Immense sodium-sulfur batteries that can store tens of megawatts of power help stabilize many utility grids around the world. In fact, the world’s largest battery, a 108 MW battery tied to an electrical grid in Abu Dhabi, is sodium-sulfur. Although these batteries store about half the energy per volume as do lithium-ion batteries, they have the advantage of using abundant, inexpensive, and non-toxic materials. “Sodium-sulfur is a dream system because it is a long-term, sustainable technology,” says Arumugam Manthiram, a chemist at the University of Texas at Austin.
Traditional sodium-sulfur batteries work at blistering temperatures of 300 °C. A chemistry that works at room temperature would be cheaper and safer but has proven tricky, with batteries lasting just a few charging cycles. Now, with the help of a novel solid electrolyte, Manthiram and his postdoctoral associate Xingwen Yu have boosted the lifetime of sodium-sulfur batteries, a big step towards practical use (Matter, 2019. DOI: 10.1016/j.matt.2019.03.008).
Originally developed by Ford in the 1960s, sodium-sulfur batteries typically use molten sodium and sulfur as the anode and cathode respectively, with a porous separator in between. The molten electrodes are expensive to operate and maintain, and can be explosive.
Researchers have been developing sodium-sulfur batteries with different electrodes that work at room temperature. But these devices face critical challenges. For example, sodium ions react with sulfur to form unwanted polysulfides that corrode the anode so that the battery stores less energy over time. Also sodium can form whiskery dendrite deposits on the anodes, which can then pierce the porous separator, short-circuiting the cell and causing fires.
Manthiram and Yu tried to solve both issues with a solid electrolyte consisting of a ceramic membrane (Na3Zr2Si2PO12) known to be an excellent sodium-ion conductor. With this electrolyte, researchers made a coin cell battery with sodium foil as the anode and cellulose nanofiber paper coated with sulfur powder as the cathode.
The researchers coated the ceramic electrolyte with an ultra-thin flexible film of a polymer that is riddled with tiny pores. The polymer acts as an elastic buffer layer that keeps the solid electrodes and electrolyte in close contact so that charges can flow easily between them. The polymer’s nanopores keep sodium polysulfides from passing through, and dendrites can’t puncture the ceramic electrolyte.
During tests, the charge-holding capacity of a conventional coin cell quickly dropped after 20 cycles. Meanwhile, the new solid-electrolyte battery’s capacity stayed the same for over 100 cycles. Manthiram thinks they can push the battery to last even longer. And that’s necessary because grid batteries need to last for thousands of charging cycles. For now, the researchers plan to make larger pouch cells and optimize the design.
Solid electrolytes have attracted wide attention for lithium- and sodium-ion batteries because they enable better electrode materials and make batteries safer, says Jun Liu, an energy storage researcher at Pacific Northwest National Laboratory. “The challenge is to make the solid electrolyte work with a particular cell chemistry,” he says. This team’s research provides new insight on how to make the sodium-sulfur chemistry work by using the “proper combination of ceramic and polymer materials.”