Crystal structure and cathode performance dependence on oxygen content of LiMn1.5Ni0.5O4 as a cathode material for secondary lithium batteries

LiMn1.5Ni0.5O4, as a 5 V 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 PO2) 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 4.7–5.0 V. The 4.1 V plateau disappeared for the samples made by the sol–gel method or annealed at a high PO2. The discharge capacity with respect to cycle number of the 5 V 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 PO2 or using the sol–gel method. Furthermore, the distortion of the Mn, Ni(12d)-O octahedral site was restrained by increasing oxygen content.