We investigated the relation between the cycle performance and crystal structural change during the charge-discharge process of LiMn1.5Ni0.5O4 as a 5 V class cathode active material, which was prepared by changing the calcination temperature using the sol-gel method. The lithium content of (Li1-xMn15Ni0.5O4)-Ni-. (x=0.5, 0.7, 1.0) was controlled by electrochemical lithium extraction. The crystal structure was determined by Rietveld analysis using powder neutron diffraction. As a result, all samples consisted of three phases (space group: P4(3)32) of different lattice constants and Ni valences. The main phase, which has the maximum percentage, was shifted to a phase with a lower lattice constant with the decreasing lithium content, and then finally Li1-xMn1.5Ni0.5O4 (x=1.0) was almost oxidized to Ni4+ by a charging process. Furthermore, LiMn1.5Ni0.5O4,, by changing the synthesis temperature, was different for a few oxidation processes; the structure of the phase at Ni3+ was not stable based on the distortion of each phase and the Madelung energy. It was suggested that these factors should provide an effective cycle performance. (C) 2004 Elsevier B.V. All rights reserved.