Development of high-voltage sodium–sulfur battery

 High-voltage sodium–sulfur battery by Chinese researchers, an alternative to lithium batteries     

Researchers at Shanghai Jiao Tong University in China have designed a new sodium-sulfur battery with higher power density and discharge capacity than before, enabling a cheaper, safer alternative to lithium-ion batteries. The researchers switched to S0/S4+ redox chemistry an non-flammable electrolyte to created high-voltage anode-free batteries which are safer. The new sodium-sulfur battery design provides a high-power energy storage solution. Team of researchers has just pulled the curtain back on a new sodium-sulfur battery design which could fundamentally change the math on energy storage. By leaning into the very chemistry which has historically made sulfur a headache for engineers, they have managed to build a cell which is incredibly cheap to make but still packs a massive energy punch. This new sodium-sulfur battery design pushes energy density to 2,021 Wh/kg.

Sulfur has always been the “white whale” of battery tech because it can theoretically hold a ton of energy. This switch to electric energy has driven increased demand for energy storage devices. Lithium-ion batteries are the most energy-dense solutions we currently know and are widely used. However, issues of thermal runaway and fire risks prevent their use in large-scale applications. Moreover, the increased demand has driven up lithium prices, making energy storage more expensive by the day. Researchers are looking for alternatives to replace lithium-ion batteries. Sodium is highly abundant and can serve as a low-cost alternative, and is part of multiple research projects globally where sodium is used in various combinations. The design, which is currently being tested in the lab, uses dirt-cheap ingredients: sulfur, sodium, aluminum and a chlorine-based electrolyte. In early trials, the battery hit energy densities over 2,000 watt-hours/kg, a figure which blows today’s sodium-ion batteries out of the water and even gives top-tier lithium cells a run for their money.

Researchers teamed up sodium with sulfur to make a high-energy-density battery. This is not the first attempt to pair sodium and sulfur. Batteries made using Na–S or S/Na2S chemistry required large amounts of sodium but delivered low voltage. The redox reaction with sulfur generates 4 valence electrons, producing a voltage of 3.6V, but replicating this reaction at room temperature was a major challenge. According to the researchers, the S/Na2S conversion reaction at the cathode yields a limited discharge voltage of less than 1.6 V, which is far lower than that of Li-ion counterparts. To overcome this, large amounts of sodium must be used at the anode, which can be 10 times or more than in a lithium battery. This defeats the purpose of using a cheaper material and also affects energy and power densities. In standard lithium-sulfur batteries, sulfur tends to create messy chemical byproducts which gunk up the works and kill the battery’s lifespan. This new approach flips the script. Instead of forcing sulfur to just accept electrons, the researchers set up a system where sulfur actually donates them.

The researchers unlocked the sodium-sulfur battery puzzle by switching to S0/S4+ redox chemistry and created high-voltage anode-free batteries. This design consists of an aluminum (Al) foil anode current collector, an S8 cathode, sodium dicyanamide (NaDCA) in a non-flammable chloroaluminate electrolyte, separated with a glass fiber. The test cells survived 1,400 charge-discharge cycles before they started losing significant capacity. Even more important is the shelf life: after sitting untouched for over a year, the battery still held onto 95% of its charge. This is a huge deal for long-term storage projects where batteries might sit idle for weeks or months. The battery uses a pure sulfur cathode and a simple piece of aluminum foil as the anode. The secret sauce is the electrolyte, which is a soup of aluminum chloride, sodium salts and chlorine. When you discharge the battery, sulfur atoms at the cathode give up electrons and react with the chlorine to form sulfur chlorides. Meanwhile, sodium ions grab those electrons and plate themselves onto the aluminum foil.

This specific chemical dance side-steps the degradation issues which usually plague sulfur batteries. A porous carbon layer keeps the reactive stuff contained, and a glass fiber separator stops the whole thing from short-circuiting. It’s a complex reaction, but the team proved it runs smoothly and reversibly. The chlorine-rich electrolyte they are using is corrosive and tricky to work with safely. Also, these numbers come from lab tests based on the weight of active materials, not a fully packaged commercial cell. Taking this from a beaker to a factory floor is going to be a massive engineering hurdle. But the real issue is the price tag. Based on the cost of the raw materials, the researchers estimate this battery could cost roughly $5/kilowatt-hour. To put that in perspective, it is less than a tenth of the cost of many current sodium batteries and miles cheaper than lithium-ion. If they can mass-produce this, it could make storing renewable energy on the grid dirt cheap.

According to the researchers, the dicyanamide anion in the electrolyte helps unlock S/SCl4 chemistry at the cathode while also improving the reversibility of sodium plating/stripping at the anode. The improved performance of this design is evident in its maximum energy density of 1,198 Wh/kg, discharge capacity of 715 mAh g−1, and power density of 23,773 W/kg. When the researchers added a Bi-COF catalyst at the cathode, the discharge capacity further increased to 1,206 mAh/g, while the energy density rose to 2,021 Wh/kg. Still, this research is a loud wake-up call. It proves that when standard materials like lithium get too expensive or scarce, getting creative with “unconventional” chemistry can open new areas we didn’t even know existed now. With an estimated cost of $5.03/kWh, the sodium-sulfur battery costs an order of magnitude less than its lithium counterparts. Safety is inherently enhanced because the electrolyte is non-flammable. However, researchers still need to work on a few issues before the battery design can be commercially available. The electrolyte, though non-flammable, is highly corrosive and difficult to handle. Additionally, it is only stable in the short term when exposed to air, while long-term stability is currently unknown. The team is, however, confident that these issues can be addressed and will help improve device safety across devices ranging from wearable to grid-scale energy storage.