LiCoO₂ degradation behavior in the high-voltage phase transition region and improved reversibility with surface coating

The degradation behaviors of bare and Al-oxide coated LiCoO₂ in the high-voltage phase transition region were investigated at the charge voltage of 4.7 V. In both materials, two voltage plateaus that indicate phase transitions from the O₃ to H1-3 and O1 phases were observed in the first charge/discharge. Bare LiCoO2 exhibited considerably decreased capacity, and increased polarization and charge transfer resistance in the cycle test, whereas these changes were remarkably suppressed in the coated LiCoO₂. The phase transitions of the coated LiCoO₂ can be assumed to be fairly reversible, since the voltage plateaus remained even after 20 cycles. After the cycle tests, stacking faults were observed throughout the bare LiCoO₂ particle. Pitting corrosion occurred on the faults, and the formation of a spinel-like layer was observed on the surface of the cycled bare LiCoO₂. The pitting corrosion caused intrinsic capacity fading by Co dissolution. The formation of the spinel-like layer also resulted in effective capacity fading due to the increased polarization. Both the pitting corrosion and the formation of the spinel-like layer were markedly suppressed by the surface coating. Therefore, a surface coating that stabilizes the electrode/electrolyte interface greatly affects the charge/discharge characteristics, even in the high-voltage phase transition region.