Hwang Sooyeon, Kim Seung Min, Bak Seong-Min, Cho Byung-Won, Chung Kyung Yoon, Lee Jeong Yong, Chang Wonyoung, Stach Eric A
Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea.
ACS Appl Mater Interfaces. 2014 Sep 10;6(17):15140-7. doi: 10.1021/am503278f. Epub 2014 Aug 20.
In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li(x)Ni(0.8)Co(0.15)Al(0.05)O2 (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °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 °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.
在这项工作中，我们利用原位透射电子显微镜（TEM）来研究处于不同充电状态（SOC）的Li(x)Ni(0.8)Co(0.15)Al(0.05)O2（NCA）阴极材料表面的热诱导分解。未充电的NCA在高达400°C时是稳定的，但随着温度升高，充电的NCA会发生显著变化。这些变化包括表面孔隙率的发展以及氧K边电子能量损失谱的变化，预边峰向更高能量损失方向移动。这些变化与从结构中释放的O2气体密切相关，也与NCA从层状结构到无序尖晶石结构，最终到岩盐结构的相变有关。尽管这些变化开始的温度强烈依赖于充电状态，但在相同充电状态的颗粒之间也存在显著差异。值得注意的是，当NCA充电至x = 0.33（这是大多数应用中的实际上限电压充电状态）时，即使在低于100°C的温度下，一些颗粒的表面也会发生形态和氧K边变化，而包含锂离子电池（LIB）的电子设备在正常运行期间可能会遇到这个温度。经历这些变化的颗粒可能极其不稳定，并且可能在比通常预期低得多的温度下引发热失控。这些结果表明，原位加热实验不仅是研究阴极材料一般热行为的独特工具，也是探索颗粒间差异的独特工具，而这些差异有时对于理解整个系统的性能至关重要。
