Sandia Demonstrates New Molten Sodium Batteries

The news is just now getting picked up in the engineering trade magazines, but in July, Sandia National Laboratories announced some impressive improvements in molten sodium batteries.   

The niche that molten sodium batteries are targeted for is power grid level energy backup.  It's a specialized niche, but the combination of safer, easier to live with and cheaper like the one they've developed is always a winner.  

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 sodium-sulfur batteries, typically operate at 520-660 degrees Fahrenheit or 270-350 degrees Celsius. Sandia’s new molten sodium battery operates at a much cooler 230 degrees Fahrenheit or 110 degrees Celsius instead.

“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.”

The challenge was that they needed to develop a new chemistry for this.  Consider an analogy to a lead-acid battery.  These batteries use a lead plate and a lead dioxide plate separated by a sulfuric acid electrolyte. 

In the new molten sodium battery, the lead plate is replaced by liquid sodium metal and the lead dioxide plate is replaced by a liquid mixture of sodium iodide and a small amount of gallium chloride. Because both sides of the battery are liquid, there are no issues with materials undergoing complex phase changes. This gives them a longer life than conventional batteries.

To create electricity in the new battery, the sodium metal produces sodium ions and electrons. On the other side of a ceramic separator which only lets sodium ions move through it, sodium ions react with the iodide ions to form molten sodium iodide salt.

In any battery, the electrons needed to do work are produced when those electrons have someplace to go.  In a car battery, the lead plate reacts with sulfuric acid to form lead sulfate and electrons. These electrons leave the battery, 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. In the molten sodium battery, the sodium metal produces sodium ions and electrons, after those electrons do their work and return to the other side, the electrons turn iodine into iodide ions. The sodium ions move across a special ceramic 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 special ceramic separator that allows only sodium ions to move from side to side, nothing else.

Now 230 degree F molten sodium doesn't seem particularly safe to be around; it's a potent positive ion that will injure you, possibly worse than the temperature will burn you.  The currently available commercial molten sodium batteries have lifetimes of 10-15 years, significantly longer than standard lead-acid batteries or lithium ion batteries. 

Sandia’s small, lab-scale sodium iodide battery was tested for eight months inside an oven. Martha Gross, a postdoctoral researcher who has worked on the laboratory tests for the past two years, conducted experiments charging and discharging the battery more than 400 times over those eight months.

Because of the COVID-19 pandemic, they had to pause the experiment for a month and let the molten sodium and the catholyte cool down to room temperature and freeze, she said. Martha was pleased that after warming the battery up, it still worked.

Dr. Gross points out that they fully expected the battery to work after it was heated up again, but it's always better to have beliefs verified by experiment.  This capacity alone means that in the case of a power plant going down and the batteries freezing, they'll work when the power (and heat) comes back. 

Leo Small (back right) and Erik Spoerke (back left) observe as Martha Gross (front) works in an argon glovebox on their lab-scale sodium iodide battery.  Sandia National Laboratories photo.

The group claims that the battery is safer than lithium ion batteries, which almost routinely seem to make headlines with some sort of fires, because of their entirely different construction.  

Erik said, “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.”

By coincidence the open cell voltage of their battery is 3.6V, right in line with common lithium ion batteries but 40% higher cell voltage of the existing molten sodium batteries.   

The team has set their next goal to reduce the cost by addressing the chemistry a bit.  The "small amount of gallium chloride" the batteries use is very expensive, quoted at 100 times the price of table salt.  The team is also working on engineering tweaks to get the battery to charge and discharge faster and more fully, Erik added.  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.  Given all of what they're looking at, they expect five to 10 years before these batteries are commercialized.  The team believes most of the work is in the commercialization side, learning how to produce them in quantity, not the technical side.