Nature of Li2O2 Oxidation in a Li鈥揙2 Battery Revealed by Operando X-ray Diffraction

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• 作者：Swapna Ganapathy ; Brian D. Adams ; Georgiana Stenou ; Maria S. Anastasaki ; Kees Goubitz ; Xue-Fei Miao ; Linda F. Nazar ; Marnix Wagemaker
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：November 19, 2014
• 年：2014
• 卷：136
• 期：46
• 页码：16335-16344
• 全文大小：684K
• ISSN：1520-5126

文摘

Fundamental research into the Li鈥揙₂ battery system has gone into high gear, gaining momentum because of its very high theoretical specific energy. Much progress has been made toward understanding the discharge mechanism, but the mechanism of the oxygen evolution reaction (OER) on charge (i.e., oxidation) remains less understood. Here, using operando X-ray diffraction, we show that oxidation of electrochemically generated Li₂O₂ occurs in two stages, but in one step for bulk crystalline (commercial) Li₂O₂, revealing a fundamental difference in the OER process depending on the nature of the peroxide. For electrochemically generated Li₂O₂, oxidation proceeds first through a noncrystalline lithium peroxide component, followed at higher potential by the crystalline peroxide via a Li deficient solid solution (Li_2鈥?i>xO₂) phase. Anisotropic broadening of the X-ray Li₂O₂ reflections confirms a platelet crystallite shape. On the basis of the evolution of the broadening during charge, we speculate that the toroid particles are deconstructed one platelet at a time, starting with the smallest sizes that expose more peroxide surface. In the case of in situ charged bulk crystalline Li₂O₂, the Li vacancies preferentially form on the interlayer position (Li1), which is supported by first-principle calculations and consistent with their lower energy compared to those located next to oxygen (Li2). The small actively oxidizing fraction results in a gradual reduction of the Li₂O₂ crystallites. The fundamental insight gained in the OER charge mechanism and its relation to the nature of the Li₂O₂ particles is essential for the design of future electrodes with lower overpotentials, one of the key challenges for high performance Li鈥揳ir batteries.