Solid‐State Redox Reactions of LiCoO2 (R3m) for 4 Volt Secondary Lithium Cells

(R3m; in hexagonal setting) was prepared and examined in nonaqueous lithium cells using 1 M propylene carbonate solution at 30°C. The oxidation of and the reduction of proceeded reversibly in the voltage region above 3.9 V. X‐ray diffraction (XRD) examinations indicated the reaction proceeded in a topotactic manner, i.e. , two‐phase reactions and a single‐phase reaction . A monoclinic phase was observed in in addition to that at about . Detailed open‐circuit voltage measurements were carried out. The open‐circuit voltage are invariable at 3.92 V for and at 4.50 V for . The two straight lines are connected smoothly by a composition‐dependent curve for , which was consistent with the XRD observations. The differences and similarities between the solid‐state redox reactions of and were discussed by comparing the structural and electrochemical data. Possible lithium ordering at for this type of material is described in terms of a superlattice in a triangular lattice of sites.