Electron microscopic characterization of electrochemically cycled LiCoO2 and Li(Al,Co)O-2 battery cathodes

Electrochemical cycling of lithium battery cathodes introduces large absolu te changes in lithium concentration that can result in microstructural dama ge and cation disorder. This damage can influence the long-term performance of battery systems, but has not to date been investigated in detail. We ha ve conducted direct observations using transmission electron microscopy (TE M) of electrochemically cycled LixCoO2 and LixAlyCo1-yO2 cathodes. A rich v ariety of electrochemical cycling-induced damage is found. Intercalation ox ide particles show widely varying degrees of damage suggesting differing de pths of cycling on a local scale. Many particles exhibit severe strain, hig h dislocation densities, and localized fracture. Moreover, in particles tha t are severely strained, electron diffraction reveals two types of cation d isorder brought about by electrochemical cycling. One is the Li and Co subs titution/vacancies on their respective octahedral layers. The second is an evolution towards spinel ordering that is not detectable by bulk analytical methods such as X-ray diffraction (XRD). Unlike previously described 'lith iated spinels,' the observed spinel ordering is characterized by tetrahedra l 8a site occupancy even at compositions near x similar to 1. Thus it is sh own that even in LiCoO2, widely considered to be the most stable of the int ercalation oxides in the layered alpha-NaFeO2 structure, electrochemical cy cling results in a transformation towards spinel cation ordering. Similar r esults are seen in LixAlyCo1-yO2. Cumulative damage of this kind may contri bute to the degradation of LiCoO2 cathodes after long-term cycling. (C) 199 9 Elsevier Science S.A. All rights reserved.