A New Look at Lithium Cobalt Oxide in a Broad Voltage Range for Lithium-Ion Batteries

The electrochemical behaviors and lithium-storage mechanism of LiCoO 2 in a broad voltage window (1.0−4.3 V) are studied by charge−discharge cycling, XRD, XPS, Raman, and HRTEM. It is found that the reduction mechanism of LiCoO 2 with lithium is associated with the irreversible formation of metastable phase Li 1+ x Co II III O 2− y and then the final products of Li 2 O and Co metal. During the charging process, the Li 2 O/Co mixture can be oxidized into CoO, and then the Li 2 O/CoO mixture can result in the formation of Co 3 O 4 in the higher-voltage region. Li x CoO y is the final product when the active material is charged to 4.3 V. During the subsequent cycles, the lithium uptake/release reactions are related to the reversible conversion of Co ↔ CoO ↔ Co 3 O 4 ↔ Li x CoO y .