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 5.2V have been studied. When a LiCoO2 cathode was charged above 4.5V, a new intermediate phase O1a was observed, before the terminal phase O1 was formed around 4.8V. 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 (5.2V for LiNi0.65Co0.25Mg0.05Ti0.05O2 and 5.0V for LiCo0.5Ni0.5O2) were increased with increasing Co content.