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

Abstract

LiCoO 2 (R3m; a = 2.82 Å , c = 14.1 Å in hexagonal setting) was prepared and examined in nonaqueous lithium cells using 1 M LiClO 4 propylene carbonate solution at 30°C. The oxidation of LiCoO 2 and the reduction of Li 1 − x CoO 2 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 ( 0 < x < 1 / 4 and 3 / 4 < x < 1 in Li 1 − x CoO 2 ) and a single‐phase reaction ( 1 / 4 < x < 3 / 4 ) . A monoclinic phase ( a = 4.91 Å , b = 2.82 Å , c = 5.02 Å , and β = 111.4 ° ) was observed in 3 / 4 < x < 1 in addition to that at about x = 0.45 ) a = 4.90 Å , b = 2.81 Å , c = 5.05 Å , and β = 108.3 ° ) . Detailed open‐circuit voltage measurements were carried out. The open‐circuit voltage are invariable at 3.92 V for 0 < x < 1 / 4 and at 4.50 V for 3 / 4 < x < 1 . The two straight lines are connected smoothly by a composition‐dependent curve for 1 / 4 < x < 3 / 4 , which was consistent with the XRD observations. The differences and similarities between the solid‐state redox reactions of LiCoO 2 and LiNiO 2 were discussed by comparing the structural and electrochemical data. Possible lithium ordering at x = 1 / 4 and 3 / 4 for this type of material is described in terms of a [ 2 × 2 ] superlattice in a triangular lattice of sites.