TEM study of electrochemical cycling-induced damage and disorder in LiCoO2cathodes for rechargeable lithium batteries

Among lithium transition metal oxides used as intercalation electrodes for rechargeable lithium batteries, LiCoO2 is considered to be the most stable in the alpha-NaFeO2 structure type. It has previously been believed that ca tion ordering is unaffected by repeated electrochemical removal and inserti on. We have conducted direct observations, at the particle scale, of damage and cation disorder induced in LiCoO2 cathodes by electrochemical cycling. Using transmission electron microscopy imaging and electron diffraction, i t was found that (i) individual LiCoO2 particles in a cathode cycled from 2 .5 to 4.35 V against a Li anode are subject to widely varying degrees of da mage; (i) cycling induces severe strain, high defect densities, and occasio nal fracture of particles; and (iii) severely strained particles exhibit tw o types of cation disorder, defects on octahedral site layers (including ca tion substitutions and vacancies) as well as a partial transformation to sp inel tetrahedral site ordering. The damage and cation disorder are localize d and have not been detected by conventional bulk characterization techniqu es such as X-ray or neutron diffraction. Cumulative damage of this nature m ay be responsible for property degradation during overcharging or in long-t erm cycling of LiCoO2-based rechargeable lithium batteries. (C) 1999 The El ectrochemical Society. S0013-4651(98)05-079-4. All rights reserved.