Byon Initiative Research Unit, RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan.
J Am Chem Soc. 2013 Jul 24;135(29):10870-6. doi: 10.1021/ja405188g. Epub 2013 Jul 12.
Understanding the lithium-oxygen (Li-O2) electrochemical reaction is of importance to improve reaction kinetics, efficiency, and mitigate parasitic reactions, which links to the strategy of enhanced Li-O2 battery performance. Many in situ and ex situ analyses have been reported to address chemical species of reduction intermediate and products, whereas details of the dynamic Li-O2 reaction have not as yet been fully unraveled. For this purpose, visual imaging can provide straightforward evidence, formation and decomposition of products, during the Li-O2 electrochemical reaction. Here, we present real-time and in situ views of the Li-O2 reaction using electrochemical atomic force microscopy (EC-AFM). Details of the reaction process can be observed at nano-/micrometer scale on a highly oriented pyrolytic graphite (HOPG) electrode with lithium ion-containing tetraglyme, representative of the carbon cathode and ether-based electrolyte extensively employed in the Li-O2 battery. Upon oxygen reduction reaction (ORR), rapid growth of nanoplates, having axial diameter of hundreds of nanometers, length of micrometers, and ~5 nm thickness, at a step edge of HOPG can be observed, which eventually forms a lithium peroxide (Li2O2) film. This Li2O2 film is decomposed during the oxygen evolution reaction (OER), for which the decomposition potential is related to a thickness. There is no evidence of byproduct analyzed by X-ray photoelectron spectroscopy (XPS) after first reduction and oxidation reaction. However, further cycles provide unintended products such as lithium carbonate (Li2CO3), lithium acetate, and fluorine-related species with irregular morphology due to the degradation of HOPG electrode, tetraglyme, and lithium salt. These observations provide the first visualization of Li-O2 reaction process and morphological information of Li2O2, which can allow one to build strategies to prepare the optimum conditions for the Li-O2 battery.
了解锂-氧（Li-O2）电化学反应对于提高反应动力学、效率和减轻寄生反应至关重要，这与增强 Li-O2 电池性能的策略有关。已经有许多原位和异位分析报告用于解决还原中间体和产物的化学物质，而 Li-O2 反应的动态细节尚未完全揭示。为此，可视化成像可以提供在 Li-O2 电化学反应过程中直接的证据，包括产物的形成和分解。在这里，我们使用电化学原子力显微镜（EC-AFM）实时原位观察 Li-O2 反应。在含有锂离子的四甘醇的高度取向热解石墨（HOPG）电极上，可以在纳米/微米尺度上观察到反应过程的细节，这代表了广泛应用于 Li-O2 电池的碳阴极和醚基电解质。在氧还原反应（ORR）中，可以在 HOPG 的台阶边缘观察到快速生长的纳米板，其轴向直径为数百纳米，长度为几微米，厚度约为 5nm，最终形成过氧化锂（Li2O2）膜。在析氧反应（OER）中，Li2O2 膜会分解，其分解电位与厚度有关。经过首次还原和氧化反应后，通过 X 射线光电子能谱（XPS）分析没有发现副产物。然而，进一步的循环会由于 HOPG 电极、四甘醇和锂盐的降解而提供意外的产物，如碳酸锂（Li2CO3）、乙酸锂和与氟有关的物质，其形态不规则。这些观察结果提供了 Li-O2 反应过程和 Li2O2 形态信息的首次可视化，这可以使人们构建策略来为 Li-O2 电池制备最佳条件。
