CoO₂, The End Member of the Li_x CoO₂ Solid Solution

While 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 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 at x values at and close to 0 for the first time. Instead of the expected destruction of the core structure of by a drastic increase in structural disorder, an increase in crystallographic quality occurred as x approached 0. For the first time, the end member phase was isolated. This phase is a hexagonal single‐layered phase (O1) believed to be isostructural with and has lattice parameters of a = 2.822 Å and c = 4.29 Å. The phase converted immediately back to a three‐layer (O3) delithiated 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.