New Insight into the Atomic Structure of Electrochemically Delithiated O3-Li(1–x)CoO2 (0 ≤ x ≤ 0.5) Nanoparticles

• Published in: Nano letters Vol. 12; no. 12; pp. 6192 - 6197
• Main Authors: Lu, Xia, Sun, Yang, Jian, Zelang, He, Xiaoqing, Gu, Lin, Hu, Yong-Sheng, Li, Hong, Wang, Zhaoxiang, Chen, Wen, Duan, Xiaofeng, Chen, Liquan, Maier, Joachim, Tsukimoto, Susumu, Ikuhara, Yuichi
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 12.12.2012
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Materials science; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals; Physics; Specific phase transitions; Structural transitions in nanoscale materials; Structure of solids and liquids; crystallography; scanning transmission electron microscopy (STEM); LiCoO2; structural evolution; Li-ion batteries; Cobalt oxide; Order-disorder transformations; Phase diagrams; Vacancies; Cobalt Lithium Oxides Mixed; Stacking sequence; Phase transformations; Digital simulation; Lithium; Electrode material; Lithium battery; Nanoparticles; Aberrations; Ordering; Scanning transmission electron microscopy; Atomic structure; Nanostructured materials; Crystal structure
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl303036e

Direct observation of delithiated structures of LiCoO2 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 LiCoO2 nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO2, 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 LiCoO2 to O1-type Li x CoO2 (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type Li x CoO2 (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-Li x CoO2 in the phase diagram of O3-LiCoO2, through which the two previously separated LiCoO2 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.