Surface chemistry study of LiCoO2 coated with alumina

摘要

The surface chemistry of the Al2O3-coated LiCoO2 prepared under heat treatment at 80-800°C was studied by SEM, XRD, IR, XPS and NMR spectroscopy combined with electrochemical testing. It is shown that the particle bulk is free from dopant. XPS A12s core-level spectrum of the coated LiCoO2 differs from that of α-Al2O3 and is associated with Al oxyhydroxide. 27Al NMR spectra of the samples prepared at 80-600°C consist of intensive asymmetric peak with maximum at 4.8-6.2 ppm (Al in an octahedral coordination free from Co in the second coordination sphere) and two less intensive ones at ～55-56 and ～61-62 ppm. On the other hand, the spectrum of the 800°C sample has no peak near 0 ppm, but has a series of peaks in the range of 26～62 ppm with an interval of about 7 ppm, characteristic of Al octahedral environment with neighboring Al number ranging from 6 to 0. This indicates that the shell consists mostly of amorphous oxyhydroxide in the first case and of LiCo(1-y)AlyO2 solid solution in the second. Electrochemical testing shows a significant increase in discharge capacity (>150 mAh/g) and improved cyclability of the coated samples heated below 600°C, while after heat treatment at 800°C, the capacity of composite cathode fades quickly.