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

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-Na_0.7CoO₂. 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 Li_0.5CoO₂, is first isolated as an intergrowth phase of O3- and T^#2-Li_0.5CoO₂. However, some deviations from ideal behavior as the O2/O3-intergrowth phase are also noted, presumably because of the existence of stacking faults