Stability and defect structure of spinels Li1 + xMn2 – xO4 – d: I. In situ investigations on the stability field of the spinel phase

The stability field of the Li–Mn–O spinel, Li1 + xMn2 – xO4 – d, was investigated as a function of temperature, T, cation composition, nLi/nMn, and oxygen partial pressure, pO2, by means of in situ X-ray diffraction (XRD) and thermogravimetry (TG). In a T-nLi/nMn phase diagram, the stability field is described by the upper and lower critical temperatures, Tc1 and TcL, respectively. Above Tc1, Li2MnO3 is formed as a second phase, and below TcL Mn2O3 is formed. Both Tc1 and TcL decrease continuously with increasing nLi/ nMn and increase with increasing pO2. The single phase region contains lithium-deficient and lithium-excess spinels, and no discontinuous change of the critical temperature curves was found at nLi/nMn = 0.5, corresponding to LiMn2O4. With the experimental data obtained in this work, a three-dimensional stability field of the Li–Mn–O spinel phase diagram is put forward to describe the relationship between T, nLi/ nMn and pO2. In addition, the upper critical temperature, Tc1, was investigated for spinels doped with Ni, Co and Mg. For all three dopants, Tc1 decreases with increasing dopant concentration.