A better way to study Li-air batteries

But their chemistry has proven frustratingly hard to understand, and researchers need to learn more before the batteries can break out of the lab and onto the market. Researchers from Yale and MIT believe that a laser analysis technique known as Raman spectroscopy could answer many of the questions that remain about the batteries, also known as Li-oxygen or Li-O₂. Their paper was published online on 31^st July by the journal ChemElectroChem. 

“Li-oxygen is superior to Li-ion, at least in theory,” said lead author Forrest S. Gittleson, a post-doctoral associate at Yale in Chemical & Environmental Engineering. “It can store at least two to three times the energy that you can store in Li-ion.”

For a battery to be rechargeable, though, the changes that convert chemical energy to electrical energy must be efficiently reversible. Among other obstacles, Li-oxygen systems have not yet demonstrated high-energy reversible cycling, so recharging is a problem. Before they can solve this, researchers need to learn more about its mechanisms.

Partly because it allows for real-time analysis, the researchers say that Raman spectroscopy might be the best tool to do this. Unlike commonly used spectroscopic and diffraction techniques, Raman spectroscopy can provide a comprehensive look at the electrochemical processes in the systems of Li-air batteries. In a study published last year, the authors observed the production of reversible lithium superoxide, but also some side products that could hinder charging. The in situ techniques of Raman spectroscopy allow researchers to directly observe the formation and decomposition pathways, and gain insight into Li-oxygen chemical mechanisms.

In use since 1928, Raman spectroscopy uses light to detect molecular vibrations in polarisable species. When a laser pulse is applied to a sample, molecules are excited to virtual energy states. These molecules then relax and emit photons, which can be detected. The behaviour of those photons allows for a ‘molecular fingerprint’.

It’s a fairly common tool, used by everyone from art conservators to authenticate artwork to law enforcement officials to detect homemade drugs. But it’s been slow to catch on in the battery research world, Gittleson said.

“Not every lab has access to Ramen spectroscopy equipment, and not everyone working on batteries realises the potential yet,” he said.

André D. Taylor, Director of the Transformative Materials & Devices Lab at Yale and senior author of the paper, said he hopes that this will change.

“This is a powerful technology, and we want to get people to see this as a viable technology,” he said.

Gittleson said there are still a lot of challenges before Li-air batteries can be put to practical use, the instability of its electrolytes among them. But with more widespread use of Raman spectroscopy, researchers will have a much better understanding of its internal chemistry, and that’s critical to advancing the development of the technology.