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 running them at high temperatures—along with a couple of other fixes—can push them to at least 150 cycles. 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.
“This is very encouraging,” says Yang Shao-Horn, a chemical physicist at the Massachusetts Institute of Technology (MIT) in Cambridge who was not involved in the work. But she and others caution that the new batteries must prove themselves over many more cycles before they’ll be considered for the mass market.
Like their lithium-ion cousins, lithium-oxygen batteries consist of two charge-storing electrodes separated by a liquid electrolyte through which lithium ions flow during charging and discharging. When discharging, lithium atoms give up electrons to the positively charged electrode, or anode, leaving positively charged lithium ions to flow through the electrolyte to the negatively charged cathode. There they react with oxygen from the air to eventually form lithium peroxide (Li2O2), a compound that over time tears apart the electrolyte. The reaction also generates an even more reactive compound called superoxide that renders multiple battery components useless.
In recent years, researchers have tried to devise electrolytes that could stand up to Li2O2 and superoxide with little success. “People were on the verge of giving up hope,” says Linda Nazar, a chemist at the University of Waterloo in Canada.
But 2 years ago, a team of U.S. researchers came up with the first hints of a breakthrough. They tested another alternative electrolyte, this one made from a combination of salts that turned into a liquid when heated. This molten salt withstood the ravages of Li2O2 and superoxide, but the battery’s carbon-based cathode still fell victim.
Now, Nazar and her colleagues have taken another step forward. They kept the molten salt electrolyte but replaced the carbon cathode with a nickel-based version. They also raised the operating temperature of the battery to 150°C. That combination, instead of producing the Li2O2 and superoxide, produces Li2O, a stable compound that doesn’t rip through the electrolyte or anything else. 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.
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.