Identifying the Structure of the Intermediate, Li2/3CoPO4, Formed during Electrochemical Cycling of LiCoPO4

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• 作者：Fiona C. Strobridge ; Rapha毛le J. Cl茅ment ; Michal Leskes ; Derek S. Middlemiss ; Olaf J. Borkiewicz ; Kamila M. Wiaderek ; Karena W. Chapman ; Peter J. Chupas ; Clare P. Grey
• 刊名：Chemistry of Materials
• 出版年：2014
• 出版时间：November 11, 2014
• 年：2014
• 卷：26
• 期：21
• 页码：6193-6205
• 全文大小：669K
• ISSN：1520-5002

文摘

In situ synchrotron diffraction measurements and subsequent Rietveld refinements are used to show that the high energy density cathode material LiCoPO₄ (space group Pnma) undergoes two distinct two-phase reactions upon charge and discharge, both occurring via an intermediate Li_2/3(Co²⁺)_2/3(Co³⁺)_1/3PO₄ phase. Two resonances are observed for Li_2/3CoPO₄ with intensity ratios of 2:1 and 1:1 in the ³¹P and ⁷Li NMR spectra, respectively. An ordering of Co²⁺/Co³⁺ oxidation states is proposed within a (a 脳 3b 脳 c) supercell, and Li⁺/vacancy ordering is investigated using experimental NMR data in combination with first-principles solid-state DFT calculations. In the lowest energy configuration, both the Co³⁺ ions and Li vacancies are found to order along the b-axis. Two other low energy Li⁺/vacancy ordering schemes are found only 5 meV per formula unit higher in energy. All three configurations lie below the LiCoPO₄鈥揅oPO₄ convex hull and they may be readily interconverted by Li⁺ hops along the b-direction.