Solid-state sodium-air batteries: a promising green energy source?

Researchers from the Pohang University of Science and Technology (POSTECH) have developed a high-energy, high-efficiency, all-solid-state sodium-air battery that can reversibly utilise sodium (Na) and air without requiring special equipment. Secondary batteries can be used in green technologies such as electric vehicles and energy storage systems.

The next-generation high-capacity secondary batteries, termed “metal–air batteries”, draw power from resources like oxygen and metals found on Earth. However, a challenge arises from the formation of carbonate — a by-product of metal and oxygen reaction involving atmospheric carbon dioxide (CO₂) and water vapour (H₂O) — which sacrifices battery efficiency. To address this, despite the name, metal–air batteries typically require additional equipment such as an oxygen permeation membrane to either purify oxygen or selectively use atmospheric oxygen.

The researchers used Nasicon, which is an Na superionic conductor and a solid electrolyte, to address the carbonate issue. Nasicon, comprising elements like Na, silicon (Si) and zirconium (Zr), serves as a solid electrolyte capable of ion movement in the solid state while demonstrating high electrochemical and chemical stability. Leveraging this solid electrolyte, the researchers protected the sodium metal electrodes from air and facilitated the breakdown of carbonate formed during electrochemical cell operation.

As a result, the reversible electrochemical reaction involving carbonate led to an increase in the cell’s energy density by increasing a working voltage while reducing the voltage gap during charging and discharging, thus enhancing energy efficiency. The all-solid-state sodium-air cell also exhibited superior kinetic capability through in-situ formed catholyte, which facilitates fast sodium ion conduction inside the electrode. The cell also operated solely on metal and air without additional special equipment for an additional oxygen filtration device.

Professor Byoungwoo Kang from the Department of Materials Science and Engineering at POSTECH said the researchers have devised a method to harness carbonate, thereby overcoming a long-standing challenge in the development of high-energy metal–air batteries. “We hope to lead the field of the next generation all-solid-state metal–air batteries, leveraging a solid electrolyte-based cell platform that remains stable in ambient conditions and offers a broad voltage range,” Kang said.

The research findings have been published in the journal Nature Communications.

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