Electrochemical and in situ synchrotron XRD studies on Al2O3-coated LiCoO2 cathode material

Using a commercial available LiCoO2 as starting material, surface-modified cathode material was obtained by coating its surface with a nanosize layer of amorphous Al2O3. Electrochemical performances and structural evolutions of the modified cathode material were characterized and compared with that of pristine LiCoO2. Specific capacity of 190 mAh/g was obtained in Li/ (Al2O3-coated LiCoO2) experimental cells when charged to 4.5 V. The relationship between the structural evolution and the electrochemical performances at overcharged state was investigated using in situ synchrotron X-ray diffraction (XRD). After charging the half-cell using uncoated LiCoO2 as cathode, the variation of the c value in hexagonal structure is significantly reduced during subsequent cycles. This reduction in c variation range is concurrently related to the capacity fading. In contrast, the variation range of c is preserved in the Al2O3-coated cathodes, and so is capacity. Based on these results, a capacity fading mechanism of LiCoO2-type cathode materials after overcharge is proposed, and an explanation of the surface coating effects is given. (C) 2004 The Electrochemical Society.