In-Situ Observation of Li-O2 Electrochemical Reactions Using Electrochemical AFM

Despite outstanding gravimetric energy density of a nonaqueous lithium-oxygen (Li–O₂) battery in regard to promising future energy storage, a true Li–O₂ electrochemical reaction has not been deeply understood. In particular, a little knowledge of dynamic Li–O₂ process of formation, growth and decomposition of Li₂O₂ has been known. Here, we present in–situ imaging of the Li–O₂ electrochemical reactions using atomic force microscopy (AFM) coupled with electrochemical tester.^[1,2]

            A highly oriented pyrolytic graphite (HOPG) and metallic lithium were employed to working and counter/reference electrodes, respectively, which were fully immersed in O₂ gas-saturated tetraethylene glycol dimethyl ether (tetraglyme) containing 0.5 M of LiTFSI (LiN(SO₂CF₃)₂). Cyclic voltammetry was carried out at a potential range of 2.0-4.5 V with a scanning of HOPG surface by an insulating AFM tip in an Ar-filled glove box, which offered almost identical condition to the actual Li-O₂ cell. In addition, the AFM tip scanning for imaging did not significantly disturb the growth/decomposition process of Li₂O₂.

            Upon the oxygen reduction reaction (ORR), the nucleation of nanoparticle (diameter = 3–10 nm) was observed at the step edge of HOPG. These nanoparticles grew up to a diameter of 200 nm and migrated to the terrace of HOPG. When the potential approached to 2.0 V, new morphology products having rectangular and elongated nanoplates were rapidly formed at the edge of HOPG, which became predominant products for further reduction, thus constructing a thick Li₂O₂ film, demonstrated by X-ray photoelectron spectroscopy (XPS). Interestingly, the size of Li₂O₂ nanoplates on the top surface of Li₂O₂ film was much smaller than the one on the HOPG, which was probably attributed to slow charge transport according to growth of thick Li₂O₂ film having the poor electronic conductivity. The Li₂O₂ was decomposed upon oxygen evolution reaction (OER), where the thick and dense Li₂O₂ film delayed its decomposition and demanded for higher oxidation potential compared to the thin and low density of Li₂O₂. For control experiments, we replaced the ether-based electrolyte and HOPG electrode with carbonate and nanoporous gold, and also extended the voltammetry cycling number. These conditions provided different morphology of products and chemical identifications, which would be discussed in the presentation.

References

[1]      Wen, R.; Hong, M.; Byon, H. R. J. Am. Chem. Soc. 2013, 135, 10870-10876.

[2]     Wen, R.; Byon, H. R. Chem. Commun. 2014, 50, 2628-2631.