Investigating Local Degradation and Thermal Stability of Charged Nickel-Based Cathode Materials through Real-Time Electron Microscopy

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• 作者：Sooyeon Hwang ; Seung Min Kim ; Seong-Min Bak ; Byung-Won Cho ; Kyung Yoon Chung ; Jeong Yong Lee ; Wonyoung Chang ; Eric A. Stach
• 刊名：ACS Applied Materials & Interfaces
• 出版年：2014
• 出版时间：September 10, 2014
• 年：2014
• 卷：6
• 期：17
• 页码：15140-15147
• 全文大小：624K
• ISSN：1944-8252

文摘

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li_xNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 掳C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O₂ gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to x = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 掳C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system.

Keywords:

lithium ion batteries; Ni-based cathode; thermal degradation; in situ transmission electron microscopy; electron energy loss spectroscopy