With the expansion of renewable energies, stationary battery storage systems are becoming increasingly important. According to calculations by the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, storage capacities of up to 100 gigawatt hours should be available in Germany by 2030 in order to cushion the yield of renewables that are subject to natural fluctuations. This corresponds to thirteen times the current battery capacity. But large-scale storage systems consisting of rechargeable lithium-ion batteries are and will remain expensive. Therefore, efficient and cost-effective solutions are sought.

This is where sodium-sulfur batteries come in handy. They have the great advantage that they do not require expensive and scarce raw materials: sodium is abundant in seawater. Sulfur is non-toxic and can be obtained via the Claus process from hydrogen sulfide, which is produced in large quantities as a waste product in petroleum refineries. Long-term stability has long been a weak point, but great progress has also been made here recently, so that a market launch seems possible.

The first sodium-sulfur battery was built in 1966 at the Ford plant in Dearborn, Michigan, but the technology had to contend with huge drawbacks: For example, the battery cells had to be operated at 300 to 340 Celsius so that sodium and sulfur, the two electrode materials, remained liquid and the solid ceramic electrolyte membrane retained its conductivity. Such high temperatures pose a safety risk during operation and cause high costs, which significantly limits the applications. For this reason, high-temperature sodium-sulfur batteries have not been able to establish themselves despite an energy density comparable to lithium batteries and a long service life.

Molybdenum disulfide slows down capacity loss

The goal of operating sodium-sulfur batteries at room temperature was therefore discussed early on. Sodium and sulfur as anode and cathode material respectively are in the solid state. Liquid sodium salts can be used as electrolytes. In 2006, Korean scientists led by Cheol-Wan Park presented a prototype with a high energy density for the first time. However, the battery quickly lost capacity and failed after only 20 charging cycles.

Above all, the sulfur cathode turned out to be a weak point right from the start. During discharge, so-called polysulfides are formed as intermediate products, which dissolve in the liquid electrolyte and are deposited on the sodium anode as a thin layer. As a result of this phenomenon, known as the shuttle effect, the battery cell quickly loses capacity because it can store less and less charge with each cycle.

An Australian-Chinese research group has now apparently solved this problem. In the laboratory of the University of Sydney, the scientists led by Shenlong Zhao produced a stable sulfur cathode. The sulfur was held together by a framework of graphene. In addition, individual molybdenum disulfide molecules were incorporated. The measure was successful. As Zhao and his colleagues report in the journal "Advanced Materials", the capacity of the sodium-sulfur cell remained largely constant even after 1000 charging cycles.

The loss was only 0.05 percent per cycle. The researchers conclude that the molybdenum disulfide optimizes the reduction of disodium tetrasulfide in disodium sulfide at the cathode, thus minimizing the shuttle effect. Next, they want to develop a small-format battery cell in pouch format. "Compared to lithium batteries, our technology has unparalleled advantages in terms of cost and environmental protection," says Shenlong Zhao.