Li1-yNi0.5Mn0.4Ti0.1O2 (y=0 and 0.5) was synthesized and characterized using X-ray diffraction, XAFS, and SQUID measurements. The samples were single-phase and adopted the alpha-NaFeO2 structure. Li1-yNi0.5Mn0.4Ti0.1O2 (y=0 and 0.5) can be represented as Li(Ni0.52+Mn0.44+Ti0.14+)O-2 and Li-0.5(Ni0.53+Mn0.44+Ti0.14+)O-2, respectively. Structural analysis demonstrated that the lattice parameter a decreased from 2.895 to 2.856 Angstrom, the lattice parameter a increased from 14.317 to 14.509 Angstrom, and the Ni-O bond length decreased from 2.06 to 1.94 Angstrom with de-lithiation. The low occupation of Ni on the 3a site was confirmed from the ferromagnetic behavior caused by the 180degrees Ni2+(3a)-O-Mn4+(3b)-O-NI2+ (3a) superexchange interaction. These results indicated that lithium de-intercalation from LiNi0.5Mn0.4Ti0.1O2 was controlled mainly by changing the valence state of Ni from 2+ to 3+. (C) 2004 Elsevier B.V. All rights reserved.