Researchers at Sandia National Laboratories have designed a new class of molten sodium batteries for grid-scale energy storage. The new battery design was shared in a research paper published today in the scientific journal Cell Reports Physical Sciences.
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 batteries, called sulfur sodium batteries, typically operate at 520-660 degrees Fahrenheit. Sandia’s new molten sodium iodide battery operates at a cooler 230°F instead.
“We have worked to reduce the operating temperature of molten sodium batteries to the lowest possible level physically,” said Leo Small, principal investigator on the project. “There are cascading cost savings that come along with lowering the battery’s temperature. You can use less expensive materials. Batteries need less insulation and the wires that connect all the batteries can be a lot thinner.”
But he added that battery chemistry operating at 550 degrees does not work at 230 degrees. One of the major innovations that allowed this lower operating temperature was the development of what he calls a catholyte. A catholyte is a liquid mixture of two salts, in this case, sodium iodide and gallium chloride.
The basics of building better batteries
The primary lead-acid battery, commonly used as a car ignition battery, has a lead plate and a lead dioxide plate with a sulfuric acid electrolyte in the middle. When the energy is discharged from the battery, the lead plate reacts with the sulfuric acid to form lead sulfate and electrons. These electrons start the car and return to the other side of the battery, where the lead dioxide plate uses the electrons and sulfuric acid to form lead sulfate and water. For the new molten sodium battery, the lead plate is replaced with liquid sodium metal, and the lead dioxide plate is replaced with a liquid mixture of sodium iodide and a small amount of gallium chloride, said Eric Spork, a materials scientist who was working. on molten sodium batteries for more than a decade.
When energy is discharged from the new battery, sodium metal produces sodium ions and electrons. On the other hand, the iodine electrons convert into iodide ions. The sodium ions move through a separator to the other side where they react with the iodide ions to form a molten sodium iodide salt. Instead of the sulfuric acid electrolyte, the middle of the battery is a special ceramic separator that only allows sodium ions to move from one side to the other, and nothing else.
“In our system, unlike a lithium-ion battery, everything is liquid on both sides,” Spurky said. “This means we don’t have to deal with issues like materials going through complex phase changes or disintegrating; they are all liquid. Essentially, these liquid-based batteries don’t have a limited life like many other batteries.
In fact, commercial molten sodium batteries have a lifespan of 10 to 15 years, which is much longer than standard lead-acid or lithium-ion batteries.
Long lasting and safer batteries
The small-sized sodium iodide battery was tested in the Sandia laboratory for eight months in an oven. Martha Gross, a postdoctoral researcher who has worked in lab testing for the past two years, has conducted experiments to charge and discharge the battery more than 400 times during those eight months.
She said that due to the COVID-19 pandemic, they had to stop the experiment for a month and let the molten sodium and catholyte cool to room temperature and freeze. Gross was glad that after heating the battery up, it still worked.
This means that in the event of a major power disturbance, such as what happened in Texas in February, sodium iodide batteries can be used, then let cool until frozen. Once the outage is over, Spoerke added, it can be heated up, recharged, and returned to normal operation without the need for a lengthy or expensive start-up process, and without degrading the battery’s internal chemistry.
Sodium iodide batteries are safer, too. Spoerke said, “A lithium-ion battery ignites when there is a malfunction inside the battery, causing the battery to overheat. We have demonstrated that this cannot happen with our battery chemistry. Our battery, if you were to take a ceramic separator, and allow the sodium metal to mix with the salts. , nothing happens. Sure, the battery stops working, but there’s no violent chemical reaction or fire.”
Small added that if an external fire swallowed a sodium iodide battery, the battery would likely crack and fail, but it shouldn’t add fuel to the fire or cause a sodium fire.
In addition, at 3.6V, the new sodium iodide battery has a 40% higher operating voltage than a commercial molten sodium battery. This voltage leads to a higher energy density, meaning that potential future batteries made with this chemistry will need fewer cells, fewer connections between cells, and a lower overall unit cost to store the same amount of electricity, Small said.
“We were really excited about how much energy we could put into the system because of the new Catholic structure that we’re reporting in this paper,” Gross added. “Molten sodium batteries have been around for decades, and they’re all over the world, but nobody ever talks about them. So being able to turn the temperature down and go back with some numbers and say, ‘This is really, really a viable system’ is pretty neat.”
The future of sodium iodide batteries
Small said the next step for the sodium iodide battery project is to continue to fine-tune and improve the cathode chemistry to replace the gallium chloride component. Gallium chloride is very expensive, 100 times more than table salt.
Spoerke added that the team is also working on various engineering tweaks to make the battery charge and discharge faster and more fully. One of the modifications previously identified to speed up battery charging was to coat the molten sodium side of the ceramic separator with a thin layer of tin.
Spoerke added that it will likely take five to ten years to bring sodium iodide batteries to market, with most of the remaining challenges being marketing, rather than technical.
“This is the first evidence of a long-lived and stable cycle of a low-temperature molten sodium battery,” Spurky said. “The magic of what we put together is that we have identified the chemistry of the salt and the electrochemistry that allows us to operate effectively at 230 degrees Fahrenheit. This low-temperature formation of sodium iodide is kind of reinventing what it means to a molten sodium battery.”
The new sodium battery was developed with support from the Department of Energy’s Office of Electric Energy Storage Program.
The team develops a stable, efficient, anode-free sodium battery
Martha M. Gross et al, A high-voltage, low-temperature molten sodium battery enabled by metal halide cathochemistry, Cell Reports Physical Sciences (2021). DOI: 10.1016 / j.xcrp.2021.100489
Submitted by Sandia National Laboratories
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