Electrochemical and In Situ X‐Ray Diffraction Studies of Lithium Intercalation in Li x CoO2

Electrochemical properties of are studied as Li is deintercalated from . High precision voltage measurements and in situ x‐ray diffraction indicate a sequence of three distinct phase transitions as varies from 1 to 0.4. Two of the transitions are situated slightly above and below and are caused by an order/disorder transition of the lithium ions. The order/disorder transition is studied as a function of temperature allowing the determination of an order/disorder phase diagram. In situ x‐ray diffraction measurements facilitate a direct observation of the effects of deintercalation on the host lattice crystal structure. The other phase transition is shown to be first order (coexisting phases are observed for ) involving a significant expansion of the parameter of the hexagonal unit cell. We report the variation of the lattice constants of with and show that the phase transition to the lithium ordered phase near is accompanied by a lattice distortion to a monoclinic unit cell with , , and . Finally we report an overall phase diagram for and .