There is a growing need for creative solutions for storing excess renewable power from sources that are intermittent by nature, like wind and solar. Molten salt batteries (commercially known as sodium-sulfur batteries) are one potential solution already in use.
They offer advantages that lithium-ion batteries don’t, albeit having a few cons of their own.
For one, they typically operate at a high 520 to 660°F (270 to 350°C).
But now, scientists at Sandia National Laboratories made a new design, rethinking the battery’s chemistry altogether to get it to work at a much lower temperature – 230°F (110°C).
The Sandia team’s molten salt battery is also cheaper and stores more energy. It could be one of the solutions for grid-scale energy storage the world needs.
When it comes to powering entire cities, storing vast amounts of energy affordably and efficiently is the name of the game. However, today’s most commonly used lithium battery technology is expensive. But molten salt batteries are a more cost-effective solution, especially one that works at a low temperature.
The lead researcher Leo Small said:
We’ve been working to bring the operating temperature of molten sodium batteries down as low as physically possible. There’s a whole cascading cost saving that comes along with lowering the battery temperature. First, you can use less expensive materials. Second, the batteries need less insulation, and third, the wiring that connects all the batteries can be a lot thinner.
One of the design innovations that enabled the lower operating temperature was developing a catholyte – a liquid mixture of sodium iodide and gallium chloride. They’re liquid also means they have an extended life compared to other types of batteries. Commercial molten sodium batteries have 10-15 years’ lifetimes, significantly longer than lithium-ion batteries or standard lead-acid batteries.
Materials scientist Erik Spoerke, who has been working on molten sodium batteries for over a decade, said:
In our system, unlike a lithium-ion battery, everything is liquid on the two sides. That means we don’t have to deal with issues like the material undergoing complex phase changes or falling apart; it’s all liquid. As a result, 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. 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 degrees Fahrenheit. This low-temperature sodium-iodide configuration is a reinvention of what it means to have a molten sodium battery.
Sandia’s small, lab-scale sodium-iodide battery withstood eight months of testing in the lab.
The experiments involved charging and discharging the battery over 400 times.
They also left the molten sodium and the catholyte to cool to room temperature and freeze for a month, then warmed up and recharged. Finally, it returned to regular operation without degrading the battery’s internal chemistry or a lengthy or costly start-up process.
In addition, sodium-iodide batteries are safer. Spoerke explained:
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.
So certainly, the battery stops working, but there’s no violent chemical reaction or fire.
If an outside fire engulfs a sodium-iodide battery, it is likely the battery will crack and fail, but it shouldn’t add fuel to the fire or cause a sodium fire.
Furthermore, the new sodium-iodide battery has a 40% higher operating voltage (3.6 volts) than a commercial molten sodium battery.
Meaning, the batteries could house fewer cells resulting in a higher energy density. It also means they’d require fewer connections between cells and, therefore, an overall lower unit cost to store an equal amount of electricity.
Postdoctoral researcher Martha Gross, who has worked on the laboratory tests, added:
We were excited about how much energy we could potentially cram into the system because of the new catholyte we’re reporting in this paper. 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 next step in their project is to continue refining the catholyte chemistry to replace the gallium chloride component and lower the battery cost further. The researchers say the technology is still five to ten years away from being market-ready.
But most of the remaining hurdles are commercialization challenges over technical challenges.