Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes: Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy

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• 作者：Phoebe K. Allan ; John M. Griffin ; Ali Darwiche ; Olaf J. Borkiewicz ; Kamila M. Wiaderek ; Karena W. Chapman ; Andrew J. Morris ; Peter J. Chupas ; Laure Monconduit ; Clare P. Grey
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：February 24, 2016
• 年：2016
• 卷：138
• 期：7
• 页码：2352-2365
• 全文大小：695K
• ISSN：1520-5126

文摘

Operando pair distribution function (PDF) analysis and ex situ ²³Na magic-angle spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy are used to gain insight into the alloying mechanism of high-capacity antimony anodes for sodium-ion batteries. Subtraction of the PDF of crystalline Na_xSb phases from the total PDF, an approach constrained by chemical phase information gained from ²³Na ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electrochemically; a-Na_3–xSb (x ≈ 0.4–0.5), a structure locally similar to crystalline Na₃Sb (c-Na₃Sb) but with significant numbers of sodium vacancies and a limited correlation length, and a-Na_1.7Sb, a highly amorphous structure featuring some Sb–Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na_3–xSb and, finally, crystalline Na₃Sb. Desodiation results in the formation of an electrode formed of a composite of crystalline and amorphous antimony networks. We link the different reactivity of these networks to a series of sequential sodiation reactions manifesting as a cascade of processes observed in the electrochemical profile of subsequent cycles. The amorphous network reacts at higher voltages reforming a-Na_1.7Sb, then a-Na_3–xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na_3–xSb without the formation of a-Na_1.7Sb. a-Na_3–xSb is converted to crystalline Na₃Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature ²³Na NMR experiments reveal significant sodium mobility within c-Na₃Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.