Li de-intercalation mechanism in LiNi0.5Mn0.5O2 cathode material for Li-ion batteries

Structural changes in Li1−yNi0.5Mn0.5O2 with Li de-intercalation were studied using X-ray diffraction, XAFS, and magnetic measurements. The Li de-intercalation from LiNi0.5Mn0.5O2 proceed mainly by a valence state change of Ni ions from Ni2+ to Ni3+ up to y=0.5 from the XANES and magnetic results. Structural analysis clarified that a second order phase transition from trigonal to monoclinic was observed between y=0.2 and 0.3 and that a small fraction of the Li ions start to migrate from the octahedral 2a site to the tetrahedral 4i site at y=0.3 and most of the Li ions occupied the tetrahedral 4i site at y=0.5 with Li de-intercalation. The EXAFS analysis clarified the valence state change and the range of phase transition during the Li de-intercalation, from the changes in the bond distance of the first neighbor for each transition metal. This Li de-intercalation mechanism is characteristic of this system and has not been reported for Li1−yMO2 (M=Co and Ni).