Argonne team finds precise control of subnanometer surface structures on Li-air cathodes affects formation of Li2O2 and performance of cell

3 November 2014

Researchers at Argonne National Laboratory, with colleagues at Hanyang University in Korea, have found that precise control of subnanometer surface structures on Li-air battery cathodes could be used as a means to improve the performance of the cells. A paper on their work was published in the journal Nature Communications.

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Li−O₂ batteries are based on the oxidation of lithium at the (lithium) metal electrode and reduction of oxygen at the air electrode to induce current flow. Lithium peroxide (Li₂O₂) is the discharging product formed on the oxygen electrode surface and is oxidized back to O₂ and Li⁺ in the following charging process. However, the insulating property of bulk Li₂O₂ and the sluggish kinetics of the Li₂O₂ oxidation reaction make it difficult to electrochemically decompose Li₂O₂ efficiently. (Earlier post.)

Further, the Argonne team notes, the active sites on cathode surfaces and their role in electrochemical reactions are difficult to ascertain because the exact nature of the sites is unknown.

In their study, the researchers used molecular beam methods to decorated defect-free carbon electrodes with size-selected catalytically active silver (Ag) clusters consisting of three, nine, or fifteen atoms (Ag₃, Ag₉ and Ag₁₅). Using various characterization methods and electrochemistry tests, they analyzed the Li₂O₂-based product that forms in Li-air batteries and evaluated battery performance.

They found that morphology, crystallinity, and other properties of the Li₂O₂ varied with silver cluster size.

• Ag₃ clusters resulted in fine particulate films built of Li₂O₂ nanoplates.

• Ag₉ resulted in rough toroid-shaped features (~500 nm long) consisting of nanorod building blocks.

• Ag₁₅ clusters resulted in large (~1,000 nm long) smooth-surfaced toroids comprising spherical nanoparticles.

The batteries with Ag₁₅ clusters exhibited higher charge capacity (~3,500 versus ~2,400 mAh/g) and lasted for ~10 charging cycles before failing. The others failed more quickly.

The results reveal dramatically different morphologies of the electrochemically grown lithium peroxide dependent on the size of the clusters. This dependence is found to be due to the influence of the cluster size on the formation mechanism, which also affects the charge process. The results of this study suggest that precise control of subnanometre surface structure on cathodes can be used as a means to improve the performance of lithium–oxygen cells.

Resources

• Jun Lu, Lei Cheng, Kah Chun Lau, Eric Tyo, Xiangyi Luo, Jianguo Wen, Dean Miller, Rajeev S. Assary, Hsien-Hau Wang, Paul Redfern, Huiming Wu, Jin-Bum Park, Yang-Kook Sun, Stefan Vajda, Khalil Amine Larry A. Curtiss (2014) “Effect of the size-selective silver clusters on ​lithium peroxide morphology in lithium–oxygen batteries” Nature Communications 5, Article number: 4895 doi: 10.1038/ncomms5895