Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
Nano Lett. 2012 Dec 12;12(12):6192-7. doi: 10.1021/nl303036e. Epub 2012 Nov 28.
Direct observation of delithiated structures of LiCoO(2) at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. The ordered Li, Co, and O columns for LiCoO(2) nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO(2), the Co-contained (003) planes distort from the bulk to the surface region and the c-axis is expanded significantly. Ordering of lithium ions and lithium vacancies has been observed directly and explained by first-principles simulation. On the basis of HAADF micrographs, it is found that the phase irreversibly changes from O3-type in pristine LiCoO(2) to O1-type Li(x)CoO(2) (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type Li(x)CoO(2) (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-Li(x)CoO(2) in the phase diagram of O3-LiCoO(2), through which the two previously separated LiCoO(2) phases, i.e. O2 and O3 systems, are connected. These new investigations shed new insight into the lithium storage mechanism in this important cathode material for Li-ion batteries.
使用具有高角环形暗场（HAADF）和环形明场（ABF）技术的球差校正扫描透射电子显微镜（STEM），在原子尺度上直接观察到了脱锂的 LiCoO(2)结构。在 ABF 显微照片中，可以清楚地识别出 LiCoO(2)纳米颗粒中有序的 Li、Co 和 O 列。在从 LiCoO(2)中提取 Li 离子后，Co 含量的（003）面从体相扭曲到表面区域，c 轴显著扩展。通过第一性原理模拟直接观察到并解释了锂离子和锂空位的有序化。根据 HAADF 显微照片，发现在第一次电化学 Li 提取后，相从原始 LiCoO(2)中的 O3 型不可逆地转变为 O1 型 Li(x)CoO(2)（x ≈ 0.50），然后再回到 O2 型 Li(x)CoO(2)（x ≈ 0.93），而不是在第一次电化学锂化后回到 O3 堆积。这是首次在 O3-LiCoO(2)相图中发现 O2-Li(x)CoO(2)的报道，通过该报道，将先前分离的 LiCoO(2)相，即 O2 和 O3 系统，连接起来。这些新的研究为锂离子电池中这种重要阴极材料的锂存储机制提供了新的见解。
