained only 46.40 percent of its initial capacity.

"We confirmed stable operating characteristics not only in small laboratory-scale cells but also in pouch cells, suggesting strong potential for application in next-generation energy storage systems where safety is critical, including large-capacity ESS," Kim said.

The findings were published in Chemical Engineering Journal, an international academic journal in materials science and chemistry, on July 26.