Studying the Reversibility of Multielectron Charge Transfer in Fe(VI) Cathodes Utilizing X-ray Absorption Spectroscopy

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• 作者：Maryam Farmand ; Stuart Licht ; David Ramaker
• 刊名：Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：October 3, 2013
• 年：2013
• 卷：117
• 期：39
• 页码：19875-19884
• 全文大小：475K
• 年卷期：v.117,no.39(October 3, 2013)
• ISSN：1932-7455

文摘

The superiron salts BaFeO₄ and K₂FeO₄ when utilized as battery cathodes both undergo a three electron charge transfer; however, they exhibit significantly different physical and electrochemical properties. K₂FeO₄ exhibits higher solid-state stability and higher intrinsic 3e^{鈥?/sup> capacity (406 mAh/g) than BaFeO₄ (313 mAh/g); however, the rate of cathodic charge transfer is considerably higher for BaFeO₄. To understand these differences, primary coin cells of alkaline batteries containing either 渭m-BaFeO₄, 渭m-K₂FeO₄, or nm-K₂FeO₄ (nm = nanometer, or 渭m = micrometer size particles) were constructed and discharged to various depths under a constant load. Discharged cathode composite were studied by ex-situ X-ray absorption measurements. The oxidation state of discharge product of the Fe local symmetry was followed by the magnitude of K-edge and pre-edge Fe 1s to 3d peak. To track structural changes, the extended X-ray absorption fine structure (EXAFS) 蠂 functions of the partially discharged cathodes were subject to linear combination fitting. The expanded BaFeO₄ lattice, or the much larger surface-electrolyte interface in the nm-K₂FeO₄ materials, significantly increased their capacities compared to 渭m-K₂FeO₄. In the case of nm-K₂FeO₄, electron density is more distributed by water intercalation about the Fe hydrous environment, which relieves the 鈥渟tress鈥?of full Fe6+ to Fe3+ reduction. The stronger Ba鈥揊eO₄ anion鈥揷ation interaction and increased lattice size apparently slows the rate of lattice rearrangement into the discharge product.}