Lithium-air batteries are a popular subject of experiment because they have the potential to replace lithium-ion cells. Compared to lithium-ion batteries, lithium-air can store up to five times more energy. The challenge is to prove the chemical reactions that provide high efficiency and improve life cycle.

Ex-situ analysis does not provide enough information, so researchers at the University of Illinois at Chicago (UIC) have taken a closer look by conducting in-situ analysis with positive results.

Lithium-Air Batteries

Lithium-air batteries store energy as chemical bonds of oxide compounds. So far, batteries of this type store and release energy from lithium peroxide, which is insoluble in aprotic electrolytes and which clogs the cathode. As the metallic anode lithium reacts with oxygen from the air, lithium ions migrate through the electrolyte. Lithium goes through oxidation at the anode while oxygen is reduced at the cathode to create current flow.

Lithium-air battery charge and discharge cycles. (Image courtesy of Na9234/Wikimedia Commons.)

Specific energy levels measure the efficiency of batteries; lithium-air can be between five and 15 times higher than lithium-ion batteries in use today. Battery scientists at the U.S. Department of Energy's Argonne National Laboratory developed a prototype that produced lithium superoxide instead of lithium peroxide as the battery discharges. Lithium superoxide can easily return to lithium and oxygen ions in order to increase efficiency and battery lifecycle.

Instead of consuming two electrons, the new battery design consumes one electron to produce the superoxide. However, it was difficult to prove that this chemical reaction took place so UCI researchers used analytical chemistry to measure the mass-to-charge ratio and ions in the gas phase.

Advanced Mass-Spectroscopy

Research associates Amin Salehi-Khojin and Mohammad Asadi developed an advanced mass spectrometer to measure in-situ electrochemical reaction products while lithium-air batteries charged and discharged. Operating at ultra-high vacuum, the apparatus is "very sensitive to the tiniest change in oxygen concentration," said Asadi.

Salehi-Khojin (right) and Asadi(left) with their specially modified electrochemical mass spectrometry instrument. (Image courtesy of UIC.)

With this advanced mass spectrometer, the UIC researchers proved how one electron per oxygen atom was produced, showing signs that lithium superoxide formed. In addition, results indicated the absence of lithium compounds as byproducts.

"This is going to be a valuable system for continuing the study of this battery and other types of metal-air batteries," said Salehi-Khojin. "Not only can we analyze the products of the electrochemical reaction; we can elucidate the reaction pathway. If we know the reaction pathway, we'll know how to design the next generation of that battery for energy efficiency and cost effectiveness."

For more information, visit the UIC website.