LiMn1.5Ni0.5O4, as a 5V cathode material, was prepared by changing synthesis method and heat treatment. We investigated the dependence of its properties, crystal structure and cathode performance on the oxygen content. The oxygen content of the samples (sol–gel method and annealed at a high P O2 ) increased in comparison with the sample prepared by the solid-state method. The charge–discharge curves of the samples (solid-state method) had plateaus at 4.1 and 4.7–5.0V. The 4.1V plateau disappeared for the samples made by the sol–gel method or annealed at a high P O2 . The discharge capacity with respect to cycle number of the 5V region in the LiMn1.5Ni0.5O4 (sol–gel method) electrodes shows an especially good cycle performance. The neutron-diffraction intensity profiles of LiMn1.5Ni0.5O4 were analyzed. From these results, the samples are assigned to the ordered form (space group P4332). The amount of the impurity phase decreases by annealing at a high P O2 or using the sol–gel method. Furthermore, the distortion of the Mn, Ni(12d)-O octahedral site was restrained by increasing oxygen content.