Synthesis and characterization of P3-type CoO2-δ

Metastable P3-type CoO2-delta oxides have been synthesized by chemically extracting lithium from the O3-type LiCoO2 with an oxidizer, NO2BF4, in acetonitrile medium. The oxygen content values determined by a redox titration indicate that the Li1-xCoO2-delta system loses oxygen during lithium extraction for 1 - x < 0.5. The oxygen content of the end member CoO2-delta decreases with the lithium extraction reaction time from 1.88 for a reaction time of 1 h to 1.72 for a reaction time of 7 days, indicating chemical instability of the system. The decrease in oxygen content with reaction time is accompanied by a slow transformation of the metastable P3-type phase to the more stable O1-type phase for lithium extraction reaction times of >2 days. The formation of the P3- and O1-type CoO2-delta with lower c-parameters from the initial O3-type LiCoO2 by a gliding of the (CoO2) sheets is attributed to the decreased charge on the oxide ions and the formation of oxygen vacancies. The P3-type CoO2-delta is metallic and reverts back to the semiconducting O3-type LixCoO2 upon lithium reinsertion, with the amount of lithium reinserted scaling with the oxidation state of cobalt in CoO2-delta.