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Improved Batteries for Large-Scale Energy Storage

Molten sodium sulphur batteries were originally designed by the Ford Motor Company in the 1960s and later sold to the Japanese spark plug manufacturer NGK. Apart from automotive applications, the batteries also have a long history of being used as energy storage media for renewable sources including solar and wind energy. Sodium sulphur batteries operate under high temperatures reaching up to 300°C and use a solid electrolyte. One electrode is molten sodium and the other molten sulphur. It is the reaction between the two elements that forms the basis for the cell reaction.

Now (2021), scientists at Sandia National Laboratories have designed a new class of molten sodium batteries for large-scale energy storage applications. One of the chief aims of their research was to reduce the operating temperature of the batteries. For this reason, the battery chemistry had to be altered as different temperatures require different materials. Among the major innovations that enabled using a lower operating temperature was the development of what the scientists called a catholyte, which is a liquid mixture of the two salts sodium iodide and gallium chloride.

In commercial molten sodium sulphur batteries the process pattern is that whenever energy is discharged from the battery, the sodium metals create sodium ions and electrons. The electrons turn iodine into iodide ions. The sodium ions move across a separator to the other side and react with the iodide ions to create molten sodium iodide salt. Instead of a sulfuric acid electrolyte, the middle of the battery is made up of a special ceramic separator that allows only sodium ions to move from one side to the other. In the new system everything is in a liquid state on both sides, which means that problems with complex phase changes of the material or material disintegration do not occur. Also, these liquid-based batteries have a longer lifespan. The sodium-iodide battery was tested for eight months inside an oven. During the experiments the battery was charged and discharged more than 400 times. Owing to the COVID-19 pandemic the experiments had to be paused for a month. As a consequence, the molten sodium and the catholyte cooled down to room temperature and froze. When the experiments recommenced the scientists initially feared that their battery might not work properly, but contrary to their expectations the battery only needed to warm up to its usual operating temperature to function properly, which proved that even if a major disruption were to occur the battery would still work.

For some time, scientists have tried to improve the efficiencies of sodium sulphur batteries. In 2018, a sodium–sulphur battery was created which worked at room-temperature and had a high electrochemical performance. This battery was also safer than its predecessors as they had used an optimised electrolyte system which contained propylene carbonate and fluoroethylene carbonate as co-solvents, as well as highly concentrated sodium salt and indium triiodide as an additive. The fluoroethylene carbonate solvent and high salt concentration reduced the solubility of sodium polysulfides and also created a solid-electrolyte interface on the sodium anode upon cycling. The experiments showed that sodium–sulphur batteries had high capacity and long cycling stability.

In 2019, scientists manufactured a nanomaterial that acted as an improved cathode for room-temperature sodium-sulphur batteries, making them suitable for large-scale energy storage. The researchers created a nanomaterial consisting of nickel sulphide nanocrystals embedded in nitrogen-doped porous carbon nanotubes which exhibited excellent performance when employed as cathodes. The new nanomaterial not only delivered superior performance, but was also suitable for large-scale production and therefore commercial.

There are numerous advantages of the new battery design: the reduced battery temperatures were responsible for great cost savings as less expensive materials were used, as well the batteries needing less insulation. Also, if an operation disruption occurred, the battery simply needed to warm up and could return to normal operation without a time-consuming or expensive start-up process, and without harm to the internal chemistry of the battery. Sodium-iodide batteries are also safer. The battery is unlikely to catch fire when there is a failure inside it which might lead to uncontrollable overheating of the battery. Also, if the ceramic separator is removed and the sodium metal brought to mix with the salts, no potentially dangerous situations can occur. And if the sodium-iodide battery is caught in an outside fire, it is unlikely that the battery will explode. Finally, the new sodium-iodide battery has a 40% higher operating voltage than a commercial molten sodium battery.

The next step of the research is to modify and improve the catholyte chemistry and to replace the gallium chloride component. The scientists are also working on methods to make the battery charge and discharge more quickly and more completely. One method to achieve this is to coat the molten sodium side of the ceramic separator with a thin layer of tin. The scientists estimate that it will take approximately five to 10 years until the battery is ready to be launched onto the market.