A Comparative Study of LiCoO2 Polymorphs: Structural and Electrochemical Characterization of O2‑, O3‑, and O4-type Phases

• Published in: Inorganic chemistry 2013-08, Vol.52 (15), p.9131-9142
• Main Authors: Yabuuchi, Naoaki, Kawamoto, Yuta, Hara, Ryo, Ishigaki, Toru, Hoshikawa, Akinori, Yonemura, Masao, Kamiyama, Takashi, Komaba, Shinichi
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic4013922

O4-type LiCoO2 as a third polymorph of LiCoO2 is prepared by an ion-exchange method in aqueous media from OP4-[Li, Na]­CoO2, which has an intergrowth structure of O3-LiCoO2 and P2-Na0.7CoO2. O4-type LiCoO2 is characterized by synchrotron X-ray diffraction, neutron diffraction, and X-ray absorption spectroscopy. Structural characterization reveals that O4-type LiCoO2 has an intergrowth structure of O3- and O2-LiCoO2 with stacking faulted domains. Three LiCoO2 polymorphs are formed from the close-packed CoO2 layers, which consist of edge-shared CoO6 octahedra, whereas the oxide-ion stacking is different: cubic in the O3-phase, cubic/hexagonal in the O2-phase, and alternate O3 and O2 in the O4-phase. Structural analysis using the DIFFaX program suggests that the O4-phase consists of approximately 30% of O12-domains, while stacking faults are not evidenced for O2-phase. The results suggest that a nucleation process for Na/Li ion-exchange kinetically dominates a growth process of ideal O4-domains because the presence of CoO2–Li–CoO2 blocks as O3-domains could be expected to prevent through-plane interaction of Na layers. Electrochemical behavior and structural transition processes for three LiCoO2 polymorphs are compared in Li cells. A new phase, OT#4-type Li0.5CoO2, is first isolated as an intergrowth phase of O3- and T#2-Li0.5CoO2. However, some deviations from ideal behavior as the O2/O3-intergrowth phase are also noted, presumably because of the existence of stacking faults.