Solid-state batteries, which use solids instead of liquids to transport ions through their core, are attracting billions in investments, thanks to their potential to reduce battery fires. Now, researchers have created a solid state sodium battery with a recording capacity to store charge and a flexible electrode that allows recharging hundreds of times. In addition, the use of sodium by the battery instead of expensive lithium could allow the development of cheaper energy storage devices for everything from small portable electronic devices to solar and wind farms.
Maria Helena Braga, a battery researcher at the University of Texas at Austin, who was not involved in the work, says that the flexibility of the electrode is particularly inventive. And even though the new batteries are not ready for commercialization, their potential for cheap production makes it likely that scientists will continue to look for them, he says.
Today, lithium-ion batteries are the most important, since they feed everything from our cell phones to our cars. But in rare and dramatic cases, their dependence on flammable liquid electrolytes has caused them to catch fire. Researchers are exploring solid-state lithium batteries to address this problem. But that does not address the cost. A recent badysis by Bloomberg New Energy Finance predicts that the demand for lithium will explode, rising by 1,530 by 2030. This could cause lithium prices to skyrocket because the metal is mined in only a handful of countries.
Sodium, a fellow alkali metal, has a similar chemical behavior and is much more abundant, which is why many research groups have manufactured solid sodium batteries in the last decade. But batteries, which use non-flammable solids to transport sodium ions from one electrode to another, tend to decompose rapidly. In a common configuration, during discharge, the sodium atoms leave an electron in an electrode (the anode), creating an electrical current that is used to do the work. The sodium ions now positively charged are then moved through a sulfur-based electrolyte that carries ions 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 electrical voltage expels the sodium ions from the cathode and causes it to contract. The ions return to the anode, where they meet electrons. But repeated swelling and shrinkage can break the brittle ceramic and cause it to detach from the solid electrolyte, killing the battery.
To address 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 that contains sodium, carbon and oxygen, they reported last year in International Edition Angewandte Chemie. The flexibility of the material allowed it to swell and contract through 400 charging cycles without breaking and losing contact with the sulfur-based electrolyte. And the cathode stored 495 watt-hours per kilogram (Wh / kg), just a little less than most conventional lithium-ion cathodes. But the researchers still had a problem. The sulfur-based electrolyte is somewhat fragile. And the operating voltage of the sodium cells destroyed the electrolyte.
Yao's team has solved this problem by redesigning the cathode. As before, the researchers used a flexible organic compound. But each molecule of its new, abbreviated PTO (for pyrene-4,5,9,10-tetraone), contains twice as many sodium ions as the previous version, which allows the battery to contain 587 Wh / kg, approximately the pair. Standard cathodes of lithium ions. Meanwhile, the flexibility of the cathode allows the battery to manage 500 cycles of charge and discharge while retaining 89% of its storage potential, approaching the performance of conventional lithium ion cells. As an additional benefit, the cell operates at a lower voltage, which keeps the electrolyte intact, the team reports today in Joule.
If other improvements in durability are followed, the non-flammable battery could find many low-voltage uses, such as powering the next generation of portable devices. But for voltage hungry applications, such as electric cars, researchers will need to increase another parameter: the difference in electrical potential (measured in voltage) between the two electrodes. Yan says his group is trying to modify its organic electrode (adding fluoride, among other things) to do just that.