We focused on LiMn1.5Ni0.5O4 with the spinel structure, and performed a supersonic-wave treatment in Zn2+ aqueous solution and a subsequent heat-treatment at lower temperature than the sintering temperature. From the X-ray diffraction, ICP, potentiometric titration and TEM-EDX measurements, it was indicated that the samples after the treatments had a single phase of the spinel structure and Zn2+ was substituted partially for the transition-metals. Such a partial substitution was also suggested by the Rietveld analysis using neutron diffractions. As for these samples, charge–discharge cycle tests were carried out at room temperature and 45 °C. These experiments demonstrated that the treatments in Zn2+ aqueous solution could improve the cycle performance although they deteriorated first discharge capacities slightly. Such an improvement of the cathode properties may be attributed to the structural change induced by the supersonic-wave treatments.