Investigation of Changes in the Surface Structure of LixNi0.8Co0.15Al0.05O2 Cathode Materials Induced by the Initial Charge

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• 作者：Sooyeon Hwang ; Wonyoung Chang ; Seung Min Kim ; Dong Su ; Dong Hyun Kim ; Jeong Yong Lee ; Kyung Yoon Chung ; Eric A. Stach
• 刊名：Chemistry of Materials
• 出版年：2014
• 出版时间：January 28, 2014
• 年：2014
• 卷：26
• 期：2
• 页码：1084-1092
• 全文大小：659K
• ISSN：1520-5002

文摘

We use transmission electron microscopy (TEM) to investigate the evolution of the surface structure of Li_xNi_0.8Co_0.15Al_0.05O₂ cathode materials (NCA) as a function of the extent of first charge at room temperature using a combination of high-resolution electron microscopy (HREM) imaging, selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS). It was found that the surface changes from the layered structure (space group R3̅m) to the disordered spinel structure (Fd3̅m), and eventually to the rock-salt structure (Fm3̅m), and that these changes are more substantial as the extent of charge increases. EELS indicates that these crystal structure changes are also accompanied by significant changes in the electronic structure, which are consistent with delithiation leading to both a reduction of the Ni and an increase in the effective electron density of oxygen. This leads to a charge imbalance, which results in the formation of oxygen vacancies and the development of surface porosity. The degree of local surface structure change differs among particles, likely due to kinetic factors that are manifested with changes in particle size. These results demonstrate that TEM, when coupled with EELS, can provide detailed information about the crystallographic and electronic structure changes that occur at the surface of these materials during delithiation. This information is of critical importance for obtaining a complete understanding of the mechanisms by which both degradation and thermal runaway initiate in these electrode materials.