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

Abstract Using synchrotron-based in situ X-ray diffraction, the structural changes of LiCoO 2 , LiCo 0.5 Ni 0.5 O 2 , and LiNi 0.65 Co 0.25 Mg 0.05 Ti 0.05 O 2 cathode materials during charge in the voltage range of 3.5 to 5.2 V have been studied. When a LiCoO 2 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 LiCo 0.5 Ni 0.5 O 2 and LiNi 0.65 Co 0.25 Mg 0.05 Ti 0.05 O 2 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 LiNi 0.65 Co 0.25 Mg 0.05 Ti 0.05 O 2 and 5.0 V for LiCo 0.5 Ni 0.5 O 2 ) were increased with increasing Co content.