By introducing some new ingredients to the flow battery, Stanford University scientists are advancing a new way to store wind and solar electricity.

The addition: liquid metal, made from sodium and potassium.

According to research published in the July 18 issue of Joule, the liquid-metal combo more than doubled the maximum voltage of conventional flow batteries. With further development, the researchers hope that the new technology will someday deliver energy to the electric grid quickly, and at normal ambient temperatures.

The flow battery has long been considered a likely option for storing intermittent renewable energy. Most conventional flow batteries store energy in two electrolyte liquids: one with a negatively charged cathode, and one with a positively charged anode.

Until now, however, the kinds of liquids capable of producing the electrical current have either been limited in their energy output, or require high temperatures and expensive chemicals.

Stanford materials science and engineering professor William Chueh, along with his PhD student Antonio Baclig and Jason Rugolo, now a technology prospector at Alphabet’s research subsidiary X Development, turned to two elements: sodium and potassium.

When mixed, the elemental combination forms a liquid metal at room temperature. The team used the fluid for the electron donor – or negative – side of the battery.

In order to use the liquid metal in the negative end of the battery, the group implemented a ceramic membrane made of potassium and aluminum oxide. The membrane separated the negative and positive materials while allowing current to flow.

The Stanford researchers’ setup more than doubled the maximum voltage of conventional flow batteries, and the prototype remained stable for thousands of hours of operation.

The higher voltage potentially reduces production costs, as the battery can store more energy for its size.

“We still have a lot of work to do,” said Baclig in a recent press release from the university, “but this is a new type of flow battery that could affordably enable much higher use of solar and wind power using Earth-abundant materials.”

Baclig spoke with Tech Briefs about the excitement of finding the right combination for flow batteries.

Tech Briefs: What inspired this work?

Antonio Baclig: The big goal is to make low-cost energy storage for the electricity grid, which would help to integrate abundant wind and solar energy resources. Batteries are already being added to the grid at a growing rate. But to achieve massive deployment, the storage costs have to come down a lot, so we felt like a new technological approach was necessary.

Tech Briefs: What led you to liquid metal?

Baclig: We were looking for energy-dense and earth-abundant liquids to use in a flow battery, which led us to the idea of a liquid metal, because the energy density of a liquid metal can be extremely high. We drew a lot of inspiration from current battery technologies, and research on them: sodium-sulfur batteries use a liquid metal (pure sodium), but at high temperature; flow batteries operate at lower temperatures. To get the best of both technologies, we needed a liquid metal at room temperature. It turns out that sodium-potassium alloy (Na-K) is a liquid metal at room temperature, and it has been used for other purposes such as a heat exchange liquid.

Tech Briefs: Why is it particularly valuable to use liquid metal?

Baclig: Liquid metals like Na-K have a few advantages. First, the energy density is extremely high, since it’s almost a pure reactive species instead of a reactive species dissolved in a nonreacting solvent, like water. Second, the voltage is very high, because the alkali metal really wants to give up its electron. A higher voltage helps with so many metrics: energy density, power density, and efficiency. Third, the metal reacts easily and conducts electrons itself, so it doesn’t require an expensive catalyst or even a carbon electrode.

Tech Briefs: How did you get the liquid metal to work in a flow battery?

Baclig: Na-K had not been used in a flow battery before because a suitable membrane had not been demonstrated. Membranes used in typical flow batteries won’t work because they would allow small amounts of the solvent through, which would react with the liquid metal. The ceramic membrane used in sodium-sulfur batteries won’t work because it reacts with the liquid metal itself in an ion-exchange process. We discovered, however, that the potassium variant of that ceramic membrane does work: it is stable to the liquid metal under the conditions we tested.

The sodium-potassium alloy is a room-temperature liquid metal that could unlock a high-voltage flow battery. (Image Credit: Antonio Baclig)

Tech Briefs: How do you envision this flow battery being used in the future?

Baclig: One of the advantages of a flow battery is the ability to scale up the amount of energy stored at only the marginal cost of the liquid and tank. This is useful for addressing the intermittent nature of a grid powered largely by wind and solar energy, for example if the wind quiets down for a few days or if it’s cloudy for a few days. In such an application you want the liquid and tank to be as low cost as possible, which means earth-abundant elements and high-energy-density liquids so the tank can be smaller — both advantages of Na-K.

Tech Briefs: What is most exciting to you about this achievement?

Baclig: Liquid metals have been used in batteries at high temperatures, but there hasn’t been a lot of work on using them at room temperature. Room temperature operation frees up a lot of design constraints. We think this type of battery could, with more work, be a promising technology.

Taking a step back, I am also excited by the thought that there are still so many battery chemistries to explore. We didn't invent any new materials in this research, but we found that the right combination of existing materials gave an exciting battery that nobody had tried before. I am motivated by the feeling that there are better ways to store energy still waiting to be discovered.

Will this combination of flow battery lead to low-cost energy storage for the electricity grid? Share your comments and questions below.

This project was funded by Stanford’s TomKat Center for Sustainable Energy, the Anthropocene Institute, the State Grid Corporation of China through Stanford’s Energy 3.0 corporate affiliate program, the National Research Foundation of Korea, the U.S. National Science Foundation and Stanford Graduate Fellowships.