New molten salt battery for grid-scale storage runs at low temp and cost

As renewable forms of power like wind and solar continue to gain prominence, there will be a need for creative solutions when it comes to storing energy from sources that are intermittent by nature. One potential solution is known as a molten salt battery, which offers advantages that lithium batteries do not, but have their share of kinks to iron out, too. Scientists at Sandia National Laboratories have come up with a new design that addresses a number of these shortcomings, and demonstrated a working molten salt battery that can be constructed far more cheaply, while storing more energy, than currently available versions.

Storing vast amounts of energy in a cheap and efficient manner is the name of the game when it comes to powering whole cities with renewable energy, and despite its many strengths, this is where expensive lithium battery technology falls short. Molten salt batteries shape as a more cost-effective solution, which use electrodes kept in a molten state with the help of high temperatures. This is something that the Sandia scientists have been working to change.

"We've been working to bring the operating temperature of molten sodium batteries down as low as physically possible," says Leo Small, the lead researcher on the project. "There's a whole cascading cost savings that comes along with lowering the battery temperature. You can use less expensive materials. The batteries need less insulation and the wiring that connects all the batteries can be a lot thinner."

In their commercial form, these batteries are known as sodium-sulfur batteries, and a few of these have been developed around the world but generally operate at 520 to 660 °F (270 to 350 °C). The Sandia team have set their sights much lower, although doing so required a rethink as the chemistries that work at high temperatures don't lend themselves well to lower temperatures.

The scientists' design consists of liquid sodium metal that sits on the opposite side of a ceramic separator material to a novel liquid mixture made of sodium iodide and gallium chloride, which the scientists call a catholyte. When the battery discharges energy, chemical reactions take place that produces sodium ions and electrons that pass through the highly-selective separator material and produce molten iodide salt on the other side.

This sodium-sulfur battery proved capable of operating at just 230 °F (110 °C), and proved its worth across eight months of testing in the lab through which it was charged and discharged more than 400 times. Further, it runs at 3.6 volts, which the scientists say is around 40 percent higher than commercially available molten salt batteries. This could equate to versions with fewer cells and therefore a higher energy density.

"We were really excited about how much energy we could potentially cram into the system because of the new catholyte we're reporting in this paper," says study author Martha Gross. "Molten sodium batteries have existed for decades, and they're all over the globe, but no one ever talks about them. So, being able to lower the temperature and come back with some numbers and say, 'this is a really, really viable system' is pretty neat."

The scientists are now turning their attention to lowering the cost of the battery, which could come from replacing the gallium chloride which is around 100 times more expensive than table salt. They say the technology is still five to 10 years away from commercialization, but working in their favor is the safety of the battery, which poses no risk of fire.

"This is the first demonstration of long-term, stable cycling of a low-temperature molten-sodium battery," says study author Erik Spoerke. "The magic of what we've put together is that we've identified salt chemistry and electrochemistry that allow us to operate effectively at 230 °F. This low-temperature sodium-iodide configuration is sort of a reinvention of what it means to have a molten sodium battery."

The research was published in the journal Cell Reports Physical Science.

Source: Sandia Labs via EurekAlert