Phase transitions and high-voltage electrochemical behavior of LiCoO2 thin films grown by pulsed laser deposition

The phase transitions and electrochemical behavior of LiCoO2 thin-film cathodes prepared by pulsed laser deposition are studied for charging voltages ranging from 4.2 to 4.9 V. Two voltage plateaus at about 4.55 and 4.62 V are observed in the charge-discharge curves. Ex situ X-ray diffraction measurements confirm structural changes and a phase transition from the O3 to the H1-3 phase when LiCoO2 is charged above 4.5 V. Thin-film LiCoO2 electrodes show stable capacity retention up to 4.5 V but significant capacity fade above 4.5 V, which we attribute to damage from strain due to phase transitions and Li gradients. Li diffusion in the LixCoO2 thin film up to 4.5 V is investigated by the potentiostatic intermittent titration technique. The chemical diffusion coefficient of Li reaches the maximum near x=0.4 and starts to decrease when the Li content is further decreased.