New phases and phase transitions observed in over-charged states of LiCoO2-based cathode materials

Using synchrotron-based in situ X-ray diffraction, the structural changes of LiCoO2, LiCo0.5Ni0.5O2, and LiNi0.65Co0.25Mg0.05Ti0.05O2 cathode materials during charge in the voltage range of 3.5 to 5.2 V have been studied. When a LiCoO2 cathode was charged above 4.5 V, a new intermediate phase O1a was observed, before the terminal phase O1 was formed around 4.8 V. In the X-ray diffraction spectra for LiCo0.5Ni0.5O2 and LiNi0.65Co0.25Mg0.05Ti0.05O2 cathode materials, Bragg peaks representing the O1 structure were also observed. The amounts of O1 structure formed at the end of charge (5.2 V for LiNi0.65Co0.25Mg0.05Ti0.05O2 and 5.0 V for LiCo0.5Ni0.5O2) were increased with increasing Co content.