Li/LiNi0.5Mn0.5O2 cells were cycled galvanostatically to 4.5 and 5.3 V at different current densities. Cycling to 5.3 V was shown to not only improve the reversible capacities and rate capability upon subsequent cycling in the voltage range of 4.5 and 2.0 V, but also to lead to the appearance of a reversible process at 4.3 V. LixNi0.5Mn0.5O2 electrodes exposed initially to 5.3 V were found to exhibit stable, reversible capacities of similar to 190 mAh/g upon subsequent cycling to 4.5 V, which were considerably higher than those of electrodes (similar to 170 mAh/g) cycled only to 4.5 V for the identical number of cycles. The changes in the crystal structure of layered O3 LixNi0.5Mn0.5O2 were investigated by synchrotron X-ray powder diffraction and electron diffraction studies. Rietveld refinement analyses of X-ray diffraction data showed that octahedral nickel migrated from the lithium layer to transition metal layer, and to tetrahedral sites occurred upon cycling to 4.5 V. Increasing the charging voltage to 5.3 V appeared to remove all of the tetrahedral nickel and further increased the occupancy of octahedral nickel in the transition metal layer. The formation of O1 [ ]Ni0.5Mn0.5O2 having hexagonal close-packed oxygen array was revealed in the electrode charged to 5.3 V by X-ray and electron diffraction. The changes in the cation arrangements of the LixNi0.5Mn0.5O2 structure are discussed with respect to lithium reactivity, reversible capacities, and lithium mobility. (c) 2007 The Electrochemical Society.