Structural changes of bare and AlPO4-coated LixCoO2 (x = 0.24 and 0.1) upon thermal annealing ≥200°C

Structural changes of bare and AlPO4-coated LixCoO2 with a coating thickness of 20 and 200 nm are investigated at x = 0.24 and 0.1 after thermal annealing at 200, 300, and 400 °C using XRD and Co K-edge XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure). Both the bare and coated cathodes exhibit faster phase transformation into spinel phases at lower annealing temperatures as x in LixCoO2 is decreased. Bare LixCoO2 cathodes exhibit phase transitions from LixCo2O4 to Co3O4 spinel as the annealing temperature is increased and the x is value decreased, which suggests a possible reaction according to (1/2)LixCo2O4 → xLi2CO3 + (1/3)Co3O4 + (2/3)O2. However, the coated cathodes sustain a LixCo2O4 phase even at 400 °C and x = 0.1. This indicates that the AlPO4 coating layer suppresses the LixCo2O4 phase decomposition into Co3O4.