Structural changes in Li1-yNi0.5Mn0.5O2 With Li de-intercalation were studied using X-ray diffraction, XAFS, and magnetic measurements. The Li de-intercalation from LiNi0.5Mn0.5O2 proceed mainly by a valence state change of Ni ions from Ni2+ to Ni3+ Up to y=0.5 from the XANES and magnetic results. Structural analysis clarified that a second order phase transition from trigonal to monoclinic was observed between Y=0.2 and 0.3 and that a small fraction of the Li ions start to migrate from the octahedral 2a site to the tetrahedral 4i site at y=0.3 and most of the Li ions occupied the tetrahedral 4i site at y=0.5 with Li de-intercalation. The EXAFS analysis clarified the valence state change and the range of phase transition during the Li de-intercalation, from the changes in the bond distance of the first neighbor for each transition metal. This Li de-intercalation mechanism is characteristic of this system and has not been reported for Li1-yMO2 (M=Co and Ni). (c) 2004 Elsevier B.V. All rights reserved.