DFT+U Calculations and XAS Study: Further Confirmation of the Presence of CoO5 Square-Based Pyramids with IS-Co3+ in Li-Overstoichiometric LiCoO2

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• 作者：Dany Carlier ; Ju-Hsiang Cheng ; Chun-Jern Pan ; Michel M茅n茅trier ; Claude Delmas ; Bing-Joe Hwang
• 刊名：Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：December 19, 2013
• 年：2013
• 卷：117
• 期：50
• 页码：26493-26500
• 全文大小：546K
• 年卷期：v.117,no.50(December 19, 2013)
• ISSN：1932-7455

文摘

LiCoO₂, one of the major positive electrode materials for Li-ion batteries, can be synthesized with excess Li. Previous experimental work suggested the existence of intermediate spin (IS) Co³⁺ ions in square-based pyramids to account for the defect in this material. We present here a theoretical study based on density functional theory (DFT) calculations together with an X-ray absorption spectroscopy (XAS) experimental study. In the theoretical study, a hypothetical Li₄Co₂O₅ material, where all the Co ions are in pyramids, was initially considered as a model material. Using DFT+U, the intermediate spin state of the Co³⁺ ions is found stable for U values around 1.5 eV. The crystal and electronic structures are studied in detail, showing that the defect must actually be considered as a pair of such square-based pyramids, and that Co鈥揅o bonding can explain the position of Co in the basal plane. Using a supercell corresponding to more diluted defects (as in the actual material), the calculations show that the IS state is also stabilized. In order to investigate experimentally the change in the electronic structure in the Li-overstoichiometric LiCoO₂, we used X-ray absorption near edge structure (XANES) spectroscopy and propose an interpretation of the O Kedge spectra based on the DFT+U calculations, that fully supports the presence of pairs of intermediate spin state Co³⁺ defects in Li-overstoichiometric LiCoO₂.