With their ability to pack roughly 10 times more energy per weight than lithium-ion batteries, Li-air batteries look amazing—at least on paper. In practice, they don’t fare too well. Preventing these batteries from failing quickly requires controlling complex and incompletely understood chemical reactions that occur during charge and discharge.
A team of electrochemists has taken a step in that direction. They made a series of Li-air battery electrodes with precisely formed nanosized features and showed that the batteries’ chemistry and performance depend intimately on the structure of those features (Nat. Commun. 2014, DOI: 10.1038/ncomms5895). The study provides insights into the chemistry that drives these batteries and shows that microscopic structuring of battery components may improve performance.
Li-air batteries have been touted as a possible power source for electric vehicles with long driving range. Yet these batteries remain far from mass production because of their short lifetimes, low numbers of charging cycles, and sluggish discharge rates. Ferreting out details of the chemistry associated with these problems—a key step to solving them—remains challenging because the exact nature of the electrochemically active sites is unknown.
So a team led by Jun Lu, Stefan Vajda, and Larry A. Curtiss of Argonne National Laboratory prepared defect-free carbon electrodes by treating the carbon surface with a thin film of alumina. Then, using previously developed molecular beam methods, the researchers decorated the electrodes with size-selected catalytically active silver clusters. The clusters consisted of three, nine, or 15 atoms. Finally, by using various characterization methods and electrochemistry tests, they analyzed the Li2O2-based product that forms in Li-air batteries and evaluated battery performance.
The analyses show that the morphology, crystallinity, and other properties of the Li2O2-based material vary with silver cluster size. Ag3 clusters yielded fine particulate films built up from nanosized plates. Electrodes with Ag9 clusters led to formation of rough toroid-shaped features (~500 nm long) consisting of nanorod building blocks. And electrodes with Ag15 clusters resulted in large (~1,000 nm long) smooth-surfaced toroids composed of spherical nanoparticles. Compared with Ag3- and Ag9-based batteries, the ones with Ag15 clusters exhibited higher charge capacity (~3,500 versus ~2,400 milliampere-hours per gram) and lasted for roughly 10 charging cycles before failing. The other batteries failed even more quickly.
It’s remarkable that small differences in cluster size yield completely different morphologies and battery performance, says Jeffrey Greeley of Purdue University. He notes that the team’s computations show that these differences can be traced to subtle differences in the energetics of lithium peroxide interaction with the various clusters. He adds, “It is entirely possible that clusters of other elements, even alloy clusters, could ultimately show even better performance.”