LiCoO2 Degradation Behavior in the High-Voltage Phase Transition Region and Improved Reversibility with Surface Coating

Abstract

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.