Direct observation of the partial formation of a framework structure for Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2 upon the first charge and discharge

The structural changes upon the first charge and discharge of a Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O-2 were investigated using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and selected-area electron diffraction (SAED). Atomic resolution STEM observations revealed that some of the transition metal (TM) atoms were transferred from the TM layers to the Li layers upon the first charge and discharge, leading to the partial formation of a framework structure. This framework structure was considered as a spinel structure based on our simulation results of the corresponding SAED pattern of the fully charged state. This framework structure was also recognized even at the early stage of the first charging process in the plateau region around 4.5 V by using the SAED patterns, indicating that the formation of this framework structure started at the same time as the electrochemical activation. (C) 2011 Elsevier B.V. All rights reserved.