Sodium Intercalation Mechanism of 3.8 V Class Alluaudite Sodium Iron Sulfate

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• 作者：Gosuke Oyama ; Oliver Pecher ; Kent J. Griffith ; Shin-ichi Nishimura ; Roberta Pigliapochi ; Clare P. Grey ; Atsuo Yamada
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：August 9, 2016
• 年：2016
• 卷：28
• 期：15
• 页码：5321-5328
• 全文大小：576K
• 年卷期：0
• ISSN：1520-5002

文摘

Alluaudite sodium iron sulfate Na_2+2xFe_2–x(SO₄)₃ is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fep>3+p>/Fep>2+p> redox reactions (3.8 V vs Na/Nap>+p>), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), p>23p>Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fep>3+p>/Fep>2+p> redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions.