Factors influencing the layered to spinel-like phase transition in layered oxide cathodes

Li0.5MO2 [M = Mn, Co, Ni, Ni1-yCoy,Ni1-yMny, and Mn1-y(Cr,Al,Mg)(y)] compositions obtained by chemically extracting lithium with Na2S2O8 from LiMO2 have been investigated by X-ray diffraction after heating at various temperatures. While Li0.5MnO2 obtained from both the monoclinic (O3 stacked layer structure! and orthorhombic LiMnO2 transforms to the spinel-like phases at ambient temperature during the lithium extraction process due to cation migration, Li0.5NiO2 needs mild heat (similar to150degreesC) to transform to the spinel-like phase. Li0.5CoO2 does not transform even after heating at 200degreesC. The decrease in the structural stability in the order Li0.5CoO2 >Li0.5NiO2 > Li0.5MnO2 is explained on the basis of crystal field stabilization energies. Additionally, the incorporation of foreign cations into the transition metal planes via cationic substitutions is found to increase the structural stability by perturbing the cooperativity among the transition metal ions and thereby suppressing the cation migration. (C) 2002 The Electrochemical Society.