The visions for the battery of the future are divided into two parts: On the one hand there is the hope for more energy and power density, on the other hand there is hope for lower costs and fewer critical raw materials. Solid-state lithium cells stand for the first vision, and sodium batteries for the second. Recent developments now give hope that one day the advantages of both technologies can be combined.
Sodium batteries themselves are nothing new. However, behind this is a whole zoo of concepts that, apart from the charge carrier sodium, only have something in common to a limited extent.
Sodium nickel chloride is experiencing a renaissance
A classic are sodium nickel chloride batteries. They have been known since the 1980s and are considered safe, robust and inexpensive. However, they have a moderate energy density and have to be operated at temperatures of around 250 to 300 °C. They used to be installed in e-cars like the early e-smarts under the brand name “Zebra”. Because they almost always had to be attached to the net to maintain their temperature, they quickly disappeared again.
In stationary use, for example as buffer storage for the power grid, they are currently experiencing a renaissance. This year, the Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) and Altech Batteries GmbH want to set up a battery factory with an annual output of 100 megawatt hours in Schwarze Pumpe, Saxony.
“The sister technology sodium-sulphur has been around on a large scale for some time, but this battery burns, which is why we decided on sodium-nickel chloride,” says Roland Weidl, site manager of the Battery Innovation and Technology Center at IKTS. “A major cost factor is manufacturing. We’ve made great progress there, and upscaling means there’s a further reduction in costs. This battery is specially optimized for stationary energy storage. That’s the crucial difference from before.”
The batteries with the brand name Cerenergy consist of steel cylinders, which also form the negative electrode. Inside is a ceramic tube as an electrolyte that is permeable to sodium ions but not to electrons. It is filled with granules of table salt and nickel and with molten chlorine aluminate, which makes contact with the electrodes. The positive electrode is also located inside the ceramic tube. When charging, the sodium ions in the salt migrate through the ceramic and form a layer of molten metallic sodium. The chloride ions combine with the nickel to form nickel chloride. When discharging, the process happens in the opposite direction. 40 of these steel cylinders are combined into modules of ten kilowatt hours each.
And above all: no lithium
Altech cites the fact that critical raw materials are not used as an advantage: no cobalt, no graphite, no copper, only a relatively small amount of nickel and, above all, no lithium. Sodium is available in abundance everywhere – for example as a component of table salt – and, unlike lithium, it can be obtained easily and inexpensively. In addition, the cells, with an estimated service life of 15 years, are more robust and around 40 percent cheaper than comparable lithium-ion batteries.
But aren’t the batteries quite inefficient because of their high temperature level? “The largest batteries in the world are batteries that work at these temperatures, but mostly nobody knows,” says Weidl. “Lithium-ion batteries have to be cooled in summer and heated in winter, here you need air conditioning. Our battery only needs to be heated, but it stays at a high temperature for a very long time due to operation and insulation.”