Pure metallic gel opens door to more powerful liquid metal batteries

Researchers at Texas A&M University have developed the first known metallic gel. Unlike everyday gels, like those used in hand sanitizers, hair products or soft contact lenses, this new material is made entirely of metals and can withstand extreme heat. The discovery could be a game changer for energy storage.

The work is published in Advanced Engineering Materials.

The gel is created by mixing two metal powders. When heated, one metal melts into a liquid, while the other stays solid and forms a microscopic scaffold. The liquid metal remains trapped inside this structure, creating a gel-like material that looks solid but contains liquid within.

Everyday gels are semi-solid materials containing an organic backbone holding liquids in place at room temperature. Unlike them, metallic gels require very high temperatures, which, depending on the metals used, can be around 1,000 degrees Celsius or 1,832 degrees Fahrenheit.

“Metallic gels have never been reported before, probably because no one thought liquid metals could be supported by an internal ultrafine skeleton,” said Dr. Michael J. Demkowicz, a professor in the Department of Materials Science and Engineering, who led the research.

“What’s surprising in this case is that when the majority component—copper—was melted into liquid, it didn’t just collapse into a puddle. That’s what pure copper would have done,” he explained.

Metallic gels made from highly reactive metals with strong electrical attraction, known as electronegativity, can be used as electrodes in liquid metal batteries (LMBs). In simple terms, these metals are very reactive and easily bond with other materials, which helps the battery work efficiently.

LMBs are special types of batteries that store and release large amounts of electrical energy. Instead of using solid materials like most batteries, they use layers of liquid metal. Because the parts are liquid, they do not wear out as quickly as regular batteries.

So far, LMBs have mainly been used in large stationary systems, such as backup power for building applications that need to keep running during a power outage. They have not been used in moving systems because the liquid inside shifts when the battery moves. This can cause a short circuit, which means the battery loses electrical power.

That is where metallic gel electrodes come in. By holding the liquid metal in place, they could make it possible to use LMBs in things that move, such as powering large ships or heavy industrial vehicles that can safely handle the heat of these batteries.

To test the idea, researchers built a small lab version of the battery using two cube-shaped electrodes. One was made from a mix of liquid calcium and solid iron, which acted as the anode, and the other from liquid bismuth and iron, which acted as the cathode.

When placed in a molten salt, a hot liquid that allows electrical charge to flow between the two, the battery worked successfully. It produced electricity, and the mostly liquid electrodes stayed in shape and kept working as intended.

The research was performed by a team led by Demkowicz and doctoral student Charles Borenstein, who is the first author on the paper.

Demkowicz and Borenstein said that what began as an exploration of the behaviors of metal composites of copper and tantalum resulted in this serendipitous discovery.

“We were just exploring different methods of processing composites by heat,” Demkowicz said. “All we wanted to do, at first, was to see: Does this even survive until one of the components melts?”

Borenstein originally put a composite of 25% tantalum and 75% copper into the furnace heated to copper’s melting point.

“Nothing happened, which I found kind of confusing,” he said, noting that the copper didn’t run out and pool. “We were pretty surprised by these results.”

After testing other percentages of both metals, he found that any combination of the metals with a volume of tantalum above 18 percent still retained the gel form.

The next step was to bring the new structure to a lab with a very high-resolution micro-CT scanner to examine the metallic gel’s interior. Although copper and tantalum are not ideal candidates for electrodes, they are for CT scanning. As anticipated, the tantalum formed a solid scaffolding structure holding the liquid copper within its lacunae.

That’s when the team shifted their research to the battery materials of iron, bismuth and calcium, and demonstrated the feasibility of the metallic gel LMB.

Demkowicz said that an LMB made for transportable applications could also employ a gel-like composite electrolyte, such as a molten salt supported by a ceramic backbone, through which the electrode’s ions could pass.

He highlighted other potential applications for LMBs, including one that he said would be especially exciting to work on: powering a hypersonic vehicle, like those under feasibility study at the Texas A&M University Consortium for Applied Hypersonics. Hypersonic vehicles operate at extremely high temperatures and could theoretically be powered by a very hot LMB.

Co-authors on the paper are Dr. Brady G. Butler and Dr. James D. Paramore, visiting professors at Texas A&M, and Dr. Karl T. Hartwig, professor emeritus at the university.

The high-resolution CT scanning was performed at the University of Texas High-Resolution X-ray Computed Tomography Facility in Austin.