Relaxation Analysis of LiNi0.5Mn1.5O4 5V Cathode Material by Means of the Rietveld Refinement

After electrochemically extracting lithium ions from LiNi0.5Mn1.5O4, we have investigated the structural variation during the relaxation process by means of X-ray diffraction coupled with the Rietveld analysis, assuming Li-rich and Li-lean spinel phases with Fd (3) over barm symmetry coexistence. For both x = 0.1 and 0.2 of LiNi0.5Mn1.5O4, mole fraction of Li-lean phase decreases with the relaxation time, although any apparent change in lattice parameter does not occur for both phases. This indicates that Li-lean phase with larger oxygen tetrahedron size, presumably favorable for lithium diffusion, is formed with excess amount during the lithium extraction, and then it transforms into Li-rich and Li-lean phases with smaller lithium concentration during the relaxation process. Slight increase in the size of oxygen tetrahedron and decrease in that of octahedron during relaxation for Li-lean phase of x = 0.1 are also consistent with above phase change. (C) The Electrochemical Society of Japan, All rights reserved.