Reducing the temperature of molten sodium batteries

July 22, 2021 // By Nick Flaherty
Reducing the temperature of molten sodium batteries
Researcher at Sandia Labs in the US have developed a 3.6V molten sodium iodide battery for renewable energy storage that runs at just 110 ºC

Researchers in the US have designed a new class of molten sodium batteries for grid-scale energy storage.

Molten sodium batteries have been used for many years to store energy from renewable sources, such as solar panels and wind turbines. However, commercially available molten sodium-sulfur batteries typically operate at 270 to 350 ºC.

The researchers at Sandia National Laboratories have developed a molten sodium-iodide battery that instead operates at 110 ºC

The sodium-iodide battery operates at 3.6V, 40 percent higher operating voltage than a commercial molten sodium battery. This voltage gives a higher energy density, and that means that potential future batteries made with this chemistry would need fewer cells, fewer connections between cells and an overall lower unit cost to store the same amount of electricity.

"We've been working to bring the operating temperature of molten sodium batteries down as low as physically possible," said 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."

However soldium-sulfur does not operate at lower temperatures, so the team developed a catholyte with a mixture of two salts, sodium iodide and gallium chloride.

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When energy is discharged from the new battery, the sodium metal produces sodium ions and electrons. On the other side, the electrons turn iodine into iodide ions. The sodium ions move across a separator to the other side where they react with the iodide ions to form molten sodium iodide salt. Instead of a sulfuric acid electrolyte, the middle of the battery is a ceramic separator that allows only sodium ions to move from side to side.

"In our system, unlike a lithium ion battery, everything is liquid on the two sides," said Erik Spoerke, a materials scientist at Sandia. "That means we don't have to deal with issues like the material undergoing complex phase changes or falling apart; it's all liquid. Basically, these liquid-based batteries don't have as limited a lifetime as many other batteries."

"This is the first demonstration of long-term, stable cycling of a low-temperature molten-sodium battery," he said. "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 (110 C) . This low-temperature sodium-iodide configuration is sort of a reinvention of what it means to have a molten sodium battery."

Sandia's small, lab-scale sodium-iodide battery was tested for eight months inside an oven with 400 charge-discharge cycles (above). The Covid-19 pandemic meant pausing the experiment for a month and let the molten sodium and the catholyte cool down to room temperature and freeze. After warming the battery up, it still worked.

This means that if a large-scale energy disruption were to occur such as the Texas snow storms earlier this year, the sodium-iodide batteries could be used, and then allowed to cool until frozen. Once the disruption was over, they could be warmed up, recharged and returned to normal operation without a lengthy or costly start-up process, and without degradation of the battery's internal chemistry.

Sodium-iodide batteries are also safer. "A lithium ion battery catches on fire when there is a failure inside the battery, leading to runaway overheating of the battery. We've proven that cannot happen with our battery chemistry. Our battery, if you were to take the ceramic separator out, and allow the sodium metal to mix with the salts, nothing happens. Certainly, the battery stops working, but there's no violent chemical reaction or fire," said Spoerke.

The next step for the sodium-iodide battery project is to continue to tune and refine the catholyte chemistry to replace the expensive gallium chloride component says Small.

The team is also working on various engineering tweaks to get the battery to charge and discharge faster and more fully. One previously identified modification to speed up the battery charging was to coat the molten sodium side of the ceramic separator with a thin layer of tin.

The team expects it will take five to 10 years to get sodium-iodide batteries to market, with most of the remaining challenges being commercialisation.

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Postdoctoral researcher Martha Gross works in an argon glove box at Sandia on a lab-scale sodium iodide battery

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