LiCoO[sub 2] Cathode Material That Does Not Show a Phase Transition from Hexagonal to Monoclinic Phase

Structural instability of LiCoO 2 can be improved by sol-gel coating of Al 2 O 3 and subsequent heat-treatments. While Al 2 O 3 phase does not exist after heat-treatments, solid solution LiCo 1-x Al x O 2 that has discretely higher Al concentration was formed at the surface up to ∼500 A inside the particle. However, heat-treatment to 700°C results in the presence of the solid solution beyond ∼500 A. The different Al concentration at the surface significantly affects the structural stability of the materials during cycling, and those prepared at 400°C do not show a phase transition from hexagonal to monoclinic phase. Disappearance of such a phase transition improves capacity retention of the cathode. Moreover, cathodes prepared at 400 and 500°C show improved layered characteristics with cation order.