Lithium-ion batteries have made continuous progress since first introduced to commercial production by Sony in 1991. But their performance is limited by the rate of diffusion of lithium ions through the solid materials that make up their anode and cathode electrodes. Thus, there has been an emphasis by battery researchers to find materials that improve this diffusion rate and also make the electrodes lighter to increase the energy density (Wh/kg) of lithium-based batteries.
What if you could replace one of the electrodes with air? That’s the concept behind the lithium air battery, estimated by battery experts to be capable of holding 5 times the electrical energy of a lithium-ion battery of the same weight. In a lithium air battery, the anode is made from lithium metal. The cathode is a permeable carbon-based surface that is covered with oxygen molecules, which comes from the surrounding air. During discharge, as the lithium oxidizes into lithium peroxide (which collects on the cathode), it releases energy in the form of electrons that can be used to power an electronic device.
Lithium ions can combine with water vapor and with carbon dioxide from the air, producing compounds which build up on the cathode, eventually coating it so it is unable to function. Still, single-use (non-rechargeable) lithium air batteries have been successfully produced and tiny lightweight models are sold to power hearing aids.
Building a rechargeable lithium air battery, however, is a different proposition. In addition to the buildup on the cathode, the lithium metal of the anode is highly reactive, particularly with water. So any water vapor in the incoming air can cause problems. Many experimental batteries operate on tanks of pure oxygen—a solution that is obviously not practical for real world applications. During charging, lithium metal also can build up spikey dendrites on its surface—some so large that they can reach the cathode and short circuit the battery.
|A schematic drawing of the lithium-air battery. (Image source: UIC and Argonne National Laboratories)|
Despite these limitations, development of lithium air batteries has continued. Recently, in a paper published in Nature, researchers from the University of Illinois at Chicago and Argonne National Laboratory detailed a few tricks that might eventually make lithium air batteries possible.
First of all, the carbon lattice structure that makes up the cathode is coated with a molybdenum disulfide catalyst to help promote the lithium oxygen reactions while suppressing reactions with other elements and compounds in the air. The electrolyte was also specially developed to help reduce those unwanted reactions.
At the anode, the lithium metal is coated with a thin layer of lithium carbonate. It allows lithium ions to travel from the anode to and from the electrolyte, while preventing unwanted compounds (such as oxygen and water vapor) from reaching the highly reactive metal. In the paper, the researchers report a battery life of up to 700 charge and discharge cycles—far more than the handful of cycles most experimental lithium air batteries achieve. “The complete architectural overhaul we performed on this battery by redesigning every part of it helped us enable the reactions we wanted to occur, and prevent and block those that would ultimately cause the battery to go dead,” said Amin Salehi-Khojin, assistant professor of mechanical and industrial engineering at the University of Illinois at Chicago and co-author of the Nature paper in a press release from the university.
The research paper states the results were obtained with “a simulated air atmosphere,” which might be an indication that there is more work to be done before rechargeable lithium air batteries are ready for prime time. The question of dendrite growth on the lithium metal surface during charging also needs to be addressed. “This first demonstration of a true lithium-air battery is an important step toward what we call ‘beyond lithium-ion’ batteries, but we have more work to do in order to commercialize it,” said Salehi-Khojin in the university release.
Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.