͔O 1.96 F 0.04 cathodes ͑x = 0͒ occurring after full delithiation and subsequent heat-treatment at 200, 300, and 400°C using X-ray diffraction ͑XRD͒, transmission electron microscopy, and X-ray absorption spectra ͑XAS͒. In both cathodes, XRD patterns at x = 0 showed a layered O3-type structure ͑CdCl 2 ͒, but subsequent thermal annealing led to a similar structure decomposition to a Co 3 O 4 -type spinel and possible new O3-Ni 1/3 Mn 1/3 O 2+␦ -type layered phases. However, the XAS spectra

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... y showed that the Li x ͓Ni 0.33 Co 0.33 Mn 0.3 Mg 0.04 ͔O 1.96 F 0.04 exhibited much improved structural stability, showing a suppression of the phase decomposition to spinel Co 3 O 4 , as evidenced by a decrease in the FT peak intensity of corner-shared CoO 6 octahedra at ϳ3.2 Å.