Solid-state batteries, which use solids instead of liquids to ferry ions through their core, are attracting billions in investment, thanks to their potential for reducing battery fires. Now, researchers have created a solid-state sodium battery with a record capacity to store charge and a flexible electrode that allows recharging hundreds of times. What’s more, the battery’s use of sodium instead of expensive lithium could enable the development of cheaper energy storage devices for everything from small wearable electronics to solar and wind farms.
Maria Helena Braga, a battery researcher at the University of Texas in Austin, who was not involved with the work, says the electrode’s flexibility is particularly inventive. And even though the new batteries aren’t ready for commercialization, their potential for cheap production makes it likely that scientists will continue to pursue them, she says.
Today, lithium-ion batteries are king, powering everything from our cellphones to our cars. But in rare, dramatic instances, their reliance on flammable liquid electrolytes has caused them to catch fire. Researchers are exploring lithium solid state batteries to address this problem. But that doesn’t address the cost. A recent analysis by Bloomberg New Energy Finance predicts that demand for lithium will explode, increasing 1500-fold by 2030. That could send lithium prices skyrocketing because the metal is mined in only a handful of countries.
Sodium, a fellow alkali metal, has similar chemical behavior and is far more abundant, so many research groups have crafted solid sodium batteries over the past decade. But the batteries, which use nonflammable solids to ferry sodium ions from one electrode to another, tend to break down quickly. In one common setup, during discharge, sodium atoms give up an electron at one electrode (the anode), creating an electric current that’s used to do work. The now positively charged sodium ions then move through an ion-ferrying sulfur-based electrolyte to the second electrode (known as a cathode), which is made of a ceramic oxide compound. When the ions arrive, the cathode swells in size. Then, when the battery is recharged, an applied electric voltage drives sodium ions out of the cathode, causing it to shrink. The ions go back to the anode, where they reunite with electrons. But the repeated swelling and shrinking can crack the brittle ceramic and cause it to detach from the solid electrolyte, killing the battery.
To tackle this problem, researchers led by Yan Yao, a materials scientist at the University of Houston in Texas, created a cathode from a flexible organic compound containing sodium, carbon, and oxygen, they reported last year in Angewandte Chemie International Edition. The material’s flexibility allowed it to swell and shrink through 400 charging cycles without breaking apart and losing touch with the sulfur-based electrolyte. And the cathode stored 495 watt-hours per kilogram (Wh/kg), just slightly less than most conventional lithium-ion cathodes. But the researchers still had a problem. The sulfur-based electrolyte is somewhat fragile. And the sodium cells’ operating voltage tore apart the electrolyte.
Yao’s team has now solved this issue by redesigning the cathode. As before, the researchers used a flexible organic compound. But each molecule of their new one, abbreviated PTO (for pyrene-4,5,9,10-tetraone), holds twice as many sodium ions as the previous version, enabling the battery to hold 587 Wh/kg, roughly on par with standard lithium-ion cathodes. Meanwhile, the cathode’s flexibility allows the battery to manage 500 charge and discharge cycles while retaining 89% of its storage potential, nearing the performance of conventional lithium-ion cells. As a bonus, the cell operates at a lower voltage, which keeps the electrolyte intact, the team reports today in Joule.
If further durability improvements follow, the nonflammable battery could find many low-voltage uses, such as powering the next generation of wearable devices. But for voltage-hungry applications, such as electric cars, researchers will need to boost another parameter: the difference in electric potential (measured in voltage) between the two electrodes. Yan says his group is trying to tweak its organic electrode—adding fluorine, among other things—to do just that.