Real-Time Crystallization of LiCoO2from β-Co(OH)2and Co3O4: Synthetic Pathways and Structural Evolution

Cobalt oxides are technologically important materials, especially when lithiated for application as Li-ion cathodes. However, several phases may crystallize during solid-state synthesis in the Li–Co–O–H system. The solid-state reactions of LiOH·H2O with both β-Co(OH)2 and Co3O4 have been investigated here through the combined use of high-resolution in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy, with a special focus on the low-temperature range (RT–600 °C). We show that several spinel phases (AB2O4 and A2B2O4 with A, B = Li, Co) are formed in the range 300–525 °C, whose unambiguous identification is only possible through the complementary use of Raman spectroscopy as their XRD patterns are almost identical. While the various structures evidenced are mostly stabilized over similar temperature ranges regardless of the initial choice of Co precursor, the lithiated spinel phases are observed at lower temperatures for samples synthesized from Co(OH)2. Moreover, their respective Li and Co fractions and crystallinity are strongly affected by the initial choice of precursor. These findings have strong implications in the preparation and optimization of both Co-based cathode active materials and Co-based coatings for other cathode active materials.