Surface and bulk structure investigation of fully delithiated bare and AlPO4-coated LixCoO2 (x = 0) cathode materials annealed between 200 and 400 °C

The surface and bulk structure of the fully delithiated bare and AlPO4-coated LixCoO2 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 CoO2 cathodes showed different phase transformation between 200 and 400 °C. The bare samples exhibited only the O1-CoO2 phase in the fully delithiated state, but the CoO2 phase was converted to the Co3O4 phase at higher temperatures. On the other hand, the AlPO4-coated LixCoO2 at x = 0 showed a mixture of phases including the major O1-type CoO2 and minor Co3O4 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 Co3O4 and were completely transformed into the thermally stable Co3O4 spinel phase at 400 °C. The coated samples, however, were completely transformed into the spinel phase alone at 200 °C. We believe that the AlPO4 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.