The electrochemical properties and crystal structure of LiMn1.5Ni0.5O4 treated with supersonic waves in an aqueous Ni-containing solution were investigated by performing charge–discharge tests, inductively coupled plasma (ICP) analysis, scanning electron microscopy (SEM), iodometry, X-ray diffraction (XRD), powder neutron diffraction and synchrotron powder XRD. The charge–discharge curve of LiMn1.5Ni0.5O4 versus Li/Li+ has plateaus at 4.1 and 4.7V. The 4.1V versus Li/Li+ plateau due to the oxidation of Mn3+/4+ was reduced by the supersonic treatment. During the charge–discharge cycling test at 25°C, the supersonic treatment increased the discharge capacity of the 50th cycle. Rietveld analysis of the neutron diffraction patterns revealed that the Ni occupancy of the 4b site in LiMn1.5Mn0.5O4, which is mainly occupied by Ni, was increased by the supersonic treatment. This result suggests that Ni2+ is partially substituted for Mn3+/4+ during the supersonic treatment.