Surface and bulk structure investigation of fully delithiated bare and AlPO 4-coated Li xCoO 2 (x = 0) cathode materials annealed between 200 and 400 °C
The surface and bulk structure of the fully delithiated bare and AlPO 4-coated Li xCoO 2 cathode materials (x = 0) between 200 and 400 °C were investigated using high resolution XRD, XAS (X-ray absorption spectroscopy), TEM, and solid-state NMR. Bare and coated CoO 2 cathodes showed different phase transformation between 200 and 400 °C. The bare samples exhibited only the O1-CoO 2 phase in the fully delithiated state, but the CoO 2 phase was converted to the Co 3O 4 phase at higher temperatures. On the other hand, the AlPO 4-coated Li xCoO 2 at x = 0 showed a mixture of phases including the major O1-type CoO 2 and minor Co 3O 4 spinel phases in the fully delithiated state. When annealing temperatures increased from 200 to 400 °C, the bare samples showed a slow phase transition to Co 3O 4 and were completely transformed into the thermally stable Co 3O 4 spinel phase at 400 °C. The coated samples, however, were completely transformed into the spinel phase alone at 200 °C. We believe that the AlPO 4 coating layer accelerated the phase transition to the spinel phase at lower temperatures. TEM and XAS results showed that structural damages were mainly confined to the cathode particle surface, and this phenomenon was only observed in the bare samples.