A new type of lithium-oxygen battery could pack 10 times more energy and last longer than its predecessors.
Lithium-ion batteries power everything from our smartphones to our cars. But one of their most promising replacements is lithium-oxygen batteries, which in theory could store 10 times more power. The only problem: They fall apart after just a handful of charging cycles.
Now, researchers have found that by tweaking the building materials and running them at high temperatures—along with a couple of other fixes—can push them to at least 150 cycles and release nearly 100 percent of its stored charge. Although they would be too hot to handle in phones, lithium-oxygen batteries the size of rail cars could one day underpin a green energy grid, storing excess wind and solar power and delivering it on demand.
Lithium-oxygen cells are made of two electrodes, an anode and a cathode, separated by a substance called electrolyte. When the battery is powering another device, oxygen molecules on the cathode combine with lithium ions from the electrolyte to form a solid compound called lithium peroxide (Li2O2). That chemical reaction releases energy. Recharging the battery breaks apart the lithium peroxide, returning oxygen and lithium to their starting positions. Over time the reaction tears apart the electrolyte giving it an extremely short lifespan. The reaction also generates an even more reactive compound called superoxide that renders multiple battery components useless.
To build a better lithium-oxygen battery, chemist Linda Nazar of the University of Waterloo in Canada and colleagues replaced the typical organic electrolyte for an inorganic molten salt, and the standard carbon-based cathode for a metal-based one.
In this new battery, oxygen combines with lithium to create lithium oxide. This chemical reaction can store 50 percent more energy than the lithium peroxide reaction. As a result, the new design can make more energy-dense batteries than the previous setup. What’s more, lithium oxide doesn’t produce the troublesome chemical by-products that lithium peroxide does. That allows the new lithium-oxygen battery to release nearly all of its stored electric charge to other devices, and recharge more times than other lithium-oxygen cells.
The batteries suffer virtually no degradation out to 150 cycles, Nazar and her colleagues report today in Science. “This shows that if we think outside the box, there is room for some forward progress,” says Betar Gallant, a mechanical engineer at MIT who was not involved in the new work.
These new batteries may eventually be used to power electric cars, but there is still “a ways to go before they can actually be used in vehicles,” Curtiss says. That’s because the new batteries have to be heated to at least 150° Celsius to work. “You’d have to find some way to heat this up when you start up the car,” he says.
But both she and Shao-Horn caution that lithium-oxygen batteries have a long way to go before they’ll have a shot at shaking up the market. Most importantly, Gallant says, the batteries will need to be tested for many more cycles to ensure they don’t fall victim to some other type of degradation that didn’t show up in the early tests. If that can happen, it might usher in a new era of battery—and green energy—technology.
Source: A high-energy-density lithium-oxygen battery based on a reversible four-electron conversion to lithium oxide. Science. Vol. 361
Hot lithium-oxygen batteries charge ahead. Science. Vol. 361