Researchers have reinvented molten sodium batteries for grid-scale energy storage.
Due to energy storage requirements currently, there has been renewed interest in molten sodium batteries for safe, reliable, typically large format storage. These batteries take advantage of sodium as an active material, employing solid-state ceramic separators, and using several different cathode chemistries in battery design. 
A lead-acid battery has a lead plate and a lead dioxide plate with a sulfuric acid electrolyte in the middle. The electrical energy produced by discharging a lead-acid battery can be attributed to the lead plate reacting with sulfuric acid to form lead sulfate and electrons. In the molten sodium batteries, 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.
The sodium metal produces sodium ions and electrons. These sodium ions move across a separator to the other side where they react with the iodide ions (formed from iodine) to form molten sodium iodide salt. The middle of the battery is a semipermeable ceramic separator that only allows sodium ions to pass through.
Molten sodium batteries are used to store energy from renewable sources, such as solar panels and wind turbines. These batteries have lifetimes of 10-15 years which is significantly longer than standard lead-acid batteries or lithium ion batteries.
However, commercially available molten sodium batteries typically operate at 520-660 degrees Fahrenheit. Researchers from the Sandia National Laboratory have designed a new molten sodium battery for grid-scale energy storage that can operate at a much cooler temperature of 230 degrees Fahrenheit. The work was reported in the journal Cell Reports Physical Science.
“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 batteries developed are safer. Erik Spoerke, a materials scientist who has been working on molten sodium batteries for more than a decade, 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.”
Moreover a 3.6V sodium-iodide battery has a higher operating voltage than commercial molten sodium batteries. According to Leo Small, this voltage leads to 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.
Spoerke 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 degrees Fahrenheit. This low-temperature sodium-iodide configuration is sort of a reinvention of what it means to have a molten sodium battery.”