CoO2, the end member of the LixCoO2 solid solution

While LiCoO2 has been widely studied in the past 15 years as a promising positive electrode material in lithium-ion batteries, suprisingly, many questions are still unanswered concerning the electrochemical characteristics of the lithium intercalation material. Among these is the existence of an end member CoO2 phase on complete lithium deintercalation. The use of dry plastic lithium-ion battery technology has allowed the construction of an in situ x-ray diffraction cell which allows structural characterization of Li2CoO2 at x values at and close to 0 for the first time. Instead of the expected destruction of the core structure of LiCoO2 by a drastic increase in structural disorder, an increase in crystallographic quality occurred as It approached 0. For the first time, the end member CoO2 phase was isolated. This phase is a hexagonal single-layered phase (O1) believed to be isostructural with CdI2 and has lattice parameters of a = 2.822 Angstrom and c = 4.29 Angstrom. The phase converted immediately back to a three-layer (O3) delithiated Li2CoO2 type phase on lithium reinsertion. Electrochemical studies show that 95% of lithium can be reinserted back into the structure on complete delithiation and reversible cycling properties are maintained when cycled back to 4.2 V.