ELECTROCHEMICAL AND INSITU X-RAY-DIFFRACTION STUDIES OF LITHIUM INTERCALATION IN LIXCOO2

Electrochemical properties of LixCoO2 are studied as Li is deintercalated from LiCoO2. High precision voltage measurements and in situ x-ray diffraction indicate a sequence of three distinct phase transitions as x varies from 1 to 0.4. Two of the transitions are situated slightly above and below x = 1/2 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 0.75 less-than-or-equal-to x less-than-or-equal-to 0.93) involving a significant expansion of the c-lattice parameter of the hexagonal unit cell. We report the variation of the lattice constants of LixCoO2 with x and show that the phase transition to the lithium ordered phase near x = 1/2 is accompanied by a lattice distortion to a monoclinic unit cell with a(Mon) = 4.865 (2) angstrom, b(Mon) = 2.806 (1) angstrom, c(Mon) = 14.420 (4) angstrom and beta = 90.77 (3). Finally we report an overall phase diagram for 0.4 less-than-or-equal-to x less-than-or-equal-to 1.0 and -10-degrees-C less-than-or-equal-to T less-than-or-equal-to 60-degrees-C.