TEM Study of Electrochemical Cycling‐Induced Damage and Disorder in LiCoO2 Cathodes for Rechargeable Lithium Batteries

Among lithium transition metal oxides used as intercalation electrodes for rechargeable lithium batteries, LiCoO{sub 2} is considered to be the most stable in the {alpha}-NaFeO{sub 2} structure type. It has previously been believed that cation ordering is unaffected by repeated electrochemical removal and insertion. The authors have conducted direct observations, at the particle scale, of damage and cation disorder induced in LiCoO{sub 2} cathodes by electrochemical cycling. Using transmission electron microscopy imaging and electron diffraction, it was found that (1) individual LiCoO{sub 2} particles in a cathode cycled from 1.5 to 4.35 V against a Li anode are subject to widely varying degrees of damage; (2) cycling induces severe strain, high defect densities, and occasional fracture of particles; and (3) severely strained particles exhibit two types of cation disorder, defects on octahedral site layers (including cation substitutions and vacancies) as well as a partial transformation to spinel tetrahedral site ordering. The damage and cation disorder are localized and have not been detected by conventional bulk characterization techniques such as X-ray or neutron diffraction. Cumulative damage of this nature may be responsible for property degradation during overcharging or in long-term cycling of LiCoO{sub 2}-based rechargeable lithium batteries.