Solid-state batteries, which use solids instead of liquids to transport ions into their hearts, attract billions of dollars in investment, thanks to their potential to reduce battery fires. Today, researchers have created a solid state sodium battery with a record capacity of charge storage and a flexible electrode that can be recharged 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, be they small portable electronic devices, solar parks or wind farms. .
Maria Helena Braga, a battery researcher at the University of Texas at Austin, who did not participate, explains that the flexibility of the electrode is particularly inventive. And even if the new batteries are not ready for commercialization, their cheap production potential suggests that scientists will continue to research them, she says.
Today, lithium-ion batteries are the rule, fueling everything from mobile phones to cars. But in rare dramatic cases, their dependence on flammable liquid electrolytes has set them on fire. Researchers are exploring lithium batteries in the solid state to solve this problem. But that does not solve the cost. A recent Bloomberg New Energy Finance analysis predicts that lithium demand will explode, it will be multiplied by 1,500 by 2030. This could drive up lithium prices soar, as the metal is being mined in only a few countries.
Sodium, another alkali metal, has similar chemical behavior and is much more abundant. This is why many research groups have designed solid sodium batteries over the past decade. But batteries, which use non-flammable solids to transport sodium ions from one electrode to another, tend to decompose quickly. In a typical configuration, during discharge, the sodium atoms give up an electron to an electrode (the anode), creating an electric current that previously worked. The now positively charged sodium ions then move through a sulfur-based electrolyte carrying ions to the second electrode (known as the cathode), which is made of a ceramic oxide compound. As the ions arrive, the cathode grows. Then, when the battery is recharged, an applied voltage causes the sodium ions out of the cathode, causing its contraction. The ions return to the anode, where they meet with electrons. But repeated swelling and shrinking can crack the fragile ceramic and detach it from the solid electrolyte, which kills the battery.
To solve this problem, researchers led by Yan Yao, a materials specialist at the University of Houston, Texas, created a cathode from a soft organic compound containing sodium, carbon, and oxygen, reported on last year Angewandte Chemie International Edition. The flexibility of the material allowed it to swell and contract over 400 charge cycles without breaking and losing contact with the sulfur electrolyte. And the cathode stored 495 watt hours per kilogram (Wh / kg), just less than most conventional lithium-ion cathodes. But researchers still had a problem. The sulfur-based electrolyte is somewhat fragile. And the operating voltage of the sodium batteries tore the electrolyte.
Yao's team solved this problem by changing the design of the cathode. As before, the researchers used a flexible organic compound. But each molecule of their new molecule, the abbreviation 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, or about standard lithium-ion cathodes. At the same time, the flexibility of the cathode allows the battery to handle 500 charge and discharge cycles while retaining 89% of its storage potential, bringing it closer to the performance of conventional lithium-ion cells. As a bonus, the cell operates at a lower voltage, which keeps the electrolyte intact, announced the team today at Joule.
If further improvements in durability ensue, 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 have to reinforce another parameter: the electrical potential difference (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.