Relaxation Analysis of LiNi_0.5Mn_1.5O₄ 5 V Cathode Material by Means of the Rietveld Refinement

Abstract

After electrochemically extracting lithium ions from LiNi_0.5Mn_1.5O₄, 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 symmetry coexistence. For both x = 0.1 and 0.2 of Li_xNi_0.5Mn_1.5O₄, 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.