This study compared the structural changes in Li-x[Ni1/3Co1/3Mn1/3]O-2 and Li-x[Ni0.33Co0.33Mn0.3Mg0.04]O1.96F0.04 cathodes (x = 0) occurring after full delithiation and subsequent heat-treatment at 200, 300, and 400 degrees C using X-ray diffraction (XRD), transmission electron microscopy, and X-ray absorption spectra (XAS). In both cathodes, XRD patterns at x = 0 showed a layered O3-type structure (CdCl2), but subsequent thermal annealing led to a similar structure decomposition to a Co3O4-type spinel and possible new O3-Ni1/3Mn1/3O2+delta-type layered phases. However, the XAS spectra clearly showed that the Li-x[Ni0.33Co0.33Mn0.3Mg0.04]O1.96F0.04 exhibited much improved structural stability, showing a suppression of the phase decomposition to spinel Co3O4, as evidenced by a decrease in the FT peak intensity of corner-shared CoO6 octahedra at similar to 3.2 angstrom. (c) 2007 The Electrochemical Society.